MGS began its second phase of aerobraking on September 23 when it executed a 4.8-second burn to lower its closest approach to Mars into the upper martian atmosphere. The atmospheric drag on the spacecraft during its closest approach to Mars on each elliptical orbit will ultimately reduce the period of the orbit to approximately two hours and will allow MGS to achieve a circular orbit around the martian poles with an altitude of 250 miles. Such an orbit is suitable for mapping the red planet with the MGS suite of scientific instruments. The mapping phase of the MGS mission is expected to begin in April, 1999 and to continue for one martian year (687 days).
By October 3, the second aerobraking sequence had reduced the period of the MGS orbit around Mars from 11 hours and 38 minutes to 10 hours and 44 minutes. The altitude of the closest approach (periapse) to Mars on that date was 73 miles, and the orbit period was decreasing by 3 minutes during each pass through the upper martian atmosphere. At its most distant point from Mars (apoapse) during each orbit, the MGS altitude was approximately 10,400 miles. It is interesting that the drag force on the MGS spacecraft has been varying by up to 100% from one aerobraking pass to the next!
The start of the second phase of MGS aerobraking was delayed twice due to technical problems. The burn to begin the second phase of MGS aerobraking was originally scheduled to be executed on September 14. Flight controllers are now planning to increase the pace of aerobraking for a short while to make up for the nine-day delay. The problems which delayed the beginning of the second phase of MGS aerobraking are described in this MGS Flight Status Report.
The MGS Radio Science Team met on September 17 at Stanford University to discuss the Team's plans for operations and data acquisition during this aerobraking phase and the upcoming mapping phase of the MGS mission. The acquisition of scientific data is not the highest mission priority during aerobraking, and the Team will not be able to collect martian meteorological data in the most desirable fashion. The Team is expecting, however, to perform radio occultation measurements of the martian atmosphere between November 20 and December 31, and to make the resulting meteorological data available to participants in this outreach program in the most timely manner possible. During this period, it will be late Spring on Mars in the northern hemisphere and late fall on Mars in the southern hemisphere. The spacecraft orbit period will be approximately six hours at the beginning of this period and four hours at the end.
It is likely that the strength of the MGS signal will be significantly compromised during much of the second aerobraking data collection period because it will not be possible to prevent modulation of the MGS radio signal with regular spacecraft telemetry information. This loss of carrier strength will limit the precision of our Team's atmospheric observations. In addition, the spacecraft may not be in its normal ("one-way") communications configuration during the radio occultations. This would complicate the processing of the raw radio occultation data and could slow our retrieval of atmospheric profiles.
The MGS Radio Science Team will be gathering the bulk of its martian atmospheric data during the mapping phase of the MGS mission. The start of the mapping phase of the mission is scheduled to begin in April, 1999. The first occultation "season" will extend from the beginning of mapping through the middle of July, 1999. During this period, the spacecraft will complete one orbit every two hours and each orbit will feature two occultation opportunities (entry and exit). The Team will acquire data during approximately one-third of the mapping occultation opportunities. It will not be possible to acquire data during every occultation because the MGS spacecraft will typically only be tracked by one of the three NASA Deep Space Network complexes here on Earth on any particular day. There will be times, however, when MGS is tracked around the clock. On these days, we should obtain 24 radio occultation data sets!
Earlier this summer, there was some question about the ability of the antenna boom to withstand the forces expected to be experienced when the MGS high-gain (dish) antenna is deployed. The forces will likely be larger than originally estimated because the damper on the antenna boom is now believed to be faulty. As a result, MGS management was investigating the possibility of delaying the deployment of the MGS high-gain antenna until some data had been collected by all of the MGS instruments and some support for other missions had been provided. Although a final decision on the high-gain antenna deployment has not yet been reached, subsequent testing of engineering models of the MGS antenna boom has shown that the boom will not be damaged when the antenna is deployed and that there is a considerable margin for safety. As a result, it appears likely that there will not be a significant delay (if there is one at all) in deployment of the high-gain antenna. This is good news for the Radio Science Team, because it means that we will be able to conduct our mapping investigation of the martian atmosphere as originally planned (although one year behind schedule!).