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Red Rover Goes to Mars
Favorable Time to Build on Experience
Mars came closer to Earth in August 2003 than it had in thousands of years. NASA decided in the summer of 2000 to take advantage of this favorable planetary geometry to send two rovers to Mars. The design began with some basics from Sojourner, the rover on NASA¡¦s 1997 Mars Pathfinder mission. Some of the carried-over design elements are six wheels and a rocker-bogie suspension for driving over rough terrain, a shell of airbags for cushioning the landing, solar panels and rechargeable batteries for power, and radioisotope heater units for protecting batteries through extremely cold martian nights. However, at 174 kilograms (384 pounds), each Mars Exploration Rover is more than 17 times as heavy as Pathfinder. It is also more than more than twice as long (at 1.6 meters or 5.2 feet) and tall (1.5 meters or
4.9 feet). Pathfinder¡¦s lander, not the Sojourner rover, housed that mission¡¦s main communications, camera and computer functions. The Mars Exploration Rovers carry equipment for those functions onboard. Their landers enfolded them in flight and performed crucial roles on arrival, but after Spirit and Opportunity rolled off their unfolded landers onto martian soil, the landers¡¦ jobs was finished. NASA¡¦s Jet Propulsion Laboratory, Pasadena, Calif., designed and built the two new rovers plus the lander and the cruise stage for each. The cruise stage provided capabilities needed during the journey from Earth to Mars. In early 2003, the hardware arrived at NASA¡¦s Kennedy Space Station in Florida for final assembly, testing and integration with Boeing Delta II launch vehicles. While the twin spacecraft were being built, scientists and engineers winnowed a list of 155 candidate landing sites to a final pair best suited to the missions¡¦ goals and safety. More than 100 Mars experts participated in evaluating the sites. They made heavy use of images and other data from NASA¡¦s Mars Global Surveyor and Mars Odyssey orbiters.
The rover project¡¦s science goal has been to assess the history of environmental conditions at sites that may once have been wet and favorable to life. Each of the two selected landing sites showed evidence detectable from orbit that it may have once been wet. For Spirit, NASA chose Gusev Crater, a Connecticut-size basin that appears to have once held a lake, judging from the shapes of the landscape. A wide channel, now dry, runs downhill for hundreds of kilometers or miles to the crater and appears to have been carved by water flowing into the crater. For Opportunity, NASA chose part of a broad plain named Meridiani Planum based on a different type of evidence for a possibly watery past. A mineral-mapping instrument on Mars Global Surveyor had identified there an Oklahoma-size exposure of gray hematite, a mineral that usually forms in the presence of liquid water.
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