Late Mission and Outreach Program News

July 27, 1999

The Mars Global Surveyor high-gain antenna was deployed from its stowed position on April 5, 1999, and regular mapping operations began on that date. In the standard mapping mode, the spacecraft completes one orbit around the red planet every two hours while a variety of instruments acquire photographs and other scientific data. The mapping phase is the principal part of the Mars Global Surveyor mission, and it is expected to continue for one complete martian year (687 days).

In the mapping configuration, two gimbals allow the high-gain dish antenna to be continuously pointed towards Earth while the MGS instruments remain fixed on Mars. It was discovered in late April that one of the two high-gain antenna gimbals is not free to rotate over its full range of motion. As a result, the high-gain antenna will be unable to track the Earth in the standard mapping mode beginning in February, 2000. A number of alternative mapping scenarios have been developed to allow the mapping phase of the mission to continue beyond next February. A final decision about which alternative mapping scheme to utilize will be made later this summer.

In the standard mapping mode, the MGS Radio Science Team acquires samples of the MGS radio transmission at stations of NASA's Deep Space Network when the spacecraft enters occultation behind Mars and when the spacecraft exits occultation. These occultations occur during most mapping orbits, and the perturbation of the radio transmission by the martian atmosphere at the time of the occultations allows Radio Science Team members to reconstruct profiles of the density, temperature and pressure of the martian atmosphere. The Team also uses radio tracking of the MGS spacecraft to model the gravity field of the red planet.

The Radio Science Team recorded data from many hundreds of MGS occultation events between April 5 and June 28. The Team has been posting results from these experiments on this Web site, and making the martian meteorological data available to participants in our outreach program. The Team is continuing to process and analyze the raw data from these experiments, and will continue to post results throughout the summer! At this time, the orbit of the MGS spacecraft is approximately face-on as viewed from Earth, so the spacecraft is never occulted by Mars and we are unable to acquire any new data. Spacecraft occultations will begin once again in December, and will then continue for the remainder of the mapping phase of the MGS mission.

The grid of occultation points on Mars which is typical of the mapping phase of the mission can be seen clearly on the map of martian weather data from April-June, 1999, on our Late Martian Weather page. Mars rotates approximately 30 degrees in the two hours that it takes MGS to complete an orbit of the red planet, so successive occultation points fall about 30 degrees (of longitude) apart. The points seems to be arranged in horizontal rows by date of acquisition, and the rows are offset diagonally from one another because the period of the MGS orbit is not synchronized with the rotation of the planet.

It is currently very late summer in the northern hemisphere of Mars, and very late winter in the south. The (northern) autumnal equinox will occur at the beginning of August! When we stopped recording atmospheric data last month, both the entry and exit occultations of the MGS spacecraft were occurring in the southern martian hemisphere. Earth and Mars have been relatively close during the first part of the mapping mission and the MGS radio signal has been quite strong!

When MGS occultations resume in December, both the entry and exit occultations of the spacecraft will occur in the northern martian hemisphere. It will be late fall there, and late spring in the martian south. As data from this part of the mapping mission are acquired and analyzed by Radio Science Team members, results will be made available to participants in the outreach program.

Team members at Stanford University moved to a new building on campus last month. The building has been named after David Packard, a very distinguished alumnus of and contributor to Stanford University. There was some interruption in our Web service at the time of the move, and we apologize for any inconvenience. The move did go more smoothly than expected, however, particularly with regard to the moving of our numerous computers and peripherals. We hope to have very little, if any, down time in the future!

Last updated: July 27, 1999
Joe Twicken /