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Voyager Interstellar Mission (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. 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. The mission objective of the VIM is to obtain useful interplanetary, and possibly interstellar, fields, particles, and waves (FPW) science data until year 2020 and beyond when the spacecraft's ability to generate adequate electrical power for continued science instrument operation will come to an end.

Introduction

In order to capitalize on this lengthy data acquisition potential, it is imperative that the spacecraft have a continuing sequence of instructions for acquiring the desired science data, and that the spacecraft High Gain Antenna (HGA) boresight remain pointed at the Earth for continuous data transmission. Because of the long mission duration and the likelihood of periodic spacecraft anomalies, it is also advantageous to continue the use of the on-board fault protection capability for automated responses to specific subsystem anomalies, and to provide an on-board sequence to continue spacecraft operation in the specific event of the future loss of command reception capability. All of these factors are considered in the VIM operating plan. The following sections describe the overall spacecraft sequencing strategy used for VIM operations, the interaction of the sequence elements with the on-board Fault Protection Algorithms (FPAs), and the planned modifications to the spacecraft configuration in response to the continual reduction in electrical power availability throughout the remainder of the mission.

Sequencing Strategy

A spacecraft sequence is the set of instructions that are stored in an on-board computer memory and control the operation of the engineering subsystems and science instruments for the purpose of gathering and transmitting science data to the ground stations. Typically, a spacecraft sequence is designed to function for a specific period of time. Some of the key factors that determine the duration of a spacecraft sequence are:

With the Voyager spacecraft being mid-1970's technology, the available on-board memory for storing spacecraft sequence information is very limited by today's standards. A total of about 1500 18-bit words are available between the two Computer Command Subsystem (CCS) memories for storing sequence instructions and HGA pointing information. This limited memory space can be a significant factor if the sequence is composed of a series of one of a kind activities. Fortunately, the science observations being made during VIM are very repetitive in nature. This allows a science observation or instrument calibration to be repeated (cyclicized), essentially for the duration of the mission, at practically the same memory word cost as a single observation or calibration. This implementation of events as a cyclic sequence that repeats an observation or calibration on a regular basis is key to the VIM sequencing strategy. Another key factor is the ability to store HGA pointing information out to the end of mission in the CCS memory. This not only simplifies the routine sequence generation support by not requiring HGA pointing information to be included in individual sequences transmitted to the spacecraft, but also enables the ability to provide science data even if command capability to the spacecraft is lost. These two sequence capabilities are referred to as the "baseline sequence."

Augmentation of the baseline sequence with non-repetitive science or engineering events use either an "overlay sequence," or a "mini-sequence." The difference between these two types of augmentation sequences is that the overlay sequence operates for a fixed interval of time, and contains all of the baseline sequence augmentations for that time interval. The mini-sequence is focused on accomplishing a single augmentation need and is not a regularly scheduled activity but is done on an as needed basis. Both types of sequences are developed and transmitted from the ground. It was envisioned that there would be one overlay sequence in every three months and it was practiced in early VIM. However, due to the difficulty in predicting and obtaining the DSN resources, an overlay load has been split into two sequences in recent years. Mini-sequences are infrequent and mainly used for anomaly investigation and resolution.

In addition, any one-time-event for the duration of the mission are implanted in long-term events table. One example is terminating the Digital Tape Recorder (DTR) utilization when the Plasma Wave science instrument (PWS) playbacks can no longer be supported. Another important function included in this table is resetting the hourly counter in the CCS about every 13.5 years so the spacecraft can continue to have sequencing capabilities.

The baseline sequence, overlay sequence, and mini-sequence, and long-term events table provide the basic operational sequence elements for normal operations while command capability with the spacecraft is available. In the event command capability is lost, another sequence element, the "Backup Mission Load (BML)," provides the mechanism for continued spacecraft data acquisition without further ground interaction. A BML is stored on-board each spacecraft and contains the necessary instructions to modify the on-going baseline sequence to maintain the continued return of basic FPW data. The BML also terminates the use of the gyros and Digital Tape Recorder (DTR) utilization when the available electrical power will no longer support their operations. Another important function included in BML is resetting the hourly counter in the CCS about every 13.5 years so the spacecraft can continue to have the sequencing capabilities.

Sequence Interaction with On-Board Fault Protection

The FPAs on the Voyager spacecraft are designed to recover the spacecraft from specific potential mission-catastrophic failures. They are implemented primarily in the CCS, while some are in the Attitude and Articulation Control Subsystem (AACS). The FPAs in the CCS are invoked by interrupts received from external sources, and followed by preprogrammed responses.

The five FPAs that currently reside in the CCS are:

If an AACSIN, CMDLOS, or RFLOSS entry occurs, the response will be integrated into on-going sequence and the commands from an FPA response and the regular sequencing activities will be interleaved. For an ERROR or PWRCHK entry, all the on-going activities will be terminated. However, some of the critical activities including the baseline sequence will be automatically restarted by a special feature called "rollback" although there will be some data loss depending on when a PWRCHK or ERROR entry occurs.

Modifications In Response to Electrical Power Limitations

Radioisotope thermoelectric generators (RTGs) provide electrical power to the Voyager spacecraft. Due to the natural radioactive decay of the plutonium oxide heat source, the electrical power provided by the RTGs is continually declining, as expected. The current rate of decay is approximately 4 watts per year. Because of the continual decline in the amount of power that is available, it is necessary to periodically reduce power consumption in order to maintain an adequate power margin. This is accomplished by turning off spacecraft power loads.

VOYAGER 1
The following lists those loads that have been turned off in VIM and the year:

The following lists those loads that are planned to be turned off:

Further responses to decreasing electrical power, beginning after 2020, will consist of either turning instruments off sequentially or turning instruments off and on in a power sharing mode to maintain an adequate power margin.

VOYAGER 2
The following lists those loads that have been turned off in VIM and the year:

The following lists those loads that are planned to be turned off:

Further responses to decreasing electrical power, beginning after 2020, will consist of either turning instruments off sequentially or turning instruments off and on in a power sharing mode to maintain an adequate power margin.

Voyager science data are available on-line at the National Space Science Data Center (NSSDC) and at the investigators' institutions.

Summary of Voyager Data Available at the Virtual Space Physics Observatory

Voyager Project Information and Science Data at NSSDC

Voyager Data at OMNIWeb Plus

Voyager Data at Virtual Energetic Particle Observatory

Voyager Data at NSSDC Anonymous ftp site

Magnetic Fields at GSFC

Low-energy Charged Particles at APL
At Fundamental Technologies
At Univ of Maryland

Plasma Science at MIT

Cosmic Ray Subsystem at GSFC(including latest Voyager 1 data from the heliopause)

Plasma Wave Subsystem

Trajectory Data at MIT

Heliospheric Trajectories at HelioWeb

High Energy Ions Energized By Bastille Day, 2000 Shock Bombard Voyager 2 at 62 AU, April 2001

A Global Merged Interaction Region and Cosmic Ray Step Decrease at 61 AU, Voyager 2, March 2001

Propagation of Bastille Day Event to Voyagers 1 & 2, January 2001

Slowdown of the Solar Wind in the Outer Heliosphere, November 2000

Solar Periodicity Observed By Voyager 1 in Distant Anomalous Cosmic Rays, September 2000

New Solar Cycle Activity Detected at 56 & 72 AU by Voyagers 1 & 2, August 2000

Interplanetary Dust Particles Detected by Voyager Plasma Wave Instruments, June 2000

Interstellar Oxygen Ions Boosted into Voyager 1 and 2 LECP Detectors, April 2000

Do Shocks Always Accelerate Ions?, March 2000

Hydrogen Nuclei from a Burst of Solar Activity, October 1999

Voyager Detects New Anomalous Component Species, June 1999

Voyager And The Motions Of The Heliospheric Boundaries, May 1999

Solar Minimum Marked By Onset of Cosmic Ray Decrease at 70 AU, April 1999

Empirical Model of the Modulation of Anomalous Cosmic Ray Oxygen, January 1999

Global Solar Cycle Variations of the Solar Wind (Voyager), December 1998

Voyager 1 Observes Large Increase in Anomalous Oxygen at Solar Minimum, September 1998

Voyager 1 Measures Magnetic Fields at Greatest Distance Ever, August 1998

Voyager Observation of K-capture Decay of Cosmic Rays, June 1998

Dust Particles Detected in Outer Solar System, May 1998

Voyager Loses Part of its Magnetic Connection with our Sun, March 1998

Solar Control of the Solar Wind Speed at 54 AU, February 1998

CRS Investigation Results, January 1998

Pre-Acceleration of Anomalous Cosmic Ray Ions at Recurrent Solar Wind Shocks, October 1997

Voyager 1 and 2 Data Show Entry of Interstellar Particles into the Solar System, August 1997

Plasma Investigation Results, July 1997

Abbreviations for Scientific Journals used in the Voyager Bibliography.

This bibliography lists scientific articles and papers coauthored by the Principal Investigators, Coinvestigators, and their associates of the eleven Voyager science investigations. They report analysis and research based on data from six planetary encounters, the intervening cruise periods, and the post-Neptune interstellar cruise. The bibliography covers publications from 1977 through the present. Annual updates will be issued as addendums to this bibliography

This listing organizes articles and papers by investigation, as shown in the table of contents, then by year of publication, and, alphabetically by author. It also identifies some (but by no means all) of the collaborative efforts of scientists from different investigations. Generally, the multiauthor articles appear according to the first author's affiliation.