The US Deep Impact spacecraft is on course to fire a probe into a comet on 4 July, scientists have announced.
Deep Impact will punch a crater in Comet Tempel 1
The missile, travelling at 100 times the speed of a bullet, is expected to blast a hole in the heart of the space rock, spraying out ice and dust.
Analysing this primordial material, which was around at the time the Solar System formed, could shed light on the origin of the planets.
The collision is set for 0552 GMT on Independence Day in America.
About a day before, the mothership will release a small copper probe, which will steer itself towards Comet Tempel 1, a ball of ice, rock and dust the size of Manhattan.
Cameras on the projectile and the mothership will record the event, and instruments on Deep Impact will analyse the gases and debris ejected.
'Out of this world'
Rick Grammier, Deep Impact project manager at the US space agency Nasa's Jet Propulsion Laboratory in Pasadena, California, called the manoeuvre "extremely challenging".
"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.
"It's a bullet trying to hit a second bullet with a third bullet, in the right place at the right time, watching the first two bullets and gathering the scientific data from that impact," he told a news briefing in Washington on Thursday.
Mission scientists said they expected to be able to solve a technical glitch that has caused the spacecraft to return blurry images from one of its instruments.
They will use a mathematical process on the images it captures after they have been transmitted to Earth.
Deep Impact has taken six months to travel some 431 million km (268 million miles) from Earth.
The mission shares a title with the 1998 Hollywood film in which astronauts attempt to stop a comet hitting the Earth.
Deep Impact aims to look under the surface of a comet, at material that has remained untouched since the birth of the planets.