Washington : A technology-laden NASA spacecraft swung into orbit around Mars, passing a critical milestone in its planned two-year mission to the planet to the joy of mission controllers.

Cheers erupted Friday at mission control in Pasadena, California, as the Mars Reconnaissance Orbiter emerged from a half-hour radio silence during its flight behind the far side of the planet - the tensely awaited sign that it had reached its planned orbit.

Less than an hour earlier, the most technologically advanced craft ever sent to Mars fired its rockets to slow down from speeds that topped 16,000 km, allowing the planet's gravity to pull it into orbit. The probe would have been lost in space if the rockets had not burned long enough.

The probe is now to fly in ever closer circles around Mars until it reaches an orbit 304 km above the planet's surface, where it will begin primary data collection in six months.

The probe is to descend into lower orbit through a process NASA calls aerobreaking, meaning the spacecraft will perform hundreds of calculated dips toward the planet's surface to slow the probe. Should the probe dip down too deep, it risks overheating.

"Aerobraking is like a high-wire act in open air," said Jim Graf, project manager for the Mars Reconnaissance Orbiter.

NASA's latest Mars probe is expected to collect 10 times more data than all previous missions to the red planet combined. It is only the third probe to ever reach the Earth's closest neighbour.

NASA is particularly interested in finding any water on the planet and tracking down past water traces to help determine if Mars ever supported life. It will also serve as preparation for future Mars missions next year and a Mars laboratory in 2009.

NASA gave the total cost of the Mars Reconnaissance Orbiter mission at $720 million.

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