with Simon Mansfield
Surveyor To Image Famous Mars Features
JPL - March 27, 1998 - NASA's Mars Global Surveyor spacecraft is about to begin a summer-long set of scientific observations of the red planet from an interim elliptical orbit, including several attempts to take images of features of public interest ranging from the Mars Pathfinder and Viking mission landing sites to the Cydonia region.
The spacecraft will turn on its payload of science instruments on March 27, about 12 hours after it suspends "aerobraking," a technique that lowers the spacecraft's orbit by using atmospheric drag each time it passes close to the planet on each looping orbit. Aerobraking will resume in September and continue until March 1999, when the spacecraft will be in a final, circular orbit for its prime mapping mission.
It will not be possible to predict on which orbit the spacecraft will pass closest to specific features on Mars until Global Surveyor has established a stable orbit and flight controllers are able to project its ground track. This process should be completed in the next few days. The exact time of observations and the schedule for the subsequent availability of photographs on the World Wide Web are expected to be announced early next week.
"Global Surveyor will have three opportunities in the next month to see each of the sites, including the Cydonia region, location of the so-called 'Face on Mars,' " said Glenn E. Cunningham, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "The sites will be visible about once every eight days, and we'll have a 30- to- 50-percent chance of capturing images of the sites each time."
Several factors limit the chances of obtaining images of specific features with the high-resolution mode of the camera on any one pass. These factors are related primarily to uncertainties both in the spacecraft's pointing and the knowledge of the spacecraft's ground track from its navigation data. In addition, current maps of Mars are derived from Viking data taken more than 20 years ago. Data obtained by Global Surveyor's laser altimeter and camera during the last few months have indicated that our knowledge of specific locations on the surface is uncertain by 1 to 2 kilometers (0.6 to 1.2 miles). As a result, the locations of the landing sites and specific features in the Cydonia region are not precisely known.
In addition, the Mars Pathfinder and Viking landers are very small targets to image, even at the closest distance possible, because they are the smallest objects that the camera can see. The Cydonia features, on the other hand, are hundreds to thousands of times larger and the camera should be able to capture some of the features in that area.
Global Surveyor's observations of the Viking and Pathfinder landing sites will provide scientists with important information from which to tie together surface observations and orbital measurements of the planet. Data from landing sites provide "ground truth" for observations of the planet made from space.
As for the "Face on Mars" feature, "Most scientists believe that everything we've seen on Mars is of natural origin," said Dr. Carl Pilcher, acting science director for solar system exploration in NASA's Office of Space Science, Washington, DC. "However, we also believe it is appropriate to seek to resolve speculation about features in the Cydonia region by obtaining images when it is possible to do so."
So far in the aerobraking process, Global Surveyor's orbit has been reduced from an initial 45-hour duration to less than 12 hours. During the aerobraking hiatus, the spacecraft will be orbiting Mars about once every 11.6 hours, passing about 106 miles (170 kilometers) above the surface at closest approach and about 11,100 miles (17,864 kilometers) at its farthest distance from the planet. The pause in aerobraking allows the spacecraft to achieve a final orbit with lighting conditions that are optimal for science observations.
Mars News at JPL NASA's Missions to Mars Viking observations of the Cydonia region
EARTH INVADES MARS
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