Roman Space Telescope
Roman Space Telescope / former WFIRST (Wide Field Infrared Survey Telescope)
WFIRST is a NASA observatory designed to perform wide-field imaging and surveys of the NIR (Near Infrared ) sky and to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The telescope has a primary mirror that is 2.4 m in diameter, and is the same size as the Hubble Space Telescope's primary mirror. WFIRST will have two instruments, the WFI (Wide Field Instrument), and the CGI (Coronagraph Instrument).
WFIRST is the top-ranked large space mission in the ”New Worlds, New Horizon Decadal Survey of Astronomy and Astrophysics,” written by the U.S. National Research Council in 2010. That study, which laid out a blueprint for ground- and space-based astronomy and astrophysics for the decade of the 2010s, rated WFIRST as the top-priority large-scale mission. NASA's current plans call for WFIRST to perform an extraordinarily broad set of scientific investigations: studying the newly-discovered phenomenon of dark energy, measuring the history of cosmic acceleration, completing the exoplanet census begun by NASA's Kepler Space Telescope and demonstrating technology for direct imaging and characterization of exoplanets. 1)
After years of preparatory studies, NASA is formally starting an astrophysics mission designed to help unlock the secrets of the universe — the WFIRST (Wide Field Infrared Survey Telescope). NASA's Agency Program Management Council, which evaluates the agency's programs and projects on content, risk management, and performance, made the decision to move forward with the mission on February 17, 2016. 2)
The Wide Field Instrument will provide a field of view of the sky that is 100 times larger than images provided by HST (Hubble Space Telescope). The coronagraph will enable astronomers to detect and measure properties of planets in other solar systems. 3)
Table 1: NASA Telescope Named For ‘Mother of Hubble’ Nancy Grace Roman 4)
Figure 1: NASA’s Wide Field Infrared Survey Telescope (WFIRST) is now named the Nancy Grace Roman Space Telescope, after NASA’s first Chief of Astronomy (image credit: NASA)
Figure 2: Scheduled to launch in the mid-2020s, the Nancy Grace Roman Space Telescope, formerly known as WFIRST, will function as Hubble’s wide-eyed cousin. While just as sensitive as Hubble's cameras, the Roman Space Telescope's 300-megapixel Wide Field Instrument will image a sky area 100 times larger. This means a single Roman Space Telescope image will hold the equivalent detail of 100 pictures from Hubble (image credit: NASA)
• Dark Energy: In 1998, scientists discovered that the expansion of the universe is accelerating, causing them to reconsider their models for the formation of the universe. In order to explain their observations, cosmologists have to assume that 3/4 of the universe is filled with mass and energy that cannot be observed through traditional means. This 'dark matter' and 'dark energy' has revolutionized cosmology. WFIRST will contribute to our understanding of the nature of dark energy by addressing two questions:
- Is cosmic acceleration caused by a new energy component or by the breakdown of General Relativity on cosmological scales?
- If the cause is a new energy component, is its energy density constant in space and time, or has it evolved over the history of the universe?
WFIRST will conduct 3 different types of surveys in order to answer these questions.
1) HLSS (High Latitude Spectroscopic Survey): The HLSS will measure accurate distances and positions of a very large number of galaxies. By measuring the changes in the distribution of galaxies, the evolution of dark energy over time can be determined. The High Latitude Survey will measure the growth of large structure of the universe, testing theory of Einstein's General Relativity.
2) SNe (Type Ia Supernovae) Survey: The SNe survey uses type Ia SNe as "standard candles" to measure absolute distances. Patches of the sky are monitored to discover new supernovae and measure their light curves and spectra. Measuring the distance to and redshift of the SNe provides another means of measuring the evolution of dark energy over time, providing a cross-check with the high latitude surveys.
3) HLIS (High Latitude Imaging Survey): The HLIS will measure the shapes and distances of a very large number of galaxies and galaxy clusters. The shapes of very distant galaxies are distorted by the bending of light as it passes more nearby mass concentrations. These distortions are measured and used to infer the three-dimensional mass distribution in the Universe. This survey will determine both the evolution of dark energy over time as well as provide another independent measurement of the growth of large structure of the universe.
Figure 3: Is the Universe expanding more slowly than previously thought? A University of Arizona-led team of astronomers found that the type of supernovae commonly used to measure distances in the universe fall into distinct populations not recognized before. The findings have implications for our understanding of how fast the universe has been expanding since the Big Bang (image credit: NASA)
Some background on Dark Energy and Dark Matter
In the early 1990s, one thing was fairly certain about the expansion of the universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it. 5)
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Is Dark Energy? More is unknown than is known. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the universe.
Figure 4: Changes in the Rate of Expansion over Time: This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart (image credit: NASA/STScI/Ann Feild)
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.
Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.
What Is Dark Matter? By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as MACHOs (Massive Compact Halo Objects). But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).
The Expanding Universe:
When astronomer Edwin Hubble discovered nearly 100 years ago that the universe was uniformly expanding in all directions, the finding was a big surprise. Then, in the mid-1990s, another shocker occurred: astronomers found that the expansion rate was accelerating perhaps due to a repulsive property called "dark energy." Now, the latest measurements of our runaway universe suggest that it is expanding faster than astronomers thought. The consequences could be very significant for our understanding of the shadowy contents of our unruly universe. It may mean that dark energy is shoving galaxies away from each other with even greater – or growing – strength. Or, the early cosmos may contain a new type of subatomic particle referred to as "dark radiation." A third possibility is that "dark matter," an invisible form of matter that makes up the bulk of our universe, possesses some weird, unexpected characteristics. Finally, Einstein's theory of gravity may be incomplete. 6)
These unnerving scenarios are based on the research of a team led by Nobel Laureate Adam Riess, who began a quest in 2005 to measure the universe's expansion rate to unprecedented accuracy with new, innovative observing techniques. The new measurement reduces the rate of expansion to an uncertainty of only 2.4 percent. That's the good news. The bad news is that it does not agree with expansion measurements derived from probing the fireball relic radiation from the big bang. So it seems like something's amiss – possibly sending cosmologists back to the drawing board.
Astronomers using NASA's Hubble Space Telescope have discovered that the universe is expanding 5 percent to 9 percent faster than expected. "This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 percent of everything and don't emit light, such as dark energy, dark matter, and dark radiation," said study leader and Nobel Laureate Adam Riess of the STScI (Space Telescope Science Institute) and The Johns Hopkins University, both in Baltimore, Maryland.
There are a few possible explanations for the universe's excessive speed. One possibility is that dark energy, already known to be accelerating the universe, may be shoving galaxies away from each other with even greater – or growing – strength.
Another idea is that the cosmos contained a new subatomic particle in its early history that traveled close to the speed of light. Such speedy particles are collectively referred to as "dark radiation" and include previously known particles like neutrinos. More energy from additional dark radiation could be throwing off the best efforts to predict today's expansion rate from its post-big bang trajectory.
The boost in acceleration could also mean that dark matter possesses some weird, unknown characteristics. Dark matter is the backbone of the universe upon which galaxies built themselves up into the large-scale structures seen today. - And finally, the speedier universe may be telling astronomers that Einstein's theory of gravity is incomplete.
Riess' team made the discovery by refining the universe's current expansion rate to unprecedented accuracy, reducing the uncertainty to only 2.4 percent. The team made the refinements by developing innovative techniques that improved the precision of distance measurements to faraway galaxies.
These measurements are fundamental to making more precise calculations of how fast the universe expands with time, a value called the Hubble constant. The improved Hubble constant value is 73.2 km/second per megaparsec (a megaparsec equals 3.26 million light-years). The new value means the distance between cosmic objects will double in another 9.8 billion years.
This refined calibration presents a puzzle, however, because it does not quite match the expansion rate predicted for the universe from its trajectory seen shortly after the big bang. Measurements of the afterglow from the big bang by NASA's WMAP (Wilkinson Microwave Anisotropy Probe) and the European Space Agency's Planck satellite mission yield predictions for the Hubble constant that are 5 percent and 9 percent smaller, respectively.
"We know so little about the dark parts of the universe, it's important to measure how they push and pull on space over cosmic history," said Lucas Macri of Texas A&M University in College Station, a key collaborator on the study. Added Riess: "If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the big bang and use that understanding to predict how fast the universe should be expanding today. However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today."
Comparing the universe's expansion rate with WMAP, Planck, and the Hubble Space Telescope is like building a bridge, Riess explained. On the distant shore are the cosmic microwave background observations of the early universe. On the nearby shore are the measurements made by Riess' team using Hubble.
"You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right," Riess said. "But now the ends are not quite meeting in the middle and we want to know why."
The Hubble observations were conducted by the Supernova H0 for the Equation of State (SH0ES) team, which works to refine the accuracy of the Hubble constant to a precision that allows for a better understanding of the universe's behavior.
Riess' team made the improvements by streamlining and strengthening the construction of the cosmic distance ladder, which astronomers use to measure accurate distances to galaxies near and far from Earth. The team compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They used these two values to calculate the Hubble constant.
Among the most reliable of these cosmic yardsticks for relatively shorter distances are Cepheid variables, pulsating stars that dim and fade at rates that correspond to their true brightness. Their distances, therefore, can be inferred by comparing their true brightness with their apparent brightness as seen from Earth.
The researchers calibrated this stellar yardstick using a basic tool of geometry called parallax, the same technique that surveyors use to measure distances on Earth. With Hubble's sharp-eyed WFC3 (Wide Field Camera 3), they extended the parallax measurements farther than previously possible, across the Milky Way galaxy, to reach distant Cepheids.
To calculate accurate distances to nearby galaxies, the team looked for galaxies containing both Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same brightness and are brilliant enough to be seen from relatively longer distances. So far, Riess' team has measured about 2,400 Cepheid stars in 19 of these galaxies, representing the largest sample of such measurements outside the Milky Way. By comparing the observed brightness of both types of stars in those nearby galaxies, the astronomers could then accurately measure their true brightness and therefore calculate distances to roughly 300 Type Ia supernovae in far-flung galaxies.
Using one instrument, WFC3, to bridge the Cepheid rungs in the distance ladder, the researchers eliminated the systematic errors that are almost unavoidably introduced by comparing measurements from different telescopes. Measuring the Hubble constant with a single instrument is like measuring a hallway with a long tape measure instead of a single 12-inch ruler. By avoiding the need to pick up the ruler and lay it back down over and over again, you can prevent cumulative errors.
The SH0ES Team is still using Hubble to reduce the uncertainty in the Hubble constant even more, with a goal to reach an accuracy of 1 percent. Current telescopes such as the European Space Agency's Gaia satellite, and future telescopes such as the JWST (James Webb Space Telescope), an infrared observatory, and the WFIRST (Wide Field Infrared Space Telescope), also could help astronomers make better measurements of the expansion rate.
Before Hubble was launched in 1990, the estimates of the Hubble constant varied by a factor of two. In the late 1990s the Hubble Space Telescope Key Project on the Extragalactic Distance Scale refined the value of the Hubble constant to within an error of only 10%, accomplishing one of the telescope's key goals. The SH0ES team has reduced the uncertainty in the Hubble constant value by 76% since beginning its quest in 2005.
Figure 5: The illustration shows the three steps astronomers used to measure the universe's expansion rate to an unprecedented accuracy, reducing the total uncertainty to 2.4 percent [image credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)] 7)
Legend to Figure 5: Astronomers made the measurements by streamlining and strengthening the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near and far from Earth.
Beginning at left, astronomers use Hubble to measure the distances to a class of pulsating stars called Cepheid variables, employing a basic tool of geometry called parallax. This is the same technique that surveyors use to measure distances on Earth. Once astronomers calibrate the Cepheids' true brightness, they can use them as cosmic yardsticks to measure distances to galaxies much farther away than they can with the parallax technique. The rate at which Cepheids pulsate provides an additional fine-tuning to the true brightness, with slower pulses for brighter Cepheids. The astronomers compare the calibrated true brightness values with the stars' apparent brightness, as seen from Earth, to determine accurate distances.
Once the Cepheids are calibrated, astronomers move beyond our Milky Way to nearby galaxies (shown at center). They look for galaxies that contain Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same amount of brightness. The astronomers use the Cepheids to measure the true brightness of the supernovae in each host galaxy. From these measurements, the astronomers determine the galaxies' distances.
They then look for supernovae in galaxies located even farther away from Earth. Unlike Cepheids, Type Ia supernovae are brilliant enough to be seen from relatively longer distances. The astronomers compare the true and apparent brightness of distant supernovae to measure out to the distance where the expansion of the universe can be seen (shown at right). They compare those distance measurements with how the light from the supernovae is stretched to longer wavelengths by the expansion of space. They use these two values to calculate how fast the universe expands with time, called the Hubble constant.
• March 30, 2020: NASA’s Wide Field Infrared Survey Telescope (WFIRST) will search for exoplanets, planets outside our solar system, toward the center of our Milky Way galaxy, where most stars are. Studying the properties of exoplanet worlds will help us understand what planetary systems throughout the galaxy are like and how planets form and evolve. 8)
- Combining WFIRST’s findings with results from NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions will complete the first planet census that is sensitive to a wide range of planet masses and orbits, bringing us a step closer to discovering habitable Earth-like worlds beyond our own.
- To date, astronomers have found most planets when they pass in front of their host star in events called transits, which temporarily dim the star's light. WFIRST data can spot transits too, but the mission will primarily watch for the opposite effect — little surges of radiance produced by a light-bending phenomenon called microlensing. These events are much less common than transits because they rely on the chance alignment of two widely separated and unrelated stars drifting through space.
Figure 6: WFIRST will make its microlensing observations in the direction of the center of the Milky Way galaxy. The higher density of stars will yield more microlensing events, including those that reveal exoplanets (image credit: NASA's Goddard Space Flight Center/CI Lab)
Figure 7: This animation illustrates two ways a gravitational microlensing event could look to an observer. At top is the way it could appear to a telescope able to resolve the features. The source star appears to move and distort as its light is warped by the closer lensing star and its planet. At bottom is a light curve showing the intensity of light from the event. As the two stars reach best alignment, the signal reaches its peak. The planet orbiting the lensing star is detectable as a brief change in brightness (image credit: NASA/GSFC Conceptual Image Lab)
- "Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods,” said David Bennett, who leads the gravitational microlensing group at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Since it’s a one-in-a-million event, the key to WFIRST finding low-mass planets is to search hundreds of millions of stars.”
- In addition, microlensing is better at finding planets in and beyond the habitable zone — the orbital distances where planets might have liquid water on their surfaces.
- Microlensing 101: This effect occurs when light passes near a massive object. Anything with mass warps the fabric of space-time, sort of like the dent a bowling ball makes when set on a trampoline. Light travels in a straight line, but if space-time is bent — which happens near something massive, like a star — light follows the curve.
- Any time two stars align closely from our vantage point, light from the more distant star curves as it travels through the warped space-time of the nearer star. This phenomenon, one of the predictions of Einstein’s general theory of relativity, was famously confirmed by British physicist Sir Arthur Eddington during a total solar eclipse in 1919. If the alignment is especially close, the nearer star acts like a natural cosmic lens, focusing and intensifying light from the background star.
- Planets orbiting the foreground star may also modify the lensed light, acting as their own tiny lenses. The distortion they create allows astronomers to measure the planet’s mass and distance from its host star. This is how WFIRST will use microlensing to discover new worlds.
Familiar and exotic worlds
- “Trying to interpret planet populations today is like trying to interpret a picture with half of it covered,” said Matthew Penny, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge who led a study to predict WFIRST’s microlensing survey capabilities. “To fully understand how planetary systems form we need to find planets of all masses at all distances. No one technique can do this, but WFIRST’s microlensing survey, combined with the results from Kepler and TESS, will reveal far more of the picture.”
- More than 4,000 confirmed exoplanets have been discovered so far, but only 86 were found via microlensing. The techniques commonly used to find other worlds are biased toward planets that tend to be very different from those in our solar system. The transit method, for example, is best at finding sub-Neptune-sized planets that have orbits much smaller than Mercury’s. For a solar system like our own, transit studies could miss every planet.
- WFIRST’s microlensing survey will help us find analogs to every planet in our solar system except Mercury, whose small orbit and low mass combine to put it beyond the mission’s reach. WFIRST will find planets that are the mass of Earth and even smaller — perhaps even large moons, like Jupiter’s moon Ganymede.
- WFIRST will find planets in other poorly studied categories, too. Microlensing is best suited to finding worlds from the habitable zone of their star and farther out. This includes ice giants, like Uranus and Neptune in our solar system, and even rogue planets — worlds freely roaming the galaxy unbound to any stars.
- While ice giants are a minority in our solar system, a 2016 study indicated that they may be the most common kind of planet throughout the galaxy. WFIRST will put that theory to the test and help us get a better understanding of which planetary characteristics are most prevalent.
Hidden gems in the galactic core
- WFIRST will explore regions of the galaxy that haven’t yet been systematically scoured for exoplanets due to the different goals of previous missions. Kepler, for example, searched a modest-sized region of about 100 square degrees with 100,000 stars at typical distances of around a thousand light years. TESS scans the entire sky and tracks 200,000 stars, however their typical distances are around 100 light-years. WFIRST will search roughly 3 square degrees, but will follow 200 million stars at distances of around 10,000 light years.
- Since WFIRST is an infrared telescope, it will see right through the clouds of dust that block other telescopes from studying planets in the crowded central region of our galaxy. Most ground-based microlensing observations to date have been in visible light, making the center of the galaxy largely uncharted exoplanet territory. A microlensing survey conducted since 2015 using the United Kingdom Infrared Telescope (UKIRT) in Hawaii is smoothing the way for WFIRST’s exoplanet census by mapping out the region.
- The UKIRT survey is providing the first measurements of the rate of microlensing events toward the galaxy’s core, where stars are most densely concentrated. The results will help astronomers select the final observing strategy for WFIRST’s microlensing effort.
- The UKIRT team’s most recent goal is detecting microlensing events using machine learning, which will be vital for WFIRST. The mission will produce such a vast amount of data that combing through it solely by eye will be impractical. Streamlining the search will require automated processes.
- Additional UKIRT results point to an observing strategy that will reveal the most microlensing events possible while avoiding the thickest dust clouds that can block even infrared light.
- “Our current survey with UKIRT is laying the groundwork so that WFIRST can implement the first space-based dedicated microlensing survey,” said Savannah Jacklin, an astronomer at Vanderbilt University in Nashville, Tennessee who has led several UKIRT studies. “Previous exoplanet missions expanded our knowledge of planetary systems, and WFIRST will move us a giant step closer to truly understanding how planets — particularly those within the habitable zones of their host stars — form and evolve.”
From brown dwarfs to black holes
- The same microlensing survey that will reveal thousands of planets will also detect hundreds of other bizarre and interesting cosmic objects. Scientists will be able to study free-floating bodies with masses ranging from that of Mars to 100 times the Sun’s.
- The low end of the mass range includes planets that were ejected from their host stars and now roam the galaxy as rogue planets. Next are brown dwarfs, which are too massive to be characterized as planets but not quite massive enough to ignite as stars. Brown dwarfs don’t shine visibly like stars, but WFIRST will be able to study them in infrared light through the heat left over from their formation.
- Kepler and
other exoplanet search efforts have discovered thousands of large
planets with small orbits, represented by the red and black dots on
this chart. WFIRST will find planets with a much wider range of masses
orbiting farther from their host star, shown by the blue dots.
- Objects at the higher end include stellar corpses — neutron stars and black holes — left behind when massive stars exhaust their fuel. Studying them and measuring their masses will help scientists understand more about stars’ death throes while providing a census of stellar-mass black holes.
- “WFIRST’s microlensing survey will not only advance our understanding of planetary systems,” said Penny, “it will also enable a whole host of other studies of the variability of 200 million stars, the structure and formation of the inner Milky Way, and the population of black holes and other dark, compact objects that are hard or impossible to study in any other way.”
- The FY2020 Consolidated Appropriations Act funds the WFIRST program through September 2020. The FY2021 budget request proposes to terminate funding for the WFIRST mission and focus on the completion of the James Webb Space Telescope, now planned for launch in March 2021. The Administration is not ready to proceed with another multi-billion-dollar telescope until Webb has been successfully launched and deployed.
- WFIRST is managed at Goddard, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC (Infrared Processing and Analysis Center) in Pasadena, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from research institutions across the United States.
• March 2, 2020: NASA's WFIRST (Wide Field Infrared Survey Telescope ) project has passed a critical programmatic and technical milestone, giving the mission the official green light to begin hardware development and testing. 9)
- The WFIRST space telescope will have a viewing area 100 times larger than that of NASA’s Hubble Space Telescope, which will enable it to detect faint infrared signals from across the cosmos while also generating enormous panoramas of the universe, revealing secrets of dark energy, discovering planets outside our solar system (exoplanets), and addressing a host of other astrophysics and planetary science topics.
- WFIRST’s design already is at an advanced stage, using components with mature technologies. These include heritage hardware – primarily Hubble-quality telescope assets transferred to NASA from another federal agency – and lessons learned from NASA’s James Webb Space Telescope – the agency’s flagship infrared observatory, targeted for launch next year.
- With the passage of this latest key milestone, the team will begin finalizing the WFIRST mission design by building engineering test units and models to ensure the design will hold up under the extreme conditions during launch and while in space.
- WFIRST has an expected development cost of $3.2 billion. Including the cost of five years of operations and science, and a ride-along technology demonstration instrument capable of imaging planets around other stars, brings the maximum cost of WFIRST to $3.934 billion.
- The FY2020 Consolidated Appropriations Act funds the WFIRST program through September 2020. The FY2021 budget request proposes to terminate funding for the WFIRST mission and focus on the completion of the James Webb Space Telescope, now planned for launch in March 2021. The Administration is not ready to proceed with another multi-billion-dollar telescope until Webb has been successfully launched and deployed.
- WFIRST is managed at Goddard, with participation by the Jet Propulsion Laboratory (JPL) in Pasadena, California, the Space Telescope Science Institute in Baltimore, the Infrared Processing and Analysis Center, also in Pasadena, and a science team comprised of members from U.S. research institutions across the country.
• January 5, 2020: Imagine a fleet of 100 Hubble Space Telescopes, deployed in a strategic space-invader-shaped array a million miles from Earth, scanning the universe at warp speed. — With NASA's WFIRST (Wide Field Infrared Survey Telescope), scheduled for launch in the mid-2020s, this vision will (effectively) become reality. 10)
- WFIRST will capture the equivalent of 100 high-resolution Hubble images in a single shot, imaging large areas of the sky 1,000 times faster than Hubble. In several months, WFIRST could survey as much of the sky in near-infrared light — in just as much detail — as Hubble has over its entire three decades.
Figure 8: This simulated image of a portion of our neighboring galaxy Andromeda (M31) provides a preview of the vast expanse and fine detail that can be covered with just a single pointing of WFIRST. Using information gleaned from hundreds of Hubble observations, the simulated image covers a swath roughly 34,000 light-years across, showcasing the red and infrared light of more than 50 million individual stars detectable with WFIRST (video credit: NASA's Goddard Space Flight Center)
- Elisa Quintana, WFIRST Deputy Project Scientist for Communications at NASA's Goddard Space Flight Center in Greenbelt, Maryland, is confident that WFIRST will have the power to transform astrophysics. "To answer fundamental questions like: How common are planets like those in our solar system? How do galaxies form, evolve, and interact? Exactly how — and why — has the universe's expansion rate changed over time? We need a tool that can give us both a broad and detailed view of the sky. WFIRST will be that tool."
- Although WFIRST has not yet opened its wide, keen eyes on the universe, astronomers are already running simulations to demonstrate what it will be able to see and plan their observations.
- This simulated image of a portion of our neighboring galaxy, Andromeda (M31), provides a preview of the vast expanse and fine detail that can be covered with just a single pointing of WFIRST. Using information gleaned from hundreds of Hubble observations, the simulated image covers a swath roughly 34,000 light-years across, showcasing the red and infrared light of more than 50 million individual stars detectable with WFIRST.
- While it may appear to be a somewhat haphazard arrangement of 18 separate images, the simulation actually represents a single shot. Eighteen square detectors, 4096 by 4096 pixels each, make up WFIRST’s Wide Field Instrument (WFI) and give the telescope its unique window into space.
- With each pointing, WFIRST will cover an area roughly 1.33 times that of the full Moon. By comparison, each individual infrared Hubble image covers an area less than 1% of the full Moon.
The Advantages of Speed
- WFIRST is designed to collect the big data needed to tackle essential questions across a wide range of topics, including dark energy, exoplanets, and general astrophysics spanning from our solar system to the most distant galaxies in the observable universe. Over its 5-year planned lifetime, WFIRST is expected to amass more than 20 petabytes (20 x 1015 bytes) of information on thousands of planets, billions of stars, millions of galaxies, and the fundamental forces that govern the cosmos.
- For astronomers like Ben Williams of the University of Washington in Seattle, who generated the simulated data set for this image, WFIRST will provide a valuable opportunity to understand large nearby objects like Andromeda, which are otherwise extremely time-consuming to image because they take up such a large portion of the sky.
- "We have spent the last couple of decades getting images at high resolution in small parts of nearby galaxies. With Hubble you get these really tantalizing glimpses of very complex nearby systems. With WFIRST, all of a sudden you can cover the whole thing without spending lots of time," Williams said.
- The ability to image such a large area will provide astronomers with important context needed to understand how stars form and how galaxies change over time. Williams explained that with a wide field, "you get the individual stars, you get the structures they live in, and the structures that surround them in their environment."
- Julianne Dalcanton of the University of Washington, who led the Panchromatic Hubble Andromeda Treasury (PHAT) program that the simulated data are based on, also believes that WFIRST's combination of ultra-telephoto and super-wide-angle capabilities will be ground-breaking. "The PHAT survey of Andromeda was a tremendous investment of time, requiring careful justification and forethought. This new simulation shows how easy an equivalent observation could be for WFIRST." WFIRST could survey Andromeda nearly 1,500 times faster than Hubble, building a panorama of the main disk of the galaxy in just a few hours.
- WFIRST's extraordinary survey speed is a result of its wide field of view, its agility, and its orbit. Williams explained that by covering more area in one field and being able to switch fields more quickly, "you're avoiding all those overheads that are associated with repointing the telescope so many times." In addition, WFIRST's orbit one million miles out will provide a view that is generally unobstructed by Earth. While Hubble is often able to collect data during only half of its low-Earth orbit 350 miles up, WFIRST will be able to observe more-or-less continuously.
Figure 9: A composite figure of the Andromeda galaxy (M31) highlights the extremely large field of view of NASA’s upcoming Wide Field Infrared Survey Telescope (WFIRST). The background consists of ground-based imagery of the main disk of the Andromeda galaxy from the Digitized Sky Survey (DSS). A photo of the full Moon from NASA’s Lunar Reconnaissance Orbiter is provided for scale: Andromeda has a diameter of about 3 degrees on the sky, while the Moon is about 0.5 degrees across. (In reality, the Moon is much smaller than Andromeda, but it is also a lot closer.) Outlined in teal is the region of Andromeda covered by the Panchromatic Hubble Andromeda Treasury (PHAT) mosaic, the largest Hubble mosaic ever created. Overlaid on the PHAT region and outlined in white is the footprint of the 18 square detectors that make up WFIRST’s Wide Field Instrument (WFI). The entire footprint covers about 1.33 times the area of the full Moon and represents the area captured in a single shot by WFIRST (0.28 square degrees of the sky). The PHAT, which covers a 61,000-light-year swath of Andromeda, consists of more than 400 composite images collected over more than 650 hours of infrared observing time between 2010 and 2013. WFIRST could cover the entire PHAT, at the same resolution, with just two pointings in less than half an hour [image credit: Background image: Digitized Sky Survey and R. Gendler, Moon image: NASA, GSFC, and Arizona State University, WFIRST simulation: NASA, STScI, and B. F. Williams (University of Washington)]
Major Survey Programs
- Because it can collect so much detailed data so quickly, WFIRST is ideally suited for large surveys. A significant portion of the mission will be dedicated to monitoring hundreds of thousands of distant galaxies for supernova explosions, which can be used to study dark energy and the expansion of the universe. Another major program will involve mapping the shapes and distribution of galaxies in order to better understand how the universe — including galaxies, dark matter, and dark energy — has evolved over the past 13+ billion years.
- WFIRST will also play an important role in the census of exoplanets. By monitoring the brightness of billions of stars in the Milky Way, astronomers expect to catch thousands of microlensing events — slight increases in brightness that occur when a planet passes between the telescope and a distant star. WFIRST's ability to detect planets that are relatively small or far from their own stars — as well as rogue planets, which don't orbit any star at all — will help fill major gaps in our knowledge of planets beyond our solar system. Although microlensing will not give us the ability to see exoplanets directly, WFIRST will also carry a coronagraph, a technology demonstration instrument designed to block enough of the blinding starlight to make direct imaging and characterization of orbiting planets possible.
- These large surveys are also expected to reveal the unexpected: strange, transient phenomena that have never before been observed. "If you cover a lot of the sky, you're going to find those rare things," explained Williams.
- Further broadening its potential impact, all of the data collected by WFIRST will be non-proprietary and immediately available to the public. Dalcanton underscored the importance of this aspect of the mission: "Thousands of minds from across the globe are going to be able to think about that data and come up with new ways to use it. It's hard to anticipate what the WFIRST data will unlock, but I do know that the more people we have looking at it, the greater the pace of discovery."
Complementing other Observatories
- WFIRST's combination of talents will be a valuable complement to those of other observatories, including Hubble and the James Webb Space Telescope. "With one hundred times the field of view of Hubble, and the ability to rapidly survey the sky, WFIRST will be an extremely powerful discovery tool," explained Karoline Gilbert, WFIRST Mission Scientist at the Space Telescope Science Institute in Baltimore, Maryland. "Webb, which is 100 times more sensitive and can see deeper into the infrared, will be able to observe the rare astronomical objects discovered by WFIRST in exquisite detail. Meanwhile, Hubble will continue to provide a unique view into the optical and ultraviolet light emitted by the objects that WFIRST discovers, and Webb follows up on."
- The simulated image is being presented at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii (4 – 8 January 2020).
- WFIRST is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, with participation by the Jet Propulsion Laboratory (JPL) in Pasadena, California; the Space Telescope Science Institute (STScI) in Baltimore, Maryland; the Infrared Processing and Analysis Center (IPAC), also in Pasadena; and a science team comprising members from U.S. research institutions across the country, as well as international and industrial partners. WFIRST's Science Operations Center (SOC) will reside at the STScI, which also runs science operations for Hubble and will be SOC and Mission Operations Center for the James Webb Space Telescope. All of the data collected by the WFIRST mission will be held in the Barbara A. Mikulski Archive for Space Telescopes (MAST) at STScI.
• September 27, 2019: There is an as-yet-unseen population of Jupiter-like planets orbiting nearby Sun-like stars, awaiting discovery by future missions like NASA’s WFIRST space telescope, according to new models of gas giant planet formation by Carnegie’s Alan Boss described in an upcoming publication in The Astrophysical Journal. His models are supported by a new Science paper on the surprising discovery of a gas giant planet orbiting a low-mass star. 11)
- “Astronomers have struck a bonanza in searching for and detecting exoplanets of every size and stripe since the first confirmed exoplanet, a hot Jupiter, was discovered in 1995,” Boss explained. “Literally thousands upon thousands have been found to date, with masses ranging from less than that of Earth, to many times the mass of Jupiter.”
- But there are still gaping holes in scientists’ knowledge about exoplanets that orbit their stars at distances similar to those at which our Solar System’s gas giants orbit the Sun. In terms of mass and orbital period, planets like Jupiter represent a particularly small population of the known exoplanets, but it’s not yet clear if this is due to biases in the observational techniques used to find them—which favor planets with short-period-orbits over those with long-period-orbits—or if this represents an actual deficit in exoplanet demographics.
- All the recent exoplanet discoveries have led to a renewed focus on theoretical planet formation models. Two primary mechanisms exist for predicting how gas giant planets form from the rotating disk of gas and dust that surrounds a young star—bottom-up, called core accretion, and top-down, called disk instability.
- The former refers to slowly building a planet through the collisions of increasingly larger material—solid dust grains, pebbles, boulders, and eventually planetesimals. The latter refers to a rapidly triggered process that occurs when the disk is massive and cool enough to form spiral arms and then dense clumps of self-gravitating gas and dust contract and coalesce into a baby planet.
- While core accretion is considered the consensus planet-formation mechanism, Boss has long been a proponent of the competing disk instability mechanism, dating back to a seminal 1997 Science paper.
- The just-published discovery by an Institute for Space Studies of Catalonia-led team of a star that’s a tenth the mass of our Sun and hosts at least one gas giant planet is challenging the core-accretion method.
- The mass of a disk should be proportional to the mass of the young star around which it rotates. The fact that at least one gas giant—possibly two—was found around a star that’s so much smaller than our Sun indicates that either the original disk was enormous, or that core accretion does not work in this system. Orbital periods for lower mass stars are longer, which prevents core accretion from forming gas giants before the disk gas disappears, as core accretion is a much slower process than disk instability, according to Boss.
- “It’s a great vindication for the disk instability method and a demonstration how one unusual discovery can swing the pendulum on our understanding of how planets form,” said one of the IEEC research team’s members, Guillem Anglada-Escudé, himself a former Carnegie postdoc.
- Boss’ latest simulations follow the three-dimensional evolution of hot disks that start out in a stable configuration. On a variety of time scales, these disks cool down and form spiral arms, eventually resulting in dense clumps representing newborn protoplanets. Their masses and distances from the host star are similar to that of Jupiter and Saturn.
- “My new models show that disk instability can form dense clumps at distances similar to those of the Solar System’s giant planets,” said Boss. “The exoplanet census is still very much underway, and this work suggests that there are many more gas giants out there waiting to be counted.”
Figure 10: The black box encapsulating Jupiter denotes the approximate region of exoplanet discovery space where Alan Boss’ new models of gas giant planet formation suggest significant numbers of exoplanets remain to be found by direct imaging surveys of nearby stars. NASA’s WFIRST mission, slated for launch in 2025, will test the technology for a coronagraph (CGI) that would be capable of detecting these putative exoplanets. - Top Right: This simulation of the disk of gas and dust surrounding a young star shows dense clumps forming in the material. According to the proposed disk instability method of planet formation, they will contract and coalesce into a baby gas giant planet (image is provided courtesy of Alan Boss)
• September 24, 2019: When a new NASA space telescope opens its eyes in the mid-2020s, it will peer at the universe through some of the most sophisticated sunglasses ever designed. 12)
- This multi-layered technology, the coronagraph instrument, might more rightly be called "starglasses": a system of masks, prisms, detectors and even self-flexing mirrors built to block out the glare from distant stars - and reveal the planets in orbit around them.
- Normally, that glare is overwhelming, blotting out any chance of seeing planets orbiting other stars, called exoplanets, said Jason Rhodes, the project scientist for the Wide-Field Infrared Survey Telescope (WFIRST) at NASA's Jet Propulsion Laboratory in Pasadena, California.
- A star's photons - particles of light - vastly overpower any light coming from an orbiting planet when they hit the telescope.
- "What we're trying to do is cancel out a billion photons from the star for every one we capture from the planet," Rhodes said.
- And WFIRST's coronagraph just completed a major milestone: a preliminary design review by NASA. That means the instrument has met all design, schedule and budget requirements, and can now proceed to the next phase: building hardware that will fly in space. It's one of a series of such reviews examining every facet of the mission, said WFIRST Project Scientist Jeffrey Kruk of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- "Every one of these reviews is comprehensive," Kruk said. "We go over all aspects of the mission, to show that everything hangs together."
- The WFIRST mission's coronagraph is meant to demonstrate the power of increasingly advanced technology. As it captures light directly from large, gaseous exoplanets, and from disks of dust and gas surrounding other stars, it will point the way to technologies for even larger space telescopes.
- Future telescopes with even more sophisticated coronagraphs will be able to generate single pixel "images" of rocky planets the size of Earth. Then the light can be spread into a rainbow called a "spectrum," revealing which gases are present in the planet's atmosphere - perhaps oxygen, methane, carbon dioxide, and maybe even signs of life.
- "With WFIRST we'll be able to get images and spectra of these large planets, with the goal of proving technologies that will be used in a future mission - to eventually look at small rocky planets that could have liquid water on their surfaces, or even signs of life, like our own," Rhodes said.
- In this way, WFIRST is a kind of pioneer. That's why NASA considers the coronagraph to be a "technology demonstration." While it is likely to generate important scientific discoveries, its main job is to prove to the scientific community that complex coronagraphs really can work in space.
- "This may be the most complicated astronomical instrument ever flown," Rhodes said.
Figure 11: An optical engineer at NASA's Jet Propulsion Laboratory, in Pasadena, California, Camilo Mejia Prada, shines a light on the interior of a testbed for an instrument called a coronagraph that will fly aboard the WFIRST space telescope (image credit: NASA/JPL-Caltech/Matthew Luem)
Why This Coronagraph Is Different
- NASA's Hubble Space Telescope, in orbit since 1990, is so far the only NASA astrophysics flagship mission to include coronagraphs - far simpler and less sophisticated versions than will fly on WFIRST.
- But by the time it launches in the mid 2020s, WFIRST will be the third such mission to include coronagraph technology. NASA's massive James Webb Space Telescope, launching in 2021, will include a coronagraph with a sharpness of vision greater than Hubble's, but without the starlight suppression capability of WFIRST.
- "WFIRST should be two or three orders of magnitude more powerful than any other coronagraph ever flown" in its ability to distinguish a planet from its star, Rhodes said. "There should be a chance for some really compelling science, even though it's just a tech demo."
- The two flexible mirrors inside the coronagraph are key components. As light that has traveled tens of light-years from an exoplanet enters the telescope, thousands of actuators move like pistons, changing the shape of the mirrors in real time. The flexing of these "deformable mirrors" compensates for tiny flaws and changes in the telescope's optics.
- Changes on the mirrors' surfaces are so precise they can compensate for errors smaller than the width of a strand of DNA.
Figure 12: When a new NASA space telescope opens its eyes in the mid 2020s, it will peer at the universe through some of the most sophisticated sunglasses ever designed (video credit: NASA Goddard, Published on 24 September 2019)
- These mirrors, in tandem with high-tech "masks," another major advance, squelch the star's diffraction - the bending of light waves around the edges of light-blocking elements inside the coronagraph.
- The result: blinding starlight is sharply dimmed, and faintly glowing, previously hidden planets appear.
- The star-dimming technology also could deliver the clearest-ever images of distant star systems' formative years - when they are still swaddled in disks of dust and gas as infant planets take shape inside.
- "The debris disks we see today around other stars are brighter and more massive than what we have in our own solar system," said Vanessa Bailey, an astronomer at JPL and instrument technologist for the WFIRST coronagraph. "WFIRST's coronagraph instrument could study fainter, more diffuse disk material that's more like the Main Asteroid Belt, the Kuiper Belt, and other dust orbiting the Sun."
- That could yield deep insights into how our solar system formed.
- Kruk said the instrument's deformable mirrors and other advanced technology - known as "active wavefront control" - should mean a leap of 100 to 1,000 times the capability of previous coronagraphs.
- "When you see an opportunity like this to really open new frontiers in a new field, you can't help but be excited by that," he said.
- Once the coronagraph technology is successfully demonstrated over the mission's first 18 months, WFIRST's coronagraph could become open to the scientific community. A "Participating Scientist Program" would invite a broader variety of observers to conduct experiments beyond the demonstration phase.
- The coronagraph's advancement through the design review milestone is part of a development schedule now moving at a fast clip. A giant camera that will also fly on the spacecraft, called the Wide-Field Instrument, cleared the same hurdle in June. It is considered the space telescope's main instrument.
- Rhodes likes to compare WFIRST to the history-making Mars Pathfinder mission. After landing on the Red Planet in 1997, the Pathfinder lander unleashed a small rover, named Sojourner, to trundle on its own around the landing site and examine nearby rocks.
- "That was a tech demo," Rhodes said. "The goal was to show that a rover works on Mars. But it went on to do some very interesting science during its lifetime. So we're hopeful the same is going to be true of WFIRST's coronagraph tech demo."
• September 13, 2019: Scientists have discovered that a mysterious pressure dubbed "dark energy" makes up about 68% of the total energy content of the cosmos, but so far we don't know much more about it. Exploring the nature of dark energy is one of the primary reasons NASA is building the Wide Field Infrared Survey Telescope (WFIRST), a space telescope whose measurements will help illuminate the dark energy puzzle. With a better understanding of dark energy, we will have a better sense of the past and future evolution of the universe. 13)
• August 28, 2019: On schedule to launch in the mid-2020s, NASA’s WFIRST (Wide Field Infrared Survey Telescope) mission will help uncover some of the biggest mysteries in the cosmos. The state-of-the-art telescope on the WFIRST spacecraft will play a significant role in this, providing the largest picture of the universe ever seen with the same depth and precision as the Hubble Space Telescope. 14)
Figure 13: The telescope for WFIRST has successfully passed its preliminary design review, a major milestone for the mission. This means the telescope has met the performance, schedule, and budget requirements to advance to the next stage of development, where the team will finalize its design (video credit: NASA Goddard)
- WFIRST is leveraging existing hardware that was transferred to NASA, and the development is much further along at this point than it would be if the telescope had originated with WFIRST. While many of the inherited components are being modified or reconfigured to function as part of the final design, the telescope is already at a very advanced stage of design.
- The WFIRST telescope will collect and focus light using a primary mirror that is 2.4 meters in diameter. While it’s the same size as the Hubble Space Telescope’s main mirror, it is only one-fourth the weight, showcasing an impressive improvement in telescope technology.
- The mirror gathers light and sends it on to a pair of science instruments. The spacecraft’s giant camera, the Wide Field Instrument (WFI), will enable astronomers to map the presence of mysterious dark matter, which is known only through its gravitational effects on normal matter. The WFI will also help scientists investigate the equally mysterious "dark energy," which causes the universe's expansion to accelerate. Whatever its nature, dark energy may hold the key to understanding the fate of the cosmos.
- In addition, the WFI will survey our own galaxy to further our understanding of what planets orbit other stars, using the telescope’s ability to sense both smaller planets and more distant planets than any survey before (planets orbiting stars beyond our Sun are called "exoplanets"). This survey will help determine whether our solar system is common, unusual, or nearly unique in the galaxy. The WFI will have the same resolution as Hubble, yet has a field of view that is 100 times greater, combining excellent image quality with the power to conduct large surveys that would take Hubble hundreds of years to complete.
- WFIRST’s Coronagraph Instrument (CGI) will directly image exoplanets by blocking out the light of their host stars. To date, astronomers have directly imaged only a small fraction of exoplanets, so WFIRST’s advanced techniques will expand our inventory and enable us to learn more about them. Results from the CGI will provide the first opportunity to observe and characterize exoplanets similar to those in our solar system, located between three and 10 times Earth’s distance from the Sun, or from about midway to Jupiter to about the distance of Saturn in our solar system. Studying the physical properties of exoplanets that are more similar to Earth will take us a step closer to discovering habitable planets.
- The team at Harris Corporation in Rochester, New York, the prime contractor for the telescope, is making significant strides in modifying the preexisting hardware for the spacecraft.
• July 2, 2019: NASA has awarded a contract to the Space Telescope Science Institute (STScI) in Baltimore, Maryland, for the Science Operations Center (SOC) of the Wide Field Infrared Survey Telescope (WFIRST) mission. WFIRST is a NASA observatory designed to settle essential questions in a wide-range of science areas, including dark energy and dark matter, and planets outside our solar system. 15)
- STScI is the science operations center for both the Hubble and upcoming James Webb Space Telescope. Its expertise with these great observatories puts the Institute in a unique position to support cutting-edge astronomical research well into the future. STScI was established in 1981 on the Johns Hopkins University campus, and is operated by the Association of Universities for Research in Astronomy (AURA).
- AURA/STScI will join a team led by NASA's Goddard Space Flight Center in Greenbelt, Maryland, which manages the WFIRST mission for NASA. The team also includes the Jet Propulsion Laboratory (JPL) in Pasadena, California; the Infrared Processing and Analysis Center (IPAC), also in Pasadena; a science team comprised of members from U.S. research institutions across the country, including STScI astronomers; and various industrial and international partners.
- The WFIRST observatory will follow on the legacy of the Hubble Space Telescope. WFIRST has the same-sized mirror, but will have a wide-field view of the universe in near-infrared light. Sharp exposures of millions of far-flung galaxies will be done in a fraction of the time that it would take with Hubble. WFIRST's deep-space view will cover 100 times the area of sky as Hubble.
• November 1, 2018: NASA has awarded a contract to Harris Corporation of Rochester, New York, for the Optical Telescope Assembly (OTA) for the agency's Wide Field Infrared Survey Telescope (WFIRST) mission. 16)
- The total value of this cost-plus-award-fee contract is approximately $195.9 million. The period of performance runs from Nov. 30 through Dec. 1, 2025.
- Harris Corporation will provide the personnel, services, materials, equipment and facilities necessary to build, refurbish or modify the OTA, as required, to meet WFIRST performance requirements. The efforts include fabricating, as necessary, aligning, testing, verifying and delivering the telescope to NASA's Goddard Space Flight Center in Greenbelt, Maryland. Harris also will provide post-delivery support for both the observatory integration and test program, as well as for in-orbit observatory checkout and commissioning.
- The work will be performed at Harris' facility in Rochester, New York, and at Goddard.
• June 20, 2018: NASA has awarded a contract to Teledyne Scientific and Imaging, LLC, Camarillo, California, for the Short Wave Infra-Red Sensor Chip Assembly (SCA) for the WFIRST (Wide Field Infrared Survey Telescope) Project. 17)
- Managed by NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, WFIRST is fully-funded for Fiscal Year 2018. Work will continue on the mission in this time period until appropriations for Fiscal Year 2019 have been determined.
- The contract is a Cost-Plus-Award-Fee contract with a value of $23,035,123. The period of performance is from June 14, 2018, through Oct. 31, 2020.
- The contract shall provide 72 Short Wave Infrared SCA devices for the WFIRST Space Flight Focal Plane Assembly. In addition, the contractor shall perform a Warm Functional Test and Cold Functional Screen Test for final space flight SCA testing and performance requirements verification.
• May 23, 2018: NASA has awarded a contract to BATC (Ball Aerospace and Technologies Corporation), Boulder, Colorado, for the primary instrument components for the WFIRST (Wide Field Infrared Survey Telescope). Called the WFI (Wide Field Instrument) Opto-Mechanical Assembly, the cost-plus-award-fee contract has a value of approximately $113.2 million. The period of performance is from May 2018 through June 2026. 18)
WFIRST is a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The telescope has a primary mirror that is 2.4 meters in diameter, and is the same size as the Hubble Space Telescope's primary mirror. WFIRST will have two instruments, the Wide Field Instrument, and the Coronagraph Instrument. 19) 20) 21)
“WFIRST has the potential to open our eyes to the wonders of the universe, much the same way Hubble has,” said John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate at Headquarters in Washington. “This mission uniquely combines the ability to discover and characterize planets beyond our own solar system with the sensitivity and optics to look wide and deep into the universe in a quest to unravel the mysteries of dark energy and dark matter.”
Figure 14: Artist's rendition of the WFIRST observatory, which will study multiple cosmic phenomena, including dark energy (image credit: NASA/GSFC)
L3Harris Technologies Inc. of Melbourne, Florida is carrying out the overall preparation of the telescope hardware to meet mission requirements, including the 2.4-meter primary mirror and the remaining optics and structures to feed the mission’s instruments. L3Harris has already conducted a successful test of the primary mirror assembly to ensure it performs at WFIRST operating temperatures. To enable the WFIRST science, the telescope will be more stable than any mission in this class and have a wide field of view that is 100 times larger than Hubble. 22)
L3Harris is responsible for some of the most important tasks to create the telescope, including assembling the primary mirror. L3Harris created hardware to accommodate and interact with the two main scientific instruments on the telescope, the Wide Field Instrument and the Coronagraph. L3Harris also conducted the successful test of the primary mirror to ensure it functions in the very cold temperatures found in space.
The telescope was initially constructed for another mission, but was declared surplus and transferred to NASA. L3Harris has worked with NASA and other partners to turn the hardware into a powerful astrophysics and universe-exploration tool. This was cost effective and expanded mission capability by providing a larger mirror than first planned.
• Launch mass: 4166 kg
• Spacecraft dry mass: 4059 kg
• Payload mass: 2191 kg
• Spacecraft power: 2.5 kW
• Communications: TT&C in S-band; data acquisition in Ka-band.
FIRST Observatory Overview 23)
The WFIRST Observatory consists of an Integrated Payload Assembly (IPA; Figure 15)and a Spacecraft(S/C). As will be discussed below, the IPAconsists of components of the Optical Telescope Assembly (OTA), an Instrument Carrier (IC), components ofthe Wide Field Instrument (WFI)and the Coronagraph Instrument (CGI).
The OTA (Optical Telescope Assembly) consists of the donated FOA (Forward Optics Assembly) integrated with an AOM (Aft Optics Module) and supported by the IC (Instrument Carrier) via FOA mounts. The OTA is controlled by its TCE (Telescope Control Electronics) mounted within the WFIRST Spacecraft. Functionally, the first two optics of the OTA’s three mirror anastigmat imaging system is in the common path for all WFIRST science modes. The existing FOA will undergo a minor optical prescription change for the WFIRST mission via optical re-polishing. The passively isolated FOA Mounts and the AOM are new hardware specifically designed for WFIRST. As a result of one of the trades described in this paper, the Tertiary Mirror of the three mirror Anastigmat is now mounted in the AOM instead of the WFI. A functional block diagram of the WFIRST payload is illustrated in Figure 17.
As shown in the block diagram, numerous on-orbit optical compensators are available to ensure the WFIRST Payload meets on-orbit alignment, optical quality, and long term optical stability requirements. The FOA (Forward Optics Assembly) secondary mirror provides a five degree-of-freedom rigid body adjustment capability. In the WFI channel the IFM (Instrument Fold Mirror) provides alignment and focus adjustment, enabling on-orbit focus diversity phase retrieval during Observatory commissioning and as required thereafter.
The flight serviceable WFI consists of a CSM (Cold Sensing Module), a WEM (Warm Electronics Module) mounted within the spacecraft (not shown), and a FCR (Facility Cryogenic Radiator) mounted to the Spacecraft’s Outer Barrel Assembly (not shown). All optical functions of the WFI are contained within the CSM.
The Transmissive Optics Selection Assembly (TOSA) consists of 10 selectable modes; seven filters covering 0.48 to 2.00 µm, a grism for spectrographic studies, a cold dark for calibration, and an engineering filter used for ground AI&T (Assembly, Integration and Test).
The Coronagraph Instrument (CGI) consists of the flight serviceable Optical Bench Assembly (CGI-OBA) and the Tertiary Collimator Assembly (TCA). The TCA optically and mechanically interfaces with the FOA to provide a collimated (infinite conjugate) input to the CGI. The TCA mounts directly to the FOA, and is hence not flight serviceable.
Early in the pre-Phase A study it was quickly realized that the combined launch loads of the WFI and CGI would exceed the heritage design capability of the donated telescope. Various configurations of an Instrument Carrier (IC) to meter the WFIRST instruments off the spacecraft deck were subsequently traded. Our selected IC architecture (Figure 18) leverages GSFC recent experience with the James Webb Space Telescope ISIM (Integrated Science Instrument Module) structure. The IC accommodates the IOA, WFI, CGI, and the Spacecraft’s Star Tracker/Inertial Reference Unit via flight serviceable mounting interfaces (Figure 16).
The IC is kinematically mounted to the WFIRST spacecraft and consists of a carbon fiber/cyanate ester composite (M55J/954-6) truss assembly to achieve high levels of structural strength and stability in a lightweight structure. This is the same material used for the JWST ISIM, with the same lay-up but with some of the composite tubes having a larger OD. Truss members are joined with a combination of titanium fittings and composite gussets and clips; again with design heritage in the JWST/ISIM; however WFIRST will use titanium fittings in lieu of invar to conserve mass. Analysis has shown that titanium works at expected IC temperatures throughout the WFIRST mission, and our analysis will be augmented by test articles currently in fabrication. Dynamic isolation of the IC from the Spacecraft is achieved using Honeywell D-struts.
WFI Optical Interface
In the original WFIRST concept presented in 2013 24) the third mirror of the three mirror anastigmat was mounted within the WFI assembly for the Wide Field Channel (AKA the WFC AOM TM in Figure 17). When the WFI was moved off of the OTA and into the IC, analysis revealed an increased sensitivity to observatory thermal and dynamic perturbations due to the metering of the WFC TM independently of the OTA. Trades supported the decision to reduce risk by moving the WFC TM out of the WFI and back to the OTA. 25)
Key outcomes of the move of the WFC TM from the WFI to the OTA included reduced OTA to WFI alignment tolerances, simplification of Payload AI&T by establishing a test point in the AI&T flow that allowed double pass interferometry of the entire image formation system (“test-as-you-fly”), and the elimination of costly support equipment intended to compensate for the absence of the TM during Payload AI&T. However, the most important outcome of the WFC TM move was not initially anticipated. The packaging of the OTA and WFI now allowed for the positioning of the WFI Mosaic Plate Assembly (AKA the WFI’s 95K focal plane) much farther outboard towards the anti-sun side of the Observatory. It was quickly realized that this could enable the largest risk reduction achieved for WFIRST since 2013, the elimination of the Observatory’s cryocooler in favor of passive cooling of the WFI cold electronics.
Passive Cooling Trade
The use of passive cryo-cooling to operate the WFC and IFC SCA s at ≤100 K was first seriously considered when the WFIRST mission orbit was changed from GEO to L2 in 2015. That change was based primarily on considerations related to SCA radiation tolerance, but it also afforded the mission with a more stable thermal environment. In particular, it offered the possibility of a true cold side of the observatory that would only be exposed to deep space.
Mechanical Cryo-cooling (MC) using a NICMOS-heritage Turbo-Brayton system was the baseline at the time, primarily due to the extremely low vibration of its non-reciprocating design, the ability of its heat transfer lines to provide cooling to focal planes located deep within the payload, the tolerance of the high temperature (nearly room temperature) MC radiator to GEO thermal loading variations, and the relatively high technological maturity of the planned implementation.
With the move to L2, a trade was conducted to evaluate the MC concept against Passive Cryo-cooling Systems (PCS) employing either cryo heat pipes or traditional thermal straps. The recommendation at the time was to retain the MC concept, as the cryo heat pipes were a less mature technology, and the temperature gradient in the thermal straps given the inboard location of the SCAs was too large to allow an acceptable cryo radiator size. However, it was specifically noted in the trade closure that a PCS option could be reconsidered if the SCAs could be located closer to a PCS radiator, and the telescope outer barrel assembly could be made to support a large (~9 m2) PCS radiator, and/or the SCA operational temperature could be increased to ~120 K.
The optical redesign activity to relocate the WFC TM offered the possibility of developing and evaluating configurations that would also move the SCAs radially outboard, much closer to the potential location of a PCS radiator (dubbed the FCR (Facility Cryogenic Radiator) which would remain as part of the observatory if/when the WFI were changed out during servicing). When this SCA relocation proved practical, the mechanical vs. passive cryo-cooling trade was reopened and resulted in a change from MC to PCS SCA cooling. The primary advantages of the PCS concept were the unlimited life and zero intrinsic vibration, and the lower cost and complexity relative to MC.
It was realized that the PCS concept needed time to mature, so the MC and PCS options were carried in parallel for ~9 months in case unanticipated PCS issues were discovered. The new optical design was specifically chosen to be compatible with either a MC or PCS approach, enabling a change back to MC later in Phase A, should it be needed. Fortunately, the PCS design has remained robust, with parasitics margins fluctuating in the 110% to 130% range despite a lowering of the SCAs operational temperature from 100 K to 95 K (to improve SCA production yield), and a reduction in the size of the FCR from its original 9.5 m2 to its current 7 m2.
Integrated Modeling Results
Integrated Modeling (IM) is analysis that includes multiple disciplines and uses a flight observatory model to provide end-to-end perturbation to performance predictions. The WFIRST Project has developed the IM processes and capabilities in the last several years to validate requirements that cannot be verified by test on the ground. The tool is also used to support system-level trades by performing observatory performance evaluation for different proposed designs. During Phase A of the mission, the IM analyses focus on the thermal elastic effects due to ground-to-orbit and on-orbit temperature changes which distort structures and lead to optical alignment and surface figure errors, and the mechanical vibration effect that is generated from the spacecraft reaction wheels and propagated through the observatory structure which also degrades the optical performance.
For thermal distortion mitigations, WFIRST employs a thermal control system with proportional heaters on the optical telescope assembly. For reducing jitter disturbances, WFIRST implements a two-stage, passive vibration isolation system, where the first stage is co-located with the reaction wheels, and the second stage is between the spacecraft and payload interface. In order to maintain stable performance, WFIRST plans to avoid moving any mechanisms (e.g. spacecraft high gain antenna actuators and instrument filter wheel) during science exposures. All other errors due to long-term material changes or dry-out effects can be compensated by flight alignment actuators as necessary.
The key stability requirements for the WFIRST Wide Field Instrument and Coronagraph Instrument (CGI) are summarized in Table 2. The CGI has internal control systems that can correct for line-of-sight (LOS) and wavefront (WFE) drift, outside of the observatory stability mitigation capabilities. In Phase A, to simplify the modeling approach, the CGI control systems are modeled as simple high-pass rejection filters. Both CGI requirements and performance predictions include these closed-loop rejection filters where appropriate, as shown in Table 2.
As part of the IM process, all prediction results include appropriate model uncertainty factors (MUFs). The MUFs are determined from heritage data and relevant past experience at this stage of the program. More extensive analyses will be performed during Phase B of the mission to ensure that the MUFs chosen are sufficient for WFIRST. With the MUFs included, the IM current best estimates as shown in Table 1 demonstrate that all key stability requirements can be met with reasonable margin. To meet the CGI LOS and WFE jitter requirements, the observatory wheel speed range is limited to 5-19 rev/sec, from the nominal ± 40 rev/sec. This wheel speed range is predicted to allow CGI to achieve its technology demonstration goals.
Launch: Roman Space Telescope is expected to be launched in 2025, from the Cape Canaveral Air Force Station, Florida. The mission is set to last about 6 years.
Orbit: The WFIRST spacecraft will be located at L2 Sun-Earth Lagrangian Point, about 1.5 million km from Earth (in the direction of the universe). The gravitational forces between the sun and Earth at this point are equal, allowing for a stable satellite orbit.
Sensor complement (WFI, CGI)
WFI (Wide Field Instrument)
WFIRST is a next-generation space telescope that will survey the infrared universe from beyond the orbit of the Moon. Its two instruments are a technology demonstration called a coronagraph, and the WFI. The WFI features the same angular resolution as Hubble but with 100 times the FOV (Field of View). Data it gathers will enable scientists to discover new and uniquely detailed information about planetary systems around other stars. The WFI will also map how matter is structured and distributed throughout the cosmos, which should ultimately allow scientists to discover the fate of the universe.
The WFI provides the Wide Field imaging and slitless spectroscopic capabilities required to perform the Dark Energy, Exoplanet Microlensing, and NIR surveys while the coronagraph instrument supports the Exoplanet high contrast imaging and spectroscopy science. The wide field instrument includes filters that provide an imaging mode covering 0.48 — 2.0 µm, and two slitless spectroscopy modes. The spectroscopy modes cover 1.0 — 1.93 µm with resolving power 450-850, and .8-1.8 µm (not yet final) with resolving power of 70-140. The wide field focal plane uses 4k x 4k HgCdTe detectors with 10 µm pixels. The HgCdTe detectors are arranged in a 6x3 array, providing an active area of 0.281 deg2. The instrument provides a sharp point spread function, precision photometry, and stable observations for implementing the Dark Energy, Exoplanet Microlensing, and NIR surveys.
Figure 19: Illustration of WFI on WFIRST (image credit: NASA)
• As of 26 June 2019, the WFI has just passed its preliminary design review, an important milestone for the mission. It means the WFI successfully met the design, schedule and budget requirements to advance to the next phase of development, where the team will begin detailed design and fabrication of the flight hardware. 26)
“This was an outstanding preliminary design review, providing a snapshot of the tremendous amount of engineering this team has accomplished in a short period of time,” said Jamie Dunn, WFIRST project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The WFI team is well on their way down the path of building a world-class instrument for NASA’s next great observatory.”
“The preliminary design review is a vital step in the mission because it takes the engineering ideas and assesses them against stringent criteria to make sure they will perform as planned,” said Goddard’s Mary Walker, instrument manager for the WFI. “This is where we find the things we need to tweak so WFIRST can advance to the next stage in its journey.”
Engineers will feed the results of the review into the next design iteration, preparing the instrument for an even more rigorous test — the critical design review, currently planned for June 2020. This will involve data from WFI engineering test units in simulated space environments, including testing at cryogenic temperatures.
Figure 20: WFIRST is a next-generation space telescope that will survey the infrared universe from beyond the orbit of the Moon. The spacecraft's giant camera, the Wide Field Instrument (WFI), will be fundamental to this exploration. Watch this video to see a simplified version of how it works (video credit: NASA's Goddard Space Flight Center)
The WFI is designed to detect faint infrared light from across the universe. Infrared light is observed at wavelengths longer than the human eye can detect. The expansion of the universe stretches light emitted by distant galaxies, causing visible or ultraviolet light to appear as infrared by the time it reaches us. Such distant galaxies are difficult to observe from the ground because Earth’s atmosphere blocks some infrared wavelengths, and the upper atmosphere glows brightly enough to overwhelm light from these distant galaxies. By going into space and using a Hubble-size telescope, the WFI will be sensitive enough to detect infrared light from farther than any previous telescope. This will help scientists capture a new view of the universe that could help solve some of its biggest mysteries, one of which is how the universe became the way it is now.
The WFI will allow scientists to peer very far back in time. Seeing the universe in its early stages will help scientists unravel how it expanded throughout its history. This will illuminate how the cosmos developed to its present condition, enabling scientists to predict how it will continue to evolve.
“We’re going to try to discover the fate of the universe,” said Goddard’s Jeff Kruk, the WFIRST project scientist. “The expansion of the universe is accelerating, and one of the things the Wide Field Instrument will help us figure out is if the acceleration is increasing or slowing down.”
One possible explanation for this speed-up is dark energy, an unexplained phenomenon that currently makes up about 68 percent of the total content of the cosmos and may be changing as the universe evolves. Another possibility is that this apparent cosmic acceleration points to the breakdown of Einstein’s general theory of relativity across large swaths of the universe.
The WFI will test these ideas by measuring matter in hundreds of millions of distant galaxies through a phenomenon called weak gravitational lensing. Massive objects like galaxies and clusters of galaxies curve space-time, bending the path traveled by light that passes nearby. This creates a distorted, magnified view of far-off galaxies behind them. Viewing those distant galaxies will show how matter is structured throughout the universe and across time.
All of the astronomical surveys that WFIRST will conduct rely on the WFI. An extremely stable optical structure is necessary to make the high-precision measurements with both the WFI and the coronagraph. Further ensuring stability, WFIRST will orbit the second Sun-Earth Lagrange point (L2). At this special location over 930,000 miles (1.5 million km) from Earth, gravitational forces balance to keep objects in steady orbits with very little assistance. The thermal stability of an observatory at L2 will provide a ten-fold improvement beyond Hubble in much of the data the WFI will gather. This degree of stability is impractical with observatories in low-Earth orbit, such as Hubble.
• May 2, 2017: WFIRST will have a 2.4 meter mirror, the same size as the Hubble. But, it will have a camera that will expand the power of that mirror. The WFI (Wide Field Instrument) is a 288 Mpixel multi-band near-infrared camera. Once it’s in operation, it will capture images that are every bit as sharp as those from Hubble. But there is one huge difference: The Wide Field Instrument will capture images that cover over 100 times the sky that Hubble does. 27)
CGI (Coronagraph Instrument)
Alongside of WFI, WFIRST will have the Coronagraphic Instrument. The Coronagraphic Instrument will advance the study of exoplanets. It’ll use a system of filters and masks to block out the light from other stars, and hone in on planets orbiting those stars. This will allow very detailed study of the atmospheres of exoplanets, one of the main ways of determining habitability. 28)
The Coronagraph Instrument (CGI) will be the first instrument on a space telescope to make use of numerically optimized, precision-fabricated coronagraph masks; large-format deformable mirrors for high-order wavefront control; a low-order wavefront sensing and control system; and EMCCD (Electron-Multiplying CCD) detectors for low-flux photon counting. 29)
In order to execute a compelling demonstration of its critical technologies on astrophysical sources, CGI will have the capability to switch between three observing modes: (i) broadband imaging with a Hybrid Lyot Coronagraph with inner working angle 3 λ/D (150 mas) in a 546—604 nm bandpass; (ii) a Shaped Pupil Coronagraph for spectroscopic imaging with a lenslet-based integral field spectrograph, at spectral resolving power R=50 in a 675—785 nm bandpass; (iii) a Shaped Pupil Coronagraph for broadband imaging of debris disks at separations ranging 6—20 λ/D in a 784—866 nm bandpass. All of these observing modes are designed to reach a flux ratio sensitivity requirement of 5 x10-8 including margins and model uncertainty factors. Error budget predictions indicate CGI may reach flux ratio sensitivities down to 5 x10-10 in imaging (100 hr, V=5), and 4 x 10-9 in spectroscopy (400 hr, V=5).
CGI is a high-contrast imager and integral field spectrograph that will enable the study of exoplanets and circumstellar disks at visible wavelengths. Ground-based high-contrast instrumentation is fundamentally limited to flux ratios of 107-8 at small working angles, even under optimistic assumptions for 30 m-class telescopes. There is a strong scientific driver for better performance, particularly at visible wavelengths. Future flagship mission concepts aim to image Earth analogues with visible light flux ratios >1010. CGI is a critical intermediate step toward that goal, with a predicted 108-9 flux ratio capability. CGI achieves that capability through improvements over current ground and space systems in several areas: 30)
• Hardware: space-qualified (TRL9) deformable mirrors, detectors, and coronagraphs
• Algorithms: wavefront sensing and control; post-processing of integral field spectrograph, polarimetric, and extended object data
• Validation of telescope and instrument models at high accuracy and precision.
Figure 21: Illustration of the CGI on WFIRST (image credit: NASA)
Key CGI components
• Two science cameras (cannot be used simultaneously):
- Imager: 10” FOV; direct imaging or polarimetry; 10% bandpass filters
- Integral Field Spectrograph (IFS): 2” FOV; R~50 spectrum; 18%bandpass filters.
- Electron Multiplying CCDs (EMCCDs) for improved signal-to-noise on faint objects.
• Visible to very near infrared wavelengths:
- 10% bandwidth: 575 nm and 825 nm; 18% bandwidth: 660 nm and760 nm
- 1% bandwidth Hα filter for IFS calibration and imaging is under consideration.
• Starlight suppression with interchangeable coronagraphic masks [31)]:
- Hybrid Lyot Coronagraph (HLC): 360º FOV, 3-9 λ/D, optimized for imaging.
- Shaped Pupil Coronagraph (SPC) “bowtie”:2 x 65º FOV, 3-9 λ/D, optimized for the broader IFS bandpasses.
- SPC “disk”: 360º FOV, 6.5-20 λ/D, optimized for imaging.
• Wavefront sensing and control at unprecedented levels of precision:
- Dedicated Low Order Wavefront Sensor (LOWFS) for Zernike modes 2-11.
- High Order Wavefront Sensing (HOWFS) using science camera images.
- Two high-actuator count deformable mirrors (DMs) for phase and amplitude control.
The current budget allows for fully commissioning three observing modes: 575nm/HLC/imaging, 760 nm/SPC bowtie [32)]3/IFS, and 825 nm/SPC disk/imaging. These modes will be tested with CGI flight hardware and software. Other combinations of filters and coronagraphic masks are possible and will be exercised at the JPL WFIRST CGI engineering testbed, though they will not be fully tested with flight hardware and software prior to launch, due to CGI Integration and Test schedule and budget constraints.
Figure 22: CGI schematic diagram (image credit: NASA)
Coronagraph designs: CGI has chosen two families of coronagraphs, Hybrid Lyot and Reflective Shaped Pupil, on the basis of their maturity, expected performance with the WFIRST obscured pupil, and low sensitivity to aberrations. New fabrication techniques have been implemented to address the tight optical tolerances. The designs must be robust against effects that were not significant at the contrast levels achieved by previous-generation coronagraphs. For example, accommodating the secondary mirror support struts pushes designs to more difficult trades between performance metrics such as IWA, throughput, bandwidth, contrast, field-of-view, and aberration sensitivities, relative to designs for unobscured apertures. Additionally, polarization-dependent aberrations and telescope tip/tilt jitter limit starlight suppression at small working angles; ongoing work is evaluating soft-edge focal plane masks to reduce sensitivity to these effects. Future flagship mission concepts are already learning from CGI experience in areas including: coronagraph designs for complex apertures, mirror coatings to minimize polarization-dependent aberrations, and lower-vibration spacecraft pointing control systems.
Wavefront Sensing and Control: State of the art ground-based adaptive optics systems control the incoming wavefront to tens of nanometers RMS. CGI must stabilize the wavefront to tens of picometers RMS, and future exo-Earth imaging missions aim for <10 pm RMS. This level of wavefront control is infeasible even on future 30 m-class ground-based facilities. The surest path to imaging Jovian and Earth analogues in reflected visible light is to move to a stable, space-based platform.
Low Order Aberrations: Tip/tilt errors will originate from slow (sub-Hz) observatory pointing drift and from structural vibrations excited by the telescope reaction wheels (1-100 Hz). Longer timescale thermal drifts in the spacecraft will be the primary contributors to errors in other low order modes. To compensate, CGI will have a dedicated Low Order Wavefront Sensor and Control system (LOWFS/C) for Zernike modes 2-11. The Zernike phase contrast wavefront sensor uses rejected starlight reflected by the coronagraph occulting masks. A fast steering mirror will correct tip/tilt jitter at frequencies <20 Hz; other modes will be corrected at 5 mHz with a combination of a dedicated focus corrector and the DMs. LOWFS images will be downlinked at full frame rate (1 kHz) for use in post-processing and in telescope model validation to inform future missions.
Lab tests of the LOWFS/C have shown promising performance, with several important tests remaining. On bright sources (V = -5), with disturbances approximating the anticipated telescope error power spectrum, the engineering testbed has demonstrated tip/tilt control to better than 0.5 mas RMS and focus sensing at 10 pm accuracy with closed-loop rejection of 20 dB/decade. Future work will verify sensing and control of other modes. In the coming year, fainter (V=5) sources will be tested; models predict sensing on V<6 stars will be photon noise-limited. Finally, we note that LOWFS optical alignment tolerances require thermal regulation to better than 0.1K over tens of hours; future observatories will need even more stringent controls.
Higher Order Aberrations: For higher-order modes, CGI will employ focal-plane wavefront sensing, using science camera images themselves; corrections will be applied to the DMs. The baseline wavefront sensing and correction scheme is pairwise probing and electric field conjugation. This scheme has been demonstrated in the engineering testbed for both the SPC (Shaped Pupil Coronagraph) bowtie and HLC (Hybrid Lyot Coronagraph) modes, achieving contrasts below 5 x 10-9 and 1 x 10-9, respectively. More recent tests have demonstrated that the control scheme may be used while flight-like tip/tilt jitter and low-order drift are injected upstream and corrected by the LOWFS/C. Future tests will verify performance of newer HLC and SPC designs on fainter (V=5) sources and with polarization-dependent low-order aberrations. One remaining challenge is instrument-model mismatch, which slows wavefront correction convergence and limits achievable contrast. Work is ongoing to improve characterization of as-built coronagraph masks, optics, and DM actuator influence functions. New algorithms that update the instrument model in situ, using feedback from previous iterations, are another promising path for improvement.
Deformable Mirrors: Achieving an annular dark hole requires two deformable mirrors to correct both the amplitude and phase components of the complex electric field. WFIRST will place one DM (Deformable Mirror) at a pupil image plane and a second out-of-plane. The DMs, from Xinetics, each have 48 x 48 electrostrictive actuators and were chosen for their relatively high level of technology readiness.
Because CGI high-order wavefront sensing is photon-starved, the DMs will be tuned on a bright reference star before slewing to the science target. The DMs are required to remain stable throughout a ~10 hr science sequence, without active control of dynamic optical aberrations higher than the Zernike modes sensed by the LOWFS. The CGI DMs must therefore be calibrated and stabilized to levels far exceeding those of ground-based applications. A three-pronged approach of precise thermal control (~3 mK stability), improved commanding (accounting for long-timescale actuator settling), and actuator-by-actuator calibration (gains, stability, and surface influence profiles) is the subject of ongoing effort.
Detector development: Both science cameras and the LOWFS will use EMCCDs (Electron Multiplying CCDs). These devices amplify the photo-electron signal at readout, giving superior signal-to-noise in the very low flux regime, relative to classical CCDs. The EMCCDs have not flown in space-based science instruments; information from CGI on detector performance, systematics, and degradation over multi-year timescales is essential for formulating future missions. The imager and IFS detectors will typically operate in high-gain “photon counting” mode, where the frame time is set so that each pixel has at most one photo-electron. This bypasses the effect of EM gain uncertainty (the “extra noise factor”) and maintains the effective quantum efficiency of the detector. Frame times <100 s also minimize contamination from cosmic rays, which are a significant noise source.
Ongoing work is addressing several hardware and software challenges. Detector degradation from high energy particle damage is a concern for operations beyond the initial 18 month technology demonstration period. Damaged pixels can have both higher dark current and more “charge traps;” the latter lead to lower effective detector quantum efficiency. Additionally, large signals, such as cosmic rays, induce “tails” of charge in adjacent pixels during high-gain read; it is estimated that this effect could contaminate 10-20% of pixels in photon counting mode. Currently, these effects are the limiting factors for IFS sensitivity. JPL is iterating with Teledyne e2v to produce new devices that address each of these effects. New devices will be tested prior to PDR, including a radiation test campaign. Additionally, CGI will build on post-processing strategies, pioneered on HST, for mitigating the impact of cosmic rays and charge traps.
Integral Field Spectroscopy: The IFS will enable the first demonstrations of atmosphere spectral retrieval at very high (>108) flux ratios, providing system-level scientific operations experience to benefit future flagship missions. The R = 50 spectra will also support the mission objective of validating integrated observatory and instrument models by capturing the chromatic behavior of the speckle noise floor in a new contrast regime. The prioritized spectroscopy demonstration filter is an 18% bandpass centered at 760 nm; however, the IFS can operate in a Δλ/λ = 20% instantaneous bandpass anywhere in the wavelength range 600 - 1000 nm.
Post-processing: Lessons learned from existing high-contrast instrumentation provide a solid foundation for processing CGI data. Techniques such as angular and reference differential imaging (ADI, RDI) will still be critical. Current baseline observing scenarios include both two-roll imaging (limited by telescope sun angle constraints) as well as target-reference chopping. In this new very high-contrast regime, speckles are affected by both phase and amplitude aberrations. Hence, existing tools that hinge on phase-only dependence must be updated . Additionally, aberrations will be polarization-dependent; therefore, polarimetry will only be possible for targets that are much brighter than the speckle floor, unless new algorithms are invented.
Algorithm development with realistic simulated images will be a high priority during Phase B. Challenges include: spectral retrieval in the presence of detector artifacts and other systematics, as well as reconstruction of extended, low surface brightness features in both polarized and unpolarized light. Incorporating LOWFS and telescope telemetry, including tracking the star position under the coronagraph and creating physically motivated modal basis sets for PSF subtraction, is an area of active research.
Performance Prediction: Validating performance of CGI pre- and post-launch will heavily rely on comparison to simulation. Good agreement has been demonstrated for lab tests of the SPC/IFS mode in several metrics: raw contrast, HOWFS/C convergence rate, and key contrast stability sensitivities. Future work will improve model predictions for other operational modes, for new wavefront control schemes, and for other, more flight-like, configurations.
Figure 23 presents model predictions for CGI imaging and IFS sensitivity, based on current lab-validated performance of the coronagraphs, wavefront control, and detectors. Telescope vibration, aberration, and thermal environments predicted by integrated modeling are assumed; model uncertainty factors of unity are used throughout. Shot noise from the planet, residual stellar PSF, zodi, and exo-zodi is included. Until post-processing algorithms are more mature, a conservative scenario is used: simple roll/reference image subtraction with an additional factor of two gain from the application of all other post-processing algorithms.
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30) Vanessa P. Bailey, Lee Armus, Bala Balasubramanian, Pierre Baudoz, Andrea Bellini, Dominic Benford, Bruce Berriman, Aparna Bhattacharya, Anthony Boccaletti, Eric Cady, Sebastiano Calchi Novati, Kenneth Carpenter, David Ciardi, Brendan Crill, William Danchi, John Debes, Richard Demers, Kjetil Dohlen, Robert Effinger, Marc Ferrari, Margaret Frerking, Dawn Gelino, Julien Girard, Kevin Grady, Tyler Groff, Leon Harding, George Helou, Avenhaus Henning, Markus Janson, Jason Kalirai, Stephen Kane, N. Jeremy Kasdin, Matthew Kenworthy, Brian Kern, John Krist, Jeffrey Kruk, Anne Marie Lagrange, Seppo Laine, Maud Langlois, Hervé Le Coroller, Chris Lindensmith, Patrick Lowrance , Anne-Lise Maire, Sangeeta Malhotra, Avi Mandell, Michael McElwain, Camilo Mejia Prada, Bertrand Mennesson, Tiffany Meshkat, Dwight Moody, Patrick Morrissey, Leonidas Moustakas, Mamadou N'Diaye, Bijan Nemati, Charley Noecker, Roberta Paladini, Marshall Perrin, Ilya Poberezhskiy, Marc Postman, Laurent Pueyo, Solange Ramirez, Clément Ranc, Jason Rhodes, A.J.E. Riggs, Maxime Rizzo, Aki Roberge, Daniel Rouan, Joshua Schlieder, Byoung-Joon Seo, Stuart Shaklan, Fang Shi, Rémi Soummer, David Spergel , Karl Stapelfeldt, Christopher Stark, Motohide Tamura, Hong Tang, John Trauger, Margaret Turnbull, Roeland van der Marel, Arthur Vigan, Benjamin Williams, Edward J. Wollack, Marie Ygouf, Feng Zhao, Hanying Zhou, and Neil Zimmerman, ”Key Technologies for the Wide Field Infrared Survey Telescope Coronagraph Instrument,” Instrumentation and Methods for Astrophysics, arXiv:1901.04050, 13 January 2019, URL: https://arxiv.org/ftp/arxiv/papers/1901/1901.04050.pdf
31) The use of an external occulting “starshade” with WFIRST is under consideration [Ziemer WP], pending guidance from the next Decadal Survey.
one SPC bowtie orientation is included in the current baseline.
Installation of the remaining two SPC bowtie masks, without full
pre-flight commissioning, is under consideration.
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 (firstname.lastname@example.org).