Voyagers

The Voyager space probes are probably two of the most famous space probes in history, for they provided us with our first detailed close-up views and scans of the outer planets: Jupiter, Saturn, Uranus, and Neptune (they did not fly by Pluto). Because of the two Voyager probes, we have gained much knowledge on the outer planets, their moons, and their rings.

Each Voyager consisted of a decahedral bus, 47 cm in height and 1.78 m across from flat to flat. A 3.66 m diameter parabolic high-gain antenna was mounted on top of the bus. The major portion of the science instruments were mounted on a science boom extending out some 2.5 m from the spacecraft. At the end of the science boom was a steerable scan platform on which were mounted the imaging and spectroscopic remote sensing instruments. Also mounted at various distances along the science boom were the plasma and charged particle detectors. The magnetometers were located along a separate boom extending 13 m on the side opposite the science boom. A third boom, extending down and away from the science instruments, held the spacecraft's radioisotope thermoelectric generators (RTG's). Two 10 m whip antennas (used for the plasma wave and planetary radio astronomy investigations) also extended from the spacecraft, each perpendicular to the other. The spacecraft was three-axis spin stabilized to enable long integration times and selective viewing for the instruments mounted on the scan platform.

Power was provided to the spacecraft systems and instruments through the use of three radioisotope thermoelectric generators. The RTG's were assembled in tandem on a deployable boom hinged on an outrigger arrangement of struts attached to the basic structure. Each RTG unit, contained in a beryllium outer case, was 40.6 cm in diameter, 50.8 cm in length, and weighed 39 kg. The RTG's used a radioactive source (Plutonium-238 in the form of plutonium oxide, or PuO2, in this case) which, as it decayed, gave off heat. A bi-metallic thermoelectric device was used to convert the heat to electric power for the spacecraft. The total output of RTG's slowly decreases with time as the radioactive material is expended. Therefore, although the initial output of the RTG's on Voyager was approximately 470 W of 30 V DC power at launch, it had fallen off to approximately 335 W by the beginning of 1997 (about 19.5 years post-launch). As power continues to decrease, power loads on the spacecraft must also decrease. Current estimates (1998) are that increasingly limited instrument operations can be carried out at least until 2020.

Communications were provided through the high-gain antenna with a low-gain antenna for backup. The high-gain antenna supported both X-band and S-band downlink telemetry. Voyager was the first spacecraft to utilize X-band as the primary telemetry link frequency. Data could be stored for later transmission to Earth through the use of an on-board digital tape recorder.

Voyager, because of its distance from Earth and the resulting time-lag for commanding, was designed to operate in a highly-autonomous manner. In order to do this and carry out the complex sequences of spacecraft motions and instrument operations, three interconnected on-board computers were utilized. The Computer Command Subsystem (CCS) was responsible for storing commanding for the other two computers and issuing the commands at set times. The Attitude and Articulation Control Subsystem (AACS) was responsible for controlling spacecraft attitude and motions of the scan platform. The Flight Data Subsystem (FDS) controlled the instruments, including changes in configuration (state) or telemetry rates. All three computers had redundant components to ensure continued operations. The AACS included redundant star trackers and Sun sensors as well.

Voyager 1 was one of a pair of spacecraft launched to explore the planets of the outer solar system and the interplanetary environment, launched after Voyager 2. Voyager 1 flew by Jupiter and Saturn only, though it could have visited Neptune and Uranus, but NASA did not want to pass up the opportunity to view Saturn's moon Titan more closely. Voyager 1 passed Jupiter on March 5, 1979, and passed by Saturn on November 13, 1980.

Originally planned as a Grand Tour of the outer planets, including dual launches to Jupiter, Saturn, and Pluto in 1976-77 and dual launches to Jupiter, Uranus, and Neptune in 1979, budgetary constraints caused a dramatic re-scoping of the project to two spacecraft, each of which would go to only Jupiter and Saturn. The new mission was called Mariner Jupiter/Saturn, or MJS. It was subsequently renamed Voyager about six months prior to launch. The re-scoped mission was estimated to cost $250 million (through the end of Saturn operations), only a third of what the Grand Tour design would have cost.

Originally scheduled to launch twelve days after Voyager 2, Voyager 1's launch was delayed twice to prevent the occurrence of problems which Voyager 2 experienced after launch. Voyager 1's launch finally happened on September 5, 1977, and was termed "flawless and accurate". Although launched 16 days after Voyager 2, Voyager 1's trajectory was the quicker one to Jupiter. On Dec 15, 1977, while both spacecraft were in the asteroid belt, Voyager 1 surpassed Voyager 2's distance from the sun. Voyager 1 then proceeded to Jupiter (making its closest approach on March 5, 1979) and Saturn (with closest approach on November 12, 1980). Both prior to and after planetary encounters observations were made of the interplanetary medium. Some 18,000 images of Jupiter and its satellites were taken by Voyager 1. In addition, roughly 16,000 images of Saturn, its rings, and satellites were obtained.

After its encounter with Saturn, Voyager 1 remained relatively quiescent, continuing to make in-situ observations of the interplanetary environment and UV observations of stars. After nearly nine years of dormancy, Voyager 1's cameras were once again turned on to take a series of pictures. On February 14, 1990, Voyager 1 looked back from whence it came and took the first "family portrait" of the solar system, a mosaic of 60 frames of the Sun and six of the planets (Venus, Earth, Jupiter, Saturn, Uranus, and Neptune) as seen from "outside" the solar system. After this final look back, the cameras on Voyager 1 were once again turned off. All of the experiments, save the photopolarimeter (which failed to operate), have produced useful data.

Voyager 1 has passed Pioneer 10 in 1998, so has become the most distant human-made object.

Voyager 2 was launched on August 20, 1977, before Voyager 1. Only eight months into the journey though, Voyager 2's primary radio stopped working, and its backup radio receiver developed a short circuit. Functioning weakly, it was still able to relay all of its scientific discoveries. Voyager 2 flew by Jupiter, Saturn, Uranus, and Neptune. It flew by Jupiter on August 7, 1979, Saturn on August 26, 1981, Uranus on January 24, 1986, and Neptune on August 8, 1989, taking advantage of a once-in-189-years alignment of the outer planets to "slingshot" its way from planet to planet.

Voyager 2 found Uranus to be solidly-colored. It showed that Uranus' magnetic axis was far off (30°) from its already 90° off rotational axis, and that the magnetic axis doesn't even pass through the center of the planet. Voyager 2 also identified Uranus as a radio source. It found ice patches on Ariel, one of its moons, and strange collages of different types of terrains on the moon Miranda. Voyager 2 also found 10 more moons and one more ring than were previously known. Neptune from Voyager 2

Voyager 2 found Neptune to have interesting weather patterns, which include clouds. Neptune was found to have an internal heat source, and, like Uranus, Neptune's magnetic axis is inclined greatly off of its rotational axis. Neptune's ring arcs were discovered to be bright patches on one ring. Voyager 2 discovered two more rings, and six more moons. Triton, a moon, was found to be cantaloupe-shaped and to have geysers. Triton was also found to have pink nitrogen snow at its poles.

Voyager 2 was the first of the two spacecraft to be launched. What was at first an auspicious launch, however, proved to be the beginning of a number of problems. The primary cause of the initial problems were attributed to commanding by the AACS, including difficulty in determining the full deployment of the science boom. These problems resulted in a delay of four days in the launch of Voyager 1 to ensure they wouldn't occur for it.

Although launched sixteen days after Voyager 2, Voyager 1's trajectory was the quicker one to Jupiter. On December 15, 1977, while both spacecraft were in the asteroid belt, Voyager 1 surpassed Voyager 2's distance from the sun.

Several months after launch, in April 1978, Voyager 2's primary radio receiver failed, automatically kicking in the backup receiver which proved to be faulty. Attempts to recover the use of the primary receiver failed and the backup receiver was used for the remainder of the mission. Although use of the backup receiver made communication with the spacecraft more difficult, engineers were able to find workarounds.

Voyager 2 proceeded with its primary mission and flew by Jupiter (closest approach on July 9, 1979) and Saturn (August 5, 1981). During these flybys, Voyager 2 obtained images roughly equal in number to Voyager 1 (18,000 at Jupiter, 16,000 at Saturn).

Voyager 2's launch date had preserved one part of the original Grand Tour design, i.e. the possibility of an extended mission to Uranus and Neptune. Despite the difficulties encountered, scientists and engineers had been able to make Voyager enormously successful. As a result, approval was granted to extend the mission, first to Uranus, then to Neptune and later to continue observations well past Neptune.

Voyager 2 made successful flybys of Uranus (January 24, 1986) and Neptune (August 25, 1989). Because of the additional distance of these two planets, adaptations had to made to accommodate the lower light levels and decreased communications. Voyager 2 was successfully able to obtain about 8,000 images of Uranus and its satellites. Additional improvements in the on-board software and use of image compression techniques allowed about 10,000 images of Neptune and its satellites to be taken. All of the experiments on Voyager 2 have produced useful data.

Jupiter has complicated atmospheric dynamics, lightning, and auroras.
Three new moons.
Jupiter has rings.
Io has active sulfur volcanoes, and these volcanoes have major effects on Jupiter's magnetosphere.
The Great Red Spot rotates once every six days, an it can survive almost indefinitely because it pulls in smaller eddies and adds their spin to its own.

Saturn has over 1000 ringlets.
Saturn's rings have braids, kinks, and spokes which have not yet been explained.
Seven new moons, including shepherd moons which keep the rings stable.
Massive jet streams which change rarely.
Saturn's magnetic poles lie exactly on its true north and south poles.
Titan, a moon, has a smoggy atmosphere, mostly composed of nitrogen, and at the surface has a density about 1.5 times Earth's at sea level.
Mimas, a moon, has a crater in it that covers about 25% of the surface.

Renamed the Voyager Interstellar Mission (VIM) by NASA in 1989 (after Voyager 2's Neptune encounter), Voyager 1 continues operations, taking measurements of the interplanetary magnetic field, plasma, and charged particle environment while searching for the heliopause (the distance at which the solar wind becomes subsumed by the more general interstellar wind). Through the end of the Neptune phase of the Voyager project, a total of $875 million had been expended for the construction, launch, and operations of both Voyager spacecraft. An additional $30 million was allocated for the first two years of VIM.

Voyager 1 is speeding away from the sun at a velocity of about 3.50 A.U./year toward a point in the sky of RA 17:28, Dec +12° (35.55° ecliptic latitude, 260.78° ecliptic longitude). Late on February 17, 1998, Voyager 1 became the most distant human-made object from the sun, surpassing the distance of Pioneer 10.

Voyager 2 continues operations, taking measurements of the interplanetary magnetic field, plasma, and charged particle environment while searching for the heliopause (the distance at which the solar wind becomes subsumed by the more general interstellar wind). It is speeding away from the sun at a velocity of about 3.13 A.U./year toward a point in the sky of RA 22:32, Dec -62° (-47.46° ecliptic latitude, 310.89° ecliptic longitude).

If nothing happens to them, NASA should be able to remain in contact with the two Voyager space probes until about 2030. Both crafts have enough hydrazine fuel (Voyager 1 should last until 2040 and Voyager 2 until 2034).

The only problem with this forecast is that the crafts' Radio-isotope Thermal Generators (RTG's) are slowly diminishing their power output. Since 2000, there has not been enough power to run the UltraViolet Spectrometer (UVS). By 2010, power output will be so low that not all of the instruments can be in operation at the same time. When that occurs, NASA will initiate a plan of taking different instruments on and off line at different times. This should keep the Voyagers in operation until 2020, and then power will be too low to maintain the crafts.

The distance of the Voyagers has given scientists a new vantage point to study objects that emit ultraviolet light. Other instruments that are currently operational are the cosmic ray subsystem, the low-energy charge particle instrument, the magnetometer, the plasma subsystem, the plasma wave subsystem, and the planetary radio astronomy instrument.