Watch the launch up to first stage separation with Nasa commentary

The Delta 2 rocket left the launchpad on schedule on Sunday at 20:21 GMT (15:21 local time).

With America's Mid-west gripped by some of the worst snow storms this century, launch officials were concerned that strong thunderstorms and an approaching cold front at the time of launch gave them only a 30-40% chance of a take-off.

The Lander will test samples of the Martian soil

The launch of the Mars Polar Lander, together with the the Mars Climate Orbiter which was successfully launched last month, form the second phase of Nasa's long-term programme for the robotic exploration of Mars.

The programme began with the groundbreaking Pathfinder mission of 1996 which sent back vivid pictures of the planet's barren surface.

Mapping the planet

Three months later, Mars Global Surveyor began to map the planet from orbit.

Jean Bennett-Powell reports on Nasa's most ambitious mission yet to the Red Planet

The Polar Lander - as its name suggests - is due to touch down on the surface of Mars, near the permanent ice cap at the Red Planet's south pole.

The purpose of the mission says Edward Stone, Director of the Jet Propulsion Laboratory, is "to understand the geology of Mars and if there is still water, or at least ice, in the soil of Mars".

Scientists hope that two additional missions planned every two years will lead to a manned voyage to Mars as early as 2015.

Water is the key

Pathfinder sent back dramatic pictures

Nasa officials are keen to stress the purpose of the mission is not to look for signs of Martian life, but to look for signs of water which they say is the key.

"On Earth, wherever there is liquid water we find life," says the Jet Propulsion Laboratory's Edward Stone. "So one of the keys to searching for evidence of past or possible current life is to look for where there was water, or where there is water."

Nasa Engineer Dave Murrow talks through animation of the Polar Lander mission

Once water flowed freely on Mars. The mission's objective is to find out what happened to it.

After its 416 million mile journey, the Lander will enter the upper reaches of the Martian atmosphere in December travelling at a break-neck 6.8km per second (15,400 mph). By the time it reaches the surface, it should be travelling at a more stately 2.5 meters per second (5.4 mph).

Robotic arm

Technicians prepare the Lander for its journey

Unlike the Pathfinder probe which used large airbags to bounce its way to halt on the surface, the polar Lander will use on-board guidance systems and retro rockets to land softly on the surface.

Shortly before touch-down it will eject two mini-probes known as Deep Space 2 that will stab into the Martian surface to analyse soil samples.

The Lander itself is equipped with a robotic arm that will collect samples of soil for testing in small ovens to check for signs of water and other gases.

Edward Stone of the Jet Propulsion Laboratory: Water is the key to life

The Lander is also equipped with a microphone that, for the first time in space history, will allow scientists to hear wind and other sounds originating on another planet.