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Voyager grand tour mission of the solar system

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. 1)


Figure 1: 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 2: 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 3: 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 3: 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 4: 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 5: 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. 2)

- 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 6: Photo of CSIRO's Parkes 64 m radio telescope (image credit: CSIRO)

Legend to Figure 6: 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 7: Photo of NASA's CDSCC (Canberra Deep Space Communication Complex), located 35 km outside Australia's capital (image credit: NASA, CSIRO)

Legend to Figure 7: 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) Dwayne Brown, Karen Fox, Calla Cofield, "NASA's Voyager 2 Probe Enters Interstellar Space," NASA/JPL News Release 18-115, 10 December 2018, URL:

2) Andrew Warren, "We're all ears as Voyager 2 goes interstellar," CSIRO news, 11 December 2018, URL:

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