PASADENA, California -- A three-legged NASA spacecraft was closing in on Mars Sunday for what scientists hope will be the first touchdown near Mars' north pole to study whether the permafrost could have supported life.

The time it takes the Phoenix Mars Lander to streak through the atmosphere and set down on the dusty surface has been dubbed "the seven minutes of terror" for good reason. More than half of the world's attempts to land on Mars have ended in failures.

"I'm a little nervous on the inside. I'm getting butterflies," Peter Smith, principal investigator from the University of Arizona, Tucson, said on the eve of the landing. "We bet the whole farm on this safe landing, and we can't do our science without this safe landing."

Phoenix is preprogrammed to plummet through the red planet's atmosphere and will rely on the intricately choreographed use of its heat shield, parachute and rockets to slow its descent from over 19,300 kilometers per hour to eight kilometers per hour.

In the ideal scenario, "we evolve out of this cocoon and spread our wings, and we turn into this beautiful butterfly on the surface," said Ed Sedivy, program manager at Lockheed Martin, which built Phoenix.

Mission controllers decided late Saturday to skip an opportunity to adjust Phoenix's flight path since the lander was well on track for its target landing site.

NASA has not had a successful soft landing in more than three decades since the twin Viking landers in 1976. The last time the space agency tried was in 1999, when the Mars Polar Lander angling for the south pole crashed after prematurely cutting off its engines.

Phoenix was built from a lander that was scrapped after the Polar Lander disaster. Engineers spent years testing Phoenix to resolve all known problems, but there are no guarantees on landing day.

"It's kind of like going to Vegas. If you have high odds, you play a number of times, eventually one of them is going to bite you," said Barry Goldstein, project manager of NASA's Jet Propulsion Laboratory. "That's what's concerns us the most."

Launched last summer, Phoenix has traveled 679 million kilometers over nearly 10 months. Its arrival to the high northern latitudes will be closely watched by a trio of Mars orbiters circling overhead. If successful, it will join the twin Mars rovers, Spirit and Opportunity, which have been exploring the equatorial plains since 2004.

The $420 million mission is led by the University of Arizona and managed by JPL.

Mars and Earth are 274 million kilometers apart on landing day, which means that it will take about 15 minutes for mission control to receive a confirmation signal from the lander that it is safe.

Phoenix is equipped with an 2.4-meter robotic arm capable of digging trenches to expose the ice, believed to be buried less than 30 centimeters deep.