It is the last 78 miles of a NASA rover's 154 million-mile journey to Mars that concerns Ravi Prakash the most.

That's because this is the first time that NASA - or anyone else - has ever tried to land something nearly so big as the 1-ton Curiosity rover on Mars, and because so much is riding on this particular mission.

"We've got to go from five times as fast as a speeding bullet - 13,000 mph - all the way to a screeching halt in seven minutes," said Prakash, a Texas City native who now works at NASA's Jet Propulsion Laboratory on the rover's lander team.

Five times the size of the Spirit and Opportunity rovers already on Mars, Curiosity is packed with scientific equipment: HD-resolution cameras that can also capture video; a laser than can ignite a spark on rocks 20 feet away to determine what they're made of; and other high-tech tools including an X-ray diffraction setup, a mass spectrometer, and a gas chromatograph.

With these devices the six-wheeled rover will be able to sample hundreds of layers of sedimentary rock, allowing scientists to understand how the surface of Mars changed over time, and providing a detailed history of the Red Planet and clues to whether life could have flourished there.

But it's got to get there safely at first.

Adding to the pressure is that NASA is not currently planning or building a next generation rover to go to Mars. More than 200 scientists attended a NASA meeting earlier this year in Houston to discuss plans for follow-up missions, but none has been chosen.

Nail-biting time

So Curiosity, itself a decade in the works, is it for a long time.

"With no sense of how or when we will follow this up, and without knowing which direction our tools and techniques are evolving - yes, landing successfully is a big deal," said Mark Lemmon, a Texas A&M University planetary scientist who will help operate the rover on Mars.

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NASA hopes are riding on Mars rover's tricky descent