Extreme Mars challenge: Entry, descent and landing
Scientists are preparing for "seven minutes of terror" as a Nasa spacecraft makes a nail-biting descent to the surface of Mars.
The Phoenix lander will begin its plunge through the Martian atmosphere on 25 May (GMT) as it attempts to land in the planet's polar north.
The craft needs to perform a series of challenging manoeuvres along the way.
It then begins a three-month mission to investigate Mars' geological history and potential habitability.
We think we've found a place that's scientifically interesting and where we have a good probability of landing successfully
Ray Arvidson, Washington University in St Louis
Water is crucial to the mission's objectives. Not only is it a pre-requisite for biology, but it has shaped the planet's geology and climate over billions of years.
Phoenix will touch down on the northern plains, which hold vast stores of water-ice just below ground.
The lander will use a 2.4m robotic arm to dig through the protective topsoil layer to this water-ice below; a scoop on the arm will lift samples of both soil and ice to the lander's deck for detailed scientific analysis.
However, much of the water-ice is thought to be frozen as hard as paving stones.
"We are looking at active processes that are taking place today," said the mission's chief scientist Peter Smith, from the University of Arizona.
"These active processes have to do with the expansion and contraction of the ice during seasonal changes."
When the planet has tilted on its axis in the past, the poles have received lots of sunshine. This could have turned water in these northern regions from ice into liquid, leaving a signature that can be detected today.
"Our highest goal is to find out whether this created a habitable zone on Mars where we might find organic materials, where we might find - periodically - the presence of liquid water, and where we might find chemical energy sources."
The robotic arm will dig below the surface
But Phoenix must first make the perilous journey to the surface.
The spacecraft will enter the top of the Martian atmosphere at almost 5.7km/s (13,000mph).
Pushing hard against the Martian air, its descent will begin to slow. A parachute will then be deployed to reduce the rate of fall still further. Finally, Phoenix will fire thrusters to bring its velocity down to about 2.4m/s (5.4mph) before its three legs touch the ground.
"We fire 26 pyrotechnic events in the last 14 minutes of the journey. Each of those has to work perfectly for the mission to come off as we planned," said Barry Goldstein, project manager from Nasa's Jet Propulsion Laboratory in Pasadena, US.
The final seven minutes to the ground should prove to be the most dramatic. Confirmation of a successful touchdown could come as early as 0053 BST (1953 EDT).
Landing on Mars is a notoriously tricky business. Nasa's new associate administrator for science Ed Weiler summed it up with a simple statistic: "Fifty-five percent of all human attempts to land robots on Mars have failed," he told journalists at a news conference in Washington DC.
Although we have fixed all the known issues with the Mars Polar Lander, there are always the unknown unknowns
Ed Weiler, Nasa
The US has a slightly better record, with five successes out of six. But the high overall failure rate has prompted scientists to talk jokingly of a "Mars Curse".
Rocks large enough to spoil the landing or prevent opening of the spacecraft's solar panels are the most dangerous hazards on the surface.
Ray Arvidson, from Washington University in St Louis, Missouri, chaired the mission's landing site working group.
He explained: "We think we've found a place that's scientifically interesting and where we have a good probability of landing successfully - that is, it is relatively safe."
In addition to having relatively few rocks, slopes in the landing area were all less than five degrees, said Professor Arvidson.
"Those are concerns we can put to bed - to the extent that you can put anything to bed on Mars," he added.
Phoenix is an apt name for the current mission, as it rose from the ashes of two previous failures.
In September 1999, the Mars Climate Orbiter spacecraft crashed into the Red Planet following a navigation error caused when technicians mixed up "English" (imperial) and metric units.
A few months later, another Nasa spacecraft, the Mars Polar Lander (MPL), was lost near the planet's South Pole.
Phoenix uses hardware from an identical twin of MPL, the Mars Surveyor 2001 Lander, which was cancelled following the two consecutive failures.
"The team has done everything possible to ensure a successful landing in a few weeks. But remember: although we have fixed all the known issues with the Mars Polar Lander, there are always the unknown unknowns," said Ed Weiler.
"[The Viking programme] was the last time we used retro-propulsion and landing legs. So it has been over 32 years since Nasa has been successful with such a landing."
He added: "I hope I have convinced you this is no trip to grandma's for the weekend."
Phoenix launched on 4 August 2007 on a Delta II rocket from Cape Canaveral Air Force Station in Florida.
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