Phoenix will be the first spacecraft to study the Martian arctic plains. Unlike NASA's mobile twin rovers, the lander will stay in one spot. It will use its robotic arm to dig into the permafrost to determine if the polar environment has the ingredients needed for life to emerge.
The robotic craft, designed to dig into the icy soil to determine if the permafrost could have supported primitive life, was expected to land under a sunny Martian sky.
Early Sunday, mission controllers decided there was no need to use their last chance to tweak the flight path, leaving them with nothing to do but wait for Phoenix to carry out the landing on its own.
Mission control hoped to receive a continuous radio signal from Phoenix throughout the descent. Because it takes about 15 minutes for radio signals to travel the 170 million miles between Mars and Earth, the earliest that controllers expected to know if Phoenix landed safely was 4:53 p.m. PDT.
Otherwise, officials said, the next opportunity would be two hours later during a pass over the landing site by the Mars Odyssey orbiter.
"What's the scariest moment for me? It's if we lose the signal during descent," said Peter Smith, the mission's lead investigator from the University of Arizona, Tucson. "If we get the signal all the way to the surface, we'll be very happy and there's going to be tremendous cheers."
Phoenix's complex, automated landing has been dubbed "the seven minutes of terror" for good reason. More than half of all nations' attempts to land on Mars have ended in failures.
"This is a jittery time," said Barry Goldstein, project manager at NASA's Jet Propulsion Laboratory.
The three-legged Phoenix was designed to use a heat shield, parachute and rockets to slow its descent speed from more than 12,000 mph to a 5 mph touchdown.
NASA has not had a successful powered 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. The Mars rover missions used parachutes and cocoons of airbags for their landings.
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 were no guarantees on landing day.
Launched last summer, Phoenix has traveled 422 million miles over nearly 10 months. Its arrival to the high northern latitudes was to be closely watched by a trio of Mars orbiters passing overhead. A successful landing would give NASA a third operating robot on the surface. The rovers Spirit and Opportunity have been exploring the equatorial plains since 2004.
"We told them they're going to have company," said the rovers' chief scientist, Steve Squyres of Cornell University. "This is sort of a distant relative of theirs. So they're pretty excited about it."
The $420 million Phoenix mission is led by the University of Arizona and managed by JPL.
Phoenix is equipped with an 8-foot-long robotic arm capable of digging trenches in the soil to expose ice that is believed to be buried inches to a foot deep.
Should it find ice, the lander will analyze dirt and ice samples for traces of organic compounds, the chemical building blocks of life. It will also study whether the ice melted at some point in Mars' history when the planet was warmer than its current harsh, cold environment.
Scientists do not expect to find water in its liquid form at the Phoenix landing site because it's too frigid. But they say if raw ingredients of life exist anywhere on the planet, they likely would be preserved in the ice.
