US space agency Nasa says its Deep Impact spacecraft is set to smash a probe into a comet on 4 July.
Deep Impact is speeding its way towards the comet Tempel 1, 83 million miles from Earth, after a 173 day journey.
Deep Impact will release a probe into the path of the comet, and then take pictures of the crash, which will make a crater as big as Rome's Coliseum.
Scientists hope they can find out what comets are made of by looking at pictures of the big bust-up.
It's thought comets haven't changed very much since the formation of the solar system.
That means the make-up of a comet could hold clues to how the solar system was made.
The impact is set to happen at around 8am on Monday.