By Molly Bentley BBC News Online, in San Francisco

Sometime this week, in an airtight chamber, Mars Rover 2 will undergo a stress test.

You'll see a view of Mars fundamentally different from anything you've seen before Dr Steven Squyres

It is already past the "shake and bake" trials that simulate the vibrations and extremes of temperature the vehicle will experience en route to the Red Planet.

Now comes the deep-freeze space simulator.

"We fill it with nitrogen to simulate the Martian atmosphere and cool it to Martian temperatures," said Steven Squyres, the principal investigator for the US space agency's (Nasa) Mars Exploration Rover (MER) Mission.

"Then we make the rover do all it's supposed to do," such as unfold itself, roll and extend its robotic arm.

With launch dates just six months away, Nasa's science team is making final preparations to send two rovers into space in an effort to understand the past environment of Mars.

Landing decision

The twin robots, each with a suite of sophisticated computers, cameras and telecommunications equipment, are decked out like field geologists to explore the Red Planet.

The rovers are tested to the limit inside chambers

The goal of the mission is to look for environmental conditions suitable for life, rather than evidence of life itself, said Dr Squyres at the American Geophysical Union (AGU) conference in San Francisco.

As all life depends on water, finding life-friendly habitats means looking for traces of water in the Martian minerals and rocks.

Choosing water friendly areas on Mars is the topic of the upcoming - and nearly the last - workshop designed to select landing sites for the two rovers.

Next month, nearly 100 scientists will gather in Pasadena to weigh the merits of a list narrowed to four.

Lessons learned

The finalists were chosen after scientists eliminated high-elevation areas with atmosphere insufficient to slow a parachute, and ruled out latitudes without enough sunshine to power the rovers' solar panels.

Sojourner: A leap forward from the Pathfinder mission

That left 5% of the planet. Of the sites proposed, Meridiani, Gusev and Isidis show surface evidence of processes involving water. Elysium was chosen for its low wind conditions.

"We want a site near areas favourable to the preservation of evidence of biological processes," said Matthew Golombeck, Mars program landing site scientist at the Jet Propulsion Laboratory (JPL). But in the end, "a safe landing is key".

It has been five years since MER's predecessor landed on Mars. The Pathfinder mission was important in validating the technology - in particular the landing techniques and the operation of a roving robot.

The success paved the way for MER's advanced field mission.

Martian tortoise

"This is a big, big leap from Pathfinder," said Dr Squyres. While Sojourner depended on the scientific equipment in its lander, the rovers carry all the instruments with them. Once the rover lands and unfolds itself, the landing pod is left as "scrap metal," said Mark Adler, MER mission manager

Landing sites still need to be decided

Only the landings will be similar. A Pathfinder-like parachute will help slow the final stages of the descent, before airbags are popped to cushion the impact. Once it hits, the lander will bounce like a crazed beach ball and perhaps roll another kilometre before stopping.

Once the lander opens, the rover unfolds itself in a "trick of reverse origami," according to Dr Adler. Then the robot is off and running - sort of.

"Pedal to the metal, it covers 5-6 centimetres a second," said Dr Squyres, who likened it to the velocity of a Galapagos tortoise.

Yet the twin rovers will cover more ground in a day - 100 meters - than Sojourner did in its entire mission. And the rovers are designed with autonomous capabilities. Once Earth transmits their daily assignments, they fulfil them on their own.

Web posting

Nasa has designed rovers that can thread their way through a Martian obstacle course.

"The rover looks at the terrain ahead of it. If it's flat, it drives forward," said Dr Squyres. "If it sees a big scary rock, it will stop and turn."

Once the rover reaches its target, the rock abrasion tool, or Rat, will be used to expose fresh rock for analysis. Then the cameras will zoom in.

One pair of the nine on board has sharper visual acuity than the human eye - the resulting images will have more than three times the resolution of anything yet seen on the planet.

"You'll see a view of Mars fundamentally different from anything you've seen before," said Dr Squyres.

The first rover launch is scheduled for 30 May with an arrival date of 4 January, 2004. Rover 2 will follow a month later.

By then, they will have proper names - from contest entries submitted by children. And public participation will continue after the landing. Nasa says it plans to post as much real-time data on the web as possible.