NASA engineers will be holding their breaths Sunday, as a digging robot attempts a precarious landing on Mars' surface.
NASA's Phoenix Lander was launched in August and has traveled 122 million miles to Mars. It is a $457 million robotic spacecraft -- equipped with a backhoe, cameras and a compact chemistry lab -- that will attempt to find out whether the cold, forbidding surface of Mars could once have been warm enough for microbial life to exist on the planet.
Phoenix is scheduled to land Sunday evening at 7:38 p.m. ET. It must first separate from its rocket and then survive a harrowing seven-minute descent at 12,600 mph. It will then slow down to 5 mph to land in one piece on the planet's unexplored north pole.
Mars has attracted more space missions than the rest of the solar system's planets, but nearly two-thirds of all Mars missions have failed in some way.
Statistics tell the story. Since 1960, the United States, Russia, England and Japan have launched a combined 36 missions to the red planet. Only 13 have succeeded.
It's called the "Mars curse," but the overwhelming odds against success don't keep scientists from trying to find out more about the first planet beyond Earth in the solar system.
Ed Weiler, NASA's space science director, understands the risk.
"This is not a trip to Grandma's for the weekend," Weiler said. "There are many risks and uncertainties and there are always the unknowns."
The memories of the failed Mars Polar Lander in 1999 are fresh in the minds of scientists. The Polar Lander was sent rocketing to Mars on Jan. 1, 1999, and was expected to land Dec. 3. Dec. 3 came and went with no signal from the Polar Lander. Engineers in the control room posted a sign with big letters, reading "MPL Phone Home."
Some missions have defied the odds, including the two Rovers, Spirit and Opportunity, which parachuted to Mars in 2003 and landed on huge air bags that cushioned the impact. They rolled out of the air bags and set off to explore. The Rovers were only supposed to run for six months -- but five years later they are still sending information back to Earth.
If Phoenix lands safely, signals confirming its arrival should reach Earth within 15 minutes, about 7:53 p.m. ET; two hours later the first images from Phoenix would be beamed back. Data from the mission should be almost continuous, thanks to the Reconnaissance Orbiter that will be tracking the Phoenix.
What makes Mars so difficult?
Dave Leestma manages the Office of Advanced Planning at the Johnson Space Center in Houston. It's his job to think ahead to how to get astronauts to Mars eventually.
He says Mars is tough to land on for several reasons.
"It has an atmosphere and it has gravity and it is a fairly big place," he said. "It doesn't have navigational aids to tell you, 'This is where you steer to land in a nice, flat area.'"
Leestma says the big challenge for NASA is developing propulsion to get a crew to Mars.
"We don't have a small nuclear reactor developed yet but my personal feeling is we can't go to Mars without nuclear power," he said. "We need to do that."
Jeff Hanley, director of the Constellation program, which will replace the space shuttle program, says NASA isn't close at all to knowing how to send humans to explore Mars.
"There is a certain amount of technical hubris involved in thinking we can just make the leap to going to a Mars mission now," Hanley said. "We have studied the risks involved of sending humans to Mars and it is about a one in three chance that we would lose the crew using today's technologies. We have a lot to learn about how do we get humans to Mars, give them a reasonable chance of mission success, reasonable chance of coming back alive. The only way we will do that is to use the moon as a proving ground."
Leestma agrees with Hanley's assessment.
"Mars is pie in the sky, but one of the primary reasons we are going back to the moon is to learn how to operate on a planet that isn't six months away," Leestma said.
Is it even necessary to send humans to Mars when it is so much cheaper and safer to send robots instead?
Neal Lane, a former science adviser in the Clinton administration who is now a senior fellow at Rice University's Baker Institute, says no one can even hazard a guess at how much a manned mission to Mars would cost, in financial -- or human -- terms.
"We have sent robots successfully and gotten a lot of information, but there are things that humans can do that robots can't," Lane said. "Humans can react in real time on the spot and synthesize information and analyze what they are seeing."
Mars, according to Lane, holds secrets that scientists would like to unlock.
"Was there water on Mars? Where there is water, there is the possibility of life forms," he said. "Is life only on Earth, or does it exist someplace else in our solar system and beyond?"