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.
Due to the natural radioactive decay of the Plutonium fuel source, the
electrical energy provided by the RTGs is continually declining. At the
beginning of 2007, the power generated by Voyager 1 had dropped to 288
w and to 289 w for Voyager 2. Both of these power levels represent
better performance than the pre-launch predictions, which included a
conservative degradation model for the bi-metallic thermocouples used
to convert thermal energy into electrical energy. 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. The following table
identifies the year when specific capabilities will end as a result of
the available electrical power limitations.
* Limited by
ability to capture 1.4 kbps data using a 70m/34m antenna array
In order to maximize the duration of the fields and particles data
acquisition capability, the first spacecraft loads to be turned off are
instrument heaters on the scan platform. As these heaters are turned
off the UVS, which is mounted on the scan platform, cools down until
the point is reached when it can no longer function.
Termination of
gyro operations ends the capability to calibrate the magnetometer
instrument with magnetometer roll maneuvers (MAGROLs). These maneuvers
are performed 6 times a year, on each spacecraft, and consist of a
spacecraft attitude maneuver of 10 successive 360 degree turns about
the roll axis. Data from a MAGROL allow the spacecraft magnetic field
to be determined and subtracted from the magnetometer science data.
This is important since the spacecraft magnetic field is larger than as
the solar magnetic field being measured. The termination of gyro
operations also means an end to the attitude maneuvers used to check
the combined calibration of the Sun Sensor and the High Gain Antenna
pointing direction for maintaining communications with the ground.
Instrument power
sharing limits the number of science instruments that can be on at any
given time. This instrument power sharing will continue until the
available power will no longer support any instrument operation. At
that time the Voyager Interstellar Mission will end.