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The Phoenix lander, due to launch to Mars in August, survived cancellation by NASA on Friday.

The lander had come under NASA鈥檚 scrutiny because it did not have adequate funds in reserve to get through its intended lifetime of about 150 days.

NASA had capped the mission鈥檚 cost at $386 million, but mission planners estimated they would exceed that limit by $31 million due to development problems and delays in selecting a landing site.

That forced NASA to conduct a 鈥榯ermination review鈥� to determine whether the mission should be allowed to proceed.

鈥淚鈥檝e been fretting about that for three months now,鈥� says Peter Smith, the mission鈥檚 principal investigator at the University of Arizona, Tucson, US.

But on Friday, NASA officials in Washington DC, US, gave the mission the go-ahead to proceed with launch preparations, as first reported by the Rocky Mountain News.

Perilous landing

Phoenix will be a stationary lander that will use a robotic arm to dig trenches near Mars鈥檚 north pole. It will examine layers of water ice beneath the surface and sniff out organic molecules.

One of the elements that has given the mission team problems is the mission鈥檚 radar altimeter, which will measure the lander鈥檚 altitude and velocity as it descends through Mars鈥檚 atmosphere.

The spacecraft inherited the altimeter 鈥� along with many other components 鈥� from NASA鈥檚 Mars Polar Lander, a spacecraft that was lost during its descent to the Martian surface in December 1999. A faulty altimeter may have been to blame.

The altimeter鈥檚 design is based on one used in F-16 fighter planes. Some software problems on the F-16 altimeter were fixed, but the altimeter for Phoenix did not get the same software upgrade.

Mission managers were concerned that the altimeter would not give the correct measurements or its signal could drop out completely, making a potential landing on Mars quite perilous.

Boulder fields

So they spent about six months fixing the device, driving up costs. Since then, an independent review team has confirmed that they have solved the problem.

A prolonged hunt for a safe landing spot has also driven up the cost of the mission. One of the early favourites for a landing site was ruled out because the agency鈥檚 Mars Reconnaissance Orbiter revealed that it was strewn with large boulders (see Boulders dash hopes for Mars landing site).

Now, the mission team has narrowed the search down to three possible landing sites, and it is set to pick a site in March. Phoenix is expected to touch down at a latitude between 65掳 and 75掳 in the northern hemisphere.

Phoenix is the first of NASA鈥檚 Scout missions, which are designed to be lower-cost ways of reaching and investigating Mars.

Mars Rovers 鈥� Mars is full of surprises; learn more in our continually updated .