Minimize Hubble

HST (Hubble Space Telescope) Mission

Background   Sensor Complement   HST Imagery    Hubble Servicing Missions    References

The HST (Hubble Space Telescope) of NASA is named in honor of the American astronomer Edwin Hubble (1889-1953), Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the big-bang theory. Hubble, the observatory, is the first major optical telescope to be placed in space, the ultimate mountaintop. Above the distortion of the atmosphere, far far above rain clouds and light pollution, Hubble has an unobstructed view of the universe. Scientists have used Hubble to observe the most distant stars and galaxies as well as the planets in our solar system. 1)

The planning for HST started in the early 1970s. The HST was launched into LEO (Low Earth Orbit) on April 24, 1990 on STS-31 (12:33:51 UTC, on Shuttle Discovery). Hubble is operational as of 2018, in its 29th year on orbit, and is one of NASA's Great Observatories. Hubble's launch and deployment in April 1990 marked the most significant advance in astronomy since Galileo's telescope. Thanks to five servicing missions and more than 25 years of operation, our view of the universe and our place within it has never been the same.


• Deployment of Hubble: April 25, 1990

• First Image: May 20, 1990: Star cluster NGC 3532

• Servicing Mission 1 (STS-61): December 1993

• Servicing Mission 2 (STS-82): February 1997

• Servicing Mission 3A (STS-103): December 1999

• Servicing Mission 3B (STS-109): February 2002

• Servicing Mission 4 (STS-125): May 2009

Spacecraft: The spacecraft has a length of 13.2 m, a mass at launch of 10,886 kg, post SM (Servicing Mission) 4 of 12,247 kg, and a maximum diameter of 4.2 m.

Orbit: LEO with an altitude of 547 km, an inclination of 28.5º, and a period of 95 minutes.

The HST (Hubble Space Telescope) of NASA features a ULE TM(Ultra-Low Expansion)primary mirror of 2.4 m diameter (f/24 Ritchey-Chretien) and a 0.3 m Zerodur secondary mirror. The HST primary mirror was a lightweighted monolithic design (824 kg) by Perkin-Elmer (now Goodrich Inc.), Danbury, CN, using a lightweight, thick egg crate core sandwiched between two plates and fused together.

The HST is the most precisely pointed instrument in spaceborne astronomy. The pointing requirements call for a continuous 24 hour target lock maintenance of 0.007 arcseconds (2 millionth degree).


Figure 1: IMAX Cargo Bay Camera view of the Hubble Space Telescope at the moment of release, mission STS-31 in April 1990 (image credit: NASA)

Some background:

The telescope's original equipment package included the Wide Field/Planetary Camera (WF/PC), Goddard High Resolution Spectograph (GHRS), Faint Object Camera (FOC), Faint Object Spectograph (FOS), and High Speed Photometer (HSP). 2) 3)

After a few weeks of operation, scientists noticed that images being sent back from Hubble were slightly blurred. While this distortion still allowed scientists to study the cosmos and make significant discoveries, it resulted in less spectacular images, and some of the original mission could not be fulfilled. An investigation finally revealed a spherical aberration in the primary mirror, due to a miscalibrated measuring instrument that caused the edges of the mirror to be ground slightly too flat. Engineers rushed to come up with a fix to the problem in time for Hubble's first scheduled servicing mission in 1993. The system designed to correct the error was designated COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR was a set of optics that compensated for the aberration and would allow all of Hubble's instruments to function normally.

In December, 1993, the crew of STS-61 embarked on a service mission to replace a number of Hubble's parts. Following intensive training on the use of new tools never used before in space, two teams of astronauts completed repairs during a record five back-to-back spacewalks. During the EVAs, COSTAR was installed and the Wide Field/Planetary Camera was replaced with the Wide Field/Planetary Camera 2, which was designed to compensate for the mirror problem. The team also performed basic maintenance on the craft, installed new solar arrays, and replaced four of Hubble's gyroscopes.

Shortly after the crew returned to Earth and the Hubble Space Telescope began returning sharp and spectacular images, NASA deemed the servicing mission a success. Astronomers could now take advantage of a fully functional space telescope, and the public was treated to breathtaking photos of stars, galaxies, nebulae, and other deep-space objects. Subsequent servicing missions improved Hubble's capabilities and performed routine repairs.

In February, 1997, the crew of STS-82 installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectograph (STIS) to detect infrared light from deep-space objects and take detailed photos of celestial objects. Servicing mission 3A in December, 1999 replaced all six of the telescope's aging gyroscopes, which accurately point the telescope at its target. STS-103 astronauts also replaced one of the telescope's three fine guidance sensors and installed a new computer, all in time to redeploy Hubble into orbit on Christmas Day. The most recent servicing mission to the spacecraft, servicing mission 3B, came aboard STS-109 in March, 2002. Columbia crewmembers installed the new Advanced Camera for Surveys (ACS), which had sharper vision, a wider field of view, and quicker data gathering than the Wide Field/Planetary Camera 2. Astronauts also replaced Hubble's solar panels with a more efficient array and conducted repairs on the NICMOS.


Figure 2: This photograph of NASA's Hubble Space Telescope was taken on the fifth servicing mission to the observatory in May 2009 (image credit: NASA)


Figure 3: Artist's view of the HST in space along with the designation of the key element locations (image credit: NASA)


Note: At this stage of the mission (2018), no attempt is being made to recover all facets of Hubble regarding the spacecraft, instrumentation and the past history (it would have required a constant accompaniment of the mission with all updates over its lifetime). Instead, some fairly recent images of the mission are presented.

The Hubble Servicing Missions are shortly described in the last chapter of this file.



HST sensor complement: (ACS, WFC3, STIS, COS, FGS, NICMOS)

The Hubble Space Telescope has three types of instruments that analyze light from the universe: cameras, spectrographs and interferometers. 4)


Figure 4: Hubble's scientific instruments analyze different types of light ranging from ultraviolet (UV) to infrared (IR). This graphic shows which wavelengths each instrument studies (image credit: NASA)



Hubble has two primary camera systems to capture images of the cosmos. Called the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3), these two systems work together to provide superb wide-field imaging over a broad range of wavelengths.

ACS (Advanced Camera for Surveys)

Installed on Hubble in 2002, ACS was designed primarily for wide-field imagery in visible wavelengths, although it can also detect ultraviolet and near-infrared light. ACS has three cameras, called channels, that capture different types of images. An electronics failure in January 2007 rendered the two most-used science channels inoperable. In 2009, astronauts were able to repair one of the channels and restored ACS's capacity to capture high-resolution, wide-field views.

WFC3 (Wide Field Camera 3)

Installed in 2009, WFC3 provides wide-field imagery in ultraviolet, visible and infrared light. WFC3 was designed to complement ACS and expand the imaging capabilities of Hubble in general. While ACS is primarily used for visible-light imaging, WFC3 probes deeper into infrared and ultraviolet wavelengths, providing a more complete view of the cosmos.


Figure 5: Astronaut Andrew Feustel prepares to install WFC3 (Wide Field Camera 3) on Hubble during Servicing Mission 4 in 2009 (image credit: NASA)



Spectrographs practice spectroscopy, the science of breaking light down to its component parts, similar to how a prism splits white light into a rainbow. Any object that absorbs or emits light can be studied with a spectrograph to determine characteristics such as temperature, density, chemical composition and velocity.

Hubble currently utilizes two spectrographs: COS (Cosmic Origins Spectrograph) and the STIS (Space Telescope Imaging Spectrograph). COS and STIS are complementary instruments that provide scientists with detailed spectral data for a variety of celestial objects. While STIS is a versatile, "all purpose" spectrograph that handles bright objects well, COS measures exceedingly faint levels of ultraviolet light emanating from distant cosmic sources, such as quasars in remote galaxies. Working together, the two spectrographs provide a full set of spectroscopic tools for astrophysical research.


Figure 6: Hubble's STIS captured a spectrum (right) of material ejected by a pair of massive stars called Eta Carinae, while the Wide Field and Planetary Camera 2 took an image of the billowing clouds of gas enveloping the stellar pair (left). The spectrum reveals that one of the lobes contains the elements helium (He), argon (Ar), iron (Fe) and nickel (Ni), image credit: NASA, ESA and the Hubble SM4 ERO Team



Hubble's interferometers serve a dual purpose — they help the telescope maintain a steady aim and also serve as a scientific instrument. The three interferometers aboard Hubble are called the FGS (Fine Guidance Sensors). The Fine Guidance Sensors measure the relative positions and brightnesses of stars.

When Hubble is pointing at a target, two of the three Fine Guidance Sensors are used to lock the telescope onto the target. For certain observations, the third Fine Guidance Sensor can be used to gather scientific information about a target, such as a celestial object's angular diameter or star positions that are ten times more accurate than those obtained by ground-based telescopes.

The Fine Guidance Sensors are very sensitive instruments. They seek out stable point sources of light (known as "guide stars") and then lock onto them to keep the telescope pointing steadily. When a light in the sky is not a point source, the Fine Guidance Sensor cannot lock on and so it rejects the guide star. Often, a rejected guide star is actually a faraway galaxy or a double-star system. Since Hubble was launched in 1990, the Fine Guidance Sensors have detected hundreds of double-star systems that were previously thought to be single stars.


Past Instruments

Only one of the instruments remaining on Hubble — the third Fine Guidance Sensor — was launched with the observatory in 1990. The rest of the instruments were installed during Hubble's five servicing missions. In addition to installing new instruments, astronauts also repaired two instruments (ACS and STIS) while visiting Hubble on Servicing Mission 4 in 2009. The NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) on Hubble is in hibernation following a cryocooler anomaly, but most of its infrared duties have since been taken over by WFC3.

Hubble's past instruments include:

• High Speed Photometer

• Faint Object Camera

• Faint Object Spectrograph

• Goddard High Resolution Spectrograph

• Wide Field and Planetary Camera

• Wide Field and Planetary Camera 2

• Fine Guidance Sensors (three).


Current Instruments

ACS (Advanced Camera for Surveys) - ACS is a third-generation imaging camera. This camera is optimized to perform surveys or broad imaging campaigns. ACS replaced Hubble's Faint Object Camera (FOC) during Servicing Mission 3B. Its wavelength range extends from the ultraviolet, through the visible and out to the near-infrared (115-1050 nm). ACS has increased Hubble's potential for new discoveries by a factor of ten.

COS (Cosmic Origins Spectrograph) - COS focuses exclusively on ultraviolet (UV) light and is the most sensitive ultraviolet spectrograph ever, increasing the sensitivity at least 10 times in the UV spectrum and up to 70 times when looking at extremely faint objects. It is best at observing points of light, like stars and quasars. COS was installed during during Servicing Mission 4 in May 2009.

STIS (Space Telescope Imaging Spectrograph) - STIS is a second-generation imager/spectrograph. STIS is used to obtain high resolution spectra of resolved objects. STIS has the special ability to simultaneously obtain spectra from many different points along a target. The STIS instrument has a mass of 318 kg and a wavelength range of 115-1000 nm.

STIS spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. Its spectrograph can be switched between two different modes of usage:

1) So-called "long slit spectroscopy" where spectra of many different points across an object are obtained simultaneously.

2) So-called "echelle spectroscopy" where the spectrum of one object is spread over the detector giving better wavelength resolution in a single exposure.

STIS also has a so-called coronagraph which can block light from bright objects, and in this way enables investigations of nearby fainter objects.

WFC3 (Wide Field Camera 3) - Wide Field Camera 3 is the main imager on the telescope. It has a camera that records visible and ultraviolet (UVIS, 200-1000 nm) wavelengths of light and is 35 times more sensitive in the UV wavelengths than its predecessor. A second camera that is built to view infrared (NIR, 850-1700 nm) light increases Hubble's IR resolution from 65,000 to 1 million pixels. Its combination of field-of-view, sensitivity, and low detector noise results in a 15-20 time improvement over Hubble's previous IR camera. WFC3 was jointly developed at GSFC, STSI (Space Telescope Science Institute) in Baltimore and Ball Aerospace & Technologies Corporation in Boulder, CO. 5)

FGS (Fine Guidance Sensor) – The FGS provides pointing information for the spacecraft by locking onto guide stars. The FGS can also function as a scientific instrument by precisely measuring the relative positions of stars, detecting rapid changes in a star's brightness, and resolving double-star systems that appear as point sources even to Hubble's cameras. Hubble has three FGSs onboard the observatory.

NICMOS (Near Infrared Camera and Multi-Object Spectrometer) – NICMOS has the ability to obtain images and spectroscopic observations of astronomical targets at near-infrared wavelengths. Although NICMOS is currently inactive, most of its functionality is replaced by Hubble's other science instruments.



HST (Hubble Space Telescope) -Status and some observation imagery

• 18 October 2018: New observations by two Arizona State University astronomers using the Hubble Space Telescope have caught a red dwarf star in a violent outburst, or superflare. The blast of radiation was more powerful than any such outburst ever detected from the Sun, and would likely affect the habitability of any planets orbiting it. 6)

- Moreover, the astronomers say, such superflares appear more common in younger red dwarfs, which erupt 100 to 1000 times more powerfully than they will when they age.

- The superflare was detected as part of a Hubble Space Telescope observing program dubbed HAZMAT (HAbitable Zones and M dwarf Activity across Time). The program surveys red dwarfs (also known as M dwarfs) at three different ages—young, intermediate, and old—and observes them in ultraviolet light, where they show the most activity.

- "Red dwarf stars are the smallest, most common, and longest-lived stars in the galaxy," says Evgenya Shkolnik, an assistant professor in ASU's School of Earth and Space Exploration and the HAZMAT program's principal investigator. "In addition, we think that most red dwarf stars have systems of planets orbiting them."

- The Hubble telescope's orbit above Earth's atmosphere gives it clear, unhindered views at ultraviolet wavelengths. The flares are believed to be powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere. When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy.

- ASU postdoctoral researcher Parke Loyd is the first author on the paper (to be published in the Astrophysical Journal) that reports on the stellar outbursts. 7)

- He says, "When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, 'Whoa.'"

- Loyd notes, "Gathering data on young red dwarfs has been especially important because we suspected these stars would be quite unruly in their youth, which is the first hundred million years or so after they form." - He adds, "Most of the potentially-habitable planets in our galaxy have had to withstand intense flares like the ones we observed at some point in their life. That's a sobering thought."


Figure 7: Violent outbursts of seething gas from young red dwarfs may make conditions uninhabitable on fledgling planets. In this artist's rendering, an active, young red dwarf (right) is stripping the atmosphere from an orbiting planet (left). ASU astronomers have found that flares from the youngest red dwarfs they surveyed — approximately 40 million years old — are 100 to 1000 times more energetic than when the stars are older. They also detected one of the most intense stellar flares ever observed in ultraviolet light — more energetic than the most powerful flare ever recorded from our Sun [image credit: NASA, ESA, and D. Player (STScI)]

Rough environment for planets

- About three-quarters of the stars in our Milky Way galaxy are red dwarfs. Most of the galaxy's "habitable-zone" planets—planets orbiting their stars at a distance where temperatures are moderate enough for liquid water to exist on their surface—orbit red dwarfs. In fact, the nearest star to our Sun, a red dwarf named Proxima Centauri, has an Earth-size planet in its habitable zone.

- However, red dwarfs—especially young red dwarfs—are active stars, producing flares that could blast out so much energy that it disrupts and possibly strips off the atmospheres of these fledgling planets.

- "The goal of the HAZMAT program is to understand the habitability of planets around low-mass stars," explains Shkolnik. "These low-mass stars are critically important in understanding planetary atmospheres." Ultraviolet radiation can modify the chemistry in a planet's atmosphere, or potentially remove that atmosphere.


Figure 8: Observations with the Hubble Space Telescope discovered a superflare (red line) that caused a red dwarf star's brightness in the far ultraviolet to abruptly increase by a factor of nearly 200 (image credit: P. Loyd/ASU)

- The observations reported in the Astrophysical Journal examined the flare frequency of 12 young (40 million year old) red dwarfs and represent just the first part of the HAZMAT program. These stars show that young low-mass stars flare much more frequently and more energetically than old stars and middle-age stars like our Sun—as evidenced by the superflare.

- "With the Sun, we have a hundred years of good observations," says Loyd. "And in that time, we've seen one, maybe two, flares that have an energy approaching that of the superflare."

- However, he says, "In a little less than a day's worth of Hubble observations of these young stars, we caught the superflare. This means that we're looking at superflares happening every day or even a few times a day."

- Could superflares of such frequency and intensity bathe young planets in so much ultraviolet radiation that they forever rule out any chance of habitability?

- According to Loyd, "Flares like we observed have the capacity to strip away the atmosphere from a planet. But that doesn't necessarily mean doom and gloom for life on the planet. It just might be different life than we imagine. Or there might be other processes that could replenish the atmosphere of the planet. It's certainly a harsh environment, but I would hesitate to call it a sterile environment."

- The next part of the HAZMAT study will be to study intermediate-age red dwarfs that are 650 million years old. Then the oldest red dwarfs will be analyzed and compared with the young and intermediate stars to understand the evolution of the high-energy-radiation environment for planets around these low-mass stars.

- Red dwarfs, which are estimated to burn as long as a trillion years, have a vast stretch of time available to eventually host evolving, habitable planets.

- "They just have many more opportunities for life to evolve, given their longevity," says Shkolnik. "I don't think we know for sure one way or another about whether planets orbiting red dwarfs are habitable just yet, but I think time will tell."

- She says, "It's great that we're living in a time when we have the technology to actually answer these kinds of questions, rather than just philosophize about them."

• 12 October 2018: This image from the NASA/ESA Hubble Space Telescope (Figure 9) reveals a spiral galaxy named Messier 95 (also known as M95 or NGC 3351). Located about 35 million light-years away in the constellation of Leo (The Lion), this swirling spiral was discovered by astronomer Pierre Méchain in 1781, and cataloged by French astronomer Charles Messier just four days later. Messier was primarily a comet hunter, and was often left frustrated by objects in the sky that resembled comets but turned out not to be. To help other astronomers avoid confusing these objects in the future, he created his famous catalog of Messier objects. 8)

- Most definitely not a comet, Messier 95 is actually a barred spiral galaxy. The galaxy has a bar cutting through its center, surrounded by an inner ring currently forming new stars. Also our own Milky Way is a barred spiral.

- As well as hosting this stellar nursery, Messier 95 is a known host of the dramatic and explosive final stages in the lives of massive stars: supernovae. In March 2016 a spectacular supernova named SN 2012aw was observed in the outer regions of one of Messier 95's spiral arms. Once the light from the supernova had faded, astronomers were able to compare observations of the region before and after the explosion to find out which star had "disappeared" — the progenitor star. In this case, the star was an especially huge red supergiant up to 26 times more massive than the Sun.


Figure 9: This Hubble image reveals a detailed view of part of the spiral galaxy Messier 95 (image credit: ESA/Hubble & NASA; CC BY 4.0)

• 08 October 2018: NASA is working to resume science operations of the Hubble Space Telescope after the spacecraft entered safe mode on Friday, 5 October , shortly after 6:00 p.m. EDT. Hubble's instruments still are fully operational and are expected to produce excellent science for years to come. 9)

- Hubble entered safe mode after one of the three gyroscopes, actively being used to point and steady the telescope, failed. Safe mode puts the telescope into a stable configuration until ground control can correct the issue and return the mission to normal operation.

- Built with multiple redundancies, Hubble had six new gyros installed during Servicing Mission-4 in 2009. Hubble usually uses three gyros at a time for maximum efficiency, but can continue to make scientific observations with just one.

- The gyro that failed had been exhibiting end-of-life behavior for approximately a year, and its failure was not unexpected; two other gyros of the same type had already failed. The remaining three gyros available for use are technically enhanced and therefore expected to have significantly longer operational lives.

- Two of those enhanced gyros are currently running. Upon powering on the third enhanced gyro that had been held in reserve, analysis of spacecraft telemetry indicated that it was not performing at the level required for operations. As a result, Hubble remains in safe mode. Staff at NASA's Goddard Space Flight Center and the Space Telescope Science Institute are currently performing analyses and tests to determine what options are available to recover the gyro to operational performance.

- Science operations with Hubble have been suspended while NASA investigates the anomaly. An Anomaly Review Board, including experts from the Hubble team and industry familiar with the design and performance of this type of gyro, is being formed to investigate this issue and develop the recovery plan. If the outcome of this investigation results in recovery of the malfunctioning gyro, Hubble will resume science operations in its standard three-gyro configuration.

- If the outcome indicates that the gyro is not usable, Hubble will resume science operations in an already defined "reduced-gyro" mode that uses only one gyro. While reduced-gyro mode offers less sky coverage at any particular time, there is relatively limited impact on the overall scientific capabilities.

• 03 October 2018: A pair of Columbia University astronomers using NASA's Hubble Space Telescope and the Kepler Space Telescope have assembled compelling evidence for the existence of a moon orbiting a gas-giant planet 8,000 light-years away. 10) 11)

- In a paper published 03 October in the journal Science Advances, Alex Teachey and David Kipping report that the detection of a candidate exomoon—that is, moons orbiting planets in other star systems—is unusual because of its large size, comparable to the diameter of Neptune. Such gargantuan moons do not exist in our own solar system, where nearly 200 natural satellites have been cataloged. 12)

- "This would be the first case of detecting a moon outside our solar system," said Kipping, an assistant professor of astronomy at Columbia. "If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets."


Figure 10: Artist's impression of the exoplanet Kepler-1625b, transiting the star, with the candidate exomoon in tow (image credit: Dan Durda)

- In looking for exomoons, the researchers analyzed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit. The researchers found one instance, in Kepler 1625b, that had intriguing anomalies.


Figure 11: Artist's impression of the exoplanet Kepler-1625b transiting the star with the candidate exomoon in tow (image credit: Dan Durda)

- "We saw little deviations and wobbles in the light curve that caught our attention," Kipping said.

- The Kepler results were enough for the team to get 40 hours of time with Hubble to intensively study the planet, obtaining data four times more precise than that of Kepler. The researchers monitored the planet before and during its 19-hour-long transit across the face of the star. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash," Kipping said. "Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured."

- In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.

- "An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit," Kipping said.

- The researchers note that in principle this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission.

- "A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation."

- The moon is estimated to be only 1.5 percent the mass of its companion planet, which itself estimated to be several times the mass of Jupiter. This value is close to the mass-ratio between the Earth and its moon. But in the case of the Earth-Moon system and the Pluto-Charon system—the largest of the five known natural satellites of the dwarf planet Pluto—an early collision with a larger body is hypothesized to have blasted off material that later coalesced into a moon. Kepler 1625b and its satellite, however, are gaseous, not rocky, and, therefore, such a collision may not lead to the condensation of a satellite.

- Exomoons are difficult to find because they are smaller than their companion planet and so their transit signal is weak; they also shift position with each transit because the moon is orbiting the planet. In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times. In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.

- The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. "Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it," Kipping said.

- Future searches will target Jupiter-sized planets that are farther from their star than Earth is from the Sun. There are just a handful of these in the Kepler database. NASA's upcoming James Webb Space Telescope could really "clean-up" in the satellite search, Kipping said. "We can expect to see really tiny moons."

• 21 September 2018: In the northern constellation of Coma Berenices (Berenice's Hair) lies the impressive Coma Cluster — a structure of over a thousand galaxies bound together by gravity. Many of these galaxies are elliptical types, as is the brighter of the two galaxies dominating this image: NGC 4860 (center of Figure 12). However, the outskirts of the cluster also host younger spiral galaxies that proudly display their swirling arms. Again, this image shows a wonderful example of such a galaxy in the shape of the beautiful NGC 4858, which can be seen to the left of its bright neighbor and which stands out on account of its unusual, tangled, fiery appearance. 13)

- NGC 4858 is special. Rather than being a simple spiral, it is something called a "galaxy aggregate", which is, just as the name suggests, a central galaxy surrounded by a handful of luminous knots of material that seem to stem from it, extending and tearing away and adding to or altering its overall structure. It is also experiencing an extremely high rate of star formation, possibly triggered by an earlier interaction with another galaxy. As we see it, NGC 4858 is forming stars so frantically that it will use up all of its gas long before it reaches the end of its life. The color of its bright knots indicates that they are formed of hydrogen, which glows in various shades of bright red as it is energized by the many young, hot stars lurking within.


Figure 12: This scene of the impressive Coma Cluster was captured by the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 (WFC3), a powerful camera designed to explore the evolution of stars and galaxies in the early Universe (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 14 September 2018: Gravity is so much a part of our daily lives that it is all too easy to forget its awesome power — but on a galactic scale, its power becomes both strikingly clear and visually stunning. 14)


Figure 13: This image was taken with the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 (WFC3) and shows an object named SDSS J1138+2754. It acts as a gravitational lens illustrating the true strength of gravity: A large mass — a galaxy cluster in this case — is creating such a strong gravitational field that it is bending the very fabric of its surroundings. This causes the billion-year-old light from galaxies sitting behind it to travel along distorted, curved paths, transforming the familiar shapes of spirals and ellipticals (visible in other parts of the image) into long, smudged arcs and scattered dashes (image credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt; CC BY 4.0)

Legend to Figure 13: Some distant galaxies even appear multiple times in this image. Since galaxies are wide objects, light from one side of the galaxy passes through the gravitational lens differently than light from the other side. When the galaxies' light reaches Earth it can appear reflected, as seen with the galaxy on the lower left part of the lens, or distorted, as seen with the galaxy to the upper right. — These data were taken as part of a research project on star formation in the distant Universe, building on Hubble's extensive legacy of deep-field images. Hubble observed 73 gravitationally-lensed galaxies for this project.

• 10 September 2018: Figure 14 is a composite image taken by Hubble on 6 June 2018 showing a fully-illuminated Saturn and its rings, along with six of its 62 known moons. The visible moons are (from left to right) Dione, Enceladus, Tethys, Janus, Epimetheus and Mimas (click here for an annotated version). Dione is the largest moon in the picture, with a diameter of 1123 km, compared to the smallest, oddly-shaped Epimetheus with a diameter around 116 km. 15)

- During Cassini's mission, Enceladus was identified as one of the most intriguing moons, with the discovery of water vapor jets spewing from the surface implying the existence of a subsurface ocean. Icy moons with subsurface oceans could potentially offer the conditions to harbor life, and understanding their origins and properties are essential for furthering our knowledge of the Solar System. ESA's JUpiter ICy moons Explorer (Juice), due to launch in 2022, aims to continue this theme by studying Jupiter's ocean-bearing moons: Ganymede, Europa, and Callisto.

- The Hubble image of Figure 15 was taken shortly before Saturn's opposition on 27 June, when the Sun, Earth and Saturn were aligned so that the Sun fully illuminated Saturn as seen from Earth. Saturn's closest approach to Earth occurs around the same time as opposition, which makes it appear brighter and larger and allows the planet to be imaged in greater detail.

- In this image the planet's rings are seen near their maximum tilt towards Earth. Towards the end of Cassini's mission, the spacecraft made multiple dives through the gap between Saturn and its rings, gathering spectacular data in this previously unchartered territory.

- The image also shows a hexagonal atmospheric feature around the north pole, with the remnants of a storm, seen as a string of bright clouds. The hexagon-shaped cloud phenomenon is a stable and persistent feature first seen by the Voyager 1 space probe when it flew past Saturn 1981. In a study published just last week, scientists using Cassini data collected between 2013 and 2017, as the planet approached northern summer, identified a hexagonal vortex above the cloud structure, showing there is still much to learn about the dynamics of Saturn's atmosphere.

- The Hubble observations making up this image were performed as part of the Outer Planet Atmospheres Legacy (OPAL) project, which uses Hubble to observe the outer planets to understand the dynamics and evolution of their complex atmospheres. This was the first time that Saturn was imaged as part of OPAL. This image was first published on 26 July.


Figure 14: A composite image taken by Hubble on 6 June 2018 showing a fully-illuminated Saturn and its rings, along with six of its 62 known moons. The visible moons are (from left to right) Dione, Enceladus, Tethys, Janus, Epimetheus and Mimas (image credit: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); CC BY 4.0)


Figure 15: This composite image, taken by the NASA/ESA Hubble Space Telescope on 6 June 2018, shows the ringed planet Saturn with six of its 62 known moons. With a diameter of 1123 km, Dione is the fourth-largest of Saturn's moons and the largest of the siblings in this family portrait. The smallest satellite in this picture is the irregularly shaped Epimetheus, with a size of 143 x 108 x 98 km. The image is a composite because the moons move during the Saturn exposures, and individual frames must be realigned to make a color portrait [image credit: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI)]

• 30 August 2018: Astronomers using the NASA/ESA Hubble Space Telescope have taken a series of spectacular images featuring the fluttering auroras at the north pole of Saturn. The observations were taken in ultraviolet light and the resulting images provide astronomers with the most comprehensive picture so far of Saturn's northern aurora. 16)

- In 2017, over a period of seven months, the NASA/ESA Hubble Space Telescope took images of auroras above Saturn's north pole region using the Space Telescope Imaging Spectrograph. The observations were taken before and after the Saturnian northern summer solstice. These conditions provided the best achievable viewing of the northern auroral region for Hubble.

- On Earth, auroras are mainly created by particles originally emitted by the Sun in the form of solar wind. When this stream of electrically charged particles gets close to our planet, it interacts with the magnetic field, which acts as a gigantic shield. While it protects Earth's environment from solar wind particles, it can also trap a small fraction of them. Particles trapped within the magnetosphere — the region of space surrounding Earth in which charged particles are affected by its magnetic field — can be energized and then follow the magnetic field lines down to the magnetic poles. There, they interact with oxygen and nitrogen atoms in the upper layers of the atmosphere, creating the flickering, colorful lights visible in the polar regions here on Earth.
Note: The auroras here on Earth have different names depending on which pole they occur at. Aurora Borealis, or the northern lights, is the name given to auroras around the north pole and Aurora Australis, or the southern lights, is the name given for auroras around the south pole.

- However, these auroras are not unique to Earth. Other planets in our Solar System have been found to have similar auroras. Among them are the four gas giants Jupiter, Saturn, Uranus and Neptune. Because the atmosphere of each of the four outer planets in the Solar System is — unlike the Earth — dominated by hydrogen, Saturn's auroras can only be seen in ultraviolet wavelengths; a part of the electromagnetic spectrum which can only be studied from space.


Figure 16: Saturn and its northern auroras (composite image), image credit: ESA/Hubble, NASA, A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI), L. Lamy (Observatoire de Paris)

- Hubble allowed researchers to monitor the behavior of the auroras at Saturn's north pole over an extended period of time. The Hubble observations were coordinated with the "Grand Finale" of the Cassini spacecraft, when the spacecraft simultaneously probed the auroral regions of Saturn. The Hubble data allowed astronomers to learn more about Saturn's magnetosphere, which is the largest of any planet in the Solar System other than Jupiter.
Note: Cassini was a collaboration between NASA, ESA and the Italian Space Agency. It spent 13 years orbiting Saturn, gathering information and giving astronomers a great insight into the inner workings of Saturn. Cassini took more risks at the end of its mission, travelling through the gap between Saturn and its rings. No spacecraft had previously done this, and Cassini gathered spectacular images of Saturn as well as new data for scientists to work with. On 15 September 2017 Cassini was sent on a controlled crash into Saturn.

- The images show a rich variety of emissions with highly variable localized features. The variability of the auroras is influenced by both the solar wind and the rapid rotation of Saturn, which lasts only about 11 hours. On top of this, the northern aurora displays two distinct peaks in brightness — at dawn and just before midnight. The latter peak, unreported before, seems specific to the interaction of the solar wind with the magnetosphere at Saturn's solstice.

- The main image presented here is a composite of observations made of Saturn in early 2018 in the optical and of the auroras on Saturn's north pole region, made in 2017, demonstrating the size of the auroras along with the beautiful colors of Saturn.

- Hubble has studied Saturn's auroras in the past. In 2004, it studied the southern auroras shortly after the southern solstice (heic0504) and in 2009 it took advantage of a rare opportunity to record Saturn when its rings were edge-on (heic1003). This allowed Hubble to observe both poles and their auroras simultaneously.

• 24 August 2018: This dramatic image from the NASA/ESA Hubble Space Telescope shows the planetary nebula NGC 3918, a brilliant cloud of colorful gas in the constellation of Centaurus, around 4,900 light-years from Earth. 17)

- In the center of the cloud of gas, and completely dwarfed by the nebula, are the dying remnants of a red giant. During the final convulsive phase in the evolution of these stars, huge clouds of gas are ejected from the surface of the star before it emerges from its cocoon as a white dwarf. The intense ultraviolet radiation from the tiny remnant star then causes the surrounding gas to glow like a fluorescent sign. These extraordinary and colorful planetary nebulas are among the most dramatic sights in the night sky, and often have strange and irregular shapes, which are not yet fully explained.

- NGC 3918's distinctive eye-like shape, with a bright inner shell of gas and a more diffuse outer shell that extends far from the nebula, looks as if it could be the result of two separate ejections of gas. But this is in fact not the case: studies of the object suggest that they were formed at the same time, but are being blown from the star at different speeds. The powerful jets of gas emerging from the ends of the large structure are estimated to be shooting away from the star at speeds of up to 350,000 km/hr.

- By the standards of astronomical phenomena, planetary nebulas like NGC 3918 are very short-lived, with a lifespan of just a few tens of thousands of years.


Figure 17: This Hubble image shows the planetary nebula NGC 3918, a brilliant cloud of colorful gas in the constellation of Centaurus. The image is a composite of visible and near-infrared snapshots taken with Hubble's Wide Field and Planetary Camera 2 (image credit: ESA/Hubble and NASA)

• 16 August 2018: Astronomers using the ultraviolet vision of NASA's Hubble Space Telescope have captured one of the largest panoramic views of the fire and fury of star birth in the distant universe. The field features approximately 15,000 galaxies, about 12,000 of which are forming stars. Hubble's ultraviolet vision opens a new window on the evolving universe, tracking the birth of stars over the last 11 billion years back to the cosmos' busiest star-forming period, which happened about 3 billion years after the big bang. 18)

- Ultraviolet light has been the missing piece to the cosmic puzzle. Now, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, astronomers have assembled one of the most comprehensive portraits yet of the universe's evolutionary history.

- The image straddles the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies, which can be seen across a broad spectrum. The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths. By comparing images of star formation in the distant and nearby universe, astronomers glean a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.

- Because Earth's atmosphere filters most ultraviolet light, Hubble can provide some of the most sensitive space-based ultraviolet observations possible.

- The program, called the Hubble Deep UV (HDUV) Legacy Survey, extends and builds on the previous Hubble multi-wavelength data in the CANDELS-Deep (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) fields within the central part of the GOODS (Great Observatories Origins Deep Survey) fields. This mosaic is 14 times the area of the Hubble Ultra Violet Ultra Deep Field released in 2014.

- The image of Figure 18 is a portion of the GOODS-North field, which is located in the northern constellation Ursa Major.


Figure 18: Astronomers have just assembled one of the most comprehensive portraits yet of the universe's evolutionary history, based on a broad spectrum of observations by the Hubble Space Telescope and other space and ground-based telescopes. In particular, Hubble's ultraviolet vision opens a new window on the evolving universe, tracking the birth of stars over the last 11 billion years back to the cosmos' busiest star-forming period, about 3 billion years after the big bang. This photo encompasses a sea of approximately 15,000 galaxies — 12,000 of which are star-forming — widely distributed in time and space. This mosaic is 14 times the area of the Hubble Ultra Violet Ultra Deep Field released in 2014 [image credit: NASA, ESA, P. Oesch (University of Geneva), and M. Montes (University of New South Wales)]

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• 10 August 2018: This Picture of the Week shows the colorful globular cluster NGC 2108. The cluster is nestled within the Large Magellanic Cloud, in the constellation of the Swordfish (Dorado). It was discovered in 1835 by the astronomer, mathematician, chemist and inventor John Herschel, son of the famous William Herschel. 19)

- The most striking feature of this globular cluster is the gleaming ruby-red spot at the center left of the image (Figure 19). What looks like the cluster's watchful eye is actually a carbon star. Carbon stars are almost always cool red giants, with atmospheres containing more carbon than oxygen — the opposite to our Sun. Carbon monoxide forms in the outer layer of the star through a combination of these elements, until there is no more oxygen available. Carbon atoms are then free to form a variety of other carbon compounds, such as C2, CH, CN, C3 and SiC2, which scatter blue light within the star, allowing red light to pass through undisturbed.


Figure 19: This image was captured by the NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys (ACS), using three different filters (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 03 August 2018: Gravitational lenses — such as this galaxy cluster SDSS J1152+3313 — possess immense masses that wrap their surroundings and bend the light from faraway objects into rings, arcs, streaks, blurs, and other odd shapes. This lens, however, is not only wrapping the appearance of a distant galaxy — it is also amplifying its light, making it appear much brighter than it would be without the lens. Combined with the high image quality obtainable with Hubble, this gives valuable clues into how stars formed in the early Universe. 20)

- Star formation is a key process in astronomy. Everything that emits light is somehow connected to stars, so understanding how stars form is key to understanding countless objects lying across the cosmos. Astronomers can probe these early star-forming regions to learn about the sizes, luminosities, formation rates, and generations of different types of stars.


Figure 20: Obtained for a research program on star formation in old and distant galaxies, this NASA/ESA Hubble Space Telescope image obtained with its Wide Field Camera 3 (WFC3) demonstrates the immense effects of gravity; more specifically, it shows the effects of gravitational lensing caused by an object called SDSS J1152+3313 (image credit: ESA/Hubble & NASA: Acknowledgement: Judy Schmidt (Geckzilla), CC BY 4.0)

• 26 July 2018: In the summer of 2018 the planets Mars and Saturn are, one after the other, in opposition to Earth. During this event the planets are relatively close to Earth, allowing astronomers to observe them in greater detail. Hubble took advantage of this preferred configuration and imaged both planets to continue its long-standing observation of the outer planets in the Solar System. 21) 22)

- Since the NASA/ESA Hubble Space Telescope was launched, its goal has always been to study not only distant astronomical objects, but also the planets within our Solar System. Hubble's high-resolution images of our planetary neighbors can only be surpassed by pictures taken from spacecraft that actually visit these bodies. However, Hubble has one advantage over space probes: it can look at these objects periodically and observe them over much longer periods than any passing probe could.

- In the last months the planets Mars and Saturn have each been in opposition to Earth — Saturn on 27 June and Mars on 27 July. An opposition occurs when the Sun, Earth and an outer planet are lined up, with Earth sitting in between the Sun and the outer planet. During an opposition, a planet is fully lit by the Sun as seen from Earth, and it also marks the time when the planet is closest to Earth, allowing astronomers to see features on the planet's surface in greater detail.
Note: The dates of opposition and closest approach differ slightly. This difference is caused by the elliptical orbit of the planets and the fact that the orbits are not in exactly the same plane.

- A month before Saturn's opposition — on 6 June — Hubble was used to observe the ringed planet . At this time Saturn was approximately 1.4 billion kilometers from Earth. The taken images show Saturn's magnificent ring system near its maximum tilt toward Earth, allowing a spectacular view of the rings and the gaps between them. Though all of the gas giants boast rings, Saturn's are the largest and most spectacular, stretching out to eight times the radius of the planet.
Note: The observations of Saturn were made as part of the Outer Planet Atmospheres Legacy (OPAL) project. OPAL is helping astronomers understand the atmospheric dynamics and evolution of the gas giant planets in our Solar System. Jupiter, Uranus and Neptune have already been observed several times as part of this project, but this is the first time Saturn was observed as part of OPAL.

- Alongside a beautiful view of the ring system, Hubble's new image reveals a hexagonal pattern around the north pole — a stable and persistent wind feature discovered during the flyby of the Voyager 1 space probe in 1981. To the south of this feature a string of bright clouds is visible: remnants of a disintegrating storm.


Figure 21: This image shows the recent observations of the planets Mars (right) and Saturn (left) made with the NASA/ESA Hubble Space Telescope. The observations of both objects were made in June and July 2018 and show the planets close to their opposition (image credit: Saturn: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); Mars: NASA, ESA, and STScI)

- While observing the planet, Hubble also managed to capture images of six of Saturn's 62 currently known moons: Dione, Enceladus, Tethys, Janus, Epimetheus, and Mimas. Scientists hypothesize that a small, wayward moon like one of these disintegrated 200 million years ago to form Saturn's ring system.

- Hubble shot the second portrait, of the planet Mars, on 18 July, just 13 days before Mars reached its closest approach to Earth. This year Mars will get as close as 57.6 million km from Earth. This makes it the closest approach since 2003, when the red planet made its way closer to us than at any other time in almost 60 000 years (opo0322).

- While previous images showed detailed surface features of the planet, this new image is dominated by a gigantic sandstorm enshrouding the entire planet. Still visible are the white polar caps, Terra Meridiani, the Schiaparelli Crater, and Hellas Basin — but all of these features are slightly blurred by the dust in the atmosphere.

- Comparing these new images of Mars and Saturn with older data gathered by Hubble, other telescopes and even space probes allows astronomers to study how cloud patterns and large-scale structures on other planets in our Solar System change over time.

• 13 July 2018: In November 2008, 14-year-old Caroline Moore from New York discovered a supernova in UGC 12682. This made her the youngest person at the time to have discovered a supernova. Follow-up observations by professional astronomers of the so-called SN 2008ha showed that it was peculiarly interesting in many different ways: its host galaxy UGC 12862 rarely produces supernovae. It is one of the faintest supernovae ever observed and after the explosion it expanded very slowly, suggesting that the explosion did not release copious amounts of energy as usually expected. 23)

- Astronomers have now classified SN 2008ha as a subclass of a Type Ia supernova, which is the explosion of a white dwarf that hungrily accretes matter from a companion star. SN 2008ha may have been the result of a partially failed supernova, explaining why the explosion failed to decimate the whole star.


Figure 22: Glowing warmly against the dark backdrop of the Universe, this image from the NASA/ESA Hubble Space Telescope shows an irregular galaxy called UGC 12682. Located approximately 70 million light-years away in the constellation of Pegasus (The Winged Horse), UGC 12682 is distorted and oddly-structured, with bright pockets of star formation (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 3 July 2018: Like a July 4 fireworks display, a young, glittering collection of stars resembles an aerial burst. The cluster is surrounded by clouds of interstellar gas and dust - the raw material for new star formation. The nebula, located 20,000 light-years away in the constellation Carina, contains a central cluster of huge, hot stars, called NGC 3603. 24)

- Appearing colorful and serene, this environment is anything but. Ultraviolet radiation and violent stellar winds have blown out an enormous cavity in the gas and dust enveloping the cluster. Most of the stars in the cluster were born around the same time but differ in size, mass, temperature and color. The course of a star's life is determined by its mass, so a cluster of a given age will contain stars in various stages of their lives, giving an opportunity for detailed analyses of stellar life cycles. NGC 3603 also contains some of the most massive stars known. These huge stars live fast and die young, burning through their hydrogen fuel quickly and ultimately ending their lives in supernova explosions.

- Star clusters like NGC 3603 provide important clues to understanding the origin of massive star formation in the early, distant universe. Astronomers also use massive clusters to study distant starbursts that occur when galaxies collide, igniting a flurry of star formation. The proximity of NGC 3603 makes it an excellent lab for studying such distant and momentous events.

- This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron.


Figure 23: This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron [image credit: NASA, ESA, R. O'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]

• 25 June 2018: For years the Hubble Space Telescope has captured crisp spectral images of exoplanets transiting their host stars. Because those images include light filtered through the exoplanets' atmospheres, they contain clues about atmospheric composition. Absorption features in such spectra have produced evidence of water, carbon dioxide, methane, and even clouds in the atmospheres of extrasolar planets. 25)

- But Hubble's workhorse detector for exoplanet atmosphere observations, the Wide Field Camera 3, collects light in only 13 wavelength bins. The James Webb Space Telescope, scheduled for a 2020 launch, will be able to resolve spectra into hundreds of bins. The abundance of data could yield far more detailed portraits of extrasolar atmospheres, but it also creates a challenge: how to decipher all that information.

- Enter Kevin Heng and his coworkers at the University of Bern in Switzerland. The researchers have now demonstrated that machine learning can be used to extract atmospheric properties from even the most complicated transmission spectra. Heng and his colleagues trained their machine on tens of thousands of model spectra that were calculated analytically for atmospheres of varying temperature, cloudiness, and composition. The machine learning algorithm plots the spectra in N-dimensional space, where N is the number of wavelength bins in each spectrum, and then identifies clusters in that multidimensional space. Model atmospheres belonging to the same cluster tend to share similar physical attributes, so when the trained machine is given a real-life spectrum to analyze, it plots the spectrum and assigns to it the physical attributes of the nearest cluster.

- Reassuringly, a test-run analysis of the gas-giant planet WASP-12b yielded results similar to those of more conventional techniques. The test was implemented in 13-dimensional space, to match Hubble's 13 spectral bins, but future implementations using more detailed spectra could include thousands of dimensions. 26)

• 25 June 2018: As if this Hubble Space Telescope picture isn't cluttered enough with myriad galaxies, nearby asteroids photobomb the image, their trails sometimes mimicking background astronomical phenomena. 27)

- The stunningly beautiful galaxy cluster Abell 370 (Figure 24) contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Located approximately four billion light years away in the constellation Cetus, the Sea Monster, this immense cluster is a rich mix of a variety of galaxy shapes.

- Entangled among the galaxies are thin, white trails that look like curved or S-shaped streaks. These are trails from asteroids that reside, on average, only about 260 million kilometers from Earth – right around the corner in astronomical terms. The trails appear in multiple Hubble exposures that have been combined into one image. Of the 22 total asteroid sightings for this field, five are unique objects. These asteroids are so faint that they were not previously identified.

- The asteroid trails look curved due to an observational effect called parallax. As Hubble orbits around Earth, an asteroid will appear to move along an arc with respect to the vastly more distant background stars and galaxies. The motion of Earth around the Sun, and the motion of the asteroids along their orbits, are other contributing factors to the apparent skewing of asteroid paths.

- All the asteroids were found manually, the majority by "blinking" consecutive exposures to capture apparent asteroid motion. Astronomers found a unique asteroid for every 10 to 20 hours of exposure time.

- These asteroid trails should not be confused with the mysterious-looking arcs of blue light that are actually distorted images of distant galaxies behind the cluster. Many of these far-flung galaxies are too faint for Hubble to see directly. Instead, in a dramatic example of "gravitational lensing," the cluster functions as a natural telescope, warping space and affecting light traveling through the cluster toward Earth.

- The study was part of the Frontier Fields program and the image, assembled from several exposures taken in visible and infrared light, was first published on 6 November 2017.


Figure 24: This image was assembled from several exposures taken in visible and infrared light. The field's position on the sky is near the ecliptic, the plane of our Solar System. This is the zone in which most asteroids reside, which is why Hubble astronomers saw so many crossings. Hubble deep-sky observations taken along a line-of-sight near the plane of our Solar System commonly record asteroid trails (image credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI) Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team)

- Every year on 30 June, the global "Asteroid Day" event takes place to raise awareness about asteroids and what can be done to protect Earth from possible impact. The day falls on the anniversary of the Tunguska event that took place on 30 June 1908, the most harmful known asteroid related event in recent history. This year, ESA is co-hosting a live webcast with the European Southern Observatory packed with expert interviews, news on some of the most recent asteroid science results, and the truth about the dinosaurs. Watch 30 June at 13:00 CEST via

• 21 June 2018: An international team of astronomers using the NASA/ESA Hubble Space Telescope and the European Southern Observatory's VLT (Very Large Telescope) has made the most precise test of general relativity yet outside our Milky Way. The nearby galaxy ESO 325-G004 acts as a strong gravitational lens, distorting light from a distant galaxy behind it to create an Einstein ring around its center. By comparing the mass of ESO 325-G004 with the curvature of space around it, the astronomers found that gravity on these astronomical length-scales behaves as predicted by general relativity. This rules out some alternative theories of gravity. 28) 29)

- Using the NASA/ESA Hubble Space Telescope and European Southern Observatory's VLT, a team led by Thomas Collett (University of Portsmouth, UK), was able to perform the most precise test of general relativity outside the Milky Way to date.

- The theory of general relativity predicts that objects deform spacetime, causing any light that passes by to be deflected and resulting in a phenomenon known as gravitational lensing. This effect is only noticeable for very massive objects. A few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass. However, the elliptical galaxy ESO 325-G004 is amongst the closest lenses at just 450 million light-years from Earth.

- Using the MUSE (Multi Unit Spectroscopic Explorer) instrument on the VLT the team calculated the mass of ESO 325-G004 by measuring the movement of stars within it. Using Hubble the scientists were able to observe an Einstein ring resulting from light from a distant galaxy being distorted by the intervening ESO 325-G004. Studying the ring allowed the astronomers to measure how light, and therefore spacetime, is being distorted by the huge mass of ESO 325-G004.

- Collett comments: "We know the mass of the foreground galaxy from MUSE and we measured the amount of gravitational lensing we see from Hubble. We then compared these two ways to measure the strength of gravity – and the result was just what general relativity predicts, with an uncertainty of only nine percent. This is the most precise test of general relativity outside the Milky Way to date. And this using just one galaxy!"

- General relativity has been tested with exquisite accuracy on Solar System scales, and the motions of stars around the black hole at the center of the Milky Way are under detailed study, but previously there had been no precise tests on larger astronomical scales. Testing the long range properties of gravity is vital to validate our current cosmological model.


Figure 25: An image of the nearby galaxy ESO 325-G004, created using data collected by the NASA/ESA Hubble Space Telescope and the MUSE instrument on the ESO's Very Large Telescope. MUSE measured the velocity of stars in ESO 325-G004 to produce the velocity dispersion map that is overlaid on top of the Hubble Space Telescope image. Knowledge of the velocities of the stars allowed the astronomers to infer the mass of ESO 325-G004. The inset shows the Einstein ring resulting from the distortion of light from a more distant source by intervening lens ESO 325-004, which becomes visible after subtraction of the foreground lens light (image credit: ESO, ESA/Hubble, NASA)

- These findings may have important implications for models of gravity alternative to general relativity. These alternative theories predict that the effects of gravity on the curvature of spacetime are "scale dependent". This means that gravity should behave differently across astronomical length-scales from the way it behaves on the smaller scales of the Solar System. Collett and his team found that this is unlikely to be true unless these differences only occur on length scales larger than 6000 light-years.

- "The Universe is an amazing place providing such lenses which we can use as our laboratories," adds team member Bob Nichol (University of Portsmouth). "It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was."

• 31 May 2018: Though it resembles a peaceful rose swirling in the darkness of the cosmos, NGC 3256 is actually the site of a violent clash. This distorted galaxy is the relic of a collision between two spiral galaxies, estimated to have occurred 500 million years ago. Today it is still reeling in the aftermath of this event. 30)

- Located about 100 million light-years away in the constellation of Vela (The Sails), NGC 3256 is approximately the same size as our Milky Way and belongs to the Hydra-Centaurus Supercluster. It still bears the marks of its tumultuous past in the extended luminous tails that sprawl out around the galaxy, thought to have formed 500 million years ago during the initial encounter between the two galaxies, which today form NGC 3256. These tails are studded with young blue stars, which were born in the frantic but fertile collision of gas and dust.

- When two galaxies merge, individual stars rarely collide because they are separated by such enormous distances, but the gas and dust of the galaxies do interact – with spectacular results. The brightness blooming in the center of NGC 3256 gives away its status as a powerful starburst galaxy, host to vast amounts of infant stars born into groups and clusters. These stars shine most brightly in the far infrared, making NGC 3256 exceedingly luminous in this wavelength domain. Because of this radiation, it is classified as a Luminous Infrared Galaxy.

- NGC 3256 has been the subject of much study due to its luminosity, its proximity, and its orientation: astronomers observe its face-on orientation, that shows the disc in all its splendor. NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers. It holds particular promise to further our understanding of the properties of young star clusters in tidal tails.

- As well as being lit up by over 1000 bright star clusters, the central region of NGC 3256 is also home to crisscrossing threads of dark dust and a large disc of molecular gas spinning around two distinct nuclei – the relics of the two original galaxies. One nucleus is largely obscured, only unveiled in infrared, radio and X-ray wavelengths.

- These two initial galaxies were gas-rich and had similar masses, as they seem to be exerting roughly equal influence on each other. Their spiral disks are no longer distinct, and in a few hundred million years' time, their nuclei will also merge and the two galaxies will likely become united as a large elliptical galaxy.

- NGC 3256 was previously imaged through fewer filters by the NASA/ESA Hubble Space Telescope as part of a large collection of 59 images of merging galaxies, released for Hubble's 18th anniversary on 24 April 2008.


Figure 26: This image, taken with the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS), both installed on the NASA/ESA Hubble Space Telescope, shows the peculiar galaxy NGC 3256. The galaxy is about 100 million light-years from Earth and is the result of a past galactic merger, which created its distorted appearance. As such, NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers (image credit: ESA/Hubble, NASA, CC BY 4.0)

• 28 May 2018: This NASA/ESA Hubble Space Telescope image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (Figure 27). The brightest galaxy within this cluster named SDSS J1156+1911 and known as the Brightest Cluster Galaxy (BCG), is visible in the lower middle of the frame. It was discovered by the Sloan Giant Arcs Survey which studied data maps covering huge parts of the sky from the Sloan Digital Sky Survey. The survey found more than 70 galaxies that look to be significantly affected by a cosmic phenomenon known as gravitational lensing. 31)

- Gravitational lensing is one of the predictions of Albert Einstein's General Theory of Relativity. The mass contained within a galaxy is so immense that it can actually warp and bend the very fabric of its surroundings (known as space-time), forcing light to travel along curved paths. As a result, the image of a more distant galaxy appears distorted and amplified to an observer, as the light from it has been bent around the intervening galaxy. This effect can be very useful in astronomy, allowing astronomers to see galaxies that are either obscured or too distant to be otherwise detected by our current instruments.

- Galaxy clusters are giant structures containing hundreds to thousands of galaxies, some with masses over one million billion times the mass of the Sun! SDSS J1156+1911 is only roughly 600 billion times the mass of the Sun, making it less massive than the average galaxy. However, it is massive enough to produce the fuzzy, greenish streak seen just below the brightest galaxy — the lensed image of a more distant galaxy.


Figure 27: This Hubble image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (image credit: ESA/Hubble & NASA; Acknowledgment: Judy Schmidt (Geckzilla))

• 17 May 2018: Ultraviolet light is a major tracer of the youngest and hottest stars. These stars are short-lived and intensely bright. Astronomers have now finished a survey called LEGUS (Legacy ExtraGalactic UV Survey) that captured the details of 50 local galaxies within 60 million light-years of Earth in both visible and ultraviolet light. 32) 33)

- The LEGUS team carefully selected its targets from among 500 candidate galaxies compiled from ground-based surveys. They chose the galaxies based on their mass, star-formation rate, and their abundances of elements heavier than hydrogen and helium. Because of the proximity of the selected galaxies, Hubble was able to resolve them into their main components: stars and star clusters. With the LEGUS data, the team created a catalog with about 8000 young clusters and it also created a star catalog comprising about 39 million stars that are at least five times more massive than our Sun.

- The data, gathered with Hubble's WFC3 (Wide Field Camera 3) and ACS (Advanced Camera for Surveys), provide detailed information on young, massive stars and star clusters, and how their environment affects their development. As such, the catalogue offers an extensive resource for understanding the complexities of star formation and galaxy evolution.

- One of the key questions the survey may help astronomers answer is the connection between star formation and the major structures, such as spiral arms, that make up a galaxy. These structured distributions are particularly visible in the youngest stellar populations.

- By resolving the fine details of the studied galaxies, while also studying the connection to larger galactic structures, the team aims to identify the physical mechanisms behind the observed distribution of stellar populations within galaxies.

- Figuring out the final link between gas and star formation is key to fully understanding galaxy evolution. Astronomers are studying this link by looking at the effects of the environment on star clusters, and how their survival is linked to their surroundings.

- LEGUS will not only allow astronomers to understand the local Universe. It will also help interpret views of distant galaxies, where the ultraviolet light from young stars is stretched to infrared wavelengths due to the expansion of space. The NASA/ESA/CSA James Webb Space Telescope and its ability to observe in the far infrared will complement the LEGUS views.


Figure 28: The glowing spiral arms of NGC 6744. This image shows the galaxy NGC 6744, about 30 million light-years away. It is one of 50 galaxies observed as part of the Hubble Space Telescope's Legacy ExtraGalactic UV Survey (LEGUS), the sharpest, most comprehensive ultraviolet-light survey of star-forming galaxies in the nearby Universe, offering an extensive resource for understanding the complexities of star formation and galaxy evolution. The image is a composite using both ultraviolet light and visible light, gathered with Hubble's Wide Field Camera 3 and Advanced Camera for Surveys (image credit: NASA, ESA, and the LEGUS team)


Figure 29: Dwarf galaxy UGCA 281. UGCA 281 is a blue compact dwarf galaxy located in the constellation of Canes Venatici. Within it, two giant star clusters appear brilliant white and are swaddled by greenish hydrogen gas clouds. These clusters are responsible for most of the recent star formation in UGCA 281; the rest of the galaxy is comprised of older stars and appears redder in color. The reddish objects in the background are background galaxies that appear through the diffuse dwarf galaxy. The image is a composite using both ultraviolet light and visible light, gathered with Hubble's Wide Field Camera 3 and Advanced Camera for Surveys (image credit: NASA, ESA, and the LEGUS team)

• 16 My 2018: Resembling a wizard's staff set aglow, NGC 1032 cleaves the quiet darkness of space in two in this image from the NASA/ESA Hubble Space Telescope (Figure 30). 34)

- NGC 1032 is located about a hundred million light years away in the constellation Cetus (The Sea Monster). Although beautiful, this image perhaps does not do justice to the galaxy's true aesthetic appeal: NGC 1032 is actually a spectacular spiral galaxy, but from Earth, the galaxy's vast disc of gas, dust and stars is seen nearly edge-on.


Figure 30: A handful of other galaxies can be seen lurking in the background, scattered around the narrow stripe of NGC 1032. Many are oriented face-on or at tilted angles, showing off their glamorous spiral arms and bright cores. Such orientations provide a wealth of detail about the arms and their nuclei, but fully understanding a galaxy's three-dimensional structure also requires an edge-on view. This gives astronomers an overall idea of how stars are distributed throughout the galaxy and allows them to measure the "height" of the disc and the bright star-studded core (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 02 May 2018: Astronomers using the NASA/ESA Hubble Space Telescope have detected helium in the atmosphere of the exoplanet WASP-107b. This is the first time this element has been detected in the atmosphere of a planet outside the Solar System. The discovery demonstrates the ability to use infrared spectra to study exoplanet extended atmospheres. 35)

- The international team of astronomers, led by Jessica Spake, a PhD student at the University of Exeter in the UK, used Hubble's Wide Field Camera 3 to discover helium in the atmosphere of the exoplanet WASP-107b. This is the first detection of its kind.

- Spake explains the importance of the discovery: "Helium is the second-most common element in the Universe after hydrogen. It is also one of the main constituents of the planets Jupiter and Saturn in our Solar System. However, up until now helium had not been detected on exoplanets - despite searches for it."

- The team made the detection by analyzing the infrared spectrum of the atmosphere of WASP-107b. Previous detections of extended exoplanet atmospheres have been made by studying the spectrum at ultraviolet and optical wavelengths; this detection therefore demonstrates that exoplanet atmospheres can also be studied at longer wavelengths.
Note: The measurement of an exoplanet's atmosphere is performed when the planet passes in front of its host star. A tiny portion of the star's light passes through the exoplanet's atmosphere, leaving detectable fingerprints in the spectrum of the star. The larger the amount of an element present in the atmosphere, the easier the detection becomes.

- "The strong signal from helium we measured demonstrates a new technique to study upper layers of exoplanet atmospheres in a wider range of planets," says Spake "Current methods, which use ultraviolet light, are limited to the closest exoplanets. We know there is helium in the Earth's upper atmosphere and this new technique may help us to detect atmospheres around Earth-sized exoplanets – which is very difficult with current technology."

- WASP-107b is one of the lowest density planets known: While the planet is about the same size as Jupiter, it has only 12% of Jupiter's mass. The exoplanet is about 200 light-years from Earth and takes less than six days to orbit its host star.

- The amount of helium detected in the atmosphere of WASP-107b is so large that its upper atmosphere must extend tens of thousands of kilometers out into space. This also makes it the first time that an extended atmosphere has been discovered at infrared wavelengths.


Figure 31: Artist's impression of WASP-107b (image credit: ESA/Hubble, NASA, M. Kornmesser, CC BY 4.0)

- Since its atmosphere is so extended, the planet is losing a significant amount of its atmospheric gases into space – between ~0.1-4% of its atmosphere's total mass every billion years.
Note: Stellar radiation has a significant effect on the rate at which a planet's atmosphere escapes. The star WASP-107 is highly active, supporting the atmospheric loss. As the atmosphere absorbs radiation it heats up, so the gas rapidly expands and escapes more quickly into space.

- As far back as the year 2000, it was predicted that helium would be one of the most readily-detectable gases on giant exoplanets, but until now, searches were unsuccessful.

- David Sing, co-author of the study also from the University of Exeter, concludes: "Our new method, along with future telescopes such as the NASA/ESA/CSA James Webb Space Telescope, will allow us to analyze atmospheres of exoplanets in far greater detail than ever before." 36)

• 19 April 2018: To celebrate its 28th anniversary in space the NASA/ESA Hubble Space Telescope took this amazing and colorful image of the Lagoon Nebula (Figure 32). The whole nebula, about 4000 light-years away, is an incredible 55 light-years wide and 20 light-years tall. This image shows only a small part of this turbulent star-formation region, about four light-years across. 37)

- This stunning nebula was first catalogued in 1654 by the Italian astronomer Giovanni Battista Hodierna, who sought to record nebulous objects in the night sky so they would not be mistaken for comets. Since Hodierna's observations, the Lagoon Nebula has been photographed and analysed by many telescopes and astronomers all over the world.


Figure 32: The observations were taken by Hubble's Wide Field Camera 3 between 12 February and 18 February 2018 (image credit: NASA, ESA, STScI, CC BY 4.0)

• 10 April 2018: This NASA/ESA Hubble Space Telescope image (Figure 33) shows a massive galaxy cluster glowing brightly in the darkness. Despite its beauty, this cluster bears the distinctly unpoetic name of PLCK_G308.3-20.2. 38)

- Galaxy clusters can contain thousands of galaxies all held together by the glue of gravity. At on39)e point in time they were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years. However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.

- One of the most interesting features of galaxy clusters is the stuff that permeates the space between the constituent galaxies: the intracluster medium (ICM). High temperatures are created in these spaces by smaller structures forming within the cluster. This results in the ICM being made up of plasma — ordinary matter in a superheated state. Most luminous matter in the cluster resides in the ICM, which is very luminous X-rays. However, the majority of the mass in a galaxy cluster exists in the form of non-luminous dark matter. Unlike plasma, dark matter is not made from ordinary matter such as protons, neutrons and electrons. It is a hypothesized substance thought to make up 80 % of the Universe's mass, yet it has never been directly observed.


Figure 33: This image was taken by Hubble's Advanced Camera for Surveys and Wide-Field Camera 3 as part of an observing program called RELICS (Reionization Lensing Cluster Survey). RELICS imaged 41 massive galaxy clusters with the aim of finding the brightest distant galaxies for the forthcoming NASA/ESA/CSA James Webb Space Telescope(JWST) to study (image credit: ESA/Hubble & NASA, RELICS)

• 02 April 2018: Astronomers using the NASA/ESA Hubble Space Telescope have found the most distant star ever discovered. The hot blue star existed only 4.4 billion years after the Big Bang. This discovery provides new insight into the formation and evolution of stars in the early Universe, the constituents of galaxy clusters and also on the nature of dark matter. 40)


Figure 34: Appearance of the most distant star (image credit: NASA & ESA and P. Kelly (University of California, Berkeley))

- The international team, led by Patrick Kelly (University of Minnesota, USA), Jose Diego (Instituto de Física de Cantabria, Spain) and Steven Rodney (University of South Carolina, USA), discovered the distant star in the galaxy cluster MACS J1149-2223 in April 2016. The observations with Hubble were actually performed in order to detect and follow the latest appearance of the gravitationally lensed supernova explosion nicknamed "Refsdal" (heic1525), when an unexpected point source brightened in the same galaxy that hosted the supernova.

- "Like the Refsdal supernova explosion the light of this distant star got magnified, making it visible for Hubble," says Patrick Kelly. "This star is at least 100 times farther away than the next individual star we can study, except for supernova explosions."

- The observed light from the newly discovered star, called Lensed Star 1 (LS1) was emitted when the Universe was only about 30 percent of its current age – about 4.4 billion years after the Big Bang. The detection of the star through Hubble was only possible because the light from the star was magnified 2000 times.

- "The star became bright enough to be visible for Hubble thanks to a process called gravitational lensing," explains Jose Diego. The light from LS1 was magnified not only by the huge total mass of the galaxy cluster, but also by another compact object of about three times the mass of the Sun within the galaxy cluster itself; an effect known as gravitational microlensing.
Note: Gravitational lensing magnifies the light from fainter, background objects, allowing Hubble to see objects it would otherwise not be able to detect. The process was first predicted by Albert Einstein and is now used to find some of the most distant objects in the Universe. Usually the lensing object is a galaxy or a galaxy cluster, but in some cases it can also be a star or even a planet. When it involves these smaller objects the process is called microlensing.

- "The discovery of LS1 allows us to gather new insights into the constituents of the galaxy cluster. We know that the microlensing was caused by either a star, a neutron star, or a stellar-mass black hole," explains Steven Rodney. LS1 therefore allows astronomers to study neutron stars and black holes, which are otherwise invisible and they can estimate how many of these dark objects exist within this galaxy cluster.

- As galaxy clusters are among the largest and most massive structures in the Universe, learning about their constituents also increases our knowledge about the composition of the Universe overall. This includes additional information about the mysterious dark matter.

- "If dark matter is at least partially made up of comparatively low-mass black holes, as it was recently proposed, we should be able to see this in the light curve of LS1. Our observations do not favor the possibility that a high fraction of dark matter is made of these primordial black holes with about 30 times the mass of the Sun", highlights Kelly.

- After the discovery the researchers used Hubble again to measure a spectrum of LS1. Based on their analysis, the astronomers think that LS1 is a B-type supergiant star. These stars are extremely luminous and blue in color, with a surface temperature between 11,000 and 14,000 degrees Celsius; making them more than twice as hot as the Sun.

- But this was not the end of the story. Observations made in October 2016 suddenly showed a second image of the star. "We were actually surprised to not have seen this second image in earlier observations, as also the galaxy the star is located in can be seen twice," comments Diego. "We assume that the light from the second image has been deflected by another moving massive object for a long time – basically hiding the image from us. And only when the massive object moved out of the line of sight the second image of the star became visible." This second image and the blocking object add another piece of the puzzle to reveal the makeup of galaxy clusters.

- With more research and the arrival of new, more powerful telescopes like the NASA/ESA/CSA James Webb Space Telescope, the astronomers suggest that with microlensing, it will be possible to study the evolution of the earliest stars in the Universe in greater detail than ever expected.

•30 March 2018: The image of Figure 35, captured by the ACS (Advanced Camera for Surveys) on the NASA/ESA Hubble Space Telescope, shows the spiral galaxy NGC 5714, about 130 million light-years away in the constellation of Boötes (the Herdsman). NGC 5714 is classified as a Sc spiral galaxy, but its spiral arms — the dominating feature of spiral galaxies — are almost impossible to see, as NGC 1787 presents itself at an almost perfectly edge-on angle. 41) 42)

- Discovered by William Herschel in 1787, NGC 5714 was host to a fascinating and rare event in 2003. A faint supernova appeared about 8000 light-years below the central bulge of NGC 5714. Supernovae are the huge, violent explosions of dying stars, and the one that exploded in NGC 5714 — not visible in this much later image — was classified as a Type Ib/c supernova and named SN 2003dr. It was particularly interesting because its spectrum showed strong signatures of calcium.

- Calcium-rich supernovae are rare and hence of great interest to astronomers. Astronomers still struggle to explain these particular explosions as their existence presents a challenge to both observation and theory. In particular, their appearance outside of galaxies, their lower luminosity compared to other supernovae, and their rapid evolution are still open questions for researchers.


Figure 35: Image of the spiral galaxy NGC 5714, captured by the NASA/ESA Hubble Space Telescope (image credit: ESA/Hubble & NASA)

• 28 March 2018: An international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories have, for the first time, uncovered a galaxy in our cosmic neighborhood that is missing most – if not all – of its dark matter. This discovery of the galaxy NGC 1052-DF2 challenges currently-accepted theories of and galaxy formation and provides new insights into the nature of dark matter. The results are published in Nature. 43) 44)


Figure 36: A ghostly galaxy lacking dark matter (image credit: NASA, ESA, and P. van Dokkum (Yale University))

- Astronomers using Hubble and several ground-based observatories have found a unique astronomical object: a galaxy that appears to contain almost no dark matter. Hubble helped to accurately confirm the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Based on these data the team discovered that NGC 1052-DF2 larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.
Note 1: The galaxy was identified with the Dragonfly Telephoto Array (DFA) and also observed by the Sloan Digital Sky Survey (SDSS). As well as the NASA/ESA Hubble Space Telescope, the Gemini Observatory and the Keck Observatory were used to study the object in more detail.

- "I spent an hour just staring at this image," lead researcher Pieter van Dokkum of Yale University says as he recalls first seeing the Hubble image of NGC 1052-DF2. "This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy."

- Further measurements of the dynamical properties of ten globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxies mass. This mass is comparable to the mass of the stars in the galaxy, leading to the conclusion that NGC 1052-DF2 contains at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all. This discovery is unpredicted by current theories on the distribution of dark matter and its influence on galaxy formation.
Note 2: Since 1884 astronomers have invoked dark matter to explain why galaxies do not fly apart, given the speed at which the stars within galaxies move. From Kepler's Second Law it is expected that the rotation velocities of stars will decrease with distance from the center of a galaxy. This is not observed.

- "Dark matter is conventionally believed to be an integral part of all galaxies – the glue that holds them together and the underlying scaffolding upon which they are built," explains co-author Allison Merritt from Yale University and the Max Planck Institute for Astronomy, Germany. And van Dokkum adds: "This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work." - Merritt remarks: "There is no theory that predicts these types of galaxies – how you actually go about forming one of these things is completely unknown."

- Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it. The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Note3: The MOND theory – Modified Newtonian Dynamics – suggests that the phenomena usually attributed to dark matter can be explained by modifying the laws of gravity. The result of this would be that a signature usually attributed to dark matter should always be detected, and is an unavoidable consequence of the presence of ordinary matter.

- Meanwhile, the researchers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2. Did a cataclysmic event such as the birth of a multitude of massive stars sweep out all the gas and dark matter? Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago play a role in NGC 1052-DF2's dark matter deficiency?

- These ideas, however, still do not explain how this galaxy formed. To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyze Hubble images of 23 ultra-diffuse galaxies – three of which appear to be similar to NGC 1052-DF2.


• April 24, 2017: Since its launch on 24 April 1990, Hubble has been nothing short of a revolution in astronomy. The first orbiting facility of its kind, for 27 years the telescope has been exploring the wonders of the cosmos. Astronomers and the public alike have witnessed what no other humans in history have before. In addition to revealing the beauty of the cosmos, Hubble has proved itself to be a treasure chest of scientific data that astronomers can access. 45) 46)

- NASA and ESA celebrate Hubble's birthday each year with a spectacular image. This year's anniversary image features a pair of spiral galaxies known as NGC 4302 – seen edge-on – and NGC 4298, both located 55 million light-years away in the northern constellation of Coma Berenices (Berenice's Hair). The pair, discovered by astronomer William Herschel in 1784, form part of the Virgo Cluster, a gravitationally bound collection of nearly 2000 individual galaxies. Such objects were first simply called "spiral nebulas," because it wasn't known how far away they were. In the early 20th century, Edwin Hubble discovered that galaxies are other island cities of stars far outside our Milky Way.

- At their closest points, the galaxies are separated from each other in projection by only around 7000 light-years. Given this very close arrangement, astronomers are intrigued by the galaxies' apparent lack of any significant gravitational interaction; only a faint bridge of neutral hydrogen gas – not visible in this image – appears to stretch between them. The long tidal tails and deformations in their structure that are typical of galaxies lying so close to each other are missing completely.


Figure 37: HST images of spiral galaxies NGC 4302 (left) and NGC 4298 (right), both located 55 million light-years away. They were observed by Hubble to celebrate its 27th year in orbit. The image in visible and infrared light brilliantly captures their warm stellar glow and brown, mottled patterns of dust [image credit: NASA, ESA, and M. Mutchler (STScI)]

- The edge-on galaxy is called NGC 4302, and the tilted galaxy is NGC 4298. These galaxies look quite different because we see them angled at different positions on the sky. They are actually very similar in terms of their structure and contents.

- From our view on Earth, researchers report an inclination of 90 degrees for NGC 4302, which is exactly edge on. NGC 4298 is tilted 70 degrees.

- In NGC 4298, the telltale, pinwheel-like structure is visible, but it's not as prominent as in some other spiral galaxies. In the edge-on NGC 4302, dust in the disk is silhouetted against rich lanes of stars. Absorption by dust makes the galaxy appear darker and redder than its companion. A large blue patch appears to be a giant region of recent star formation.

Figure 38: This animation zooms through the Virgo Cluster of nearly 2,000 galaxies into tight Hubble Space Telescope images of spiral galaxies NGC 4302 (left) and NGC 4298 (right) in visible and infrared light. Located approximately 55 million light-years away, the starry pair offers a glimpse of what our Milky Way galaxy would look like to an outside observer [image credit: NASA, ESA, and G. Bacon, J. DePasquale, and Z. Levay (STScI) Acknowledgment: A. Fujii; Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO; B. Franke (Focal Point Observatory); and M. Mutchler (STScI)]

- A typical spiral galaxy has arms of young stars that wind outward from its center. The bright arms are regions of intense star formation. Such galaxies have a central bulge and are surrounded by a faint halo of stars. Many spiral galaxies also have bars that extend from the central bulge to the arms.

- The edge-on NGC 4302 is about 87,000 light-years in diameter, which is about 60 percent the size of the Milky Way. It is about 110 billion solar masses, approximately one-tenth of the Milky Way's mass.

- The tilted NGC 4298 is about 45,000 light-years in diameter, about one third the size of the Milky Way. At 17 billion solar masses, it is less than 2 percent of the Milky Way galaxy's 1 trillion solar masses.

- The Hubble observations were taken between 2 - 22 January, 2017 with the WFC3 (Wide Field Camera 3) instrument in three visible light bands.


Hubble's 25th anniversary on orbit on April 24, 2015

From planets to planetary nebula, and from star formation to supernova explosions, the NASA/ESA Hubble Space Telescope has captured a wealth of astronomical objects in its 25-year career. The montage of Figure 39 presents 25 images that sample the space telescope's rich contribution to our understanding of the Universe around us. 47) 48) 49)

The NASA/ESA Hubble was launched into orbit by the Space Shuttle on 24 April 1990 (12:33:51 UTC). It was the first space telescope of its kind, and has surpassed all expectations, providing a quarter of a century of discoveries, stunning images and outstanding science.

The anniversary image (Figure 40) is bursting with silver anniversary fireworks, showing off a giant young star cluster known as Westerlund 2, sparkling with the light of about 3000 stars. Hubble's sharp vision resolves the dense concentration of stars in the central cluster, which measures only about 10 light-years across.

A new anniversary image of Hubble is released every year and shown in Figure 39.

At the center of the collage is Star cluster Westerlund 2, the image released on the occasion of Hubble's 25th anniversary.

Top row (from left to right): Interacting galaxies , Abell 2218 , Comet ISON , Jupiter , Green filament in the Teacup galaxy , Star formation in 30 Doradus , Interacting galaxies Arp 273

Second row: Saturn , The Ring Nebula , "Mystic Mountain" in the Carina Nebula , Crab Nebula

Third row: The horsehead nebula , Carina Nebula , Planetary nebula NGC 6302 , Star formation in M17

Fourth row: Globular cluster NGC 121 , "Pillars of creation" , Ring galaxy AM 0644-741,

Fifth row: Colliding galaxies Arp 272, Star cluster NGC 602, Hubble Ultra Deep Field, Mars, Variable star RS Puppis , Orion Nebula

Table 1: Links to the Hubble anniversary images in Figure 39 (Ref. 47)


Figure 39: Collage of 25 images representing Hubble's rich contribution to our understanding of the Universe (image credit: NASA/ESA)


• This glittering tapestry of young stars flaring into life in the star cluster Westerlund 2 has been released to celebrate the NASA/ESA Hubble Space Telescope's 25th year in orbit and a quarter of a century of discoveries, stunning images and outstanding science. 50)


Figure 40: NASA unveils Celestial Fireworks as Official Image for Hubble's 25th Anniversary on April 24, 2015. The image was acquired with WFC-3 (Wide Field Camera-3) piercing through the dusty veil shrouding the stellar nursery in near-infrared light, giving astronomers a clear view of the nebula and the dense concentration of stars in the central cluster. (image credit: NASA, ESA, STScI) 51) 52) 53)

Legend to Figure 40: The sparkling centerpiece of Hubble's anniversary fireworks is a giant cluster of about 3,000 stars called Westerlund 2, named for Swedish astronomer Bengt Westerlund who discovered the grouping in the 1960s. The cluster resides in a raucous stellar breeding ground known as Gum 29, located 20,000 light-years away from Earth in the constellation Carina.

The giant star cluster is only about two million years old, but contains some of the brightest, hottest and most massive stars ever discovered. Some of these are carving deep cavities in the surrounding material through their intense ultraviolet light and the high-speed charged particles contained in their stellar winds.

This image is a testament to Hubble's observational power and demonstrates that, even with 25 years of operations under its belt, its story is by no means over. Hubble has set the stage for the James Webb Space Telescope – scheduled for launch in 2018 – but will not be immediately replaced by this next-generation observatory, instead working alongside it. Now, 25 years after launch, is the time to celebrate Hubble's future potential as well as its remarkable history.

• November 2, 2015: Eerie, dramatic new pictures from NASA's Hubble Space Telescope show newborn stars emerging from "eggs" - not the barnyard variety - but rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs," appropriately enough, in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens (Figure 41). 54)

- "For a long time astronomers have speculated about what processes control the sizes of stars - about why stars are the sizes that they are," said Jeff Hester of Arizona State University, Tempe, AZ. "Now in M16 we seem to be watching at least one such process at work right in front of our eyes."

- Striking pictures taken by Hester and co-investigators with Hubble's Wide Field and Planetary Camera 2 (WFPC2) resolve the EGGs at the tip of finger-like features protruding from monstrous columns of cold gas and dust in the Eagle nebula (also called M16 - 16th object in the Messier catalog). The columns - dubbed "elephant trunks" - protrude from the wall of a vast cloud of molecular hydrogen, like stalagmites rising above the floor of a cavern. Inside the gaseous towers, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings.

- Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space - a process called "photoevaporation. "The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.

- "It's a bit like a wind storm in the desert," said Hester. "As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns."

- Some EGGs appear as nothing but tiny bumps on the surface of the columns. Others have been uncovered more completely, and now resemble "fingers" of gas protruding from the larger cloud. (The fingers are gas that has been protected from photoevaporation by the shadows of the EGGs). Some EGGs have pinched off completely from the larger column from which they emerged, and now look like teardrops in space.

- By stringing together these pictures of EGGs caught at different stages of being uncovered, Hester and his colleagues from the Wide Field and Planetary Camera Investigation Definition Team are getting an unprecedented look at what stars and their surroundings look like before they are truly stars.

- "This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation," Hester emphasized. "In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed."

- "In a few cases we can see the stars in the EGGs directly in the WFPC2 images," says Hester. "As soon as the star in an EGG is exposed, the object looks something like an ice cream cone, with a newly uncovered star playing the role of the cherry on top."

- Ultimately, photoevaporation inhibits the further growth of the embyronic stars by dispersing the cloud of gas they were "feeding" from. "We believe that the stars in M16 were continuing to grow as more and more gas fell onto them, right up until the moment that they were cut off from that surrounding material by photoevaporation," said Hester.


Figure 41: Gas Pillars in the Eagle Nebula (M16): Pillars of Creation in a star-forming region (image credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University))


Super Nova SN 1987A in the Large Magellanic Cloud

Thirty years ago, on 23 February 1987, the light from a stellar explosion marking the death of a massive star arrived at Earth to shine in Southern Hemisphere skies. Located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, SN 1987A was the closest observed supernova to Earth since the invention of the telescope. Studying it for the last 30 years has revolutionized our understanding of the explosive death of massive stars. 55)

- In operation since 1990, the NASA/ESA Hubble Space Telescope has observed the supernova remnant many times, as highlighted in this montage of Figure 42. The images show its evolution between 1994 and 2016, and highlight the main ring that blazes around the exploded star.


Figure 42: Hubble follows the evolution of an expanding supernova remnant over three decades (image credit: NASA, ESA and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)

A new wide-field image(Figure 43) was also taken by Hubble in January 2017 to mark the 30 year anniversary. By observing the expanding remnant material over the years, Hubble has helped to show that the material within the ring was likely ejected 20,000 years before the actual explosion took place.

The initial burst of light from the supernova initially illuminated the rings. They slowly faded over the first decade after the explosion, until a fast-moving shell of gas ejected during the supernova slammed into the central ring, sending a powerful shockwave through the gas, heating it to searing temperatures and generating strong X-ray emission.

This caused clumps of denser gas within the ring to light up like a string of pearls, seen as the increasing number of bright spots, which are now fading again. As the shock wave continues to move through the shells ejected by the dying star in its final throes of life, who knows what new details will be revealed?

Since its launch in 1990 Hubble has observed the expanding dust cloud of SN 1987A several times and this way helped astronomers to create a better understanding of these cosmic explosions.

Supernova 1987A is located in the center of the image amidst a backdrop of stars. The bright ring around the central region of the exploded star is composed of material ejected by the star about 20,000 years before the actual explosion took place. The supernova is surrounded by gaseous clouds. The clouds' red color represents the glow of hydrogen gas.

The colors of the foreground and background stars were added from observations taken by Hubble's WFPC2 ( Wide Field Planetary Camera 2).


Figure 43: This new image of the supernova remnant SN 1987A was taken by the NASA/ESA Hubble Space Telescope in January 2017 using its WFC3 (Wide Field Camera 3), image credit: NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)

Left: For comparison, a picture taken with the WFPC-1 camera in wide field mode, on November 27, 1993, just a few days prior to the STS-61 servicing mission. The effects of optical aberration in HST's 2.4-meter primary mirror blur starlight, smear out fine detail, and limit the telescope's ability to see faint structure. Both Hubble images are "raw;" they have not been subject to computer image reconstruction techniques commonly used in aberrated images made before the servicing mission.

Target Information of M100: The galaxy M100 (100th object in the Messier Catalog of non-stellar objects) is one of the brightest members of the Virgo Cluster of galaxies. The galaxy is in the spring constellation Coma Berenices and can be seen through a moderate-sized amateur telescope. M100 is spiral shaped, like our Milky Way, and tilted nearly face-on as seen from earth. The galaxy has two prominent arms of bright stars and several fainter arms. Though the galaxy is estimated to be tens of millions of light-years away, Hubble reveals the sort of detail only seen previously (with ground based telescopes) in neighboring galaxies that are ten times closer. Before HST, astronomers could only see such a level of detail in roughly a dozen galaxies in our Local Group. Now, with Hubble's improved vision, the portion of the universe which can be studied with such clarity has grown a thousand fold. Only the future will tell what revelations await as Hubble's spectacular vision is applied to a host of fascinating and important questions about the universe and our place in it.



Hubble Servicing Missions

HST (Hubble Space Telescope) was launched April 24, 1990 on Shuttle flight STS-31. However, after Hubble's deployment, scientist realized that the telescope's primary mirror had a flaw called "spherical aberration." The outer edge of the mirror was ground too flat by a depth of 4 μm. This aberration resulted in images that were fuzzy because some of the light from the objects being studied was being scattered. After the amount of aberration was understood, scientists and engineers developed WFPC2 (Wide Field/Planetary Camera 2) and COSTAR (Corrective Optics Space Telescope Axial Replacement), which were installed in Hubble during the first space shuttle servicing mission in 1993. - Without periodic onboard servicing, HST would have been a disaster and would not have produced all of the great science it has. 56) 57) 58)

All Hubble Servicing Missions were conducted by berthing the Hubble Spacecraft inside the Shuttle Payload Bay. The Shuttle resources SRMS/OBSS (Shuttle Remote Manipulator Arm/Orbiter Boom Sensor System), SSRMS (Space Station Remote Manipulator System, Canadarm2 ), communication system, etc. were used to conduct the inspections and repairs of Hubble.

The servicing missions involved intensive coordination between NASA's Kennedy Space Center in Florida, Johnson Space Center in Houston, and Goddard Space Flight Center in Greenbelt, Maryland. Preparation activities included astronaut training at all three centers; simulations of telescope operations during the mission at the Space Telescope Operations Control Center (STOCC) at Goddard; testing and preparing instruments and hardware for flight at Goddard; and preparing the space shuttle for launch, flight and landing at Kennedy.

During the missions, operations took place primarily at Johnson and in Goddard's STOCC. Johnson's Mission Control Center (MCC) monitored every aspect of the space shuttle and astronauts, including spacewalks, procedures and schedules, crew activities and health, and in-cabin and cargo bay systems and experiments. The STOCC ground crew handled telescope operations, sending commands to Hubble to place the instruments into "safe hold" (hibernation), close the aperture door (which covers the precious optical components), and perform maneuvers to position the telescope for grappling by the shuttle's robotic arm, operated by astronauts to bring Hubble into the shuttle's payload bay.

After each new system part or science instrument was installed, STOCC personnel performed tests to make sure each instrument and component had power and operated as it should. During the astronauts' sleep cycles, the STOCC team performed more detailed tests on each newly installed component to determine if additional service from astronauts would be required.

After all the servicing tasks had been performed via a three- to five-day series of spacewalks, STOCC controllers and Johnson Mission Control prepared the telescope for release. Often this also involved using the shuttle's thrusters to climb into a higher orbit before releasing Hubble.

The astronaut crew then used the shuttle's robotic arm to slowly raise Hubble from the payload bay and out into space, where controllers at STOCC opened Hubble's aperture door and made sure the telescope was functioning on its own. Returning Hubble to full science observations after a servicing mission usually took a few months.


SM-1 (Servicing Mission-1)

The primary goal of Servicing Mission 1 was to restore Hubble's vision. Because Hubble's primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed. Astronauts on space shuttle Endeavour's STS‐61 mission spent five days tuning it up. They installed two new devices - WFPC2 and COSTAR.

The first Hubble repair mission was launched Dec. 2, 1993 on STS-61 (Endeavour, landing on 13 Dec. 1993 at KSC). Installation of COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR deployed corrective optics in front of three of Hubble's first-generation instruments – the Faint Object Camera, the Goddard High Resolution Spectrometer, and the Faint Object Spectrograph. 59) 60) 61)
In addition, SM-1 included the installation and replacement of other components including: solar arrays, solar array drive electronics, magnetometers, two rate sensor units, two gyroscope electronic control units, etc.

After SM-1, Hubble became operational transmitting stupefying images of supernovas, gigantic explosions that marked the death of a star and revealed mysterious black holes in the center of virtually all galaxies. Thanks to these observations, delivered with 10 times the clarity of the most powerful telescopes on Earth, astronomers have been able to confirm that the universe is expanding at an accelerating rate and to calculate its age with greater precision as an estimated 13.7 billion years.

The shuttle flight of 1993 was one of most challenging and complex manned missions ever attempted. During a record five back-to-back space walks totaling 35 hours and 28 minutes, two teams of astronauts completed the first servicing of the Hubble Space Telescope (HST). In many instances, tasks were completed sooner than expected and a few contingencies that did arise were handled smoothly. 62)


Figure 44: STS-61 Crew photo with Commander Richard O. Covey, Pilot Kenneth D. Bowersox, Payload Commander F. Story Musgrave and Mission Specialists Kathryn C. Thornton, Claude Nicollier, Jeffrey A. Hoffman and Tom Akers (image credit: NASA)

WFPC2 significantly improved ultraviolet performance over WFPC1, the original instrument. In addition to having more advanced detectors and more stringent contamination control, it also incorporated built-in corrective optics.


Figure 45: WFPC2 in the enclosure Image credit: NASA)

• This comparison image of the core of galaxy M100 shows the dramatic improvement in the Hubble telescope's view of the universe of the universe after the first Hubble Servicing Mission in December 1993. The new image (right) was taken with the second generation Wide Field and Planetary Camera (WFPC2), which was installed during the STS-61 Hubble Servicing Mission. 63)


Figure 46: This comparison image of the core of the galaxy M100 shows the dramatic improvement in Hubble Space Telescope's view of the universe after the first servicing mission in December 1993. The original view, taken a few days before the servicing mission, is on the left (image credit: NASA, Ref. 58)

Legend to Figure 46: This picture beautifully demonstrates that the camera's corrective optics compensate fully for the optical aberration in Hubble's primary mirror. With the new camera, the Hubble explored the universe with unprecedented clarity and sensitivity, and fulfilled its most important scientific objectives for which the telescope was originally built. 64)

Right: The core of the grand design spiral galaxy M100, as imaged by Hubble Space Telescope's Wide Field Planetary Camera 2 in its high resolution channel. The WFPC-2 contains modified optics that correct for Hubble's previously blurry vision, allowing the telescope for the first time to cleanly resolve faint structure as small as 30 light-years across in a galaxy which is tens of millions of light years away. The image was taken on December 31, 1993.

• December 2, 2013. Although the SM-1 mission was a triumph, and it marked the beginning of the Hubble telescope's long and illustrious career, astronaut Jeffrey Hoffman recalled the SM-1 events at a symposium to mark the 20th anniversary of STS-61. "There were a lot of people who doubted that we could accomplish all the things we had set out to, but here we were at the end of the fifth of five [spacewalks], and we had accomplished 13 of the 12 tasks that had been assigned to us — so we were, justifiably, very happy." 65)


SM-2 (Servicing Mission-2)

The second Hubble service flight was on STS-82 (Feb. 11-21, 1997). 66) The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing to probe the most distant reaches of the universe. The replacement of the failed or degraded spacecraft components increased efficiency and performance. The newly installed instruments were: STIS (Space Telescope Imaging Spectrograph), and NICMOS (Near Infrared Camera and Multi-Object Spectrometer).

• NICMOS enabled Hubble to observe infrared wavelengths (0.8-2.5 µm), crucial for viewing very distant optical sources that have lost energy traveling across most of the visible universe and now radiate in the infrared band. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.

• STIS could take detailed pictures of celestial objects and hunt for black holes. Both instruments featured technology that wasn't available when scientists designed and built the original Hubble instruments in the late 1970s. STIS's two-dimensional detectors have allowed the instrument to gather 30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble. These were capable of only looking at one place at a time.

One of the greatest advantages to using STIS is in the study of supermassive black holes. STIS searches for massive black holes by studying the star and gas dynamics around galactic centers. It measures the distribution of matter in the universe by studying quasar absorption lines. It also uses its high sensitivity and spatial resolution to study star formation in distant galaxies and perform spectroscopic mapping of solar system objects.

The astronauts also installed a refurbished FGS (Fine Guidance Sensor), one of three essential instruments used to keep Hubble steady while viewing objects and to calculate celestial distances; a Solid State Recorder to replace one of Hubble's data recorders; and a refurbished, spare Reaction Wheel Assembly, part of the Pointing Control Subsystem.


Figure 47: STS-82 Crew photo with Commander Kenneth D. Bowersox, Pilot Scott J. Horowitz, Mission Specialists Mark C. Lee, Steven A.Hawley, Gregory J. Harbaugh, Steven L. Smith and Joseph R. Tanner (image credit: NASA)


SM-3 (Servicing Mission-3)

Scheduled for June 2000, the third mission to the Hubble Space Telescope was originally planned to carry out preventive repairs. However, urgency to address the failure of Hubble's third gyroscope led NASA managers to split SM3 into two parts (SM3A and SM3B), scheduling an early servicing mission (SM3A) for December 1999. 67)

The unexpected failure of the fourth of Hubble's six gyroscopes on 13 November 1999, with SM3A already planned, caused NASA to place Hubble into safe mode. Unable to conduct science without at least three working gyros, Hubble went into a sort of protective hibernation until 19 December 1999, when a crew of astronauts aboard the Discovery Space Shuttle flew to its rescue and replaced all the gyroscopes.

Since the second Servicing Mission in February 1997, three of the gyroscopes had failed and caused some concern among NASA officials. Additionally, NASA deemed necessary the replacement of one of Hubble's three Fine Guidance Sensors (FGS). Both devices are part of Hubble's advanced pointing control system, and as such, they keep the telescope steady during observations.


Figure 48: STS-103 Crew photo with Commander Curtis L. Brown, Pilot Scott J. Kelly, Mission Specialists Steven L. Smith, C. Michael Foale, John M. Grunsfeld, Claude Nicollier and Jean-Francois Clervoy (image credit: NASA) 68)

SM-3A (Servicing Mission-3A)

The Hubble service flight on STS-103 took place Dec. 19-27, 1999. Objective: replacement of gyroscopes (after the third of Hubble's six gyroscopes failed), a fine guidance sensor and a S/C computer. Installation of six voltage/temperature kits for the S/C batteries. Installation of a new transmitter, solid-state recorder (12 Gbit), and thermal insulation blankets. 69) 70)

What was originally conceived as a mission of preventive maintenance turned more urgent on November 13, 1999, when the fourth of six gyros failed and Hubble temporarily closed its eyes on the universe. Unable to conduct science without three working gyros, Hubble entered a state of dormancy called safe mode. Essentially, Hubble "went to sleep" while it waited for help.

STS-103 restored the Hubble Space Telescope to working order and upgraded some of its systems, allowing the decade-old observatory to get ready to begin its second scheduled decade of astronomical observations (Ref. 68).

The first few days of the 8-day mission, the crew prepared for the rendezvous and capture of the Hubble Space Telescope and the three maintenance spacewalks to follow. After a 30-orbit chase Commander Brown and Kelly maneuvered the orbiter to a point directly beneath Hubble, then moved upward toward it. Mission Specialist Clervoy grappled Hubble using the orbiter's robotic arm and placed it on the Flight Support System in the rear of Discovery's cargo bay.

EVA No. 1: Mission Specialists Steven Smith and John Greenfield conducted the mission's first spacewalk. The two made numerous repairs, including replacing the telescope's three Rate Sensor Units — each containing two gyroscopes. They also installed six Voltage/Temperature Improvement Kits between Hubble's solar panels and its six 10-year-old batteries. The kits, the size of cell telephones, were designed to prevent any overheating or overcharging of those batteries. A few minor objectives were left undone, such as taking close-up photos of the Voltage/Temperature Improvement Kits. The 8-hour, 15-minute space walk was second to the longest space walk from Endeavour on STS-49 in May 1992. A few minor problems helped account for the length of the space walk. The astronauts had difficulty in removing one of the old RSUs, and opening valves and removing caps on the NICMOS. The tasks were eventually completed.

EVA No. 2: During the mission's second space walk, Mission Specialists Michael Foale and Claude Nicollier installed a new advanced computer — 20 times faster than Hubble's old one — and a new, 250 kg fine guidance sensor. This 8-hour, 10 minute space walk was the third longest in history. With all major activities accomplished, controllers reported that power was reaching both of the new pieces of equipment. "The brains of Hubble have been replaced," said Mission Specialist Grunsfeld. About 30 minutes later, Hubble began thinking with those new brains.

EVA No. 3: Smith and Grunsfeld again teamed up to make the mission's third and final space walk. Like the first two, it also lasted more than 8 hours, making it the fourth longest in history. The team installed a transmitter that sends scientific data from Hubble to the ground. It replaced one that failed in 1998. The astronauts used special tools developed for the task because transmitters, usually very reliable, were not designed to be replaced in orbit. Smith and Grunsfeld also installed a solid state digital recorder, replacing an older mechanical reel-to-reel recorder.

Hubble was released from Discovery's cargo bay on Christmas Day.

The Hubble team has left the telescope far more fit and capable than ever before. The new, improved, and upgraded equipment included six fresh gyroscopes, six battery voltage/temperature improvement kits, a faster, more powerful, main computer, a next-generation solid state data recorder, a new transmitter, an enhanced fine guidance sensor, and new insulation.


Figure 49: Hubble berthed in the Space Shuttle bay during Servicing Mission 3A. Astronauts Steven L. Smith, and John M. Grunsfeld, appear as small figures in this wide scene photographed during EVA (Extravehicular Activity), image credit: NASA/ESA


SM-3B (Servicing Mission-3B)

A routine servicing mission to HST took place Mar. 1- 11, 2002 on STS-109 (Columbia). Installation of ACS (Advanced Camera for Surveys), built by Ball Aerospace for NASA and consisting of three cameras in the spectral range of 0.12-1.0 μm. The WFC (Wide Field Camera) uses a CCD area array of 16 Mpixel (4096 x 4096). The second is a HRC (High Resolution Camera) using a 1024 x 1024 CCD array and a high sensitivity in the UV. The third camera, the SBC (Solar-Blind Camera), is a far-ultraviolet, pulse-counting array that has a relatively high throughput at 121 nm. SA-3 (Solar Array-3) installation and PCU (Power Control Unit). Installation of a new experimental cryocooler for NICMOS (70 K cooling to revive its IR vision, and extend its life by several years). 71) 72) 73) 74)


Figure 50: Illustration of the ACS instrument configuration (image credit: NASA)

Solar Array 3 (SA3) Installation: Four large flexible solar array (SA) panels (wings) provide power to the observatory. During SM1, the original arrays were replaced by SA2 and have powered Hubble for over 8 years. Radiation and debris take their toll on sensitive electronics, which will be replaced to ensure uninterrupted service for the remainder of the mission.

The new solar arrays (SA3) are rigid arrays, which do not roll up and therefore are more robust. Hubble gets a brand new look with its latest set of solar wings. Although one-third smaller than the first two pairs, the power increase was between 20 and 30 percent. They are less susceptible to extreme temperatures and their smaller-sized will reduce the effects of atmospheric drag on the spacecraft.


Figure 51: The Hubble Space Telescope (HST) heads back toward its normal routine, after a week of servicing and upgrading by the STS‐109 astronaut crew in 2002 (image credit: NASA, Ref. 75)


SM-4 (Servicing Mission-4)

The launch of SM-4 or flight STS-125 on Space Shuttle Atlantis, took place on May 11, 2009 (landing on May 24, 2009) with seven astronauts aboard (RMS capture, repair and upgrade of the 11,000 kg HST spacecraft at an orbital altitude of 560 km). Five spacewalks are required to refurbish Hubble with state-of-the-art science instruments designed to improve the telescope's discovery capabilities. The goal of the long overdue service mission is to extend the star-gazer's life by at least five years (the 2003 Columbia disaster that saw the shuttle disintegrate as it re-entered Earth's atmosphere, killing all seven crew members was the main reason for the long delay).


Figure 52: The STS‐125 crew members take a moment to pose for a crew photo before a training session in the Space Vehicle Mockup Facility at NASA's Johnson Space Center. From the left are astronauts Mike Massimino, Michael Good, both mission specialists; Gregory C. Johnson, pilot; Scott Altman, commander; Megan McArthur, John Grunsfeld and Andrew Feustel, all mission specialists (image credit: NASA)


Figure 53: This graphic depicts the location of the STS‐125 payload hardware (image credit: NASA)

The priorities of the servicing mission are: 75)

• Three Rate Sensor Unit (gyroscope) removal and replacement (only two of the six gyros are currently in operation)

WFC3 (Wide Field Camera 3). WFC3 replaces WFPC2 (Wide Field Planetary Camera 2). Use of 4 k x 2 k CCD e2v detector array providing full-frame imaging. — The WFPC2 was originally installed in the first Hubble servicing mission in 1993, and was nicknamed "the camera that saved Hubble" because its special optics were able to overcome the spherical aberration in the telescope's main mirror.

- The WFC3 is configured as a two‐channel instrument. Its wide‐wavelength coverage with high efficiency is made possible by this dual‐channel design using two detector technologies. The incoming light beam from the Hubble telescope is directed into WFC3 using a pick‐off mirror, and is directed to either the Ultraviolet‐Visible (UVIS) channel or the Near‐Infrared (NIR) channel. The light‐sensing detectors in both channels are solid‐state devices. For the UVIS channel a large format CCD (Charge Coupled Device), similar to those found in digital cameras, is used. In the NIR detector the crystalline photosensitive surface is composed of mercury, cadmium and tellurium (HgCdTe). 76)

- The high sensitivity to light of the 16 megapixel UVIS CCD, combined with a wide field of view (160 x 160 arcsec), yields about a 35‐times improvement in discovery power versus Hubble's current most sensitive ultraviolet imager, the ACS High Resolution Channel. The NIR channel's HgCdTe detector is a highly advanced and larger (one megapixel) version of the 65,000 pixel detectors in the current near‐infrared instrument, NICMOS. The combination of field‐of‐view, sensitivity, and low detector noise results in a 15‐20 x enhancement in capability for WFC3 over NICMOS.

- An important design innovation for the WFC3 NIR channel results from tailoring its detector to reject infrared light (effectively "heat") longer in wavelength than 1700 nm. In this way it becomes unnecessary to use a cryogen (e.g., liquid or solid nitrogen) to keep it cold. Instead the detector is chilled with an electrical device called a Thermo‐Electric Cooler (TEC). This greatly simplifies the design and will give WFC3 a longer operational life.


Figure 54: The Wide Field Camera 3 is inspected and readied for flight aboard STS‐125 (image credit: NASA)


Figure 55: Overview of the WFC3 instrument (image credit: NASA) 77)

- Science instrument C&DH (Command & Data Handling) system swap out (replacement of a unit that failed in Sept. 2008)

- COS (Cosmic Origins Spectrograph) installation and replacement of COSTAR of SM-1. COS is a medium resolution spectrograph specifically designed to observe in the near and mid ultraviolet spectral range. COS was designed with one overriding objective in mind: to collect as many ultraviolet photons of light as possible and hence make possible the effective study of the huge, dark reservoir of gas that exists between the galaxies both near and far — t he so-called "cosmic web" of matter which represents the largest-scale structure in the universe.


Figure 56: Illustration of the COS configuration (image credit: NASA)

- Battery module replacement installation (Bays 2 and 3). This is the first battery replacement in 19 years.

- Fine Guidance Sensor 2 removal and replacement (it is one of three sensors that help point and lock the telescope on targets)

- Repair of ACS (Advanced Camera for Surveys): ACS has been inoperable since January 2007, when its backup power supply system failed. Replacement of the entire electronics box, which will be powered by a separate low‐voltage power supply.

One piece of new technology is an ASIC, 78) 79) that enables an entire circuit board's worth of electronics to be condensed into a very small package. It will be a part of the new CCD in the CEB (CCD Electronics Box) that will be installed to repair the failed ACS instrument. - The ASIC design is the same as the one already developed and tested for the JWST (James Webb Space Telescope) mission. However, the electronics packaging for Hubble is different because of the different operating conditions such as temperature and electronics environments.

- New Outer Blanket Layer installation (Bays 8, 5 & 7)

- Reboost of the HST spacecraft altitude.

- Reboost of the HST spacecraft altitude.


EVA1: The first spacewalk of the mission, performed by astronauts John Grunsfeld and Drew Feustel lasted a little over 7 1/2 hours. They successfully installed the new Wide Field Camera 3 science instrument and a new Science Instrument Command and Data Handling Unit. Both WFC-3 and the SI C&DH passed their "aliveness" tests, which essentially means the devices powered on correctly. The WFC-3 also passed its functional test, meaning the capabilities of the instrument itself were tested. The SI C&DH unit has also received an initial OK on its functional test, pending final review of data sent down to the ground. 80)


Figure 57: Andrew Feustel hauls the new WFC3 on the robotic arm, to install the camera on Hubble. (image credit: NASA)

EVA2: The second EVA of the mission provided some challenges to astronauts Michael Good and Mike Massimino. However, they achieved all the objectives for this spacewalk, it just took them awhile — 7 hours and 56 minutes. They installed three Rate Sensor Units (RSUs), with a pair of gyros in each, and the first of two new battery module units.


Figure 58: Astronaut Michael Good works with the Hubble Space Telescope in the cargo bay of the Earth-orbiting Space Shuttle Atlantis along with Mike Massimino (image credit: NASA)

EVA3: The third EVA of the mission went like clockwork as Grunsfeld and Feustel teamed up again. They removed the COSTAR (Corrective Optics Space Telescope Axial Replacement) and installed in its place the new COS (Cosmic Origins Spectrograph). They also completed an unprecedented repair of the Advanced Camera for Surveys replacing an electronic card and installed a new electronics box and cable.

To do the repairs on ACS, Grunsfeld removed 32 screws from an access panel to replace the camera's four circuit boards and installed a new power supply. The two astronauts used specially designed tools to do a job that was never intended to be done on orbit. But they did it, and with efficiency.

Engineers at Goddard have already performed "aliveness" tests on both COS and ACS to verify they have electrical power. However while a functional test of the ACS indicated success in reviving the instrument's heavily used wide-field channel, officials said early Sunday that it appears the repairs failed to resolve power problem with the camera's stricken high-resolution channel and it appears "down for the count."


Figure 59: STS-125 astronauts John Grunsfeld and Andrew Feustel work together on EVA 3 to navigate the exterior of the Hubble Space Telescope on the end of the remote manipulator system arm, controlled from inside Atlantis' crew cabin (image credit: NASA)

On May 18, 2009, the astronauts successfully completed all the EVAs for the Hubble Servicing Mission, accomplishing all the mission goals. 81) 82)



Figure 60: This image depicts the release of the Hubble Space Telescope on Flight Day 9 (image credit: NASA)


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52) "Celestial Fireworks celebrate Hubble's 25th Anniversary," ESA, April 23, 2015, URL:

53) "Hubble Space Telescope Celebrates 25 Years of Unveiling the Universe," STCcI, April 23, 2015, News release: STScI-2015-12, URL:

54) "Embryonic Stars Emerge from Interstellar "Eggs"," Hubblesite, News Release number: STScI-1995-44, 2 Nov. 2015, URL:

55) "The evolution of SN 1987A," ESA, Space Science Image of the Week, February 27, 2017, URL:

56) Edwin P. Hubble (Nov. 20, 1889-Sept. 28, 1953) was an American astronomer. He profoundly changed astronomers' understanding of the nature of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the distance of that galaxy from the Milky Way. This became known as Hubble's law, and would help establish that the universe is expanding.

57) Note: When originally planned in 1979, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5 year interval between servicing missions. In 1985, contamination and structural loading concerns associated with return to Earth aboard the Shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15 year design life.

58) "Hubble Servicing Missions Overview," NASA Hubble Space Telescope, 3 August 2017, URL:

59) "Corrective Optics Space Telescope Axial Replacement," NASA Facts, June 1993, URL:

60) "NASA's Optical Verification Program," NASA Facts, November 1993, URL:

61) "HST Servicing Mission Observatory Verification," NASA Facts, June 1993, URL:


63) "Picture Perfect: Hubble's New Improved Optics Probe the Core of a Distant Galaxy," Hubblesite, Release 1025 of January 13, 1994, URL:

64) "M100 Galactic Nucleus: Pictures of Galaxy M100 with Hubble's Old and New Optics," Hubblesite, News release ID: STScI-1994-01, Release Date: Jan 13, 1994, URL:

65) Denise Chow, "Saving Hubble: Astronauts Recall 1st Space Telescope Repair Mission 20 Years Ago,", 2 December 2013, URL:


67) "Servicing Mission 3A," ESA, URL:



70) "Hubble Space Telescope, Servicing Mission 3A, Media Reference Guide," Prepared for NASA by Lockheed Martin, URL:


72) "Advanced Camera for Surveys (ACS)," Hubble Facts, URL:

73) "Servicing Mission 3B - Another refurbishment for Hubble," ESA, Hubble Space Telescope, URL:

74) "Hubble's instruments: ACS - Advanced Camera for Surveys," ESA, Hubble Space telescope, URL:

75) "Space Shuttle Mission STS-125, The Final Visit to Hubble," NASA Press Kit, URL:

76) "Hubble Space Telescope – Wide Field Camera 3," NASA Facts, FS-2015-3-256-GSFC, URL:

77) "Shuttle Mission STS-125 Atlantis," NASA, URL:

78) "Hubble to Receive High-Tech JWST Technology," May 8, 2009, URL:

79) Timothy J. Cole, "On-Orbit Repair of Satellites using Fastener Capture Plates to Eliminate Debris," 2011 IEEE Aerospace Conference, Big Sky, MT, USA, March 5-12, 2011

80) Nancy Atkinson, "Hubble Servicing Mission 4 in Pictures, Part 1," Universe Today, May 17, 2009, URL:

81) Nancy Atkinson, "Super-Tools Essential to Hubble Mission Success," Universe Today, May 18, 2009, URL:

82) Nancy Atkinson, "Gallery: Behind the Scenes Images of the Final Hubble Servicing Mission," Universe Today, April 1, 2015, URL:


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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