Final preparations are being made to crash a European spacecraft on to a comet, bringing a dramatic end to the £1 billion Rosetta mission.
Final commands will be uploaded on Thursday into the orbiter to line the probe up for its one-way trip on to the rugged surface of comet 67P/Churyumov-Gerasimenko.
Rosetta is due to carry out its "collision manoeuvre" at 9.50pm, UK time at an altitude of about 19 km (12 miles).
From that moment, there will be no turning back. The spacecraft will head straight for the comet, hitting the ground at around 11.40am UK time on Friday.
Confirmation of the spacecraft's death is not expected until around 12.20am because of the time it takes for radio signals from Rosetta to reach Earth.
Despite travelling at just 1.1mph - walking pace - the craft is not designed for landing and will not survive.
Rosetta will remain crumpled and lifeless on the surface of the comet as the object, a dirty chunk of ice and dust measuring 2.8 miles across, carries it on repeated circuits of the solar system that may continue for millions of years.
The decision to crash the spacecraft was taken because the comet is now heading so far from the Sun that soon its solar panels will not be able to generate enough power to keep it functioning.
Scientists hope to obtain stunning images and valuable data in the final moments before impact.
Speaking on a European Space Agency (Esa) YouTube hangout, Rosetta flight director Andrea Accomazzo said: "From an energy point of view it will be a soft landing.
"But Rosetta's not designed to land, so there will be some energy dissipation. For sure, Rosetta will bounce or tumble on the surface of the comet, but will not bounce back into orbit."
He added: "We could have abandoned the spacecraft .. but this is not what we want to do."
The big risk faced by scientists is that there will not be time for all the information they hope to obtain to be "downlinked" to Earth.
Rosetta reached comet 67P/Churyumov-Gerasimenk on August 6 2014, after a 10-year journey through the solar system.
Three months later, on November 12, the spacecraft deployed a tiny lander, called Philae, which bounced on to the comet surface before coming to rest in a dark crevice.
Philae's precise location remained unknown until September 2 this year when Rosetta spotted the craft lying crookedly at a site on the rubber duck-shaped comet's smaller lobe, later named Abydos.
All contact with Philae was lost in July after Esa switched off Rosetta's radio link with the lander.
On Friday, Rosetta will follow Philae down to the comet's smaller lobe, targeting the Ma'at region that is littered with boulders and deep active pits - some more than 100 metres across - known to produce jets of gas and dust.