Spacecraft to arrive at Mars -- and it won't be alone

(CNN) -- NASA says its latest Mars-exploring spacecraft is on track to fire up its thrusters and enter orbit this Sunday night, completing a 10-month journey of 442 million miles.

NASA's MAVEN craft will live up to its formal name -- the Mars Atmosphere and Volatile Evolution craft -- by helping scientists figure out how ancient Mars changed so dramatically into the planet we know today.

It will be the first mission devoted to studying the upper Martian atmosphere as a key to understanding the history of Mars' climate, water and habitability.

"The evidence shows that the Mars atmosphere today is a cold, dry environment, one where liquid water really can't exist in a stable state," said Bruce Jakosky, MAVEN principal investigator, during a mission preview briefing Wednesday at NASA headquarters in Washington. "But it also tells us when we look at older surfaces, that the ancient surfaces had liquid water flowing over it."

So where did the planet's water and carbon dioxide go?

Jakosky said MAVEN will help unravel that mystery by using its scientific instruments to measure the composition and escape of gases in the Martian atmosphere.

MAVEN will study the top of the atmosphere to determine the extent to which losing gas to space might have been the driving mechanism behind climate change, Jakosky said.

Before MAVEN can begin its year of exploration, it has to be inserted into orbit around Mars. That is set to happen at about 10 p.m. ET September 21. The craft's six thruster engines will fire and burn for 33 minutes to slow the spacecraft down so it can get pulled into orbit around Mars.

MAVEN will have company out near Mars, manmade and otherwise.

India's first mission to the Red Planet, the Mars Orbiter Mission, is set to arrive a few days after MAVEN does. The director of NASA's Planetary Science Division, Jim Green, says the United States and India are interested in cooperating as their crafts gather data about the planet.

There's a visitor of the cosmic kind, too.

Comet Sliding Spring, which was discovered last year, will be closest to Mars about four weeks after MAVEN arrives.

The comet is going to miss Mars by about 81,000 miles, said Jakosky.

"I'm told that the odds of having an approach that close to Mars are about one-in-a-million years," he said, adding that dust from the comet carries only a "relatively minimal" risk to the spacecraft.

MAVEN will take advantage of the rare flyby by observing the comet itself, as well as its effect on the Martian atmosphere.