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Selecting Targets on Mars to Imageby Peter Thomas
February 22, 1998
Mars Global Surveyor is currently in the aerobraking phase: trimming its long elliptical orbit down to a small, circular one by passing through the atmosphere about 75 miles above Mars each orbit. For much of the fall and winter (on Earth, northern hemisphere!) these orbits were over 24 hours long, now they are about 18 hours. On most orbits the camera has been able to take a few images near closest approach to Mars.
I spent the second half of January in San Diego, CA at Malin Space Science Systems where the camera was built and is controlled, helping pick targets to image day-by-day. The process is this: MGS makes a close pass, slows down slightly, then the Jet Propulsion Laboratory in Pasadena, CA tracks the spacecraft for a few hours, then sends out data to predict where the next close periapse groundtrack will be (each aerobraking pass is slightly uncertain in its results, so the exact time of the next pass may be off by a few seconds, which means predicting more than one orbit ahead gets real uncertain). With those data, people at Malin Systems then pick targets for the narrow-angle camera based on global digital maps made with Viking data taken in the late 1970s.
The MGS data are 10-50 times as good resolution in most cases. The targeted images have to meet a data budget for transmission back to Earth and the exposures have to be set; that can be tricky! Then the commands are checked, sent to the Jet Propulsion Lab and then on to the spacecraft. About 18 hours later the pictures are in hand! Because the orbit period is now well less than a day, the time of day of all this work shifted constantly.