A dust storm raging across the southern hemisphere of the red planet has slowed the progress of Mars Global Surveyor's aerobraking over the last two weeks. Although the altitude of the spacecraft's atmospheric passes lies well above the height of any potential encounter with dust, the storm traps heat and causes a tremendous increase in air pressure at all altitudes.
The possible occurrence of these storms was expected because Mars is approaching summer in the southern hemisphere, and this time of year marks the start of the traditional dust storm season. As a consequence of these historical trends and data returned from Surveyor's scientific instruments, the flight team's atmospheric advisory group issued a forecast in late November of a possible onset of dust storms.
Surveyor's first direct encounter with the effect of these storms occurred early on the morning of November 28th when the spacecraft encountered a 133% increase in atmospheric density during an aerobraking pass on orbit #51. Shortly afterward, Flight Operations Manager Joe Beerer gave the order for the spacecraft to perform a short firing of its thruster rockets to raise the altitude of the orbit's low point. Later that morning, Project Manager Glenn Cunningham ordered a second maneuver as a precautionary measure after receiving a storm warning from the atmospheric advisory group.
In total, the two post-Thanksgiving maneuvers raised the altitude of the orbit's low point by 4.35 miles (7 km). This increase was designed to lower the air pressure experienced by the spacecraft by a factor of 2.7 on subsequent passes through the atmosphere. Since then, Surveyor has been aerobraking at altitudes up to 6.2 miles (10 km) higher than the baseline plan. The higher aerobraking altitudes resulted in a lower air pressure and guarded against further atmospheric blooming due to dust storm activity.
According to Dr. Richard Zurek of the atmospheric advisory group, the dust storm once covered an area equal to the southern Atlantic Ocean, but now appears to be fading in intensity. The flight team is continuing to monitor conditions in the Martian atmosphere and has begun to return the spacecraft to its normal pace of aerobraking. However, despite the slower than normal progress over the last two weeks, aerobraking operations during this time has trimmed nearly 1,240 miles (2,000 km) from the orbit's high point, and decreased the period of revolution around Mars by 1.9 hours.
After a mission elapsed time of 400 days from launch, Surveyor is 192.72 million miles (310.15 million kilometers) from the Earth and in an orbit around Mars with a high point of 24,468 miles (39,378 km), a low point of 79.0 miles (127.1 km), and a period of 29.6 hours. The spacecraft is currently executing the P63 command sequence, and all systems continue to perform as expected. The next status report will be released on Wednesday, December 24th.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109