Dec. 10, 2018: NASA's Voyager 2 spacecraft has entered interstellar space. The spacecraft joins its twin—Voyager 1—as the only human-made objects to enter the space between the stars.
|Launch Date||Aug. 20, 1977|
|Launch Site||Cape Canaveral, Florida, USA | Launch Complex 41|
|Destination||Our Solar System, Jupiter, Saturn, Uranus, Neptune, Beyond Our Solar System|
|Status||Successful—Extended Mission in Progress|
|Alternate Names||1977-076A, Mariner Jupiter/Saturn B, 10271|
Voyager 1 and 2 were designed to take advantage of a rare planetary alignment to study the outer solar system up close. Voyager 2 targeted Jupiter, Saturn, Uranus and Neptune. Like its sister spacecraft, Voyager 2 also was designed to find and study the edge of our solar system beyond the orbits of the planets.
Voyager 2 has operated in space for more than 40 years as of Aug. 20, 2017.
Voyager 2 is the only spacecraft to study all four of the solar system's giant planets at close range. It is now exploring the outermost reaches of where the solar wind and the Sun's magnetic field dominate space. In September 2007, it crossed the termination shock (where the speed of the solar wind drops below the speed of sound) at 84 AU (about 13 billion km from the Sun, more than twice the distance to Pluto). Since then, Voyager 1 has been operating in the heliosheath environment, a region about 40 to 50 AU (3.7 billion to 4.7 billion km) thick where the solar wind mixes with the interstellar wind.
During the Jupiter leg of its journey, Voyager 2 was to explore the giant planet, its magnetosphere and moons in greater detail than had the Pioneer spacecraft that preceded it. Voyager 2 was not only to study Jupiter, but to use it as a springboard to Saturn, using the gravity-assist technique.
Voyager 2 succeeded on all counts. It returned spectacular photos of the entire Jovian system, and time-lapse movies made from its images of Jupiter showed how the planet had changed since Voyager 1's visit. Its images of Io revealed changes in the moon's surface and the persistence of its volcanic eruptions. The spacecraft resolved the streaks Voyager 1 had shown on Europa into a collection of cracks in a thick and remarkably smooth icy crust. It also discovered a 14th moon and revealed a third component to the planet's rings.
Voyager 2 was to become the third spacecraft to visit Saturn. Its mission there was to follow up on the pictures and data returned by Voyager 1.
Voyager 2 gave us another close-range look at Saturn and its moons. Using its photopolarimeter, an instrument that had failed on Voyager 1, Voyager 2 was able to observe the planet's rings at much higher resolution and to discover many more ringlets. It also provided more detailed images of the ring spokes and kinks, and of the F-ring and its shepherding moons. Finally, it employed a gravity-assist maneuver at Saturn to help it reach its next destination, Uranus.
Following its flybys of Jupiter and Saturn, Voyager 2 was to become the first spacecraft to visit Uranus. Voyager 2 remains the only spacecraft to have flown by Uranus. The planet displayed little detail, but gave evidence of an ocean of boiling water about 800 km below the cloud tops. Curiously, the average temperature of its Sun-facing pole was found to be the same as that of the equator. The spacecraft discovered 10 new moons, two new rings, and a strangely tilted magnetic field stronger than that of Saturn. A gravity assist at Uranus propelled the spacecraft toward its next destination, Neptune.
Voyager 2 is the only human-made object to have flown by Neptune. In the closest approach of its entire tour, the spacecraft passed less than 5,000 km above the planet's cloud tops. It discovered five moons, four rings, and a "Great Dark Spot" that vanished by the time the Hubble Space Telescope imaged Neptune five years later. Neptune's largest moon, Triton, was found to be the coldest known planetary body in the solar system, with a nitrogen ice "volcano" on its surface. A gravity assist at Neptune shot Voyager 2 below the plane in which the planets orbit the Sun, on a course which will ultimately take the spacecraft out of our solar system.
After Voyager 2's flyby of Neptune, NASA formally renamed the entire project (including both Voyager spacecraft) the Voyager Interstellar Mission (VIM). Some year before 2030, Voyager 2 is expected to cross the heliopause-the outer boundary of the vast region of space dominated by the solar wind and the Sun's magnetic field-and reach interstellar space. In that sense, it can be said that the spacecraft will be able to sample what space is like beyond our solar system. (If we define the solar system as the Sun and everything that primarily orbits the Sun, however, Voyager 2 will remain will emerge from the Oort cloud in another 14,000 to 28,000 years.
As the spacecraft's power supply dwindles, it will need to begin shutting down its instruments. Sometime in 2025 or after, there will be insufficient electricity to power even one instrument, and Voyager 2 will continue its eternal journey among the stars in silence.
Aug. 20 1977: Launch
July 9, 1979: Jupiter Flyby (Closest Approach)
Aug. 26, 1981: Saturn Flyby (Closest Approach)
Jan. 24, 1986: Uranus Flyby (Closest Approach)
Aug. 25, 1989: Neptune Flyby (Closest Approach)
An alignment of the outer planets that occurs only once in 176 years prompted NASA to plan a grand tour of the outer planets, consisting of dual launches to Jupiter, Saturn, and Pluto in 1976-77 and dual launches to Jupiter, Uranus, and Neptune in 1979. The original scheme was canceled for budgetary reasons, but was replaced by Voyager 1 and 2, which accomplished similar goals at significantly lower cost.
The two Voyager spacecraft were designed to explore Jupiter and Saturn in greater detail than the two Pioneers (Pioneers 10 and 11) that preceded them had been able to do. Each Voyager was equipped with slow-scan color TV to take live television images from the planets, and each carried an extensive suite of instruments to record magnetic, atmospheric, lunar, and other data about the planets. The original design of the spacecraft was based on that of the older Mariners. Power was provided by three plutonium oxide radioisotope thermoelectric generators (RTGs) mounted at the end of a boom.
Although launched about two weeks before Voyager 1, Voyager 2 exited the asteroid belt after its twin and followed it to Jupiter and Saturn. The primary radio receiver failed on 5 April 1978, placing the mission's fate on the backup unit, which has been used ever since. A fault in this backup receiver severely limits its bandwidth, but the mission has been a major success despite this obstacle. All of the experiments on Voyager 2 have produced useful data.
Voyager 2 began transmitting images of Jupiter on 24 April 1979 for time-lapse movies of atmospheric circulation. They showed that the planet's appearance had changed in the four months since Voyager 1's visit. The Great Red Spot had become more uniform, for example.
The spacecraft relayed spectacular photos of the entire Jovian system, including its moons Amalthea, Io, Callisto, Ganymede, and Europa, all of which had also been imaged by Voyager 1, making comparisons possible. Voyager 2's closest encounter with Jupiter was at 22:29 UT on 9 July 1979 at a range of 645,000 km.
Voyager 1's discovery of active volcanoes on Io prompted a 10-hour volcano watch for Voyager 2. Though the second spacecraft approached no closer than a million kilometers to Io, it was clear that the moon's surface had changed and that six of the volcanic plumes observed earlier were still active.
Voyager 2 imaged Europa at a distance of 206,000 km, resolving the streaks seen by Voyager 1 into a collection of cracks in a thick covering of ice. No variety in elevation was observed, prompting one scientist to say that Europa was "as smooth as a billiard ball." An image of Callisto, studied in detail months later, revealed a 14th satellite, now called Adrastea. It is only 30 to 40 km in diameter and orbits close to Jupiter's rings. As Voyager 2 left Jupiter, it took an image that revealed a faint third component to the planet's rings. It is thought that the moons Amalthea and Thebe may contribute some of the material that constitutes the ring.
Following a midcourse correction two hours after its closest approach to Jupiter, Voyager 2 sped to Saturn. Its encounter with the sixth planet began on 22 August 1981, two years after leaving the Jovian system, with imaging of the moon Iapetus.
Once again, Voyager 2 repeated the photographic mission of its predecessor, although it flew 23,000 km closer to Saturn. The closest encounter was at 01:21 UT on 26 August 1981 at a range of 101,000 km. The spacecraft provided more detailed images of the ring spokes and kinks, as well as the F-ring and its shepherding moons. Voyager 2's data suggested that Saturn's A-ring was perhaps only 300 m thick. It also photographed the moons Hyperion, Enceladus, Tethys, and Phoebe.
Using the spacecraft's photopolarimeter (the instrument that had failed on Voyager 1), scientists observed a star called Delta Scorpii through Saturn's rings and measured the flickering level of light over the course of 2 hours, 20 minutes. This provided 100-m resolution, which was 10 times better than was possible with the cameras, and many more ringlets were discovered.
After Voyager 2 fulfilled its primary mission goals with its flybys of Jupiter and Saturn, mission planners set the spacecraft on a 4.5-year journey to Uranus, during which it covered 33 AU (about 5 billion km). The geometry of the Uranus encounter was designed to enable the spacecraft to use a gravity assist to help it reach Neptune. Voyager 2 had only 5.5 hours of close study during its flyby, the first (and so far, only) human-made spacecraft to visit the planet Uranus.
Long-range observations of Uranus began on 4 November 1985. At that distance, the spacecraft's radio signals took approximately 2.5 hours to reach Earth. Light conditions were 400 times less than terrestrial conditions. The closest approach took place at 17:59 UT on 24 January 1986 at a range of 71,000 km.
The spacecraft discovered 10 new moons, two new rings, and a magnetic field (stronger than that of Saturn) tilted at 55 degrees off-axis and off-center, with a magnetic tail twisted into a helix that stretches 10 million km in the direction opposite that of the Sun.
Uranus, itself, displayed little detail, but evidence was found of a boiling ocean of water some 800 km below the top cloud surface. The atmosphere was found to be 85 percent hydrogen and 15 percent helium (26 percent helium by mass). Strangely, the average temperature of 60 K (-351.4 degrees Fahrenheit, -213 degrees Celsius) was found to be the same at the Sun-facing south pole and at the equator. Wind speeds were as high as 724 km per hour.
Voyager 2 returned spectacular photos of Miranda, Oberon, Ariel, Umbriel, and Titania, the five larger moons of Uranus. In a departure from Greek mythology, four of Uranus' moons are named for Shakespearean characters and one-Umbriel-is named for a sprite in a poem by Alexander Pope. Miranda may be the strangest of these worlds. It is believed to have fragmented at least a dozen times and reassembled in its current confused state.
Following the Uranus encounter, the spacecraft performed a single midcourse correction on 14 February 1986 to set it on a precise course to Neptune. Voyager 2's encounter with Neptune capped a 7-billion-km journey when on 25 August 1989, at 03:56 UT, it flew about 4,950 km over the cloud tops of the giant planet, closer than its flybys of the three previous planets. As with Uranus, it was the first (and so far, only) human-made object to fly by the planet. Its 10 instruments were still in working order at the time.
During the encounter, the spacecraft discovered five new moons and four new rings. The planet itself was found to be more active than previously believed, with winds of 1100 km per hour. Hydrogen was found to be the most common atmospheric element, although the abundant methane gives the planet its blue appearance. Voyager data on Triton, Neptune's largest moon, revealed the coldest known planetary body in the solar system and a nitrogen ice volcano on its surface.
The spacecraft's flyby of Neptune set it on a course below the ecliptic plane that will ultimately take it out of the solar system. After Neptune, NASA formally renamed the entire project (including both Voyager spacecraft) the Voyager Interstellar Mission (VIM).
Approximately 56 million km past the Neptune encounter, Voyager 2's instruments were put into low-power mode to conserve energy. In November 1998, twenty-one years after launch, nonessential instruments were permanently turned off. Six instruments are still operating. Data from at least some of the instruments should be received until at least 2025. Sometime after that date, power levels onboard the spacecraft will be too low to operate even one of its instruments.
Launch Vehicle: Titan IIIE-Centaur (TC-7 / Titan no. 23E-7 / Centaur D-1T)
Spacecraft Mass: 2,080 kilograms (822 kilograms mission module)
- imaging system
- ultraviolet spectrometer
- infrared spectrometer
- planetary radio astronomy experiment
- plasma particles experiment
- low-energy charged-particles experiment
- plasma waves experiment
- cosmic-ray telescope
Siddiqi, Asif A. Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, NASA, 2002.