Scientists at NASA's Jet Propulsion Laboratory must first endure what they've described as the "seven minutes of terror" as the Mars Phoenix Lander attempts a risky descent.
"It's like sending a child to kindergarten on the first day," said Ray Arvidson, professor of earth and planetary sciences at Washington University in St. Louis, and chair of the landing-site working group.
Of 11 spacecrafts sent to Mars, only five have been successful. And many of the scientists working on Phoenix remember firsthand the terror - and grim defeat - when the Mars Polar Lander crashed as it tried to land in 1999.
But scientists this time are "cautiously optimistic," said Deborah Bass, deputy project scientist for the mission. "The team that has analyzed the entry descent and landing system is absolutely excellent, and I believe they have tested every humanly possible component," she said, adding, "But it's the unknown unknowns, the things we haven't thought about that could end up getting us."
The Mars Phoenix Lander is named after the mythical firebird "Phoenix," which legend says rises from the ashes of its deceased ancestors. The Phoenix spacecraft was remade from parts of a shuttle originally designed for a cancelled 2001 space mission, and reinvented to dig up ice from the Martian ground.
For a safe touchdown, a series of events must unfold precisely in the seven minutes between the moment Phoenix enters the Mars atmosphere and the moment it hits the ground.
After plunging into the atmosphere faster then the speed of sound, friction from the craft's heating air shield will slow the freefall, and then a parachute will unfurl. Just above 1,000 meters, its three legs will pop out and its nine rocket engines will ignite, further slowing the descent. And then, if all goes as planned, it should land at a latitude of 68 degrees North, above the planet's Arctic Circle.
Time and place of descent were strategically chosen. Few rocks should make for a safe landing and the weather now on Mars is warm, with the sun at its highest. Sun is vital for the lander's solar panels to power the machine.
The main goal of the mission is to examine the soil and the ice just inches below the planet's surface to determine whether it is and ever was habitable, by searching for signs of microbial life. Using a built-in scoop and drill, the lander will dig up soil and ice and analyze the samples.
Scientists will then examine and measure the pH of the soil. If it's too acidic or too alkaline, it may never have been conducive to life, Bass said. They will look for organic molecules, namely carbon, hydrogen and oxygen, as well as more signs of past liquid water.
The twin rovers, Spirit and Opportunity, which have been exploring Mars for four years now, have already discovered rocks that were formed in the presence of water, Arvidson said.
"Mars was warm and wet for sure early in geologic time," Arvidson said. "It raises strongly in my mind the probability that it had the right stuff for life. I'd be surprised if life didn't start on that planet."
It's even possible, scientists say, that the polar region could have been an ocean once.
With Sunday's approach, Arvidson and other scientists are cycling into Mars time to prepare for the next stage of the mission, assuming the shuttle lands unharmed. Mars takes slightly longer than earth to rotate on its axis, making a Mars day 37 minutes longer than an earth day, which over time, results in a strange sort of jet lag.
And in the meantime, all they can really do is wait.
"Other spacecraft that have landed on Mars have been much more southern," Bass said. "This place will be a different kind of landing. We don't expect it to look like anything we've seen before."
