The craft shut itself down once during its journey when a cosmic ray struck its computer memory, which was the way it was designed to behave in such an event, Goldstein said. He added that a scheduled course correction was canceled last week because of smooth sailing but that NASA plans to fine-tune Phoenix's aim this weekend.

The real test will come during landing. Just before 5 P.M. Pacific time on May 25, the Phoenix lander is scheduled to separate from its rocket-powered "cruise stage" and dive into Mars's atmosphere at a speed of 12,600 miles (20,300 kilometers) per hour relative to the Martian surface.

What comes next is referred to as "seven minutes of hell." After four minutes, the craft will have slowed to 1,100 miles (1,170 kilometers) per hour under the protection of a heat shield. The lander will deploy a parachute at a distance of 7.8 miles (12.6 kilometers) from the surface, then jettison its heat shield, flip over to face its thrusters toward the planet and finally fire them in short, coordinated bursts, touching down at 4:53 P.M. Pacific time (taking into account the 15-minute communication lag between Mars and Earth).

Goldstein said his biggest worry is a three-second communications delay following the separation of the craft, after which Phoenix will begin transmitting to Earth via the orbiting Odyssey and MRO craft. He said that in an emergency, NASA has arranged with the European Space Agency to use its Mars Express orbiter to relay signals. Once Phoenix has landed, it will go silent for an hour and a half as it unfolds its solar panels and powers up, at which point Goldstein said it could send back its first images of the landing site.

Sample collection is set to begin during touchdown, even before the solar panels unfold. The vehicle's sampling tool will stay open during landing to catch the dust that Phoenix will surely kick up, said Peter Smith, Phoenix principal investigator at the University of Arizona in Tucson. (That same dust might interfere with the unfolding process, which therefore is slated to begin 15 minutes after landing.)

As soon as two days later, Phoenix could deploy its robotic arm to begin scraping away the five- or six-inch (13- to 15-centimeter) layer of loose topsoil using a backhoe-like motion. If a blade on the edge of the scraper proves too weak to chip away at the uncovered ice, a drill the size of a pinky finger will flick down to slice and slip a few shreds of ice into the sampler.

Phoenix has eight single-use ovens that will slowly bake soil and ice samples separately to 1,800 degrees F (1,000 degrees C) and sniff the resulting vapors for evidence of water, carbon-rich compounds and sulfates. The lander will also mix a soil sample with water carried from Earth to test the soil's chemistry in the presence of liquid water.

To assess whether liquid water flowed over the region during the heat spells Mars experiences every 50,000 years, an onboard microscope will examine cracks in the subsurface ice that should form during cold times. If liquid water flowed across the slits during the summer, it would pool in them; otherwise they would be filled with dust and sand, Smith said.

Phoenix's primary mission will last three months. Unlike the rovers, which have had their missions extended repeatedly, Phoenix will have at most until early 2009 before the sun sets on Mars's north pole for the winter. During that time, Phoenix might succumb to the harsh Martian winter temperatures.

Goldstein noted, however, that Phoenix would still have a chance. If it powered up again in the spring, its computers would resume communication with Earth—and Phoenix would become more like Lazarus.

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