Final preparations are being made to crash a European spacecraft on to a comet, bringing a dramatic end to the 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 (7.50am Friday AEST), at an altitude of about 19km.
From that moment, there will be no turning back. The spacecraft will head straight for the comet, hitting the surface about 11.40am UK time on Friday.
Despite travelling at 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 chunk of ice and dust measuring 4.5km across, continues on circuits of the solar system.
The spacecraft is being crashed because the comet is heading so far from the sun that soon the craft's 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 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.
"We could have abandoned the spacecraft ... but this is not what we want to do."
The big risk faced by scientists is there will not be time for all the information they hope to obtain to be transmitted 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, Philae, which bounced on to the comet surface before coming to rest in a dark crevice.
Philae's exact location remained unknown until September 2 this year, when Rosetta detected the craft at a site on the comet's smaller lobe, later named Abydos.
All contact with Philae was lost in July after the space agency 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, which is littered with boulders and deep active pits known to produce jets of gas and dust.
© PAA 2018