On November 2, 2006
the Mars Global Surveyor spacecraft reported back a malfunction in one of its solar panel
s which provide the craft with onboard power. Following a commanded repositioning, telemetry
indicated that one of the gimbal
ed panels was reporting error conditions, indicating that it was not properly positioned. This apparently caused the MGS to switch to backup systems for rotating the panel just before reaching the point of eclipse
, a regular period where the spacecraft moved to a postion where Mars
blocked it from the Sun. Normally, according to engineers overseeing the mission, the solar panels perform an automatic 200-degree reset during the period of eclipse so as to be properly positioned to catch solar radiation when the spacecraft emerges from the dark. Something apparently went wrong, however, for no communications were heard when it emerged from the eclipse (MGS is in a 2-hour orbit of Mars).
On November 5, 2006 a weak radio signal was heard, but it was too weak to sustain a telemetry link and was soon lost. The spacecraft then fell silent, and has been silent up until the time of this writeup (November 21st). Engineers and mission controllers aren't sure whether the spacecraft is dead in orbit from fault or lack of power, or whether its antennae are pointing in directions which do not permit radio contact with Earth.
The situation is somewhat complex, and akin to having you and a friend trying to find each other in a vast forest. You've set up meeting points on various hills from which you can signal each other using mirrors at particular times - but who should move from hill to hill if they don't see anyone? Who should sit tight? The analogy isn't perfect, of course, because we're not moving, but it's close. The spacecraft has two constraints, in that it has two 'directions' it has to worry about. The first is the direction of its communications antennae, which must be aligned with their targets (Earth, or a relay station such as the Mars Reconnaissance Orbiter, another Deep Space Network node, or a Mars Rover). The other is the direction of the spacecraft's solar panels, which must be kept aligned with the Sun in order to power the spacecraft's systems, since the batteries do not contain more than a few hours of power.
If the spacecraft detects a failure in one of the panels, it is programmed to rotate the entire spacecraft so that the failed panel(s) face the sun, so as to ensure that it gets power. This will break communications with earth, since the antennas are set at a relative angle that assumes the panels and antenna should be configured with the spacecraft at optimum angle for everything working. In this case, the spacecraft will remain with the failed panel facing the Sun for 72 minutes, then switch attitudes so that the low-gain antennas face Earth - what's called a 'Sun-Comm Power' attitude. If only one of the panels has failed, this should allow the spacecraft to still get some solar power, as well as communicate with home, since the properly-set panel will be aligned, and the low-gain channels will allow it to send and receive telemetry, if not science data. If it finds its power dropping, or less efficient in this attitude, it will (as far as I can tell) alternate between these two attitude while it tries to make contact with home again.
The problem is that the longer it goes without talking to Earth, the larger its position error gets. Eventually, it will end up 'lost' and only end up facing Earth by (highly improbable) chance. If anything else has gone wrong, or if its own position drifts beyond its onboard systems' ability to correct, it will eventually be unable to keep its solar panels pointed at the Sun. At that point, it will rapidly power down as its batteries drain, to true spacecraft 'death.'
Efforts were made to locate the MGS spacecraft visually using cameras on the newer Mars Reconnaissance Orbiter. If found, it is hoped that imagery will allow scientists to determine the alignment of the various dishes and solar panels on the spacecraft, giving them important insight into what the spacecraft is or was doing when it ceased to communicate. Although analysis is incomplete, so far those efforts have been unsuccessful. The next planned attempt to contact the spacecraft involves commanding the MGS to perform a relay contact with the Opportunity Mars Rover, a mission it performed several times, and then commanding Opportunity to listen carefully for any signal from the MGS. This is considered the best possible option, since the potential error in the MGS' position has increased continually since contact was lost. Due to the orbital mechanics of the rover relay mission, the MGS and the rover are designed to 'search' for each other for the full duration of the MGS' pass from horizon-to-horizon, increasing the possibility of finding a signal from the MGS even if estimates of its position are off.
Attempts to resume contact from Earth continue, utilizing the Deep Space Network.
The NASA review board studying the loss concluded (as of April 2007) that the root cause of the loss of MGS was human error. The causal chain was as follows. An update to the onboard computers performed in June 2006 contained a mis-addressed write to memory. This miswrite distrubed two critical spacecraft functions. The intended data write contained the positioning information for the high-gain communications antenna in the event of a problem - data which would have allowed it to properly align with Earth. This data was apparently written into a space which contained the spacecraft solar panel positioning instructions, which meant that two pieces of error-recovery data were compromised.
On November 2, a command was sent to move one of the solar panels. Due to the error in positioning software, that caused the panel to try to move farther than it was able, and MGS (wrongly) interpreted the result to mean that the panel was jammed, when in fact it was simply unable to reach the instructed attitude due to the software error above. As a result, the spacecraft went into what appears to be the above SUN-COMM mode, but due to the error in alignment data (also described above) it ended up with one of its two onboard batteries facing the sun. This caused the battery to overheat from direct exposure, and this physical warming of the battery caused the spacecraft to assume that the battery was in fact charged. Thus it stopped trying to charge its batteries. However, they weren't charged, merely hot - and the spacecraft lost power and was unable to determine that it should resume charging the batteries. It would have ceased functioning in 10 or 12 hours.