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Planetary News: Mars Global Surveyor (2006)Mars Global Surveyor Falls SilentAll Other NASA Spacecraft at Mars to Assist Recovery EffortsBy Emily LakdawallaNovember 15, 2006
On November 2, 2006, a mere five days before the tenth anniversary of Mars Global Surveyor's launch, an onboard malfunction caused the oldest operational Mars spacecraft to lose radio contact with Earth. Contact was briefly restored on November 5, but the spacecraft hasn't been heard from since. Mission controllers are now exploring a complicated array of scenarios for what could have caused the spacecraft to stop transmitting to determine whether the spacecraft is presently alive but mute, or dead in orbit. Plans for the coming days include attempts to capture images of the errant spacecraft from Mars Reconnaissance Orbiter and to communicate with it using the Mars Exploration Rovers. A Balky Solar PanelThe first sign of a problem with Mars Global Surveyor came in the form of error messages from one of the spacecraft's two solar panels, according to mission project manager Thomas Thorpe. "We began seeing error counts after we had moved the solar panel, and those error counts indicated that the gimbal was having a problem going to the direction it was commanded to." The spacecraft switched to backup systems before its orbital path took it into eclipse, where the bulk of Mars blocks the spacecraft's view of the Sun. "During eclipse, the panels are normally moved through a 200-degree rewind," Thorpe said. This fact would end up complicating the recovery efforts. "When we came out of eclipse, there was a very, very weak signal. It was not strong enough to have telemetry. We normally have just the carrier on when we first come out of eclipse because the Radio Science team makes occultation measurements, both on ingress and egress, and that carrier was down 42 decibels from where it normally is. So that was a drastic reduction in signal strength." Something happened behind Mars as Mars Global Surveyor attempted to rotate its solar panels -- but there are a variety of possibilities, as Thorpe explains. "One likely scenario involves a special mode when the software detects a failure of one of the panels. If it can't move [the panel], it turns the entire spacecraft so that that failed panel is faced on to the Sun at its last known position. Since you don't know the attitude of the spacecraft, or the attitude of the panel when it failed, since it [attempted to] turn through such a large angle during eclipse, the spacecraft could be turned at any odd angle." If this is indeed what happened, then the problem may simply have been that the spacecraft's orientation in space, while a good one for maintaining solar power, did not have the communications antennas pointed anywhere close to Earth. The spacecraft should only have spent 72 minutes in this configuration, Thorpe says. After that, "it switches for 30 minutes to what's called Sun-Comm Power. In that mode it turns its low-gain transmitter and low-gain receiver towards Earth. The panels then are supposed to be face-on to the Sun, but of course the failed panel could be at any attitude. That's good for communications, but unfortunately if the failed panel is turned away from the Sun, the good panel is not enough to keep the spacecraft at a positive energy balance. That means that all the power could drain out of the batteries within a matter of a few orbits. But the fact that we saw a signal on Sunday, three days later, suggests that if that happened, [the failed panel] isn't at that bad an attitude, because the spacecraft was still around three days later. But then we lost that signal after four orbits. It went away. And we haven't heard from it since." The longer that mission controllers go without a signal from the spacecraft, the harder the recovery effort becomes, because the range of possible states and positions of the spacecraft multiplies over time. "It's frustrating," Thorpe says, "because you don't know if you're getting commands in, and it's just the transmitter that's not working; or if the commands are not getting in because the spacecraft has either had an electronic malfunction or it's turned at some odd attitude that makes the commands hard to get in. It's a difficult process because the spacecraft, first of all, is moving in a 2-hour orbit which changes its attitude relative to Earth constantly. Then, depending on which mode it's gone to, to safe itself, it could have any one of three receivers pointed towards Earth. And then when it senses a problem, especially an electronic problem, it starts cycling through its various backup components. But when you send a command to the spacecraft you have to know which receiver is supposed to receive it, and which components are supposed to be affected. We continue to send commands to different components, to turn them on and off. We're looking at a detailed fault tree of possible things that could have gone wrong, and what the state of the spacecraft could be." Funny Timing
The loss of contact with Mars Global Surveyor happened at nearly the same time as a number of other milestones at Mars. In addition to being close to the tenth anniversary of Mars Global Surveyor's launch, it was also just after Mars (and all the spacecraft operating there) exited solar conjunction, when communications become difficult; and it happened just as Mars Reconnaissance Orbiter, the newest arrival at Mars, prepared to enter its primary phase of science operations. However, none of these events had anything to do with the problem, according to Thorpe. The event that would seem on face value to be most likely to trigger a problem -- solar conjunction -- did not even affect Mars Global Surveyor as much as it affected all the other Mars spacecraft. "Unlike other spacecraft," Thorpe says, "we've been through five solar conjunctions and we never stopped transmitting data through solar conjunction. We did not have recorder playbacks, but we have a real-time mode where camera images or other science data could be transmitted. [This time] we did turn off the camera during solar conjunction, but the other science instruments were on and sending data each day for about 10 hours. Even on the day of closest ray-path approach to the Sun [October 23], we were getting data from the spacecraft. We didn't try to command it, but even though the data was noisy it was interpretable and useful, so we never lost touch with the spacecraft all the way through solar conjunction." The first loss of signal occurred on November 2. Recovery Plans
In addition to attempts to contact Mars Global Surveyor from Earth, there are also plans involving all of NASA's other operational spacecraft at Mars. The most sensational of these will be attempts to observe Mars Global Surveyor using Mars Reconnaissance Orbiter. "We're going to attempt to take a picture of the spacecraft using the HiRISE camera. We'd like to be able to see some features on the spacecraft. We'd like to see the orientation of the solar panels. That will tell us what mode we're in. Plus, the angle of that one failed panel will help us determine what direction we should move it, or whether or not the spacecraft is completely out of power. So once we know that mode, we can begin reducing the number of commands [we attempt], and continue trying to rouse it with something appropriate to its current configuration. It could be just an electronics failure internal to the spacecraft, and the spacecraft has a lot of redundant components. We don't feel that it's in jeopardy, if it's getting enough power." The Mars Reconnaissance Orbiter recovery efforts will initially involve imaging Mars Global Surveyor using the relatively low-resolution, but wide field-of-view, Optical Navigation Camera. This small camera was an engineering experiment on the orbiter, designed to take images of Mars, its moons, and background stars that were used to fine-tune the orbiter's approach to Mars. Mars Global Surveyor will only occupy one pixel in these images, but where that pixel is in relation to Mars and background stars will enable Mars Global Surveyor's navigational team to determine its position more accurately, Thorpe says.
"The spacecraft has been orbiting now for a couple of weeks without us knowing exactly where it is, and it tends to drift from where we think it is due to orbit perturbations. The navigation images, which we'll get on Friday, will help us determine the pointing for HiRISE on Monday." HiRISE will attempt imaging from a range of about 200 kilometers [120 miles], from which distance it can achieve 15-centimeter (6-inch) resolution, Thorpe says. This will yield enough detail to tell the Mars Global Surveyor team the position and orientation of the spacecraft, its communications antennas, and its solar panels. This kind of imaging has been done before, in fact by Mars Global Surveyor. Its Mars Orbiter Camera caught 2001 Mars Odyssey and ESA's Mars Express in motion last year. But that feat was accomplished by a navigational team with eight years of experience flying their craft; HiRISE and Mars Reconnaissance Orbiter are just now beginning their operational lifetimes at Mars. A separate effort at recovery will involve the Mars Exploration Rovers, Thorpe says. "Next week, we're going to send a command to the Mars Relay Beacon. This is an antenna on our spacecraft that's independent of the low-gain and high-gain transmitters." The Mars Relay Beacon can be commanded to attempt a signal to the Spirit and Opportunity rovers. If the rovers receive that signal, they'll relay the data they receive back to Earth through 2001 Mars Odyssey. "The purpose of that would be, one, to see if the spacecraft is still alive -- to see if it has sufficient power -- and two, it would give us some information about the attitude of the spacecraft, because the relay would have to occur when we're passing overhead and that particular part of the spacecraft is pointed towards the rovers." Mars Global Surveyor's FutureProvided that Mars Global Surveyor is still power-positive, Thorpe says that it should be in fine condition. "The spacecraft is perfectly capable of operating in an autonomous mode for years. We may lose track of where it is on its orbit because we don't have any signal from it, but in terms of its ability to sense the Sun, keep its attitude stable, and get power, it's perfectly capable of doing that [autonomously]." Should Mars Global Surveyor be recovered but be unable to move one of its solar panels, it will still be possible for it to perform scientific observations. "In the worst case, we could always operate in a Magellan mode, where you look at the Sun for a good part of the orbit and then, for selected targets, you turn the spacecraft so you can take data from the surface of the planet." ("Magellan mode" refers to the Magellan Venus mission, which spent its entire operational lifetime turning to Earth and back to Venus once each orbit because its high-gain communications antenna was the same instrument that it used to map the surface of Venus.) These maneuvers could be performed with Mars Global Surveyor's reaction wheels, without any significant cost in fuel. Fuel is actually the least of Mars Global Surveyor's worries, Thorpe explains. "We've found ways of operating the spacecraft so that it minimizes the use of attitude fuel. We were using about 3 grams per day, roughly a kilogram per year, and we have about 10 kilograms of fuel left. Fuel isn't our problem. We don't have to unload the reaction wheels very frequently. In fact, it's been over a year since we unloaded the reaction wheels on the Y axis."
There is still plenty of science that Mars Global Surveyor could perform, should it be recovered; its mission was extended for a fourth time on October 1. "What we proposed to do for the coming years was to gather a set of science data that builds on previous studies," Thorpe says. For example, ongoing research with the Mars Orbiter Camera involves the continued search for evidence of recent activity on Mars such as the formation of new gullies, boulder trails, and impact craters. The very longevity of Mars Global Surveyor has allowed science investigations that the other orbiters can't yet match. "The weather data set was gathered consistently over the past nine years, so it is easy to use to compare what changes are taking place seasonally, and what are the longer changes that are taking place year-to-year, such as the carbon dioxide cap diminishing over the south pole." No other orbiter at Mars can capture the daily global views that Mars Global Surveyor collected. But what will happen if the recovery efforts don't pan out? "Initially you have a lot of very enthusiastic and diligent people exploring what could have happened. But after a while people tire and kind of run out of energy and then you start thinking, well, you know, we've tried everything, so people begin to get discouraged, I think, that they may not get it back. We will go on for weeks and weeks trying different things at a more leisurely pace. It's a challenging task, because of the changing geometry and all the possibilities, so it'll be a while before we can say we've exhausted every possibility." If we've truly seen the end of Mars Global Surveyor, it will remain in orbit for a minimum of 40 more years before its orbit decays into Mars' atmosphere, Thorpe says. "We have to satisfy planetary quarantine requirements, which, for our spacecraft, was 50 years in orbit with 0.99999 probability of staying above the atmosphere. We're higher than that minimum altitude. At one time it was planned to do a raise maneuver, but the models of Mars' gravity have been updated since the early days, and now we've demonstrated that we're in a safe orbit. So at the end of the mission, if we have control of it, we would put the spacecraft in an orientation that would reduce what little friction there is due to the atmosphere at this altitude even further. But in our normal operating mode, if we leave the spacecraft there, even if it starts tumbling, it will last for at least 40 more years. Eventually, yes, the orbit will decay and it will crash into the planet." Thorpe isn't completely discouraged yet, however, especially since what they know of the way that this problem began definitely allows the possibility that the spacecraft could be recovered to a condition in which it can continue doing science. The three-pronged attack of commanding the spacecraft directly, imaging the spacecraft from orbit, and relaying a command through it to the rovers will continue over the coming weeks, until they have either recovered Mars Global Surveyor or exhausted all possibilities for its recovery. |
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