alcaray wrote:
Right now, I'm trying to figure out what they intend to do with an accelerometer on the orbiter. Is it for the aerobraking stage, to glean info about the upper atmosphere?

Alex writes:
Yes, like MGS and 2001 Mars Odyssey, MRO 2005 will conduct active aerobraking (AB) to circularize and lower its orbit. MRO AB operations are baselined to last ~4-6 months, with periapsis altitudes ranging from 90 to 150
km altitude. NASA solicited Facility Team proposals from the science community to support MRO acceleromter data analysis (which in turn support engineering operations) to help with modelling the Martian upper atmosphere (in near real time) during AB operations. As was seen with MGS and 2001 Mars Odyssey, AB models can vary with season (Ls), altitude, and periapsis location. Note that MRO's periapsis during AB will occur almost exclusively over the northern polar regions, the site of past dynamic atmospheric density fluctuations. Also, the acclerometer data will help in the study of circulation patterns in the upper Martian atmosphere. Although the accelerometer is nominally an engineering instrument, the chosen Facility Team members will undoubtedly, as was the case on MGS, publish their data in science journals.

alcaray wrote:
Is it so sensitive that it can be used for gravity mapping?

Alex writes:
No, the atmospheric (i.e., aerodynamic) forces on the spacecraft during AB would overwhelm any gravity acclerations. That is why gravity measurements, through Doppler radio tracking, are conducted during "quiet" periods (i.e., when the spacecraft has reached mapping orbit and when there are a minimum of propulsive and/or aerodynamic forces operating on the spacecraft). Think of it this way: Attach an acclerometer to your arm and take measurements while extending your arm through the open window of your car, which is moving down the highway at 70 mph. If you can separate a genuine gravity signal of the Earth from that dataset I'll personally nominate you for a Nobel Prize in Physics ;-)

alcaray wrote:
Tidal forces from Phobos & Diemos???

Alex writes:
No; see the last sentence above and replace "Earth" with "Moon."

Alex R. Blackwell
University of Hawaii