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AeroBraking Resumes
JPL - November 3, 1997 - After a several-week hiatus in aerobraking, the Surveyor flight team will resume lowering the spacecraft's orbit beginning November 7th. This effort will proceed at a more gradual pace as compared to the original plan, and will extend the amount of time needed to complete aerobraking by eight to 12 months.
The decision to resume aerobraking came after intensive engineering analysis, computer simulations, and laboratory tests with representative hardware to characterize the current condition of one of the spacecraft's two solar panels, which began to flex more than expected during Surveyor's lowest dip into the Martian atmosphere on October 6th.
Under normal circumstances, the spacecraft's two 3.5-meter- long (11-foot) solar panels remain fixed during each aerobraking pass through the upper atmosphere of Mars. One of the panels, which did not fully deploy and latch after launch, moved past its latched position and has shown slight movement during the spacecraft's last three passes through the Martian atmosphere in early October.
"The investigation of the unexpected motion of the unlatched panel led us to identify a secondary source of damage in the yoke, a piece of structure that connects the solar panel to the spacecraft," said project manager Glenn Cunningham. "This secondary source of damage was a result of the failure of the damper arm that jammed in the panel's hinge joint shortly after launch when the solar panels were initially deployed."
Mechanical stress analysis tests on replicas of the solar panel show that the yoke, a triangular-shaped device constructed from aluminum honeycomb material sandwiched between two sheets of graphite epoxy, probably fractured on one surface when the panel was deployed. The analysis further suggests that the fractured surface, with increased pressure on the panel during aerobraking, began to pull away from the aluminum honeycomb beneath it.
"After sufficient time to study the observed motion, we concurred that it is possible to perform additional aerobraking at a slower rate, without putting undue stress on the solar panel in question," said Cunningham. The new plan will reinstate atmospheric passes at a slightly higher altitude than before. This increase in aerobraking altitude will reduce the air resistance pressure experienced by the solar arrays to 0.00003 pounds per square inch (0.2 Newtons per square meter), a level one-third as strong as proposed by the original mission plan.
According to the project manager, the new plan will increase the amount of time needed to complete aerobraking and change Surveyor's final mapping orbit, but should not have a significant impact on the spacecraft's ability to accomplish the mission's science objectives. Members of the flight team are currently studying mission options to collect scientific data from a wide variety of final orbits.
"There are several other desirable orbits for us to consider in the next several weeks that will give us global coverage of the planet and yield all of the science data we expected to return," Cunningham said. "In the meantime, the instruments are performing marvelously, and we will continue gathering new science data as we begin to reduce the spacecraft's altitude and bring it down into the upper Martian atmosphere."
The latest result is a spectacular color image of the giant volcano Olympus Mons. This monstrous peak is the largest in the solar system and reaches an altitude three times higher than Mount Everest. At its base, the volcano covers a surface area equal to that contained in Arizona. To view the image, please visit the following page on the Surveyor web site
After a mission elapsed time of 358 days from launch, Surveyor is 178.66 million miles (287.53 million kilometers) from the Earth and in an orbit around Mars with a high point of 28,046 miles (45,135 km), a low point of 107 miles (173 km), and a period of 35.4 hours. The spacecraft is currently executing the P31 command sequence, and all systems continue to perform as expected.
EARTH INVADES MARS
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