Tim Furniss/LONDON
NASA's Mars Global Surveyor (MGS) orbiter may be restricted in its $250 million mapping mission of the red planet by a problem on one of the spacecraft's twin solar panels, which were to be used to control the orbit.
The MGS was demonstrating the first operational use of an aerobraking technique, which is designed to save on-board propellant by using instead the solar panels as "brakes" in the Martian upper atmosphere. The manoeuvre had to be suspended on 14 October.
The problem centres on one panel, which had only partially deployed and remained locked in that position after launch in November 1996. The panel appears to have freed itself and then relocked into a new position during the spacecraft's fifteenth orbit.
The cause of the movement in the panel appears to lie in a pocket of atmosphere encountered by the spacecraft on 6 October, which was around twice the expected density.
The position in which the errant panel is now set is more advanced than originally intended, and NASA says that the unit has displayed signs of "flapping".
NASA believes that the problem may not lie in the hinge mechanism of the panel, but could be a structural failure of some kind.
As a safety precaution while the problem is studied, the craft's engine was fired, to raise the 35h-period orbit to 170km, having reached a low point of 121km.
It appears to NASA that the final 12h period orbit of the spacecraft, intended to be a 378km circular Sun-synchronous orbit, providing an entire coverage of the Red Planet, will have to be altered. restricting photographic and science coverage.
Different types of orbit are being studied to ensure that maximum results can be attained, possibly by flying in a higher orbit than planned originally, but one which would still be Sun-synchronous, ensuring that the MGS always observes Mars at the same lighting angle on every pass over surface locations.
If aerobraking is considered too dangerous for the structural integrity of the panels, the engine may have to be fired to place it into an elliptical orbit, reducing observations of some parts of the planet.
Source: Flight International
