Orbiter prepares for its close-up of Mars

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.

Armed with this package, scientists say they expect to gain deeper insights into the history of water on the planet, pick spots for a lander slated to arrive in 2008 and a rover in 2009, and provide an orbiting relay station for experiments on the surface to communicate results back to scientists on Earth. The mission, including launch and support for operations and science during its first 5-1/2 years, carries a total price tag of $720 million.

Once aerobraking begins, Dr. Keating and his team will mine data for clues about the atmosphere. One immediate scientific payoff: using motion sensors at least 100 times more sensitive than any previously flown to Mars, the team will look for subtle changes in the spacecraft's velocity as successive orbits draw it deeper into the atmosphere. The idea is to map changes in the atmosphere's density with altitude.

"Every time we look at it, we see something new," says Keating, who has conducted similar work with the Mars Global Surveyor and Mars Odyssey, which are still operating.

Previous experiments, for example, showed that the planet's famous dust storms boost the atmosphere's density far from the storms themselves. That means engineers must move orbiters higher above the planet or risk seeing them tumble to the surface under the twin grips of increased atmospheric drag and gravity.

And the density information - especially at the higher altitudes at which the craft can take measurements - can yield more-refined clues regarding how quickly hydrogen escapes from the atmosphere. That's a piece of the puzzle that could help solve the mystery of Mars' missing water.

"I've gone through quite a few failures, and you kind of get ready for something screwing up; there are so many ways to screw up," Keating says. With the MRO now safely at the planet, however, it's time to savor the moment. "It's incredible," he says.