with Simon Mansfield

Surveyor Braking Nearly Twice A Day
JPL - February August 24, 1997 - For the second consecutive month, conditions in the Martian atmosphere have remained calm, and aerobraking progress continues to proceed at a slightly faster than normal pace. As of today, Surveyor is completing one revolution around Mars every 15.7 hours. This orbit period is 93 minutes shorter than that predicted for this time prior to the winter holidays last year.

The ability of the flight team to maintain the current level of aerobraking progress will depend on continued cooperation from the Martian atmosphere. Dr. Richard Zurek of the atmospheric advisory group reports that data from Surveyor's instruments indicates a slowly increasing amount of dust in the Martian air over the last month. Consequently, the flight team will continue to remain cautious because dust storms have a great potential to slow aerobraking progress.

Historically, most global dust storms tend to occur during summer in the southern hemisphere on Mars. Although summer on the red planet began on February 7th, there is no indication at this time of the onset of another major dust storm similar to the one experienced over the Thanksgiving weekend in 1997.

In other news this week, the flight team sent commands to the spacecraft on Wednesday to power off the Mars Orbiter Camera and Thermal Emission Spectrometer science instruments. The reason for this decision is that aerobraking operations and associated activities consume the majority of time during a single orbit. With the time of revolution around Mars shrinking orbit by orbit, there is no longer enough time to conduct both aerobraking and science operations.

Despite the power off of the science instruments, the radio science team continues to collect data. This collection is made possible by the fact that the spacecraft currently passes behind Mars on every orbit as seen from the Earth. During this time, communications with the Earth is lost because Mars blocks the radio signal from the spacecraft. However, just before Surveyor enters this occultation zone, the radio signal passes through the thin Martian atmosphere on its way to Earth. An analysis of the distortion of the signal's strength and tone as it fades enables the radio science team to determine the atmospheric properties at specific locations on Mars.

Science data collection by all of the instruments will resume in late March when the period of revolution around Mars has shrunk to 11.6 hours. At that time, aerobraking will be temporarily suspended by raising the low point of the orbit out of the atmosphere. This plan will allow for a concentrated period of science data collection during the spring and summer months of this year. Aerobraking will resume in September, and Surveyor will reach its Mars mapping orbit in late March or early April 1999.

After a mission elapsed time of 470 days from launch, Surveyor is 213.04 million miles (342.86 million kilometers) from the Earth and in an orbit around Mars with a high point of 14,566 miles (23,442 km), a low point of 73.8 miles (118.8 km), and a period of 15.7 hours. The spacecraft is currently executing the P140 command sequence, and all systems continue to perform as expected. The next status report will be released on Friday, March 13th.


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