No major mission activities occurred this week onboard the Mars Global Surveyor spacecraft. Back at the Jet Propulsion Laboratory in Pasadena, California, the project management has made a decision not to attempt any more efforts to free debris that is currently keeping the -Y-side solar array slightly out of position. This solar panel is currently deployed and fully functional, but is 20.5 degrees from its proper position.
The flight team believes that the position discrepancy was caused when a damper shaft in the array's deployment mechanism broke shortly after launch. This damper is a device that was installed to minimize the mechanical shock of deployment by slowing the motion of the array during deployment. The flight team theorizes that the broken shaft caused the damper arm to wedge into the hinge joint connecting the solar panel to the spacecraft.
An important aspect of this position discrepancy is that the solar panels will be used at Mars not only to produce electrical power, but also to help the spacecraft attain its final mapping orbit. Over the course of a four-month period following Mars orbit insertion, Surveyor will be dipped into the upper Martian atmosphere on every orbit. During these atmospheric passes, air resistance generated by the solar panels will slow the spacecraft and gradually lower its orbit. Surveyor will use this "aerobraking" technique to lower the high point of its orbit from an initial 56,000 kilometer altitude to just under 400 kilometers.
For the last few months, the flight team has been considering several options to free the debris and allow the panel to latch and lock into its proper position. One idea involved a short firing of Surveyor's main rocket engine to provide a small force to dislodge the damper arm. However, such efforts will not be necessary because an extensive analysis has indicated that aerobraking with the -Y solar panel slightly out of position is feasible with a few minor modifications to the original plan.
One of the minor changes involves rotating the panel into a position where the front side will face into the air flow instead of the back side. This orientation will keep the unlatched panel from folding up on itself when it encounters the air flow during aerobraking. Because the front side contains the silicon cells that produce electricity, it is more fragile than the back side and cannot tolerate as much heating from the air flow. As a result, the flight plan will be modified so that Surveyor aerobrakes at a slightly slow pace than previously planned.
After a mission elapsed time of 162 days from launch, Surveyor is 76.20 million kilometers from the Earth, 44.32 million kilometers from Mars, and is moving in an orbit around the Sun with a velocity of 24.59 kilometers per second. This orbit will intercept Mars 146 days from now, slightly after 6:00 p.m. PDT on September 11th (01:00 UTC, September 12th). The spacecraft is currently executing the C6 command sequence, and all systems continue to be in excellent condition.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109