The lowest point of Mars Global Surveyor's aerobraking orbit has been raised temporarily and aerobraking has been suspended while the flight team analyzes data to understand why one of the spacecraft's two solar panels, which did not fully deploy, exhibited unexpected motion during a recent dip through the upper Martian atmosphere.
The spacecraft's current 35-hour orbit around Mars, which was taking it down to 121 kilometers (75 miles) above the Martian surface during each of its closest passes over the planet, has been raised to 170 kilometers (105 miles). The orbit was raised Oct. 12 by the operations team at NASA's Jet Propulsion Laboratory, Pasadena, CA, and Lockheed Martin Astronautics, Denver, CO, by performing a brief, 2.3-meter-per-second (5.15- mile-per-hour) propulsive burn at the farthest point of the spacecraft's orbit around Mars. The panel's performance has had no effect on spacecraft power.
"We're taking a hiatus from aerobraking for the next few weeks while we study data to try to model and understand the apparent deflection of the solar panel that never fully deployed and latched in place after launch," said Glenn E. Cunningham, Mars Global Surveyor project manager at JPL. "This delay in the aerobraking process will probably change the spacecraft's final mapping orbit from the originally planned 2 p.m. passage over the planet's equator in local Mars time to another time, and we are studying several other orbits that will give us nearly the same outstanding science results."
Preliminary data from the panel indicate that it has moved past what would have been its fully deployed and latched position, Cunningham said. In addition, the panel has shown some movement rather than maintaining its rigid position during aerobraking. These changes occurred during the spacecraft's fifteenth closest approach over the Martian surface, on Oct. 6, when the density of the Martian atmosphere doubled unexpectedly.
During the next few weeks, the Mars Global Surveyor flight team will leave the spacecraft's orbit in the current, 35-hour revolution around Mars, which will not take the spacecraft through the upper atmosphere of Mars, while they analyze data and simulate conditions in the Martian atmosphere to understand the behavior of the solar panel. This hiatus also means the spacecraft's solar panels will not be reconfigured for each close pass over Mars, but will remain in the normal cruise position.
"We can't explain yet what has happened," Cunningham said. "We saw the unlatched panel move past the latched-up position, and it remains past that point now. By raising the spacecraft's orbit above the upper atmosphere, the panel should not shift further because it will not be exposed to the aerodynamic forces of the Martian atmosphere."
Several other mapping orbits are available to Mars Global Surveyor to carry out its science objectives. The flight team will explore alternatives in the next few weeks to accomplish the lowest orbit possible and achieve a "sun synchronous" orbit that will allow Global Surveyor to fly over the Martian equator at the same local solar time each orbit. These sun synchronous orbits are designed so that the spacecraft's instruments always see Mars at the same lighting angle on every pass over the surface.
"As we step back from aggressive aerobraking temporarily, we will have the opportunity to study the situation until we fully understand it," Cunningham said. "We will take advantage of this opportunity to return some spectacular data from the camera and laser altimeter. The thermal emission spectrometer and magnetometer/electron reflectometer also will continue to collect data while we remain in this holding pattern."
The Mars Global Surveyor atmospheric advisory group reported that the Martian atmosphere has more than doubled in thickness in the last week. Global Surveyor is designed to withstand more than a 50 percent increase in atmospheric density, but began showing movement in the solar panel last week, during the fifteenth periapsis on Oct. 6.
Additional information about the Mars Global Surveyor mission is available on the World Wide Web by accessing JPL's Mars news site at http://www.jpl.nasa.gov/marsnews or the Global Surveyor project home page at http://mars.jpl.nasa.gov .
Mars Global Surveyor is part of a sustained program of Mars exploration, known as the Mars Surveyor Program. The mission is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft. JPL is a division of the California Institute of Technology, Pasadena, CA.
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