For the second time in a year, a simple math problem associated with a NASA spacecraft has caught the American space agency by surprise.
First it was the $125-million Mars Climate Orbiter, lost in September when a failure to convert English to metric units in crucial navigational measurements sent the spacecraft too close to the Red Planet as it prepared to go into orbit.
Now its Terra, a $1.3-billion satellite launched Dec. 18 on a six-year mission to monitor the Earths oceans, atmosphere and continents.
For exactly a week, the satellite has orbited the Earth in "safe-hold" since its main control computer shut down.
The culprit?
A math problem of a singularly seasonal nature.
The week-old shutdown occurred at 7:43 a.m. GMT - precisely a minute before the winter solstice, when the sun is farthest south of the equator.
Kevin Grady, the Terra project manager at NASAs Goddard Space Flight Center in Greenbelt, Md., said the timing was no coincidence.
In fact, when the glitch appeared so close to the solstice it provided NASA with an important clue: Terra had been stumped by a mathematical issue they had not considered in designing, building and testing the satellite, Grady said.
At the moment of the solstice, Terras sophisticated on-board navigational system apparently attempted to take the arc sine of a number less than negative one. For those who remember high school or college trigonometry, such a number can only be between negative one and positive one.
"While updating the suns position in the navigational software, it came upon a mathematical step you cannot do," Grady said. "So the trigonometry just broke down. You cant take the arc sine of number less than negative one."
Pre-launch testing did not catch the bug, which can be fixed.
"We ran the simulations for days and days. Apparently we never did one through the winter solstice," Grady said.
Predictably, the glitch should reoccur every six months, during both the winter (Dec. 21 or 22) and summer (June 21 or 22) solstices - if left unremedied.
NASA now hopes to use a software patch to work around the problem, and bring the main computer back up by Tuesday. The new codes should eliminate any worries about future solstices as well, Grady said.
Since the main computer shutdown, the spacecraft has remained otherwise in almost perfect health, with its backup processor working fine.
The spacecraft, however, has also experienced regular problems with its high-gain antenna. Grady said proton radiation associated with the South Atlantic anomaly could be the root cause of the glitches, called single-event upsets. Again, software can be written to bypass the problem, which has reoccurred at regular intervals since launch.
"It looks like there is an easy fix around it," Grady said.
NASA intends the unmanned Terra satellite to be the flagship of its Earth Observing System. With its suite of five instruments, Terra will measure the delicate interplay of the worlds oceans, land masses and atmosphere.
NASA will begin to turn the school bus-sized spacecrafts instruments on by mid-January, when it will begin acquiring scientific data.
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