Mars Global Surveyor arrived at Mars on the evening of September 11th and achieved a very precise 45 hour long elliptical orbit around the planet. During the spacecraft's third closest approach to the planet, the MGS science instruments had the opportunity acquire a significant amount of new information relative to the surface, atmosphere and interior of Mars. Among these new findings is the observation that the northern hemisphere area called Elysium is extremely flat, and that there are strong remnant magnetic fields near the surface rather than a global magnetic field like the Earth's. (See http://mars.jpl.nasa.gov)
You may have heard that MGS has encountered some difficulty with aerobraking and that's true. After twelve aerobraking drag passes we've pull up out of the Mars atmosphere to assess the unexplained motions of one of the solar panels that we've observed and to be sure that the spacecraft is safe and that we preserve our ability to acquire more great science data at Mars. We've decided to take a two week hiatus from aerobraking to do this. The big effect from the time lost due to the hiatus is that we will use a different mapping orbit later on, and we are working on defining what that orbit will be.
In the mean time, we have taken opportunity afforded us by the hiatus in aerobraking to take a lot more significant science data at each periapsis pass. The Laser Altimeter has been turned back on and the Mars Orbiter Camera, and Thermal Emission Spectrometer will have great opportunities for science acquisition. The current periapsis altitude is two times closer to the surface of Mars than the altitude would be during the originally planned mapping orbit so the surface resolution is much greater.
The spacecraft, except for the uncertainty concerning the -Y solar panel, and all its instruments are working exceptionally well, and we continue to have great expectations for the discoveries that this mission will return.
The Mars Surveyor Operations Project began, on September 16th, moving the lowest point of the MGS's orbit from 250 km (155 miles) down into the Martian atmosphere in order to begin aerobraking. Aerobraking is the process that uses the drag on the spacecraft caused by skimming through the top of the atmosphere to reduce the period of the orbit. Aerobraking worked just fine! After twelve passes through the upper atmosphere, we reduced the orbital period from its initial 45 hours down to 35 hours on the way to the target of 2 hours.
You may recall that when MGS was launched, the -Y solar panel did not completely deploy and latch properly. It ended up about 20 degrees away from being folded out completely flat. After a lot of analysis and test, we determined the best way to do aerobraking, which uses the solar panels to provide most of the drag surface area, was the turn the -Y panel around. Thus, the atmospheric drag pressure would push on it during aerobraking so that we were compressing the small piece of metal that we believe is trapped in the solar panel's hinge joint. Then, as the drag pressures on the panel increase as we got deeper into the atmosphere, the panel might move further toward the fully folded out position and even latch.
On Tuesday, October 1st, at periapsis or closest approach to Mars number 12 where the altitude was 110 km (68 miles), we observed the panel move about 14 degrees closer to fully unfolded - good! Then, on Monday, October 6th, we had an aerobraking drag pass (periapsis 15) where the Martian atmosphere became more dense than we had expected for that orbit, although well within the capabilities of the spacecraft, and we observed the panel move past where we thought it should latch and stay past that position by about one degree - not so good!
Atmospheric density variations like observed in periapsis 15 are expected as the seasons change on Mars, and it is probably not the result of a great dust storm. Our flight operational process is designed to accommodate these kinds of variations, and so, to reduce the dynamic pressure that the atmosphere puts on the solar panels, we moved the spacecraft's orbit up 11 km (7 miles) to 121 km (75 miles), to where we thought the pressure would be fairly light on the panel. That would also give us a few days to understand why the panel might have moved past its latch point. As it turned out, the pressure was small as we expected, but the -Y solar panel continued to show us some unexpected motion during the next two drag passes. There has never be any concern about the electrical power that the panel produces.
Thus, on Saturday evening, October 11th, we decided to take the spacecraft completely out of Martian atmosphere because we are concerned that we don't know why the panel moves as it does and we were concerned that we might have some kind of life limiting mechanism involved. The closest point to Mars is now about 172 km (107 miles) and the orbital period is about 35 hours.
We are taking a two week hiatus from aerobraking, and in the process we have forfeited our ability to reach the orbital conditions that would result in the sun synchronous circular mapping orbit with an equator crossing at 2 pm local solar mean time. There are, however, other orbital conditions that we can reach that will yield exceptional science return from Mars.
During the hiatus, the Mars Surveyor Operations Project team, at JPL and Lockheed Martin Astronautics, is studying the observed motion of the solar panel, and will understand how the original launch failure model must be modified to accommodate the new motion. With that information, we will then plan how to start aerobraking again in a configuration that will not stress the panel, and given the intensity of aerobraking that is possible, we, with the MGS science investigators, will determine the best mapping mission to target for. The 2 pm orbit was a compromise among the science investigators. We will work for another compromise orbit that yields the best results for whatever orbital capabilities we can achieve. In the best case, it will be a circular orbit at some other local time. In the worst case, it will be an elliptical orbit with a period of about 16 hours.
We are planning on a Monday, October 27th, a decision milestone on the plans for and safety of returning to aerobraking.
Glenn E. Cunningham Project Manager Mars Global Surveyor Mars Surveyor Operations