NASA's Mars Pathfinder mission

Heading into the home stretch of spacecraft assembly, NASA's Mars Pathfinder lander -- a tetrahedral-shaped spacecraft weighing 351 kilograms (772 pounds) and standing about 1 meter (3.2 feet) tall -- was mated today with its companion rover, Sojourner, just as it will fly to Mars later this year.

The lander and rover, in development at NASA's Jet Propulsion Laboratory, crossed a major engineering milestone with full integration after a year of rigorous testing of components making up the spacecraft's landing gear, said Brian Muirhead, Pathfinder flight system manager. Subsystems included a parachute, measuring 12.7 meters (41 feet) in diameter, three small, rocket-assisted deceleration thrusters to help the spacecraft brake through the Martian atmosphere, and giant, multi-lobed air bags to cushion its landing.

Now with its rover mounted and secured by cables to an inside petal, Pathfinder will be folded up to undergo integration testing in the next several days, Muirhead said. In the weeks ahead, the spacecraft will next be attached to the inside of its backshell and then be encased in a Viking-derived heatshield.

"This is actually the first and last time that we will see Pathfinder and Sojourner completely assembled until just before launch," Muirhead said. "It's exciting to see the spacecraft in full flight configuration, and to know that we have set a new standard for JPL and the world in the development of interplanetary spacecraft."

Currently residing in JPL's spacecraft assembly clean room, Pathfinder will be delivered to JPL's 25-foot space simulator in March for spin-balance, acoustic and thermal vacuum testing, added Robert Manning, flight system chief engineer. Over the summer, the spacecraft will be taken apart again for final pyro and electrical testing before its components are prepared for shipping on Sept. 1 to Cape Canaveral, Fla.

Pathfinder is designed to place a low-cost delivery system on the surface of Mars, demonstrating a new and unconventional atmospheric entry and landing approach. The spacecraft will be launched on Dec. 2 from Cape Canaveral, Fla., and spend seven months cruising to Mars. Landing on an ancient flood basin known as Ares Vallis, Pathfinder will touch down on July 4, 1997.

Twenty-four hours before Mars arrival, the spacecraft will turn approximately 7 degrees to its entry attitude and continue to descend, Manning said. Hitting the thin upper atmosphere at more than 27,000 kilometers per hour (about 17,000 miles per hour), the lander's heat shield will slow the craft to about 1,450 kilometers per hour (900 miles per hour) in about two minutes. An onboard computer will sense the slow-down in speed and eject a large parachute.

Seconds later, the heat shield, still red hot from the heat of entry, will be released and the lander will be separated from the backshell on a bridle. Because the rarefied atmosphere of Mars is only 1/100th as dense as Earth's, the parachute will slow the lander to about 250 kilometers per hour (155 miles per hour). A few seconds before impact, a giant cocoon of air bags will be inflated and the rockets will fire to literally stop the lander in mid-air and slow it to less than 72 kilometers per hour (45 miles per hour).

Landing four hours before sunrise, Pathfinder will bounce along the Martian surface like a huge beach ball before coming to a halt. The craft will spend the next three hours deflating and retracting its air bags, standing itself upright and unfolding its petals to expose the 10-kilogram (22-pound) Sojourner rover. Daylight will give Sojourner the solar power it will need to power up, rise to its full height and drive off one of the two exit ramps onto the Martian surface.

Although Pathfinder is considered an engineering demonstration, it will accomplish a focused set of science investigations with a stereo, multi-colored lander imager, atmospheric instruments that will be used as a weather station after landing, and an autonomous rover capable of measuring the composition of rocks and surface materials near the landing site. Sojourner will also perform mobility tests and image its surroundings. One of its first jobs will be to image the lander, so that scientists and engineers can determine the lander's condition and study the local terrain.

The Pathfinder lander will carry out most of its engineering objectives within the first few hours after landing, then be used to take panoramic images of the Martian landscape and support rover activities. The lander, the first in NASA's Discovery program of low-cost planetary spacecraft with highly focused science goals, has a mission lifetime of at least 30 days. Sojourner is expected to rove the surface of Mars for a minimum of seven days.

The Mars Pathfinder mission is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, D.C.

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