Nail-biter for NASA as Mars Phoenix Lander set to land on the Red Planet

(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 from the past or present.

"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 Mars Phoenix Lander was wrapping up its 296-day, 422-million-mile journey, 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.