After a nine-month, 422-million-mile trip from Earth that began last August, NASA's Phoenix Mars Lander will catch up to the red planet within hours to begin its three-month science mission.

Its mission is series of soil analysis projects that involve digging lightly into the Mars surface to study the history of water, ice and life potential in the planet's soil. But before those soil analysis projects can even get under way, the Phoenix has to successfully land on the planet's surface, which, as NASA knows by experience, isn't a sure thing.

More than eight years ago, in December 1999, NASA's Mars Polar Lander project came to a disastrous end when the craft's descent engines shut down early as it prepared to land on the Martian surface. The lander went of control from a high altitude, crashed and was destroyed.

Since that mission, changes were made in the design of the Phoenix, the next-generation lander that is expected to touch down on Mars at 7:38 p.m. US EDT Sunday. Once the Phoenix lands, NASA won't know if the mission was successful until 15 minutes and 20 seconds later. That's how long it will take for radio signals to get back to Earth with confirmation of a safe landing. The radio signals move at the speed of light and will travel approximately 171 million miles to reach Earth.

The system improvements on the lander stem from the results of a NASA review board, which investigated the failure of the Mars Polar Lander (MPL) mission, said David Spencer, the Phoenix deputy project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

One key change centered on the lander's 16 descent rockets, or thrusters, that are used to gently lower the spacecraft to the planet's surface in the northern hemisphere of Mars, Spencer said.

The most probable cause of failure in the MPL mission, according to the NASA review board, was a physical "jolt" that occurred as the landing gear unfolded and the lander began its descent. The jolt tripped a touchdown sensor mounted on one of the lander's three landing gear footpads, as the legs were deployed about 700 meters above the surface.

When software controlling the descent engine thrusting came online, it ... saw that the touchdown sensor said we had already touched down, and so it erroneously ... shut off the descent engines" too early.

"The sensors weren't supposed to be activated at that point, "but because of a testing issue, that problem was not uncovered" until it was too late, Spencer said. "Fast-forward to Phoenix -- we've redone all of that testing."

Another problem that was uncovered has also been fixed. It was discovered that a set of electrical connectors between the lander and its protective shell, called the cruise module, could bind up because of the cold of space and fail to separate when the lander is set to detach and descend to the surface. The module, essentially a container that holds the lander, carries a propulsion system, guidance system and the communications hardware that gets the vehicle from Earth to Mars. NASA added small heaters to the connectors so that the disconnection works reliably, Spencer said.

Because of the time delay because of the millions of miles between the planets, "there's no way you can joy-stick the spacecraft" to control its second-by-second approach and landing on the Mars surface. The entire approach and descent has to be coordinated ahead of time, leaving no room for instant midcourse corrections, Spencer said.