(CNN) -- Despite the success of NASA's Spirit and Opportunity missions, directors remained anxious Sunday over the first-ever landing of a probe near Mars' north pole to find signs of life.

1 of 2 "I do not feel confident. But in my heart I'm an optimist, and I think this is going to be a very successful mission," said principal investigator Peter Smith, an optical scientist with the University of Arizona. "The thrill of victory is so much more exciting than the agony of defeat."

The Mars Phoenix Lander was wrapping up its 296-day, 422 million-mile journey Sunday, with about a 50-50 chance of a successful touchdown on the arctic plains of the Red Planet, NASA officials said.

"There's nothing else to do but watch," said Phoenix project manager Barry Goldstein of NASA's Jet Propulsion Lab in Pasadena, California.

Goldstein's teams decided to waive an opportunity to tweak the Phoenix flight path, saying that the spacecraft was on the right track.

The mission of the Phoenix is to analyze the soils and permafrost of Mars' arctic tundra for signs of life, either past or present.

But first, everyone on the team has to get the lander on the ground -- an event dubbed "seven minutes of terror" by the Mars exploration community.

Seven minutes is all it takes for a spacecraft traveling nearly 13,000 mph to hit the Martian atmosphere, slam on the brakes and reach the ground. During that time, onboard computers will be working at a manic pace as the spacecraft deploys its parachute, jettisons its heat shield, extends its three legs, releases the parachute and finally fires its thrusters to bring it down for a soft landing.

It's risky business. Historically, 55 percent of all Mars missions have ended in failure. Watch the challenges the mission faces »

The twin to the Mars Polar Lander spacecraft, Phoenix was supposed to travel to Mars in 2001 as the Mars Surveyor spacecraft. They were originally part of the "better, faster, cheaper" program, formulated by then-NASA Administrator Dan Goldin to beef up planetary exploration on a lean budget.

But Polar malfunctioned during its entry and descent into Mars' atmosphere in 1999 and crashed. Technical investigations later concluded that as many as a dozen design flaws or malfunctions doomed the spacecraft.

The failure of that mission, as well as another spacecraft called the Mars Climate Orbiter the same year, led NASA to put future missions on hold and rethink the "better, faster, cheaper" approach. Mars Surveyor went to the warehouse. iReport.com: Send your photos, video of space

"The trouble is somebody forgot the 'better' part," said Weiler. "By pushing the 'faster and cheaper' part so hard, engineers were forced to make decisions that weren't necessarily the best and right decisions. And that led to both the failures of the Mars Climate Orbiter and ultimately the Mars Polar Lander and eventually the entire Mars program."

But all was not lost. In 2003, Smith proposed a plan to re-engineer the Mars Surveyor and fly it on a mission to look for signatures of life in the ice and dirt of Mars far North. Mars Phoenix, literally and figuratively, rose from the ashes of Surveyor. Learn about NASA's past missions to mars »

Engineers set to work, testing and retesting the onboard system to ferret out and fix all the flaws they could find.

"We always have to be scared to death," said Barry Goldstein, project manager. "The minute we lose fear is the minute that we stop looking for the next problem."

The team is also concerned about the Phoenix landing system. NASA has not successfully landed a probe on Mars using landing legs and stabilizing thrusters since the Viking missions in the late 1970s. The other three successful Mars landings -- Pathfinder in 1997 and the Spirit and Opportunity rovers in 2004 -- used massive airbags that inflated around the landing craft just before landing to cushion the impact.

"I love airbags," said Weiler. "We got three success stories with airbags, but you don't invent science by continuing to do what you know how to do."

The Phoenix doesn't have airbags because the lander is too big and heavy for them to work properly. NASA will have to figure out how to land reliably with thrusters and landing legs in order to fly even larger spacecraft in the future.

"We landed on Mars with rockets and legs twice with Viking. It's not impossible by definition, we have proof of it," Weiler said. "Humans will have to land on landing legs. Eventually, we want to send humans there, obviously."

Assuming the landing is successful, the researchers will quickly begin gathering data.

The Phoenix landing site is targeted for the far northern plains of Mars, near the northern polar ice cap. Data from the Mars Odyssey spacecraft indicate large quantities of ice there, probably in the form of permafrost, either on the surface or just barely underground.

Phoenix is equipped with a robotic arm capable of scooping up ice and dirt to look for organic evidence that life once existed there, or even exists now.

"We are not going to be able to answer the final question of is there life on Mars," said Smith, the optical scientist. "We will take the next important step. We'll find out if there's organic material associated with this ice in the polar regions. Ice is a preserver, and if there ever were organics on Mars and they got into that ice, they will still be there today."

"Follow the water" has become the unifying theme of NASA's Mars exploration strategy.

In 2004, the rover Opportunity found evidence that a salty sea once lapped the shores of an area near Mars' equator called Meridiani Planum. Astrobiologists generally agree that it's best to look for life in wet places.

"There is no life on Earth where there isn't water," Weiler said. "However, where there's water, you find life, especially if you have organics, organic material and energy."

Smith believes that the mission probably will find organic footprints of life.


"Whether it's in the northern plains, I have no idea. But the universe is an immense place. In our Milky Way, we find hundreds of planets, and those are just in the nearby stars," he said. "So there must be huge astronomical numbers of planets in the universe."

So are we really alone? "I suspect not," Smith said.

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The Phoenix has landed. A press briefing with photos is expected at midnight. You can watch it live on NASATV if you have Dish Network. It's also live at the NASA Web site: http://www.jpl.nasa.gov

Some of the first photos....


Shown here is one of the first images taken by NASA's Phoenix Mars Lander of one of the octagonal solar panels, which opened like two handheld, collapsible fans on either side of the spacecraft. Beyond this view is a small slice of the north polar terrain of Mars.


This image, one of the first captured by NASA's Phoenix Mars Lander, shows the vast plains of the northern polar region of Mars. The flat landscape is strewn with tiny pebbles and shows polygonal cracking, a pattern seen widely in Martian high latitudes and also observed in permafrost terrains on Earth. The polygonal cracking is believed to have resulted from seasonal freezing and thawing of surface ice.


This image, one of the first captured by NASA's Phoenix Mars Lander, shows the vast plains of the northern polar region of Mars. The flat landscape is strewn with tiny pebbles and shows polygonal cracking, a pattern seen widely in Martian high latitudes and also observed in permafrost terrains on Earth. The polygonal cracking is believed to have resulted from seasonal freezing and thawing of surface ice.


This image shows a polygonal pattern in the ground near NASA's Phoenix Mars Lander, similar in appearance to icy ground in the arctic regions of Earth.

More photos... http://www.nasa.gov/phoenix/