Nasa probe strikes Comet Tempel 1
The washing machine-sized "impactor" collided with Comet Tempel 1 at a relative speed of 37,000km/h, throwing up a huge plume of icy debris.
The probe's mothership, the Deep Impact spacecraft, watched the event from a safe distance, sending images to Earth.
"We hit it just exactly where we wanted to," said an ecstatic Dr Don Yeomans, a Nasa mission scientist.
"The impact was bigger than I expected, and bigger than most of us expected. We've got all the data we could possibly ask for."
Comets - giant "dirty snowballs", as some have called them - are believed to contain materials that have remained largely unchanged since the formation of the Solar System 4.6 billion years ago.
Scientists hope that by getting "under the skin" of Comet Tempel 1, they can gain new information on the Solar System's original composition and perhaps even how life emerged in our corner of the Universe.
"We are in the business of opening new frontiers in the exploration of space. When we analyse the data, we will have a whole new insight into the Universe."
The collision occurred at just after 0550 GMT at a distance of about 133 million km from Earth.
"Before we knew so little about the comet nucleus; we had little idea of what the surface looked like."
"It was like mosquito hitting a 747. What we've found is that the mosquito didn't splat on the surface, it's actually gone through the windscreen."
It will take a few days for all the data from Deep Impact's observations to download, and scientists will then spend several months interpreting it.