The Voyager program is a series of U.S. unmanned space missions that consists of a pair of unmanned scientific probes, Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s. Although they were officially designated to study just Jupiter and Saturn, the two probes were able to continue their mission into the outer solar system. They are currently on course to eventually exit the solar system. These probes were built at JPL and were funded by NASA.
Both missions have gathered large amounts of data about the gas giants of the solar system, of which little was previously known. In addition, the spacecraft trajectories have been used to place limits on the existence of a hypothetical post-Plutonian Planet X.
The Voyager probes were originally conceived as part of the Mariner program, and designated Mariner 11 and Mariner 12, respectively. They were then moved into a separate program named Mariner Jupiter-Saturn, later retitled Voyager because it was felt that the probes' designs had moved sufficiently far from the Mariner family that they merited a separate name. Voyager is essentially a scaled-back version of the Grand Tour program of the late 1960s and early 1970s. The Grand Tour's plan was to send a pair of probes to fly by all the outer planets; it was scaled back because of budget cuts. However, in the end, Voyager fulfilled all the Grand Tour flyby objectives except for Pluto, which at the time was considered a planet by the IAU.
Of the pair, Voyager 2 was launched first. Its trajectory was designed to take advantage of an unusually convenient alignment of the planets allowing the inclusion of Uranus and Neptune fly bys in the probe's mission. Voyager 1 was launched after its sister probe, but on a faster trajectory which enabled it to reach Jupiter and Saturn sooner at the expense of visiting the outer planets.
In the 1990s, Voyager 1 overtook the slower traveling Pioneer 10 to become the most distant human made artifact in space. It will keep that record for at least several decades; even the fast (at launch) New Horizons probe will not catch up with it since its final speed will be less than Voyager 1's. Voyager 1 and Pioneer 10 are also the most widely-separated man-made objects in the universe because they are traveling in roughly opposite directions from the sun.
Periodic contact has been maintained with both probes to monitor conditions in the outer expanses of the solar system. The crafts' radioactive power sources are still producing electrical energy, fueling hopes of locating the solar system's heliopause. In late 2003, Voyager 1 began sending data that seemed to indicate it had crossed the termination shock, but interpretations of this data are in dispute. It is now believed that the termination shock was crossed in December 2004, with the heliopause an unknown distance ahead.
Due to budget cuts prompted by President George W. Bush's Vision for Space Exploration, it was questionably stated that the probes were to be deactivated and abandoned as early as October 2005, before they would have observed the heliopause. However, the program continues to be funded into 2009.
As of April 2007, Voyager 1 was over 15.2 terameters (15.2 × 10 meters, or 15.2 × 10 km, 101.4 AU, or 9.4 billion miles) from the Sun, and has thus entered the heliosheath, the termination shock region between the solar system and interstellar space (or the interstellar medium), a vast area where the Sun's influence has given way to that of the Milky Way galaxy in general.
As of September 2006, Voyager 2 is at a distance of around 80.5 AU (approximately 12 terameters) from the Sun, deep in the scattered disc, and traveling outward at roughly 3.3 AU a year. It is more than twice as far from the Sun as Pluto is. On December 10, 2007, instruments on board Voyager 2 sent data back to Earth indicating that the Solar System is asymmetrical. It has also reached the termination shock, about 10 billion miles from where Voyager 1 first crossed it.
The identical Voyager spacecraft are three-axis stabilized systems that use celestial or gyro referenced attitude control to maintain pointing of the high-gain antennas toward Earth. The prime mission science payload consisted of 10 instruments (11 investigations including radio science). Only five investigator teams are still supported, though data is collected for two additional instruments. The Flight Data Subsystem (FDS) and a single eight-track digital tape recorder (DTR) provide the data handling functions. The FDS configures each instrument and controls instrument operations. It also collects engineering and science data and formats the data for transmission. The DTR is used to record high-rate Plasma Wave Subsystem (PWS) data. The data is played back every six months.
The Imaging Science Subsystem, made up of a wide angle and a narrow angle camera, is a modified version of the slow scan vidicon camera designs that were used in the earlier Mariner flights. The Imaging Science Subsystem consists of two television-type cameras, each with 8 filters in a commandable Filter Wheel mounted in front of the vidicons. One has a low resolution 200 mm wide-angle lens with an aperture of f/3 (Wide Angle Camera), while the other uses a higher resolution 1500 mm narrow-angle f/8.5 lens (Narrow Angle Camera).
Unlike the other onboard instruments, operation of the cameras is not autonomous, but is controlled by an imaging parameter table residing in one of the spacecraft computers, the Flight Data Subsystem (FDS).
The computer command subsystem (CCS) provides sequencing and control functions. The CCS contains fixed routines such as command decoding and fault detection and corrective routines, antenna pointing information, and spacecraft sequencing information. The Voyager spacecraft have three RCA 1802 CPUs running at 6.4 MHz. These CPUs sent to space were operating at full military specification temperatures (-55 to +125 °C).
The Attitude and Articulation Control Subsystem (AACS) controls the spacecraft orientation, maintains the pointing of the high-gain antenna towards Earth, controls attitude maneuvers, and positions the scan platform.
Uplink communications is via S band (16-bit/s command rate) while an X band transmitter provides downlink telemetry at 160 bit/s normally and 1.4 kbit/s for playback of high-rate plasma wave data. All data is transmitted from and received at the spacecraft via the 3.7-meter high-gain antenna.
Electrical power is supplied by three radioisotope thermoelectric generators (RTGs). They are powered by plutonium-238 (distinct from the Pu-239 isotope used in nuclear weapons) and provided approximately 470 W at 30 volts DC when the spacecraft was launched. Plutonium-238 decays with a half-life of 87.74 years, so RTGs using Pu-238 will lose a factor of 1 - 0.5 = 0.78% of their power output per year. In 2006, 29 years after launch, such an RTG would produce only 470 W × 2 ~= 373 W — or about 79.5% — of its initial power. However, the bi-metallic thermocouples that convert heat into electricity also degrade, so the actual power will be even lower. As of August 11 2006, the power generated by Voyager 1 and Voyager 2 had dropped to 290 W and 291 W respectively, about 60% of the power at launch. This is better than the pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off, eliminating some spacecraft capabilities.
As of the present date, the Voyager 2 and Voyager 1 scan platforms, including all of the platform instruments, have been powered down. The ultraviolet spectrometer (UVS) on Voyager 1 was active until 2003, when it too was deactivated. Gyro operations will end in 2010 for Voyager 2 and 2011 for Voyager 1. Gyro operations are used to rotate the probe 360 degrees six times a year to measure the magnetic field of the spacecraft, which is then subtracted from the magnetometer science data.
The two Voyager spacecraft continue to operate, with some loss in subsystem redundancy, but retain the capability of returning scientific data from a full complement of Voyager Interstellar Mission (VIM) science instruments. Both spacecraft also have adequate electrical power and attitude control propellant to continue operating until around 2020, when the available electrical power will no longer support science instrument operation. At that time, science data return and spacecraft operations will cease. It is possible that one or both craft may have enough RTG energy to last until 2025, but there is only a small probability of this.
Voyager Interstellar Mission
The Voyager primary mission was completed in 1989 with the close flyby of Neptune by Voyager 2. The Voyager Interstellar Mission (VIM) is a mission extension which began when the two spacecraft had already been in flight for over 12 years. The Heliophysics Division of the NASA Science Mission Directorate conducted a Heliophysics Senior Review in 2008. The panel found that the VIM, "is a mission that is absolutely imperative to continue" and that VIM "funding near the optimal level and increased DSN support is warranted."
Voyager Golden Record
Voyager 1 and 2 both carry with them a golden record that contains pictures and sounds of Earth, along with symbolic directions for playing the record and data detailing the location of Earth. The record is intended as a combination time capsule and interstellar message to any civilization, alien or far-future human, that recovers either of the Voyager craft. The contents of this record were selected by a committee chaired by Carl Sagan.
Fiction and popular culture
The Voyager program's discoveries during the primary phase of its mission, including striking never-before-seen close up color photos of the major planets, were regularly documented by both print and electronic media outlets. As a result, the Voyager program, especially at the high points of its mission, has seen significant public limelight. There are a number of references to the Voyager program or to the particular probes themselves within popular culture.
Published in July 2009.
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