A satellite that has been hurtling toward Mars for the past 10 months slammed on the brakes Sunday night, gliding into the red planet's gravity field to spend a year studying its atmosphere — and hopefully collect evidence that Mars might once have supported life.
On a mission managed from NASA Goddard Space Flight Center in Greenbelt, the MAVEN spacecraft neared completion Sunday night of a 442 million-mile journey by firing six thrusters in reverse and being pulled into Mars' gravity field.
The $671 million mission will study whether Mars' wisp of an atmosphere has always been that way or is constantly thinned by a barrage of solar winds, as scientists suspect. It will also closely monitor how the atmosphere interacts with a comet that, coincidentally, will make a historically close pass by Mars next month.
"We're trying to understand the context in which life might have existed" on Mars, said Bruce Jakosky, the mission's principal investigator, in a news conference Wednesday. "By understanding the processes by which the atmosphere changed, we're understanding the history of the habitability of Mars."
MAVEN — an acronym for Mars Atmosphere and Volatile Evolution — joins a cadre of robots and satellites collaborating to build on past discoveries that indicate rivers once flowed across Mars' iron-laced crust.
Scientists spent 11 years planning and designing MAVEN and have monitored its progress on a trip around the sun since it launched in November.
Along its journey, teams at NASA Goddard and at the University of Colorado Boulder twice performed maneuvers to slightly adjust MAVEN's trajectory, and have otherwise minded it for regular updates to make sure the spacecraft is still in one piece.
Its arrival at the edge of Mars' gravitational field marks the beginning of a new phase. As early as Tuesday, MAVEN will begin collecting bits of data before the science portion of its mission begins in earnest in early November.
The spacecraft, shaped like an old-fashioned tube television with two wing-like solar panel arrays extending from its sides, is carrying six scientific instruments that will study the charged particles that bombard the planet from the sun and encircle it in its ionosphere.
Scientists theorize that the atmosphere, about 100 times thinner than Earth's and almost entirely composed of carbon dioxide, was once thicker.
"The main science goal is to try to understand how the Martian climate might have changed over time and how it might have lost its atmosphere to space," said Jared Espley, a planetary scientist at NASA Goddard on the MAVEN team. "The Martian atmosphere could have been blown away one molecule at a time into interplanetary space."
A magnetometer built at Goddard will explore that hypothesis, observing Mars' magnetic field to measure how many particles are streaming away from the planet and determine whether they are being lost to space or flowing around the world, Espley said. Other instruments will look at the theory in different ways.
NASA has sent more than two dozen missions to Mars since the 1960s. Mariner 9 first discovered what looked like river beds and canyons on the Martian surface in the 1970s, suggesting the presence of water. And in 2003, Mars Odyssey found large amounts of water frozen just beneath the crust, confirmed by the Phoenix lander in 2008.
Other ongoing NASA missions include rovers Curiosity and Opportunity and satellites Odyssey and the Mars Reconnaissance Orbiter.
Questions still remain about what happened to all of the water. MAVEN aims to help answer whether water that might have once flowed on Mars remains underground, or was lost via the atmosphere into space.
"This is another piece of the puzzle," said David Mitchell, MAVEN's project manager at Goddard.
MAVEN will start orbiting Mars in a 35-hour loop, but over the next month will adjust into a 4.5-hour orbit, Mitchell said. The elliptical orbit will be about 90 miles from the Mars surface, and at several points during the mission will drop as low as 77 miles above the surface. While specialists in Greenbelt manage the mission, the Colorado team led by Jakosky is conducting the science mission.
Meanwhile, another mission will join MAVEN this week — India's first mission to Mars is set to enter the planet's orbit Wednesday. At $74 million, the Mars Orbiter Mission, called Mangalyaan, costs a tenth of MAVEN and aims to study the planet's surface and mineral composition and scan its atmosphere for methane, a chemical strongly tied to life on Earth.
The scientists also will get some help from Mother Nature, when Comet C/2013 A1, better known as Comet Siding Spring, grazes the Martian atmosphere Oct. 19 at a distance of just 82,000 miles, far closer than any known comet has come to Earth.
Scientists originally feared the cloud of dust and gas emanating from the comet's nucleus might be a hazard for MAVEN, but now don't expect any damage and instead see a rare opportunity to study both the comet and its interactions with the planet's atmosphere.
The comet's close passage means its coma — the haze of gas, dust and debris surrounding its nucleus — will enshroud Mars. That could simulate the effects of solar winds on the atmosphere over a much longer period. MAVEN is set to observe the comet in ultraviolet light for three days as it approaches, and for two days as it speeds away. To be safe, the spacecraft will hibernate in a sort of safe mode for the three hours around the comet's closest approach.
The other active Mars missions also will watch the comet, with the two rovers capturing images of it from the ground and the reconnaissance orbiter from space.
"It's literally a once-in-a-lifetime event to have a comet basically envelop another planet," Espley said. "It's just like a perfect test case for the primary science we're trying to do, yet different than normal solar storms we're hoping to observe."
Reuters contributed to this article.