A year and a half after twin robot rovers thrilled space fans with their high jinks on Mars, US National Aeronautics and Space Administration (NASA) is heading there again.

A fourth Mars orbiter is set to blast off on Wednesday, carrying some of the most sophisticated science instruments ever sent into space. Circling the Red Planet, the Mars Reconnaissance Orbiter will scan the desolate surface in search of sites to land more robotic explorers in the next decade.
 
"It's time we start peeling back the onion layer and start looking at Mars from different vantage points," said project manager James Graf of the Jet Propulsion Laboratory in Pasadena.

Like the three current spacecraft flying around Mars including a European orbiter the latest probe will seek evidence of water and other signs that the planet could have hosted life. The US$720 million mission, which launches from Cape Canaveral, will also serve as a communications link to relay data to the earth.

Its powerful camera can snap the sharpest pictures yet of the planet's rust-coloured surface, with six times higher resolution than past images.

NASA took its first close-up pictures of Mars in 1965 when the Mariner 4 spacecraft zipped past the planet and snapped fewer than two dozen photos.

Since then, numerous probes that have landed, orbited or passed the planet have shot tens of thousands more images. But only about 2 per cent of the planet has been viewed at high resolution.

"There are many unanswered questions about Mars," project scientist Richard Zurek said.

The two-ton reconnaissance orbiter will be NASA's last Mars orbiter this decade. Belt-tightening forced the space agency to cancel a US$500 million mission planned for 2009.

However, two more landing attempts are set during the next four years. Scientists hope to use the orbiter's detailed mapping to scout safe landing sites for the Phoenix Mars and Mars Science Laboratory missions slated for 2007 and 2009, respectively.

The information gleaned by the spacecraft could also help scientists decide where to send a lander during the next decade to return the first samples of Martian rocks and soil to the earth.

The stationary Phoenix lander will use a long robotic arm to explore the icy plains of the planet's north pole. Later, the mobile Mars Science Laboratory will analyze rocks and soil in finer detail than the rovers Spirit and Opportunity, which have uncovered geologic evidence of past water activity since parachuting to opposite ends of Mars last year.

The solar-powered rovers are still trekking across the Martian surface, even though scientists had not expected the six-wheeled machines to last more than three months in the hostile Martian environment.

(China Daily August 9, 2005)