By Paul Rincon
BBC News science reporter
Nasa scientists are preparing for the impact of a probe they have launched into the path of a comet in a bid to uncover secrets of the Solar System.
A washing machine-sized 372kg (820lbs) "impactor" is due to smash into Comet Tempel at 0552 GMT on Monday.
The aim is to blast out material frozen inside the comet since the Solar System was formed 4.6 billion years ago.
Scientists hope to get new insights into the Solar System's composition and the chemical building blocks of life.
The impactor itself will shatter as the comet crashes into it at a relative velocity of 37,000 km/h (23,000 mph), but an onboard camera will record the approach in the last minutes before collision.
Deep Impact, the spacecraft which carried and ejected the impactor, will take pictures and gather data from the collision and its aftermath, 133 million kilometres (83 million miles) from Earth.
The Hubble, Chandra, Spitzer and XMM Newton space observatories will be trained on Tempel 1 for the collision, as will ground-based telescopes worldwide.
'Bullet hitting bullets'
The impact is expected to excavate a crater more than 25m (80ft) deep and 100m (330ft) across, ejecting ice, dust and gas and exposing "pristine" material.
Rick Grammier, Deep Impact project manager, likened the mission to "a bullet trying to hit another bullet with a third bullet in the right place at the right time".
But despite the complexities, team members are confident they will not miss.
"The spacecraft are perfectly healthy and where they are supposed to be," Andy Dantzler, head of Nasa's solar system program, told a news conference on Sunday.
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 4.6 billion years," mission scientist Jessica Sunshine said.
"Like any good geologist would, we want to hit it (Tempel 1) with a hammer and see what's inside," Dr Sunshine added.
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 Mr Grammier.
The Deep Impact mission has been named after the Hollywood film in which a comet hits the Earth with devastating effect.
Nasa says there is no risk of the impacter knocking the comet into a dangerous orbit, likening it to a mosquito hitting an airliner. But scientists say information from this mission could inform future strategies for deflecting a comet.
Earlier this week, Deep Impact saw a major outburst from Tempel 1
"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 when approaching the Sun.