London, Nov 3 : Scientists have selected about half a dozen landing sites for NASA's Mars Science Laboratory (MSL) rover, which is due to be launched by the end of 2009. The rover, the largest ever to travel to Mars, will try to find out whether the red planet was once conducive to life.

The rover should touch down on Mars in October 2010. It will study the make-up of organic compounds, and how Martian rocks and soil have evolved over time. It will also explore the role of water in the planet's history and measure the radiation hazard from the Sun and energetic charged particles from the galaxy called cosmic rays.

"In a nutshell, MSL is going after the question of habitability on Mars," said John Grant, a geologist at the National Air and Space Museum in Washington, DC.

Scientists have considered more than 50 possible landing sites for the rover. At a workshop last week in Pasadena, California, they narrowed them down to six. All six sites appear to have clay minerals, which scientists think must have formed in prolonged contact with liquid water.

The six favoured sites are Mawrth Vallis, Nili Fossae, Jezero Crater, Southwest Meridiani, Holden Crater and Terby Crater.

But both Holden and Terby craters, which lie in the southern hemisphere, could turn out to be unsuitable because the rover would arrive during the local winter. As a result, it might have to hibernate during the coldest spells because it couldn't function properly.

According to Grant, it's not clear yet how big a problem this would be. "If operations were reduced by 50 per cent, for instance, that would be a real hit," he said. "But those things are being worked on very hard as we speak and we'll have a much better sense in the coming couple of months," he added.

But there might be other problems as well. "Because the ideal site has to be flat and not prone to high winds, the spacecraft can't land effectively on a very steep incline," said Grant. "The numbers and sizes of rocks on the surface are also issues. You don't want the rover to land on a very large rock sticking up a metre above the surface," he added.

Scientists will spend the next nine months studying images from NASA's Mars Reconnaissance Orbiter to find out more about the six sites. Engineers will also examine which sites are safest to land on. In case all the six prime sites turn out to be unsuitable, the teams will also study four other "purgatory" sites.

Scientists expects that another workshop in six to nine months will narrow the landing site down to a certain latitude range, then a single site will be selected by October 2008, a year before launch. (ANI)