A NASA spacecraft designed to investigate how Mars lost its water is expected to put itself into orbit around the Red Planet on Sunday after a 10-month journey.
After traveling 442 million miles (711 million km) from Earth, the Mars Atmosphere and Volatile Evolution, or MAVEN, probe faces a do-or-die burn of its six braking rockets beginning at 9:37 p.m. EDT/0137 GMT.
If successful, the thruster burns will trim enough speed for MAVEN to be captured by Mars' gravity and fall into a looping orbit.
Over the next six weeks, as engineers check MAVEN's nine science instruments, the spacecraft will maneuver itself into an operational orbit that comes as close as 93 miles (150 km) and as far away as 3,853 miles (6,200 km) from Mars' surface.
Unlike previous Mars orbiters, landers and rovers, MAVEN will focus on the planet's atmosphere, which scientists suspect was once far thicker than the puny envelope of mostly carbon dioxide gas that surrounds it today.
Denser air would be needed for water to pool on the surface. While no water appears there today, Mars is covered with ancient river channels, lakebeds and chemical evidence of a warmer, wetter past.
"Where did the water go? Where did the CO2 (carbon dioxide) go from that early environment?" MAVEN lead science Bruce Jakosky, of the University of Colorado, asked reporters this week. "It can go two places: down in the crust or up to the top of the atmosphere where it can be lost to space," he said.
MAVEN's focus is the latter. The spacecraft, built by Lockheed Martin, will spend a year monitoring what happens when the solar wind and other charged particles hit the upper layers of Mars' atmosphere, stripping it away.
By studying the atmosphere today, scientists expect to learn about the processes involved and then use computer models to extrapolate back in time. Ultimately, scientists want to learn if Mars had the right conditions for life to evolve.
MAVEN, said Jakosky, will tell them "the boundary conditions that surround the potential for life."
MAVEN will join a fleet of two U.S. orbiters, two U.S. rovers and a European orbiter currently working at Mars. India's first Mars probe is due to arrive on Wednesday. (Reporting by Irene Klotz; Editing by Daniel Wallis; and Rosalind Russell)
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