The Voyager 2 spacecraft is an unmanned interplanetary space probe launched on August 20, 1977. Identical in form to its sister Voyager program craft Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic (the plane of the Solar System) so that it could be sent to Uranus and Neptune by means of gravity assist during the 1981 encounter at Saturn. Because of this trajectory, Voyager 2 could not see the moon Titan up close as its twin had, but the probe did become the first and only spacecraft to travel to Uranus and Neptune, thus completing the Planetary Grand Tour, a rare geometric arrangement of the outer planets that only occurs once every 176 years.
Voyager 2 is perhaps the most productive space probe yet deployed, visiting four planets and their moons, including two primary visits to previously unexplored planets, with powerful cameras and a multitude of scientific instruments, at a fraction of the money later spent on specialized probes such as the Galileo spacecraft and the Cassini-Huygens probe. Along with Pioneer 10, Pioneer 11, Voyager 1 and New Horizons, Voyager 2 is an interstellar probe.
Voyager 2 was originally planned to be Mariner 12, part of the Mariner program.
Voyager 2 was launched on August 20, 1977, from Cape Canaveral, Florida, aboard a Titan IIIE/Centaur carrier rocket. Ground crews became engrossed in a launch problem with Voyager 1 and forgot to send an important activation code to Voyager 2. This caused the probe to shut down its main radio receiver. Fortunately, ground crews were able to establish contact through the spacecraft's back-up radio receiver. In the process of doing all of this, a fuse was blown in the power supply of the primary receiver, permanently disabling it, and thus the back-up receiver had to be relied upon for the entire mission.
The closest approach to Jupiter occurred on July 9, 1979. It came within 570,000 km (350,000 miles) of the planet's cloud tops. It discovered a few rings around Jupiter, as well as volcanic activity on the moon Io.
The Great Red Spot was revealed as a complex storm moving in a counterclockwise direction. An array of other smaller storms and eddies were found throughout the banded clouds.
Discovery of active volcanism on the satellite Io was easily the greatest unexpected discovery at Jupiter. It was the first time active volcanoes had been seen on another body in the solar system. Together, the Voyagers observed the eruption of nine volcanoes on Io, and there is evidence that other eruptions occurred between the Voyager encounters.
Europa displayed a large number of intersecting linear features in the low-resolution photos from Voyager 1. At first, scientists believed the features might be deep cracks, caused by crustal rifting or tectonic processes. The closer high-resolution photos from Voyager 2, however, left scientists puzzled: The features were so lacking in topographic relief that as one scientist described them, they "might have been painted on with a felt marker." Europa is internally active due to tidal heating at a level about one-tenth that of Io. Europa is thought to have a thin crust (less than 30 kilometers or 18 miles thick) of water ice, possibly floating on a 50-kilometer-deep (30 mile) ocean.
The closest approach to Saturn occurred on August 26, 1981.
While passing behind Saturn (as viewed from Earth), Voyager 2 probed Saturn's upper atmosphere with its radio link to measure temperature and density profiles. Voyager 2 found that at the highest levels (7 kilopascals pressure) Saturn's temperature was 70 kelvins (−203 °C), while at the deepest levels measured (120 kilopascals) the temperature increased to 143 kelvins (−130 °C). The north pole was found to be 10 kelvins cooler, although this may be seasonal (see also Saturn Oppositions).
After the Saturn flyby, the camera platform on Voyager 2 locked up briefly, putting plans to officially extend the mission to Uranus and Neptune in jeopardy. Fortunately, the mission team was able to fix the problem — caused by overuse that temporarily depleted its lubricant — and the probe was given the go-ahead to examine Uranus.
Uranus is the third largest planet in the solar system. It orbits the Sun at a distance of about 2.8 billion kilometers (1.7 billion miles) and completes one orbit every 84 years. The length of a day on Uranus as measured by Voyager 2 is 17 hours, 14 minutes. Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history. Given its odd orientation, with its polar regions exposed to sunlight or darkness for long periods, scientists were not sure what to expect at Uranus.
Voyager 2 found that one of the most striking influences of Uranus' sideways position is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. The presence of a magnetic field at Uranus was not known until Voyager's arrival.
Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles, which are almost uniformly gray in color.
A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of ultraviolet light, a phenomenon dubbed "dayglow." The average temperature is about 60 kelvins (−350 degrees Fahrenheit/−213 degrees Celsius). Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature at the cloud tops.
The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the solar system. Detailed images from Voyager's flyby of the moon showed huge fault canyons as deep as 20 kilometers (12 miles), terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when the moon was fractured by a violent impact.
All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects.
The closest appoach to Neptune occurred on August 25, 1989. Since this was the last major planet Voyager 2 could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, as with Voyager 1's encounter with Saturn and its moon Titan.
The probe also discovered the Great Dark Spot, which has since disappeared, according to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it was later postulated to be a hole in the visible cloud deck.
For many years, beginning in the late 19th century, it was widely believed that an unseen planet (dubbed "Planet X") was influencing Uranus and Neptune, as their observed and predicted positions differed. This belief brought about the 1930 discovery of Pluto, and persisted as it became increasingly clear that Pluto does not have the mass to account for these discrepancies. When Voyager 2 passed Neptune, it became possible to precisely measure Neptune's mass. Neptune turned out to be 0.5% less massive than previously believed, a difference comparable to the entire mass of Mars. When Uranus and Neptune's orbits were recalculated using the most accurate figure, it became clear that the erroneous mass figure - not the gravity of an unseen planet - caused the orbital discrepancies which had long vexed the astronomical community.
With the 2006 decision of the International Astronomical Union to reclassify Pluto as a "dwarf planet", the 1989 flyby of Neptune by Voyager 2 became the point when every planet in the solar system had been visited at least once by spacecraft.
Escaping the solar system
Since its planetary mission is over, Voyager 2 is now described as working on an interstellar mission, which NASA is using to find out what the solar system is like beyond the heliosphere. Like Voyager 1 around three years before, Voyager 2 crossed into the heliosheath, the last section of the heliosphere before interstellar space, in October 2007.
As of September 2008, Voyager 2 was at a distance of around 87.03 AU (13.019 billion km, 8.077 billion miles, or 0.0013761 ly) from the Sun , deep in the scattered disc, and traveling outward at roughly 3.28 AUs per year.It is more than twice as far from the Sun as Pluto, and far beyond the perihelion of 90377 Sedna, but not yet beyond the outer limits of the orbit of Eris.
On August 30, 2007, Voyager 2 passed the termination shock into the heliosheath, approximately 1 billion miles (1.6 billion km) closer to the Sun than Voyager 1 did. This is due to the local interstellar magnetic field of deep space. The southern hemisphere of the solar system's heliosphere is being pushed in.
Each Voyager carries a gold-plated audio-visual disc in the event that either spacecraft is ever found by intelligent aliens. The disc carries images of Earth and its lifeforms, a range of scientific information, and a medley, "Sounds of Earth", that includes the sounds of whales, a baby crying, waves breaking on a shore and a variety of music.
Voyager 2 is expected to keep transmitting until at least after 2025, over 48 years since launch.
On November 30, 2006, a command was incorrectly decoded by the spacecraft as an instruction to turn on heaters associated with the probe's magnetometer. The heaters remained on until December 4, 2006, resulting in extremely high temperatures above 130 °C (266 °F), and in sensor rotation away from the correct orientation. It has not been possible to fully diagnose and correct for the damage to the Voyager 2 magnetometer, although efforts to do so are ongoing.
Published - July 2009
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