PASADENA, Calif. — After a nearly 10-month journey, a NASA spacecraft will land softly today on the northern polar region of Mars, if all goes as planned.

The Phoenix Mars Lander is set to touch down in a broad, shallow valley in the Martian arctic plains believed to hold a vast supply of underground ice. Phoenix’s job during the 90-day mission is to excavate the soil and ice to study whether the site might have supported microbial life.

The stakes are especially high: Fewer than half of the world’s attempts to land on the Red Planet have succeeded. “I’m getting a real case of heebie-jeebies,” Joe Guinn, mission manager at NASA’s Jet Propulsion Laboratory said on the eve of the landing.

Phoenix is the first to attempt to land in Mars’ high northern latitudes. The lander will rely on its heat shield, parachute and a dozen thrusters to slow itself from 12,000 mph to 5 mph. The risky descent takes about seven minutes.

NASA has not had a successful powered landing in more than 30 years since the twin Viking landers in 1976. The last time NASA tried was in 1999 when the Mars Polar Lander prematurely cut off its engines and crashed into the south pole.

The weather looks ideal for landing, said Peter Smith, principal investigator of the University of Arizona, Tucson, which leads the $420 million mission.

If successful, Phoenix will join two other spacecraft on the Martian surface — the rovers Spirit and Opportunity — which landed in 2004 and have been exploring opposite sides of the equatorial plains.

Unlike the twin rovers, Phoenix is designed to stay in one spot and extend its long robotic arm to dig trenches in the permafrost. It has an onboard laboratory to heat the soil and analyze the vapors for traces of organic compounds, an essential ingredient for life.