The main idea is to see what it is made of – but a successful strike will feed into planning for saving the Earth from fatal collisions.
The hardware for the Deep Impact mission will be lifted off from Cape Canaveral at 13.47 local time today (18.47 GMT).
Then, attention will switch to European preparations to get pictures back from Titan, a mysterious moon of Saturn, on Friday. Until now, the farthest-flung landing man has attempted was on Venus. Saturn is five times further away and the journey has taken seven years.
Both missions are designed to add to understanding of the beginnings of our universe, four-and-a-half billion years ago. The comet chosen for Deep Impact, Tempel 1, was probably formed about then, from ice and dust, and should be  relatively unchanged inside.
Titan is thought to have formed around the same time, in much the same way as Earth. But it is permanently hidden by cloud, so it is the most mysterious large object in our solar system.
The interplanetary craft Cassini cut its ties with the Titan lander Huygens – both named after astronomers who specialised in Saturn – on Christmas Day. Since then, the Huygens team, led by John Zarnecki of Britain's Open University, has only been able to pray that it is on course, because its transmitters are shut down, to conserve battery power.
On Friday morning, just before hitting the surface of Titan, it will come to life and start sending readings and pictures back to Cassini. It will be another two hours or more before Cassini is in a position to send the information back to Earth.
Even at the speed of light, it will take 67 minutes for the signals to travel the distance, so it will be Friday evening before the scientists get the messages they have been looking forward to for years.
They are hoping to be told Titan has an ocean of condensed gases. If it has, Huygens is quite likely to disappear under a giant wave. But Professor Zarnecki has said his attitude will be: "A great way to go!"
The discovery of an ocean, added to the readings just before landing, would justify the £250m cost of Huygens as far as scientists are concerned. But if they are lucky, the little machine will carry on transmitting for another two hours.
Meanwhile, the Deep Impact journey will be just beginning. It will continue until early July, when the main ship lobs a sort of smart cannonball weighing a third of a tonne (370 kilos) into the path of Tempel 1.
The projectile has its own guidance system and will keep itself centred in the path of the comet until it is run into, at 23,000 miles an hour (38,000 kph), while the mother ship photographs the collision and analyses the debris and gases thrown off.
The comet is 2.5 miles across and seven miles (11.6 km) long and the collision will have the energy of 4.5 tonnes of TNT.
The sun will reflect off the debris, causing a flash which might be visible to the naked eye on Earth, 83 million miles away, and will be watched by thousands of professional and amateur telescopes.
Even so, according to Nasa, it will have about as much impact on the course of the comet as a mosquito hitting a Boeing 747. But measurements will be taken, all the same.
A Nasa scientist said: "If we ever have to deflect a comet, this is the kind of observation that would be needed."
Kevin Yates, a spokesman for the UK's Near Earth Objects Information Centre, which watches for threats from space objects, agreed: "In this case, the impact is probably not going to alter the orbit of the comet drastically. But there will be interest in any effect it does have.
"An attempt to deflect a comet or asteroid is already in the planning stage. But the main interest is in the interior composition of Tempel 1."
chris.benfield@ypn.co.uk