US: Scientists will tomorrow attempt to blast a hole in a distant comet using a 39-inch wide projectile fired from a spacecraft.
If it succeeds, the Deep Impact mission will be a technological triumph for mission controllers at the US space agency, Nasa.
The aim is to smash out a chunk of the icy surface of comet Tempel 1, 133 million miles from earth, to see what lies beneath.
By analysing the comet's interior and the cloud of dust and ice thrown out by the explosion, scientists hope to answer fundamental questions about the formation of the solar system.
But hitting a target less than 3.7 miles wide from a distance of 537,000 miles is no easy feat.
Controllers will steer both spacecraft and impactor to the comet from their mission base at Nasa's Jet Propulsion Laboratory in Pasadena, California.
At precisely the right time, they will release the 820-pound copper and aluminium projectile into the path of the comet, which is hurtling through space at 40,000km/h (23,000mph).
An autonomous computer on board the impactor will take over navigating two hours before the collision, making last minute corrections by firing thrusters around the rim of the craft.
"We are really threading the needle with this one," said Rick Grammier, Deep Impact project manager at JPL.
"In our quest for a great scientific payoff, we are attempting something never done before at speeds and distances that are truly out of this world."
The projectile and Tempel 1 will come together in spectacular fashion at about 6.52am tomorrow. Scientists expect to see a blast equivalent to exploding 4.8 tons of TNT.
The crater left by the impact could range in size from a large house to a football stadium, depending on the comet's composition. It might be anything from two storeys deep to the height of a 14-storey office block.
The Deep Impact probe will have about 13 minutes in which to take images and analyse the crater before it risks being struck by a blizzard of ejected particles.
Ground based telescopes around the world, including some operated by British astronomers, will turn their lenses towards Tempel 1 to witness the event.
The two-metre robotic Faulkes telescope in Hawaii will transmit images to centres across the UK, including the Thinktank in Birmingham, the Armagh Planetarium in Northern Ireland, the Royal Observatory in Edinburgh, and the Spaceguard Centre near Knighton in Wales.
Schoolchildren across Britain will have the opportunity to see the images from Tempel 1 unfold on computers.
Deep Impact principal investigator Dr Michael Ahearn, from the University of Maryland in College Park, US, said: "The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science.
"With the information we receive after the impact, it will be a whole new ball game. we know so little about the structure of cometary nuclei that almost every moment we expect to learn something new."
The Deep Impact spacecraft was launched on January 13th from Cape Canaveral, Florida, and has travelled a distance of 431 million kilometres (268m miles).