Story highlights Why did ancient Mars change so dramatically? MAVEN sent to get answers

MAVEN stands for Mars Atmosphere and Volatile Evolution craft

It's scheduled to enter orbit around the Red Planet on Sunday night

Mars will be having plenty of other visitors: a spacecraft from India and a comet

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?

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Photos: Photos: Mars MAVEN mission Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – A new NASA spacecraft called MAVEN, short for Mars Atmosphere and Volatile Evolution, will help scientists figure out what happened to the red planet's atmosphere. Its elliptical orbit will allow it to pass through and sample the entire upper atmosphere of Mars. This drawing shows MAVEN orbiting Mars. Hide Caption 1 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – Is this ancient Mars? An artist shows how Mars might have looked billions of years ago. MAVEN will be the first mission devoted to exploring and understanding the Martian upper atmosphere. Scientists hope it will solve the mystery of the red planet's missing air and water. Hide Caption 2 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – Engineers and technicians work on MAVEN in November at NASA's Kennedy Space Center in Florida. "After 10 years of working on this, I can't tell you how excited I am to see this finished spacecraft ready to go," said the mission's principal investigator, Bruce Jakosky. Hide Caption 3 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – Workers in November get MAVEN ready to be placed inside the nosecone that will protect it during launch. NASA says the project will cost $671 million. Hide Caption 4 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – Reporters and photographers wear protective gear as they get a look at the MAVEN spacecraft. Wingtip to wingtip, MAVEN is the same length of a school bus -- 37.5 feet. Hide Caption 5 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – A transporter moves MAVEN to the launchpad November 8. Three other active spacecrafts currently orbit Mars: Mars Odyssey (launched in 2001), Mars Express (launched by the European Space Agency in 2003), and the Mars Reconnaissance Orbiter (launched in 2005). Hide Caption 6 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – MAVEN is NASA's 10th Mars orbiter to be launched. Three didn't make it to orbit. Hide Caption 7 of 8 Photos: Photos: Mars MAVEN mission Mars MAVEN spacecraft – An Atlas V rocket launches MAVEN into space November 18 from Cape Canaveral Air Force Station, Florida. Hide Caption 8 of 8

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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.