Shortly before 7am, a copper and aluminium "torpedo" the size of a washing machine smashed into the icy surface of a comet in a spectacular flash of white light.
The projectile had earlier been fired from the American space agency Nasa's Deep Impact spacecraft into the path of comet Tempel 1, which is hurtling through space at 23,000 miles per hour.
Mission controllers at Nasa's Jet Propulsion Laboratory in Pasadena, California, clapped and cheered as the first images of the collision were beamed back to Earth.
Scientists hope the hole blasted in the comet, which maybe the size of a football pitch, will reveal information about the origins of the solar system never obtained before.
The mission, which involved hitting a target less than 3.7 miles wide from a distance of 537,000 miles, appears to have exceeded all expectations.
British scientists watching via a live link to a conference room in London announced that Deep Impact had been an unqualified success.
They watched spellbound as a camera on the impactor showed the approaching white surface of the comet and revealed fine details such as craters never observed before.
After the impact, the mother craft sent back dramatic pictures of an enormous flare expanding outwards from the bottom of the comet.
The explosion, equivalent to the energy released by 4.8 tonnes of TNT, could also be seen by Earth-based telescopes, including a number manned by British astronomers.
The blast was so bright that scientists expect it to be visible to the naked eye from Earth.
Just after sunset tonight it should appear as a faint new star low in the south-west to the left of the planet Jupiter.
Dr Andrew Coates, from the Mullard Space Laboratory at University College London, said: "This is one of the most audacious experiments that's been undertaken ever.
"It's a fantastic day for cometry exploration. In terms of historical events, this is the first large-scale experiment since Apollo on a solar system object."
Colleague Professor Iwan Williams, from Queen Mary College, London, said: "What we've seen so far is absolutely fantastic.
"It's obviously very, very big - much bigger than any of us expected the plume to be. Much more material has been thrown out than we expected."
The white flare is believed to be caused by fragments of icy debris caught in sunlight.
It will be some time before scientists know precisely what damage the impactor has inflicted on the comet.
Much depends on the object's composition. Early indications are that the comet's outer crust is unexpectedly fragile.
"The fact that we've seen a brighter than expected flash may be an indication that the crust is weaker than we expected and the impactor has gone down inside the comet," said Dr Coates.
Scientists have compared the event to a "mosquito hitting a 747 passenger jet".
They say there is no risk of Tempel 1 being deflected and endangering the Earth.
But data from the mission may help scientists work out the best way to stop a comet that really does pose a threat.
Comets contain material that has remained unchanged since the formation of the solar system 4.6 billion years ago.
They are also thought to have delivered water to the Earth, as well as complex organic molecules that may have helped trigger life.
Monica Grady, professor of planetary and space sciences at the Open University, said: "We've seen pictures of the outside of comets before but this is the first time that we're going to actually look inside a comet.
"We're going to see the pristine material that formed the solar system 4.6 billion years ago and that's never been seen before.
"If we want to understand how the solar system was put together, we have to understand these building blocks, and the building blocks of life are part of comets."
She said one of the most exciting episodes in the mission was the sight of Tempel 1 looming closer and closer as the impactor homed in on its target.
"We saw some really amazing images," she said. "OK, it was a grey, fuzzy blob, but you could see there were craters on the surface and that's absolutely fantastic. We had no real idea of what the surface would look like."
Professor Keith Mason, director of the Mallard Space Science Laboratory, said: "Never before have we penetrated the nucleus of a comet. The resulting data should provide us with the most comprehensive set of scientific measurements ever obtained of a comet - unprecedented information on the genesis of our solar system."
The Deep Impact spacecraft was launched on January 13 from Cape Canaveral, Florida, and has travelled a distance of 268 million miles.
To capture images and analyse the composition of the comet, it carried a high resolution camera and infra-red spectrometer.
UK-operated telescopes in Hawaii, Australia and the Canary Islands are also observing and studying the event, and transmitting images to centres across Britain.
Children at King's School, Canterbury, are being given the chance to operate the Faulkes robot telescope at Maui, Hawaii, and process the raw data.
The blast is also being monitored by the European Space Agency's Rosetta spacecraft, now on a 10-year voyage to another comet, Churyumov-Gerasimenko. It will aim to land a probe on the comet in 2014.
Prof Williams, one of the Rosetta scientists, said: "At a distance of some 50 million miles from Tempel 1, Rosetta will be in the most privileged position to observe the event from space.
"Rosetta's instruments will measure the composition of the crater and its ejected material - a cloud of dust and gas that is expected to expand and reach its maximum brightness about 10 hours after impact."
Tempel 1, which is about half the size of Manhattan Island in New York, was first spotted in 1867 by the German astronomer Ernst Wilhelm Tempel from an observatory in Marseilles, France.
It orbits the sun every five and a half years. Like other comets, Tempel 1 is effectively a dirty snowball of ice and dust.
Space expert and broadcaster Dr Heather Couper, who watched the live images, said: "I am delighted. It was a bullseye - a fantastic effort by the navigation team."
She revealed that children from the Kings School obtained the first pictures of the event from an Earth-based telescope.
"It's also a fantastic success from a British angle, in that the first images of the impact visible from the ground were seen on the Faulkes telescope in Hawaii, basically operated by British schoolchildren," she said.
"That's very exciting. This is just the beginning of what is going to unfold itself as really a search for our origins."Reuse content