NASA's Phoenix lander has run over budget because of problems with the radar altimeter the spacecraft will use for its landing on Mars in May 2008, although program officials are now confident that they have fixed the situation.

Drop tests conducted roughly a year ago revealed that the commercial aircraft radar altimeter chosen for the mission would not perform adequately to guide Phoenix to a safe landing in the red planet's northern latitudes. The altimeter was carried over from the failed Mars Polar Lander mission, which dated back to the "faster, better, cheaper" era of NASA mission planning.

The program formed a tiger team last summer to tackle the problem. Although it was too late to switch to another radar, the firmware and software have been revised. More recent drop tests held in October 2006 were successful, according to Phoenix Principal Investigator Peter Smith.

"We really feel like we've broken the back of this serious problem," Smith said at a meeting of NASA Mars program personnel in Arlington, Va., Jan. 9.

Review on Jan. 26

On Jan. 26, the Phoenix program will participate in a review with NASA headquarters to discuss the radar, the choice of landing site, and the latest estimates of the budget overage. Doug McCuistion, NASA's head of Mars exploration, told The DAILY he expects the overrun to be in the double-digit millions of dollars, all of which must be offset by cutting the budgets of other Mars exploration efforts.

Phoenix will dig into the martian ground with a robotic arm to analyze water in the soil. The team was focused on a particular candidate landing site until this past October, when new high-resolution imagery from the Mars Reconnaissance Orbiter (MRO) revealed that the site featured numerous large boulders.

Because the team wants to land where rocks are relatively infrequent and no taller than 30 centimeters, attention has shifted to other sites. This required some additional analysis that added a few hundred thousand dollars to the mission's price tag, McCuistion said.

During a recent thermal vacuum test, the surface coating of the spacecraft's backshell cracked, but this shouldn't delay final delivery to Kennedy Space Center, Smith said, which is scheduled for mid-May. The mission's launch window opens Aug. 3.

Scout program

Phoenix was competitively selected for funding by NASA's Scout program. The Scout program recently selected the two finalist missions that will vie for the chance to launch to Mars in 2011 - the MAVEN (Mars Atmosphere and Volatile EvolutioN) and The Great Escape (TGE) missions. The teams have received $2 million grants to further refine their proposals in anticipation of a final mission selection at the end of this year.

Since both missions are aeronomy missions, NASA will be able to forego the expense of including an aeronomy experiment on the Mars Science Orbiter, which is being planned for launch in 2013.