Spacecraft closes in on Mars landing


A space probe designed to make the first direct measurements of water on another planet headed into its final approach to Mars today, bolstering hopes of learning if Earth's neighbour has the right chemistry for life.

Before the spacecraft, named Phoenix, can dig up ice samples to analyze, it must touch down safely, a prospect that is giving flight control teams and scientists a bad case of nerves. Landing is expected at 11:53 pm Irish time.

"This is not an easy thing to do, I have to say. We bet the whole farm on this safe landing and we can't do our science without the safe landing," lead scientist Peter Smith told reporters at NASA's Jet Propulsion Laboratory in Pasadena, which oversees the mission.

"I'm nervous," added Joe Gunn, Phoenix mission manager. "I'm getting a real case of heebie-jeebies now."

The United States has successfully landed five of six probes on Mars, but historically 55 per cent of all Mars missions have failed.

Phoenix has had a relatively smooth ride since it was launched into space on Aug. 4, 2007, but its most challenging tasks lie ahead. After traveling 420 million miles, Phoenix has to transform from a spacecraft speeding along at 12,600 mph to a freestanding science station on the northern polar region of Mars.

The transition must happen fast and perfectly. Fourteen minutes before touchdown, Phoenix must shed key components used during its cruise through space, including its power and communications systems, as it prepares to blast through Mars' atmosphere.

Seven minutes before touchdown, the probe reaches the atmosphere, where friction will begin slowing it down. During the descent, a shield will absorb most of the heat, generating enough energy in the process to power a city the size of Topeka, Kansas, for seven minutes.

When Phoenix reaches about 1,100 mph, which in Mars' atmosphere is about 1.5 times the speed of sound, its parachute must deploy. In quick order, the heat shield will be jettisoned, three folded landing legs will open and the spacecraft's backshell must fall away.

Finally, 12 small thruster rockets will pulse to slow Phoenix to about 5 mph and guide it gently to the surface.

Flight controllers, 170 million miles away, will be helpless to do anything if Phoenix runs into trouble. At that distance, radio signals traveling at the speed of light take 15 minutes to reach Earth.

"There's no second chance," said Smith. "That's our fate."