Science Investigations

There are currently five science investigation teams participating in the VIM.

• Magnetic field investigation
• Low energy charged particle investigation
• Plasma investigation
• Cosmic ray investigation
• Plasma wave investigation

The science teams for these investigations are currently collecting and evaluating data on the strength and orientation of the Sun's magnetic field; the composition, direction and energy spectra of the solar wind particles and interstellar cosmic rays; the strength of radio emissions that are thought to be originating at the heliopause, beyond which is interstellar space; and the distribution of hydrogen within the outer heliosphere.

There are six operating instruments on-board each Voyager spacecraft.  These instruments directly support the five science investigation teams.

These five instruments are:

• MAG Magnetic field investigation
• LECP Low energy charged particle investigation
• PLS Plasma investigation (Voyager 2 only)
  
• CRS Cosmic ray investigation
• PWS Plasma wave investigation

In addition, data is being collected from the Ultraviolet Spectrometer Subsystem (UVS).

While there are no science investigation teams associated with this instrument, the captured data is made available to interested scientists.

Science Data Acquisition Strategy

Science data are returned to earth in real time at 160 bps. Real time data capture uses 34 meter Deep Space Network (DSN) resources with the project goal to acquire at least 16 hours per day of real time data per spacecraft. This goal is not always achieved due to the competition for DSN resources with prime mission projects and other extended mission projects.

Once a week per spacecraft, 48 seconds of high rate (115.2 kbps) PWS data are recorded onto the Digital Tape Recorder (DTR) for later playback.  An additional 48 seconds are recorded each week on Voyager 1.  These data are played back to Earth once every 6 months per spacecraft and require 70 meter DSN support for data capture. After transmission of the data (either real time or recorded) to JPL, it is processed and made available in electronic files to the science teams located around the country for their processing and analysis.

Spacecraft Lifetime

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 2025 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 2008, the power generated by Voyager 1 had dropped to 285 w and to 287 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.

	VOYAGER 1	VOYAGER 2
Power Off Plasma (PLS) Subsystem.	2007-032
PLS Heater	2007-130
Power Off Planetary Radio Astronomy Experiment (PRA)	2008-015	2008-052
Terminate scan platform and Ultra Violet (UV) observations	~EOY 2010	1998-316
Termination of Data Tape Recorder (DTR) operations	~2015*	2007-248**
Termination of gyro operations	~2016	~2015
Initiate instrument power shutdown	~2020***	~2020***
Can no longer power any single instrument	No earlier than 2025	No earlier than 2025

* 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,

** Voyager 2 DTR operations was no longer needed due to a failure on the high waveform receiver on the Plasma Wave Subsystem (PWS) on June 30th, 2002.

*** Science Instruments power shutdown order has not been determined.

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 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.

The following experiments are expected to be operating by 2020:

Voyager 1: Low-Energy Charged Particles, Cosmic Ray Subsystem, Magnetometer and Plasma Wave Subsystem.

Voyager 2: Low-Energy Charged Particles, Cosmic Ray Subsystem, Magnetometer, Plasma Wave Subsystem and Plasma Subsystem.