Voyagers Interstellar Mission: MISSION OBJECTIVE

The mission objective of the Voyager Interstellar Mission (VIM) is to extend the NASA exploration of the solar system beyond the neighbourhood of the outer planets to the outer limits of the Sun's sphere of influence, and possibly beyond.

This extended mission is continuing to characterise the outer solar system environment and search for the heliopause boundary, the outer limits of the Sun's magnetic field and outward flow of the solar wind. Penetration of the heliopause boundary between the solar wind and the interstellar medium will allow measurements to be made of the interstellar fields, particles and waves unaffected by the solar wind.

MISSION CHARACTERISTICS

The VIM is an extension of the Voyager primary mission that was completed in 1989 with the close flyby of Neptune by the Voyager 2 spacecraft. Neptune was the final outer planet visited by a Voyager spacecraft. Voyager 1 completed its planned close flybys of the Jupiter and Saturn planetary systems while Voyager 2, in addition to its own close flybys of Jupiter and Saturn, completed close flybys of the remaining two gas giants, Uranus and Neptune.

At the start of the VIM, the two Voyager spacecraft had been in flight for over 12 years having been launched in August (Voyager 2) and September (Voyager 1), 1977. Voyager 1 was at a distance of approximately 40 AU (Astronomical Unit - mean distance of Earth from the Sun, 150 million kilometres) from the Sun, and Voyager 2 was at a distance of approximately 31 AU.

As of January 2001, Voyager 1 was at a distance of 12 Billion Kilometres (80 AU) from the sun and Voyager 2 at a distance of 9.4 Billion kilometres (62.7 AU).

Voyager 1 is escaping the solar system at a speed of about 3.6 AU per year, 35 degrees out of the ecliptic plan to the north, in the general direction of the Solar Apex (the direction of the Sun's motion relative to nearby stars). Voyager 2 is also escaping the solar system at a speed of about 3.3 AU per year, 48 degrees out of the ecliptic plane to the south.

It is appropriate to consider the VIM as three distinct phases: the termination shock, heliosheath exploration, and interstellar exploration phases. The two Voyager spacecraft began the VIM operating, and are still operating, in an environment controlled by the Suns magnetic field with the plasma particles being dominated by those contained in the expanding supersonic solar wind. This is the characteristic environment of the termination shock phase. At some distance from the Sun, the supersonic solar wind will be held back from further expansion by the interstellar wind. The first feature to be encountered by a spacecraft as a result of this interstellar wind/solar wind interaction will be the termination shock where the solar wind slows from supersonic to subsonic speed and large changes in plasma flow direction and magnetic field orientation occur. Passage through the termination shock ends the termination shock phase and begins the heliosheath exploration phase. While the exact location of the termination shock is not known, it is very possible that Voyager 1 will complete the termination shock phase of the mission between the years 2001 and 2003 when the spacecraft will be between 80 and 90 AU from the Sun. Most of the current estimates place the termination shock at around 85 ± 5 AU. After passage through the termination shock, the spacecraft will be operating in the heliosheath environment which is still dominated by the Suns magnetic field and particles contained in the solar wind. The heliosheath exploration phase ends with passage through the heliopause which is the outer extent of the Suns magnetic field and solar wind. The thickness of the heliosheath is uncertain and could be tens of AU thick taking several years to traverse. Passage through the heliopause begins the interstellar exploration phase with the spacecraft operating in an interstellar wind dominated environment. This interstellar exploration is the ultimate goal of the Voyager Interstellar Mission.

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 seven operating instruments on-board each Voyager spacecraft although the Plasma instrument on Voyager 1 is not returning useful data. Five of 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
• CRS Cosmic ray investigation
• PWS Plasma wave investigation

In addition, there are data being collected from two science instruments that do not have official science investigation teams associated with them. These instruments are:

• PRA Planetary Radio Astronomy Subsystem
• UVS Ultraviolet Spectrometer Subsystem

While there are not science investigation teams associated with these instruments, 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. 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 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 2001, the power generated by Voyager 1 had dropped to 315 w and to 319 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 maximise 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 manoeuvres (MAGROLs). These manoeuvres are performed 6 times a year, on each spacecraft, and consist of a spacecraft attitude manoeuvre 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 manoeuvres 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.