Voyager grand tour mission of the solar system
• March 27, 2019: By all means, Voyager 1 and Voyager 2 shouldn't even be here. Now in interstellar space, they are pushing the limits of spacecraft and exploration, journeying through the cosmic neighborhood, giving us our first direct look into the space beyond our star. 1)
But when they launched in 1977, Voyager 1 and Voyager 2 had a different mission: to explore the outer solar system and gather observations directly at the source, from outer planets we had only seen with remote studies. But now, four decades after launch, they've journeyed farther than any other spacecraft from Earth; into the cold, quiet world of interstellar space.
Originally designed to measure the properties of the giant planets, the instruments on both spacecraft have spent the past few decades painting a picture of the propagation of solar events from our Sun. And the Voyagers' new mission focuses not only on effects on space from within our heliosphere — the giant bubble around the Sun filled up by the constant outflow of solar particles called the solar wind — but from outside of it. Though they once helped us look closer at the planets and their relationship to the Sun, they now give us clues about the nature of interstellar space as the spacecraft continue their journey.
The environment they explore is colder, subtler and more tenuous than ever before, and yet the Voyagers continue on, exploring and measuring the interstellar medium, a smorgasbord of gas, plasma and particles from stars and gas regions not originating from our system. Three of the spacecraft's 10 instruments are the major players that study how space inside the heliosphere differs from interstellar space. Looking at this data together allows scientist to piece together our best-yet picture of the edge of the heliosphere and the interstellar medium. Here are the stories they tell.
On the Sun Spot, we have been exploring the various instruments on Voyager 2 one at a time, and analyzing how scientists read the individual sets of data sent to Earth from the far-reaching spacecraft. But one instrument we have not yet talked about is Voyager 2's Magnetometer, or MAG for short.
During the Voyagers' first planetary mission, the MAG was designed to investigate the magnetospheres of planets and their moons, determining the physical mechanics and processes of the interactions of those magnetic fields and the solar wind. After that mission ended, the Voyager spacecraft studied the magnetic field of the heliosphere and beyond, observing the magnetic reach of the Sun and the changes that occur within that reach during solar activity.
Getting the magnetic data as we travel further into space requires an interesting trick. Voyager spins itself around, in a calibration maneuver that allows Voyager to differentiate between the spacecraft's own magnetic field — that goes along for the ride as it spins — and the magnetic fields of the space it's traveling through.
Figure 1: Illustration of NASA's Voyager spacecraft, with the Magnetometer (MAG) instrument and its boom displayed (image credit: NASA's Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
The initial peek into the magnetic field beyond the Sun's influence happened when Voyager 1 crossed the heliopause in 2012. Scientists saw that within the heliosphere, the strength of the magnetic field was quite variable, changing and jumping as Voyager 1 moved through the heliosphere. These changes are due to solar activity. But once Voyager 1 crossed into interstellar space, that variability was silenced. Although the strength of the field was similar to what it was inside the heliosphere, it no longer had the variability associated with the Sun's outbursts.
Figure 2: Magnetometer (MAG) data taken from Voyager 1 during its transition into interstellar space in 2012 (image credit: NASA's Goddard Space Flight Center/Jet Propulsion Laboratory)
This graph shows the magnitude, or the strength, of the magnetic field around the heliopause from January 2012 out to May 2014. Before encountering the heliopause, marked by the orange line, the magnetic strength fluctuates quite a bit. After a bumpy ride through the heliopause in 2012, the magnetic strength stops fluctuating and begins to stabilize in 2013, once the spacecraft is far enough out into the interstellar medium.
The Cosmic Ray Subsystem
Much like the MAG, the CRS (Cosmic Ray Subsystem) was originally designed to measure planetary systems. The CRS focused on the compositions of energetic particles in the magnetospheres of Jupiter, Saturn, Uranus and Neptune. Scientists used it to study the charged particles within the solar system and their distribution between the planets. Since it passed the planets, however, the CRS has been studying the heliosphere's charged particles and — now — the particles in the interstellar medium.
The CRS measures the count rate, or how many particles detected per second. It does this by using two telescopes: the High Energy Telescope, which measures high energy particles (70MeV) identifiable as interstellar particles, and the Low Energy Telescope, which measures low-energy particles (5MeV) that originate from our Sun. You can think of these particles like a bowling ball hitting a bowling pin versus a bullet hitting the same pin — both will make a measurable impact on the detector, but they're moving at vastly different speeds. By measuring the amounts of the two kinds of particles, Voyager can provide a sense of the space environment it's traveling through.
Figure 3: Illustration of NASA's Voyager spacecraft, with the Cosmic Ray Subsystem (CRS) highlighted (image credit: NASA's Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
Figure 4: Scientists compared data from Voyager 1 with its 2012 crossing of the heliopause to watch for clue for when Voyager 2 would cross. In November 2018, the first clues came from the Cosmic Ray Subsystem! (image credit: NASA's Jet Propulsion Laboratory/NASA Headquarters/Patrick Koehn)
These graphs show the count rate — how many particles per second are interacting with the CRS on average each day — of the galactic ray particles measured by the High Energy Telescope (top graph) and the heliospheric particles measured by the Low Energy Telescope (bottom graph). The line in red shows the data from Voyager 1, time shifted forward 6.32 years from 2012 to match up with the data from Voyager around November 2018, shown in blue.
CRS data from Voyager 2 on Nov. 5, 2018, showed the interstellar particle count rate of the High Energy Telescope increasing to count rates similar to what Voyager 1 saw then leveling out. Similarly, the Low Energy Telescope shows a severe decrease in heliospheric originating particles. This was a key indication that Voyager 2 had moved into interstellar space. Scientists can keep watching these counts to see if the composition of interstellar space particles changes along the journey.
The Plasma Instrument
The PLS (Plasma Science) instrument was made to measure plasma and ionized particles around the outer planets and to measure the solar wind's influence on those planets. The PLS is made up of four Faraday cups, an instrument that measures the plasma as it passes through the cups and calculates the plasma's speed, direction and density.
The plasma instrument on Voyager 1 was damaged during a fly-by of Saturn and had to be shut off long before Voyager 1 exited the heliosphere, making it unable to measure the interstellar medium's plasma properties. With Voyager 2's crossing, scientists will get the first-ever plasma measurements of the interstellar medium.
Figure 5: Illustration of NASA's Voyager spacecraft, with the PLS (Plasma Science) instrument displayed (image credit: NASA's Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
Scientists predicted that interstellar plasma measured by Voyager 2 would be higher in density but lower in temperature and speed than plasma inside the heliosphere. And in November 2018, the instrument saw just that for the first time. This suggests that the plasma in this region is getting colder and slower, and, like cars slowing down on a freeway, is beginning to pile up around the heliopause and into the interstellar medium.
And now, thanks to Voyager 2's PLS, we have a never-before-seen perspective on our heliosphere: The plasma velocity from Earth to the heliopause.
Figure 6: With Voyager 2 crossing the heliopause, scientists now have a new view of solar wind plasma across the heliosphere (image credit: NASA's Jet Propulsion Laboratory/ Michigan Institute of Technology/John Richardson)
These three graphs tell an amazing story, summarizing a journey of 42 years in one plot. The top section of this graph shows the plasma velocity, how fast the plasma across the heliosphere is moving, against the distance out from Earth. The distance is in astronomical units; one astronomical unit is the average distance between the Sun and Earth, about 93 million miles (150 million km). For context, Saturn is 10 AU from Earth, while Pluto is about 40 AU away.
The heliopause crossing happened at 120 AU, when the velocity of plasma coming out from the Sun drops to zero (seen on the top graph), and the outward flow of the plasma is diverted — seen in the increase in the two bottom graphs, which show the upwards and downward speeds (the normal velocity, middle graph) and the sideways speed of the solar wind (the tangential velocity, bottom graph) of the solar wind plasma, respectively. This means as the solar wind begins to interact with the interstellar medium, it is pushed out and away, like a wave hitting the side of a cliff.
Looking at each instrument in isolation, however, does not tell the full story of what interstellar space at the heliopause looks like. Together, these instruments tell a story of the transition from the turbulent, active space within our Sun's influence to the relatively calm waters on the edge of interstellar space.
The MAG shows that the magnetic field strength decreases sharply in the interstellar medium. The CRS data shows an increase in interstellar cosmic rays, and a decrease in heliospheric particles. And finally, the PLS shows that there's no longer any detectable solar wind.
Now that the Voyagers are outside of the heliosphere, their new perspective will provide new information about the formation and state of our Sun and how it interacts with interstellar space, along with insight into how other stars interact with the interstellar medium.
Voyager 1 and Voyager 2 are providing our first look at the space we would have to pass through if humanity ever were to travel beyond our home star — a glimpse of our neighborhood in space.
• December 10, 2018: For the second time in history, a human-made object has reached the space between the stars. NASA's Voyager 2 probe now has exited the heliosphere - the protective bubble of particles and magnetic fields created by the Sun. 2)
Figure 7: This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018 (image credit: NASA/JPL-Caltech)
Comparing data from different instruments aboard the trailblazing spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on 5 November. This boundary, called the heliopause, is where the tenuous, hot solar wind meets the cold, dense interstellar medium. Its twin, Voyager 1, crossed this boundary in 2012, but Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space.
Figure 8: NASA's Voyager 2 enters interstellar space (image credit: NASA/JPL-Caltech)
Voyager 2 now is slightly more than 11 billion miles (18 billion km) from Earth. Mission operators still can communicate with Voyager 2 as it enters this new phase of its journey, but information – moving at the speed of light – takes about 16.5 hours to travel from the spacecraft to Earth. By comparison, light traveling from the Sun takes about eight minutes to reach Earth.
The most compelling evidence of Voyager 2's exit from the heliosphere came from its onboard Plasma Science Experiment (PLS), an instrument that stopped working on Voyager 1 in 1980, long before that probe crossed the heliopause. Until recently, the space surrounding Voyager 2 was filled predominantly with plasma flowing out from our Sun. This outflow, called the solar wind, creates a bubble – the heliosphere – that envelopes the planets in our solar system. The PLS uses the electrical current of the plasma to detect the speed, density, temperature, pressure and flux of the solar wind. The PLS aboard Voyager 2 observed a steep decline in the speed of the solar wind particles on 5 November. Since that date, the plasma instrument has observed no solar wind flow in the environment around Voyager 2, which makes mission scientists confident the probe has left the heliosphere.
Figure 9: At the end of 2018, the cosmic ray subsystem aboard NASA's Voyager 2 spacecraft provided evidence that Voyager 2 had left the heliosphere. There were steep drops in the rate of heliospheric particles that hit the instrument's radiation detector. At the same time, there were significant increases in the rate at which particles that originate outside our heliosphere (also known as galactic cosmic rays) hit the detector (image credit: NASA/JPL-Caltech/GSFC)
Legend to Figure 9: The graphs show data from Voyager 2's CRS, which averages the number of particle hits over a six-hour block of time. CRS detects both lower-energy particles that originate inside the heliosphere (greater than 0.5 MeV) and higher-energy particles that originate farther out in the galaxy (greater than 70 MeV).
In addition to the plasma data, Voyager's science team members have seen evidence from three other onboard instruments – the cosmic ray subsystem, the low energy charged particle instrument and the magnetometer – that is consistent with the conclusion that Voyager 2 has crossed the heliopause. Voyager's team members are eager to continue to study the data from these other onboard instruments to get a clearer picture of the environment through which Voyager 2 is traveling.
Figure 10: The set of graphs on the left illustrates the drop in electrical current detected in three directions by Voyager 2's plasma science experiment (PLS) to background levels. They are among the key pieces of data that Voyager scientists used to determine that Voyager 2 entered interstellar space, the space between stars, in November 2018. The disappearance in electrical current in the sunward-looking detectors indicates the spacecraft is no longer in the outward flow of solar wind plasma. It is instead in a new plasma environment — interstellar medium plasma. The image on the right shows the Faraday cups of the PLS. The three sunward pointed cups point in slightly different directions in order to measure the direction of the solar wind. The fourth cup (on the upper left) points perpendicular to the others (image credit: NASA/JPL-Caltech)
"There is still a lot to learn about the region of interstellar space immediately beyond the heliopause," said Ed Stone, Voyager project scientist based at Caltech in Pasadena, California.
Together, the two Voyagers provide a detailed glimpse of how our heliosphere interacts with the constant interstellar wind flowing from beyond. Their observations complement data from NASA's Interstellar Boundary Explorer (IBEX), a mission that is remotely sensing that boundary. NASA also is preparing an additional mission – the upcoming Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2024 – to capitalize on the Voyagers' observations.
"Voyager has a very special place for us in our heliophysics fleet," said Nicola Fox, director of the Heliophysics Division at NASA Headquarters. "Our studies start at the Sun and extend out to everything the solar wind touches. To have the Voyagers sending back information about the edge of the Sun's influence gives us an unprecedented glimpse of truly uncharted territory."
While the probes have left the heliosphere, Voyager 1 and Voyager 2 have not yet left the solar system, and won't be leaving anytime soon. The boundary of the solar system is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun's gravity. The width of the Oort Cloud is not known precisely, but it is estimated to begin at about 1,000 astronomical units (AU) from the Sun and to extend to about 100,000 AU. One AU is the distance from the Sun to Earth. It will take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond it.
Figure 11: This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU (image credit: NASA/JPL-Caltech)
The Voyager probes are powered using heat from the decay of radioactive material, contained in a device called RTG (Radioisotope Thermal Generator). The power output of the RTGs diminishes by about four watts per year, which means that various parts of the Voyagers, including the cameras on both spacecraft, have been turned off over time to manage power.
"I think we're all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone," said Suzanne Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "This is what we've all been waiting for. Now we're looking forward to what we'll be able to learn from having both probes outside the heliopause."
Voyager 2 launched on 20 August 1977, 16 days before Voyager 1 (launch on 5 September 1977), and both have traveled well beyond their original destinations. The spacecraft were built to last five years and conduct close-up studies of Jupiter and Saturn. However, as the mission continued, additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible. As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left Earth. Their two-planet mission became a four-planet mission. Their five-year lifespans have stretched to 41 years, making Voyager 2 NASA's longest running mission.
The Voyager story has impacted not only generations of current and future scientists and engineers, but also Earth's culture, including film, art and music. Each spacecraft carries a Golden Record of Earth sounds, pictures and messages. Since the spacecraft could last billions of years, these circular time capsules could one day be the only traces of human civilization.
Voyager's mission controllers communicate with the probes using NASA's Deep Space Network (DSN), a global system for communicating with interplanetary spacecraft. The DSN consists of three clusters of antennas in Goldstone, California; Madrid, Spain; and Canberra, Australia.
The Voyager Interstellar Mission is a part of NASA's Heliophysics System Observatory, sponsored by the Heliophysics Division of NASA's Science Mission Directorate in Washington. JPL built and operates the twin Voyager spacecraft. NASA's DSN, managed by JPL, is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. The CSIRO (Commonwealth Scientific and Industrial Research Organization), Australia's national science agency, operates both the CDSCC (Canberra Deep Space Communication Complex), part of NASA's DSN, and the Parkes Observatory of CSIRO, which NASA has been using to downlink data from Voyager 2 since 8 November.
Australia's national science agency, CSIRO, is supporting NASA's Voyager 2 spacecraft as it enters interstellar space.
• On 8 November 2018, CSIRO's Parkes radio telescope joined NASA's CDSCC (Canberra Deep Space Communication Complex), part of NASA's Deep Space Network, to receive unique and historic data from Voyager 2. This provides a clearer picture of the environment through which Voyager 2 is travelling. The Parkes telescope will continue to receive downlink data into early 2019. 3)
- NASA has engaged the Parkes telescope to support receiving this historic data from Voyager 2 while CDSCC is busy with communications for other deep space missions that are making their own important encounters during this period, such as New Horizons' flyby of the most distant object yet to be explored by a spacecraft, coming up on New Year's Day.
- Because of Voyager 2's location and distance from Earth, CDSCC and the Parkes telescope are the only facilities in the world that are capable of having contact with the spacecraft.
- Voyager 2 isn't able to record its data on board – it transmits it directly from the instruments back to Earth – making it essential to receive as much of this vital data as possible.
- CSIRO Chief Executive Dr Larry Marshall said CSIRO was here to solve the greatest challenges with science. "So we're proud to help NASA solve the scientific challenge of capturing this once in a lifetime opportunity as Voyager 2 ventures into interstellar space," Dr Marshall said.
- "Our team at Parkes has partnered with NASA on some of humanity's most momentous steps in space, including the landing of the Mars Rover Curiosity and, almost fifty years ago, the Apollo 11 Moon landing.
- CSIRO Director of Astronomy and Space Science Dr Douglas Bock explained how the additional support from Parkes would track Voyager 2. "The Canberra Deep Space Communication Complex, which CSIRO operates on behalf of NASA, has been providing command, telemetry and control for the twin Voyager spacecraft since their launch in 1977," Dr Bock said.
- "NASA has engaged our 64 m Parkes radio telescope to 'combine forces' with CDSCC's 70 m antenna, Deep Space Station 43 (DSS43), to capture as much scientifically valuable data as possible during this critical period.
- "The Parkes telescope will be tracking Voyager 2 for 11 hours a day while the spacecraft is observable from Parkes. CDSCC's DSS43 will also track Voyager 2 for a number of hours both before and after Parkes, expanding the available observation time. - This is a highlight of CSIRO's decades' worth of experience operating large, complex spacecraft tracking and radio astronomy infrastructure."
Figure 12: Photo of CSIRO's Parkes 64 m radio telescope (image credit: CSIRO)
Legend to Figure 12: The Parkes radio telescope is located outside the town of Parkes in the central-west region of New South Wales, about 380 km from Sydney. It's one of three instruments that make up the Australia Telescope National Facility. Parkes is one of the largest single-dish telescopes in the southern hemisphere dedicated to astronomy. It started operating in 1961, but only its basic structure has remained unchanged. The surface, control system, focus cabin, receivers, computers and cabling have all been upgraded – some parts many times – to keep the telescope at the cutting edge of radio astronomy. The telescope is now 10,000 times more sensitive than when it was commissioned.
Figure 13: Photo of NASA's CDSCC (Canberra Deep Space Communication Complex), located 35 km outside Australia's capital (image credit: NASA, CSIRO)
Legend to Figure 13: CDSCC is part of NASA's Deep Space Network (DSN) which connects scientists around the world with their robotic spacecraft exploring the Solar System and beyond. Since 2010, CSIRO has partnered with NASA to manage the Canberra facility on their behalf. The DSN also has two other station complexs located near Madrid, Spain and Goldstone, California.The Canberra facility is dominated by a massive 70 m dish, the largest steerable parabolic antenna in the southern hemisphere. Along with three additional 34 m dishes across the site, they transmit and receive data from over 40 missions in deep space, including both of NASA's Voyager spacecraft.
1) Susannah Darling, Rob Garner, "The Voyage to Interstellar Space," NASA, 27 March 2019, URL: https://www.nasa.gov/feature/goddard/2019/the-voyage-to-interstellar-space
2) Dwayne Brown, Karen Fox, Calla Cofield, "NASA's Voyager 2 Probe Enters Interstellar Space," NASA/JPL News Release 18-115, 10 December 2018, URL: https://www.jpl.nasa.gov/news/news.php?feature=7301
3) Andrew Warren, "We're all ears as Voyager 2 goes interstellar," CSIRO news, 11 December 2018, URL: https://www.csiro.au/en/News/News-releases/2018/Were-all-ears-as-Voyager-2-goes-interstellar
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 (email@example.com).