with Simon Mansfield

Duststorm Slows Surveyor's Progress
JPL - Dec 12, 1997 - A massive dust storm that has enveloped the southern hemisphere of Mars has further slowed the progress of Mars Global Surveyor's aerobraking over the last two weeks. Although the altitude of the spacecraft's atmospheric passes are 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 by Surveyor, 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 120% 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.

The next status report will be released on Wednesday, December 24th.


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