Orbiter Prepares for Its Close-Up of Mars ; the NASA Craft Began Its Orbit around Mars Friday. It Will Study the Planet's Climate and Geology

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For earthbound explorers sending robotic surrogates to Mars, the commute to the red planet is short but perilous. Since 1960, roughly two-thirds of 35 Mars missions ended as duds.

This helps explain why a control room filled with white-knuckled flight engineers erupted in cheers when mission No. 36, the Mars Reconnaissance Orbiter (MRO), reached the planet in fine form on Friday.

Yet white-knuckle time isn't over. At the end of this month, flight engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., will begin to slow the craft using Mars' atmosphere as a brake. The technique will gradually shift the craft from its current, oblong orbit into a nearly circular path some 180 miles above the surface.

It will be touch-and-go - literally - until the craft reaches its final orbit in October, notes Gerald Keating, a planetary scientist at The George Washington University in Washington who leads the MRO team studying the Martian atmosphere's structure. And while most of the science instruments aboard the craft don't begin their work in earnest until November, the science actually begins when the orbiter begins aerobraking. During 500 orbits, the craft will tiptoe ever deeper into the atmosphere, returning data that are expected to help answer the question: What happened to all the water Mars appears to have once had?

For now, however, the flight team is savoring its success. "We finally earned our O," said Howard Eisen, an ebullient flight- system manager, referring to the spacecraft's third initial during a post-arrival briefing on Friday. By one account, the feat resembled an archer in Los Angeles hitting a bullseye in Des Moines with both the archer and target moving at tens of thousands of miles per hour. The craft arrived on time, with a significant amount of unused fuel, required no last-minute tweaks to its course, and overshot its planned encounter speed by less than half a mile per hour.

Launched from Cape Canaveral on Aug. 12, 2005, the spacecraft carries the most potent package of instruments and communications gear ever sent to the red planet. For teams associated with three of the instrument packages, the mission has the quality of a phoenix: The instruments are refined versions of experiments lost when two previous orbiter missions failed in the 1990s. …

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