A NASA spacecraft released a probe early yesterday, setting it on a collision course with a speeding comet. Scientists hope the ambitious mission will offer the first peek inside one of these mysterious icy bodies.

Deep Impact released its barrel-sized "impactor" at 06:07 GMT on a suicide journey that is expected to climax 24 hours later when the comet Tempel 1 smashes into it. The high-speed crash should be visible from parts of the Western Hemisphere.

However, mission scientists have acknowledged the project's difficulties. Among the challenges is making sure the probe stays on course as it hurtles towards the comet without guidance from mission control. There is also the chance that the cloud of gas and dust surrounding the comet's nucleus may damage the spacecraft and probe and prevent data transmissions back to Earth. Comets contain the frozen primordial ingredients of the solar system and studying them could provide clues to how the sun and planets formed.

NASA says an impact will not significantly change the comet's orbital path around the sun, so the US$333 million experiment poses no danger to Earth.

The 372-kilogram copper probe successfully separated from the mother ship to set the stage for the collision with the comet, according to mission control at NASA's Jet Propulsion Laboratory in Pasadena. Electrical wires connecting the spacecraft broke, springing free the probe.

Workers in the mission control room at the jet propulsion laboratory erupted in applause shortly after the separation. "The release went very well," said project manager Rick Grammier. "Half of the hurdles are over."

Scientists are counting on the collision to carve a stadium-sized crater in Tempel 1, a pickle-shaped comet half the size of Manhattan now about 130 million kilometers from Earth. No explosives are needed since the energy from the impact will be similar to detonating nearly 5 tons of TNT.

It is the first attempt by the US space agency to catch a glimpse of the pristine core of a comet.

Comets are blobs of ice and dust that orbit the sun and were born about 4.5 billion years ago - nearly the same time as the solar system itself. When a cloud of gas and dust condensed to form the sun and planets, comets formed from what was left over. Studying them could shed light on how the solar system formed.

After its release, the battery-powered probe began an 800,000-kilometer plunge towards the sunlit side of Tempel 1. Meanwhile, the mother ship took its first picture of the separated probe. It then fired its thrusters to slightly change course and stake out a front-row seat 7,000 kilometers from the collision, which is expected to occur around 05:52 GMT today.

The probe will switch to autopilot two hours before the encounter, relying on computer software and thrusters to steer itself into the path of the onrushing comet. If the probe's manoeuvres are off, the comet could miss and the mission would fail.

As Tempel 1 closes in at a relative speed of 37,000 kilometer pe hour, the probe is expected to beam back unprecedented pictures of its target in near real-time until it is run over.

The mother ship will record the crash and resulting crater with its high-resolution telescope. About 15 minutes after impact, the craft will make its closest flyby of the comet nucleus, approaching within 310 miles. Scientists expect it will be bombarded with flying debris and will stop taking pictures, turning on its dust shields for protection.

(China Daily via agencies July 4, 2005)