After a nearly 10-month journey, a NASA spacecraft will land softly today on the icy fringe of Mars’ north pole — if all goes as planned — and resume the search for life on our heavenly neighbor.

Touching down on Mars is a risky business; only about half of the attempts have succeeded. But, assuming the Phoenix lander does arrive upright and in one piece after 422 million miles since its launch in August, the craft will begin digging into the soil and ice beneath it to look for signs that life once existed — or could have existed — in the barren landscape.

“I’m getting a real case of heebie-jeebies,” mission manager Joe Guinn said on the eve of the landing — the first attempt in Mars’ high northern latitudes.

Phoenix’s study of the northern permafrost “takes the next step in Mars’ exploration by determining whether this region, which may encompass as much as 25 percent of the Martian surface, is habitable,” said the University of Arizona’s Peter Smith, the mission’s principal investigator.

In particular, he said, Phoenix will try to determine whether the ice holds any carbon-based organic material — a component of all known life — and whether the ice ever melted. “As we dig down, we hope to find signs that the soil has mixed with the ice, which means there was once liquid water,” Smith said. “And if we find organics, that will be a major discovery.”

Phoenix is scheduled to land about 4 p.m. PDT, joining NASA’s robotic rovers Spirit and Opportunity on the Martian surface. But its descent will be very different. When the twin rovers touched down in 2003, they were surrounded by air bags that cushioned their landings and sent them bouncing across the surface. Once they stopped, the bags deflated and the rovers emerged.

Phoenix will descend more traditionally, slowed by a parachute and its thrusters. But many space scientists and engineers will be holding their breath today as it descends. It will take about 15 minutes for its radio signal to reach Earth with the news — either that it made it safely or that another spacecraft has been destroyed in the attempt.

Also unlike the two mobile and remarkably long-lived rovers, Phoenix is designed to plant itself in one spot and start digging with a Canadian-built robotic arm as much as 3 feet deep into the soil and, scientists hope, through subsurface layers of ice. The lander has a set of miniature but sophisticated chemistry labs to analyze the soil and ice that the robotic arm pulls aboard Phoenix.

The spacecraft is aiming for a landing farther north than any other on Mars, in an area with ice and perhaps signs of once-liquid water. NASA’s search for life in the solar system is based on the theme of “follow the water,” the assumption that life needs water to exist.

Although the temperatures will be well below freezing on the surface — minus-28 to minus-100 degrees Fahrenheit — scientists think the temperatures, and the ice, could be warmer below the surface and may once have even been liquid. The region is low and flat, and some theorize that it may have held an ocean long in the past.

The Phoenix spacecraft has traveled a long and circuitous path, and not just on its journey of 422 million miles through space. Phoenix uses equipment built for a spacecraft that was supposed to be launched in 2001, which was scratched because of the Polar Lander’s crash.

In 2002, when NASA’s Mars Odyssey orbiter detected substantial amounts of water ice lying just beneath the surface, however, a reconfigured Phoenix was selected to fly to Mars and dig for the ice in the northern polar region. It will also set up a $37 million Canadian weather station.

The $457 million mission is not designed to find life itself — except in the unlikely event that it is large and visible via camera — because the process of testing for organic materials will destroy the samples to the point that life forms would not be identifiable. Instead, Phoenix will investigate whether the conditions for life exist. The search involves two sets of chemistry experiments: testing for carbon-based organic compounds created by biological or chemical processes, and testing for inorganic minerals that make life possible.

“If we can find nitrates, carbonates and the like in the soil, then we’ll know the planet is much friendlier for life,” said Samuel Kounaves of Tufts University, lead scientist for the inorganic experiments. “These minerals have been missing in all the Mars missions, but they should be there.”

Smith is optimistic that landers eventually can find signatures of life on Mars, if they exist. He said he recently spoke with a researcher who removed a cubic centimeter of ice from a long-frozen Siberian lake and found DNA remnants from a variety of life forms.

“It’s amazing how much organic material gets frozen in ice,” Smith said.

The solar-powered Phoenix is expected to function for about three months. If it finds signs of organic material and nutrients, the Mars Science Laboratory — with a much larger and more sophisticated rover, scheduled for launch next year — may be sent to a similar site. The European Space Agency is planning a lander, the Exo-Mars, for 2013, to search for signs of past or present life.

Life on Mars has captivated mystics, science-fiction writers and astronomers for centuries. More than 130 years ago, an astronomer named Giovanni Schiaparelli first trained his telescope on Mars from Milan, thinking he saw seas, continents and strange “canali” that he mapped meticulously. Imaging Mars — and exploring it — has been the goal of countless modern space missions, and most have failed.

• The Soviet Union launched its first probe, called Marsnik 1, toward the planet in 1960, and it didn’t even reach Earth’s orbit. Since then, Soviet and, later, Russian spacecraft have been launched as either orbiters or landers, and most have missed their target, vanished or landed, and signaled for only seconds before falling silent.

• In November 1964, the United States sent up its first Mars mission, called Mariner 3, to fly past the planet and send back pictures, but its solar panels never opened, and it never got there, ending in orbit around the sun.

• Seven months later, an identical spacecraft, Mariner 4, finally made it. It flew more than 6,000 miles above Mars’ surface, sped past it and sent back 22 images — the first close-up visuals of Mars ever seen.

• Later, the famed Mariner 9 orbited Mars for nearly a year in 1971, while it waited for a huge dust storm on the surface to calm. Then it returned 7,329 photos, revealing the huge volcano Olympus Mons, the enormous canyon system Valles Marineris and many visual traces of long-gone flowing water.

• Next in the Mars drama came the two NASA Viking spacecraft of 1976, the first missions to land on the surface successfully. Each Viking carried a robotic scoop to transfer soil into its ambitious suite of automated biology experiments, as scientists sought to identify living organisms in the soil.

• In 1992, NASA launched the orbiter Mars Observer, but it vanished in space just at the end of its 10-month voyage to the planet.

• Four years later, Mars Global Surveyor reached Martian orbit and for nine years transmitted more than 240,000 images and millions of other instrument readings before its batteries failed. It was lost in space.

• In July 1997, the world’s first Martian rover, called Sojourner, trundled onto the planet’s surface from its lander Pathfinder to test the chemistry of nearby rocks and explore the neighborhood. It did so spectacularly, until its battery died after three months in the freezing cold.

• Failures marked America’s next two missions: The Mars Climate Orbiter disappeared on arrival in October 1999, and three months later, the ambitious Mars Polar Lander, carrying two small surface penetrators called Deep Space 2, apparently crashed.

• Two years later, Mars Odyssey flew into Martian orbit, and it is still there, transmitting more than 350,000 close-up images of the surface and relaying to Earth all the science information from the two hardy Mars rovers still roaming on the surface. Those roving robot vehicles, Spirit and Opportunity, landed on Mars in January 2004 on missions designed to last only three months, but they’re still working hard and making new discoveries every day more than four years later.

• And now comes Phoenix, heading for the Martian far north for the first time in space history. Landing is due at 4 p.m. PDT today.

— San Francisco Chronicle