Previous missions to Mars
A look at NASA's many Mars missions over the past four decades, listed by launch date:
November 1964, Mariner 3: Spacecraft loses contact after takeoff. Mariner 4 launches three weeks later and in July 1965 becomes the world's first spacecraft to take close-up pictures of Mars, returning 21 photos.
February 1969, Mariner 6: Spacecraft flies past Mars in July 1969 and returns 75 photos.
March 1969, Mariner 7: Spacecraft flies past Mars in August 1969 and returns 126 photos.
May 1971, Mariner 8: Orbiter fails on launch. Mariner 9 orbiter arrives at Mars in November and operates until October 1972.
August 1975, Viking 1: Orbiter reaches Mars in June 1976 followed by Viking 1 lander the next month. Orbiter operates until 1980; lander lasts until 1982.
September 1975, Viking 2: Orbiter arrives at Mars in August 1976, and Viking 2 lander touches down the following month. Lander lasts until 1980; orbiter operates until 1987.
September 1992, Mars Observer: Craft loses communication in August 1993 just before going into orbit around Mars.
November 1996, Mars Global Surveyor: Orbiter reaches Mars in September 1997. Continues to operate.
December 1996, Mars Pathfinder: Lander and Sojourner rover land on Mars in July 1997. Last transmission September 1997.
December 1998, Mars Climate Orbiter: Lost on arrival in September 1999.
January 1999 Mars Polar Lander: Spacecraft and accompanying Deep Space 2 microprobes lose contact during arrival at south pole of Mars in December 1999.
March 2001, Mars Odyssey: Spacecraft reaches orbit October 2001. Continues to operate.
June 2003, Mars rover Spirit: Craft sets down in the Gusev Crater region in January 2004. It has been examining rocks that suggest a wet and violent history in Mars' early life. Continues to operate.
July 2003, Mars rover Opportunity: Craft parachutes to Meridiani plains three weeks after Spirit's touchdown. It has also found geological evidence of past water activity. Continues to operate.
Aug. 10, 2005, Mars Reconnaissance Orbiter: Craft entered orbit in March 2006. It has sent back more than 25,000 images and 3,500 radar observations. Continues to operate.
Los Angeles Like a miner prospecting for gold, NASA hopes its latest robot to Mars hits pay dirt when it lands Sunday near the red planet's north pole to conduct a 90-day digging mission.
The three-legged Phoenix Mars lander fitted with a backhoe arm is zeroing in on the unexplored arctic region where a reservoir of ice is believed to lie beneath the Martian surface.
Phoenix lacks the tools to detect signs of alien life - either now or in the past. However, it will study whether the ice ever melted and look for traces of organic compounds in the permafrost to determine whether life could have emerged at the site.
Before this robotic geologist can excavate the soil, it must first survive a nail-biting plunge through the Martian atmosphere. Despite the rousing success of NASA's twin Mars rovers, which landed in 2004, more than half of the world's attempts to land on the planet have failed.
"It's kind of like first-day jitters," said Ed Sedivy, program manager at Lockheed Martin Corp., which built Phoenix. "There's a lot of excitement, but there's also some nervousness."
Launched last summer from Cape Canaveral, Fla., Phoenix has traveled 422 million miles for Sunday's touchdown.
The spacecraft's main tool is an 8-foot aluminum-and-titanium robotic arm capable of digging trenches 2 feet deep. Once ice is exposed - believed to be anywhere from a few inches to a foot deep - the lander will use a powered drill bit at the end of the arm to break it up.
"It'll be a construction zone," said mission co-leader Ray Arvidson of Washington University in St. Louis. He predicts the ice will be "as hard as a sidewalk."
The excavated soil and ice bits will then be brought aboard Phoenix's science lab. It will be baked in miniature ovens and the vapors analyzed for organic compounds, the chemical building blocks of life.
The last time NASA did tests for organics was on a hunt for extraterrestrial life in 1976 with the twin Viking spacecraft. No conclusive signs of life were found.
On this mission, Phoenix will also probe whether the underground ice ever melted during a time when Mars was warmer and wetter. If Phoenix finds salt or sand deposits, it might be evidence of past flowing water.
Phoenix's landing target - a broad shallow valley in the high northern latitudes comparable to Greenland or northern Alaska on Earth - was chosen because if organic compounds existed, they're more likely to have been preserved in ice. Researchers do not expect to find water in its liquid form at the site because it's too cold.
"The polar region is a great preserver," said principal scientist Peter Smith of the University of Arizona, Tucson. "Just as in your kitchen you preserve your food in the freezer, so the planet preserves organic materials and the history of life ... inside of the ice."
Landing is tough
On Sunday, Phoenix will punch through the Martian atmosphere at more than 12,000 mph.
Over the next seven minutes, it will use the atmosphere's friction and a parachute to slow to 5 mph. Seconds before touchdown, Phoenix will fire its thrusters for what scientists hope will be a soft landing. If all goes well, ground controllers expect to hear a signal at 6:53 p.m. CDT.
Smith calls the entry the "seven minutes of terror."
"Try holding your breath for seven minutes," he said. "It's plenty of time to get very nervous."
The last time NASA tried a soft landing on Mars, it ended in disaster. In 1999, the Mars Polar Lander was angling for the south pole when it prematurely shut off its engine and tumbled to its death.
If successful, Phoenix would join the twin rovers Spirit and Opportunity on the Martian surface. Together, the rovers have traveled more than 10 miles in their four years exploring opposite sides of the equator. They have uncovered geologic evidence that water once flowed at or near the surface of ancient Mars.
Unlike the six-wheeled rovers, Phoenix will stay in one spot. The cost of the mission is $420 million. Phoenix will communicate with Earth through the two NASA orbiters circling the planet.