Europe's pioneering spacecraft Rosetta headed for a crash landing Friday on the comet it has stalked for two years, a dramatic end to a 12-year odyssey to demystify our Solar System's origins.
Sent by ground controllers on a leisurely, 14-hour freefall, Rosetta was engaged in a last-gasp spurt of science-gathering on the 19-kilometer (12-mile) journey to its icy comet tomb.
"Next stop #67P!" the European Space Agency (ESA) tweeted late Thursday, using a shortened version of Comet 67P/Churyumov-Gerasimenko's full name.
The craft was programmed to terminate its historic quest at about 1040 GMT on Friday, joining long-spent robot lander Philae on the comet for a never-ending journey around the Sun.
With the comet zipping through space at a speed of over 14 kilometres (nine miles) per second, Rosetta was programmed to make a "controlled impact" at human walking speed, about 90 cm (35 inches) per second.
Mission scientists expect it will bounce and tumble about before settling -- but the craft's exact fate will never be known as it was instructed to switch off on impact.
Rosetta was never designed to land.
Confirmation of the mission's end is expected in Darmstadt around 1120 GMT when Rosetta's signal, which takes 40 minutes to travel, vanishes from ground controllers' computer screens.
But before then, it is expected to upload valuable data as it sniffs the comet's gassy coma, or halo, measures its temperature and gravity and takes pictures from closer than ever before.
- Pieces of a puzzle -
"We're all very excited," Rosetta project scientist Matt Taylor told AFP at the mission control centre in Darmstadt.
"In the final descent, we will get into a region that we have never sampled before. We've never been below two kilometers and that region is where the coma, the comet atmosphere, becomes alive, it's where it goes from being an ice to a gas."
Another highlight of the final hours will be a one-off chance to peer into mysterious pits dotting the landscape for hints as to what the comet's interior might look like.
The first-ever mission to orbit and land on a comet was approved in 1993 to explore the birth of our Solar System 4.6 billion years ago.
Rosetta and lander probe Philae travelled more than six billion kilometers over 10 years to reach 67P in August 2014.
Philae was released onto the comet surface in November of that year, bouncing several times, then gathering 60 hours of on-site data which it sent home before entering standby mode.
Comets like 67P are thought to contain primordial material preserved in a dark space deep freeze.
Insights gleaned from the 1.4-billion-euro ($1.5-billion) project have shown that comets crashing into an early Earth may well have brought amino acids, the building blocks of life.
Comets of 67P's type, however, certainly did not bring water, scientists have concluded.
While the mission ends Friday for ground controllers, scientists expect to be analysing Rosetta's data for "years if not decades" to come, said Taylor.
"We've got this massive puzzle, all the pieces are everywhere and we need to put them together."