Probe heads for comet collision
By Paul Rincon
BBC News science reporter
The washing machine-sized 372kg "impactor" should smash into Comet Tempel at 0552 GMT on Monday morning.
By studying the comet's interior, scientists aim to learn more about the formation of our Solar System and the chemical building blocks of life.
Its mission will be short-lived but is certain to be spectacular.
Spacecraft instruments and ground-based telescopes will record the event, as a hole the size of a football stadium is blasted in the comet.
The cosmic fireworks are scheduled for 0552 GMT on 4 July - American Independence Day.
Deep Impact, the spacecraft which carries and ejects the impactor will take pictures and gather data from the collision and its aftermath, 133m kilometres (83m miles) from Earth.
Comets are porous balls of ice and rock hailing from the frigid outer boundaries of our Solar System. Periodically, some journey inwards, looping around the Sun.
Like other comets, Tempel 1 contains "pristine" material unchanged since the Solar System formed. This is hidden beneath an outer crust.
"These materials have not seen the light of day for 4.6bn years," mission scientist Jessica Sunshine told reporters at a news conference in Pasadena, California. But scientists still know very little about the composition of this material.
"Like any good geologist would, we want to hit it (Tempel 1) with a hammer and see what's inside," Dr Sunshine added.
The impactor itself will shatter as the comet crashes into it at 37,000 km/h (23,000 mph), but an onboard camera will record the approach in the last minutes before collision.
Cometary impacts early in Earth history are thought to have first brought water to our planet.
They might also have seeded it with the chemical building blocks required for life. "We want to find out what those materials were," explained Rick Grammier, Deep Impact project manager.
Mr Grammier likened the mission to "a bullet trying to hit another bullet with a third bullet in the right place at the right time". Despite the complexities, team members are confident they won't miss.
Mission manager Dave Spencer said: "We've got good navigation, a good idea of the comet's trajectory and we've been correcting for that all along.
"We're well prepared to position ourselves for this train that's going to run over us."
The impact is expected to excavate a crater more than 25m deep and 100m across, ejecting ice, dust and gas and exposing pristine material.
The Hubble, Chandra, Spitzer and XMM Newton space observatories will be trained on Tempel 1 for the collision, as will ground-based telescopes worldwide.
The mission could inform strategies for deflecting a comet, should one threaten to strike Earth.
"If you are worried about defending Earth from possible impactors, it's a whole lot easier to change the course of something if you know what it is you're changing the course of," said Dr Mike A'Hearn, Deep Impact principal investigator.
This week, astronomers got a preview of the pyrotechnics to come when Tempel 1's nucleus released a short-lived blizzard of particles and gas.
These outbursts seem to occur as comets heat up approaching the Sun.