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Voyager program

A pair of unmanned scientific probes, Voyager 1 and Voyager 2, ostensibly sent to study Jupiter and Saturn, using an advantageous planetary alignment of the late 1970s. However, the mission planners always had in the back of their minds a continued mission, and Voyager 2 also examined Uranus and Neptune. Both missions revealed large amounts of information about the gas giants of the solar system. In addition, the spacecraft have been used to place limits on the existence of a hypothetical post-Plutoian planet X.

As of 2002, periodic contact is kept with both probes to monitor conditions in the outer reaches of the solar system, with the hope that the craft's radioactive power sources will hold up (with judicious power rationing) to locate the distance to the heliopause.

These probes were built at JPL funded by NASA. Attached to each of them is a copy of the Voyager Golden Record.

The Voyager spacecraft (NASA)
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Voyager Spacecraft

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 are collected for two additional instruments.

The Flight Data Subsystem (FDS) and a single 8-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. Data are played back every six months.

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 Attitude and Articulation Control Subsystem (AACS) controls 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-bits/sec command rate) while an X-band transmitter provides downlink telemetry at 160 bits/sec normally and 1.4 kbps for playback of high-rate plasma wave data. All data are transmitted from and received at the spacecraft via the 3.7 meter high-gain antenna (HGA).

Electrical power is supplied by three radioisotope thermoelectric generators (RTGs). The current power levels are about 315 watts for each spacecraft. As the electrical power decreases, power loads on the spacecraft must be turned off in order to avoid having demand exceed supply. As loads are turned off, some spacecraft capabilities are eliminated.

To date, the entire Voyager 2 scan platform, including all of the platform instruments, has been powered down. All platform instruments on Voyager 1, except the UVS, have been powered down. The Voyager 1 scan platform was scheduled to be powered down in late 2000, but will be left on at the request of the UVS investigator (with the concurrence of the Science Steering Group) to investigate an unexpected excess in UV from the upwind direction. The PLS experiment on Voyager 1 is currently turned off to accommodate UVS observations.

The two Voyager spacecraft continue to operate, with some loss in subsystem redundancy, but still capable of returning science data from a full complement of 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 this time science data return and spacecraft operations will end.

Spacecraft electrical power is supplied by Radioisotope Thermoelectric Generators (RTGs) that provided approximately 470 W of 30 volt DC power at launch. Plutonium 238 decays with a radioactive half-life of approximately 85 years, so RTGs using it lose a factor of <math>1 - \sqrt[85]{0.5}</math> or ca. 0.81% of the power per year. 23 years after launch, such an RTG would produce only 470 W × 0.991923 ~= 390 W — or roughly 83% — of the initial power. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well; at the beginning of 2001, the power generated by Voyager 1 had dropped to 315 W and to 319 W for Voyager 2, so the thermocouples work at about 80%. Both of these power levels represent better performance than the pre-launch predictions, which included a conservative degradation model for the thermocouples. As the electrical power becomes less and less, power loads on the spacecraft must be turned off in order to avoid having demand exceed supply. As loads are turned off spacecraft capabilities are eliminated.

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Voyager is also the name of a series of unmanned probes to Mars which were planned between 1966 and 1968 for launch in 1974-75. The probes were to be launched by a Saturn I[?] rocket and were to be precursors for a manned Mars landing. The funding for this program was cut in 1968, and the mission goals were adopted by the similar Viking program.

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