WASHINGTON, USA - NASA was gearing up on Saturday for what it called 'the scariest seven minutes of the mission' as it prepared to land the Phoenix spacecraft on Mars' frigid north pole.

After traveling 679 million kilometres through the cosmos, on Sunday the probe will have that amount of time to decelerate from 21,000kmh to a mere float to manage a safe touchdown on the Red Planet's arctic region.

With the nearly five decades of Mars exploration fraught with failures - about half of the three-dozen tries has crashed, disappeared or missed the planet altogether - there is little room for error.

'This is not a trip to grandma's house. Putting a spacecraft safely on Mars is hard and risky,' said Mr Ed Weiler, associate administrator for Nasa's Science Mission Directorate.

Given the long distance, the US space agency's Jet Propulsion Laboratory (JPL) in Pasadena, California, which controls the mission, will have to wait an agonising 15 minutes for the radio signal confirming the safe landing to reach Earth.

Nasa's US$420 million (S$570 million) probe will become the first spacecraft to land on the Martian arctic surface, digging into the polar ice in a new three-month mission searching for signs of life on Mars.

Phoenix will enter the top of the Martian atmosphere at around 2331 GMT, use a thermal shield to slow its entry into the atmosphere and then deploy a parachute to reduce its speed.

It will then fire up its thrusters to slow to eight kph and land on its three legs on the circumpolar region known as Vastitas Borealis - akin to northern Canada in Earth's latitude.

'I'm sure as Sunday gets nearer, we'll start to get more and more nervous.

We all understand that landing safely on Mars is one of the most challenging parts of our mission,' Phoenix Project Scientist Leslie Tamppari said on Thursday.

'We do believe that it's a risk worth taking,' said mr Fuk Li, manager of Nasa's Mars Exploration programme, 'because I think that the science the mission will return with will be outstanding and we will open up a new chapter on how we understand Mars to be.'

One minute after Phoenix confirms arrival, its radio will go silent for 20 minutes to save its batteries before deploying its two solar antennas.

The first images from Phoenix will reach Earth two hours later.

'The Phoenix mission not only studies the northern permafrost region, but takes the next step in Mars exploration by determining whether this region, which may encompass as much as 25 percent of the Martian surface, is habitable,' for future manned missions to Mars, said Mr Peter Smith, Phoenix principal investigator at the University of Arizona.

The probe will work under harsh conditions with temperatures ranging between minus 73 degrees Celsius to minus 33 degrees.

Nasa wants to assess whether the Martian arctic ever had conditions favorable to microbial life. The probe will also help determine if a primitive life form was ever or is still present on Mars.

'The Arctic region is a place where there is a lot of things we are learning about the Earth,' said Smith.

'One is, the climate change for our planet is written into the ices in the Arctic region on the Earth, where the history of life (microbes, molecules and cells) is preserved in its purest form.' Researchers 'are wondering if this is true on Mars,' he said.

Phoenix is equipped with a camera and a 2.35-metre robotic arm that can dig as deep as one meter to find ice and heat up samples to detect carbon and hydrogen molecules, essential elements of life.

With its two solar panels unfurled, Phoenix is five metres wide and 1.52 metres long. It weighs 350kg, including 25kg of scientific instruments.