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NASA鈥檚 Phoenix spacecraft will experience a harrowing few minutes on 25 May when it hits Mars鈥檚 atmosphere and attempts to land safely on the surface 鈥� without any airbags to cushion its fall.

Phoenix launched in August 2007 on a mission to Mars鈥檚 icy north polar region. Changes in the Red Planet鈥檚 tilt may have allowed the abundant ice there to melt as recently as 100,000 years ago, raising the tantalising possibility that microscopic life forms could once have eked out an existence in the region. Life might even be present there now in a dormant state.

The lander will dig down as much as 50 centimetres below the surface, collecting samples of soil and ice to better understand the region鈥檚 past climate and check for carbon-containing molecules that could be associated with life.

But first, the spacecraft must make it to the surface in one piece. A host of critical manoeuvres have to go off just as planned in order to get the spacecraft safely to the surface, all of them designed to occur automatically. If it is successful, it will be the first probe since the Viking missions more than 30 years ago to land safely without airbags.

鈥淭his is no trip to grandma鈥檚 for the weekend 鈥� there are many, many risks and uncertainties,鈥� says Ed Weiler, chief of NASA鈥檚 science mission directorate at the agency鈥檚 headquarters in Washington, DC, US. 鈥淏ut as someone once said a long time ago about NASA, we do these things not because they are easy but because they are hard, and the scientific payoff will be well worth the risk.鈥�

Critical timing

If all goes according to plan, about 14 minutes prior to landing, the probe will swivel around in space so its heat shield faces forward. Five minutes later, the spacecraft will hit Mars鈥檚 atmosphere at more than 20,000 kilometres per hour, causing the shield to heat up to more than 1400 掳C.

In another 4 minutes, with Phoenix about 13 kilometres above the Martian surface, a parachute will deploy to decelerate the spacecraft. The timing of this is critical 鈥� if it occurs too early, aerodynamic forces will rip the parachute to shreds.

A few seconds later, the heat shield will be jettisoned, the spacecraft will extend its legs, and it will begin using radar to measure its altitude and speed.

Rocky hazard

With 43 seconds left to landing, the spacecraft will have slowed down to 200 kilometres per hour. With the ground 1 kilometre below it, the probe will cut loose its parachute, relying on thrusters to slow it down further.

It must also manoeuvre away from its parachute to prevent the chute from falling on top of Phoenix after landing.

Rocks are the biggest hazard for the actual touchdown, which will occur at a speed of 9 kilometres per hour. Anything more than about 40 centimetres tall could punch through Phoenix鈥檚 underside and cause damage, says the mission鈥檚 chief scientist, Peter Smith of the University of Arizona in Tucson, US.

Fortunately, team members estimate that there are not many 鈥渄angerous鈥� rocks at the landing site, nicknamed Green Valley. That estimate is based on counts of the smallest rocks 鈥� spanning 1 metre 鈥� that NASA鈥檚 Mars Reconnaissance Orbiter can see.

Unfolding panels

If all goes well, the Phoenix team expects a signal confirming Phoenix鈥檚 safe landing to arrive at the Jet Propulsion Laboratory in Pasadena, California, on 25 May at 1653 PDT (26 May at 0053 GMT).

Assuming the spacecraft touches down without incident, the next worry is whether any big rocks lie nearby. Rocks taller than 60 centimetres in the spacecraft鈥檚 immediate vicinity could prevent the spacecraft鈥檚 solar panels 鈥� the spacecraft鈥檚 only source of power 鈥� from unfolding properly.

Another issue is whether the lander legs end up on top of any rocks. That would tilt the lander and alter the amount of solar power it can collect. 鈥淚f it tilts towards the south, it might actually improve the power situation,鈥� Smith told New 杏吧原创. 鈥淚f it鈥檚 towards the north, it might degrade the power.鈥�

Icy shell

If it can begin operating, it should beam back images right away. 鈥淲e should have pictures the first day we land,鈥� Smith says. 鈥淲e don鈥檛 expect a lot, but even one picture tells a pretty good story. And the next day we get back our first panorama.鈥�

Phoenix will land in springtime in Mars鈥檚 northern hemisphere, when sunlight is abundant and temperatures are relatively high.

After 25 June, the Sun will start getting lower in the sky from Phoenix鈥檚 location. This will reduce solar power for the spacecraft, and eventually it will have too little to keep itself from freezing. The mission is supposed to last for 90 days.

Ultimately, the plunging temperatures during winter will cause carbon dioxide to freeze out of the atmosphere, entombing Phoenix in a seasonal layer deep enough to completely bury the lander.

Astrobiology 鈥� Learn more in our out-of-this-world .

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