US, British and European scientists are training their sights for an unprecedented demonstration of long-distance marksmanship: a direct hit on a comet 133 million km away, moving at a speed of 37,000kph.
At precisely 6:52am tomorrow, a copper bullet the size of a washing machine dropped from the US space agency NASA's spacecraft Deep Impact will collide with a billion-tonne lump of ice, dust and chemicals called Tempel 1.
The smash, at 10km a second, should blast a hole at least as big as a house, and popnel will fly out into space to be recorded by Deep Impact, programmed to pass within 500km of the cosmic drive-by shooting. For Americans, it will be the ultimate July 4 fireworks display.
The drama will be monitored by the Hubble telescope, other spacecraft, a battery of Earth-based optical telescopes from Arizona to Hawaii, and by thousands of amateurs, for whom Tempel 1 is little more than a faint dot in the night sky.
The collision will be the first probe into the interior of a comet, a ghostly lump of builder's rubble left over from the making of the solar system 4.5 billion years ago. The eruption of dust and ice could reveal some of the secrets of the planets and the Earth's oceans, and details of the original organic chemistry from which life must have been formed.
But the operation will be a tricky one.
"We really are threading the needle with this one," said Rick Grammier, the mission's project manager at NASA's Jet Propulsion Laboratory in Pasadena.
"In our quest of a great scientific payoff, we are attempting something never done before, at speeds and distances that are truly out of this world," he said.