Scientists are making final preparations for their nail-biting attempt to land a European spacecraft on the surface of a comet more than 300 million miles away.
Just after 9am – slightly later than originally planned – the Philae lander will separate from the orbiting Rosetta mothership and begin its historic descent from an altitude of 14 miles.
European Space Agency (ESA) controllers will be holding their breath during the seven hours it will take for the spider-like probe to reach the surface of comet 67P/Churyumov-Gerasimenko, a 2.5 mile-wide lump of ice and dust hurtling through space at around 40,000mph.
Touchdown is expected at 4.02pm UK time but will not be confirmed until half an hour later. It will take that long for a radio signal from the lander to travel 316 million miles to Earth.
If the mission succeeds it will be the first time any man-made object has made a controlled landing on a comet.
Nine years ago the US space agency Nasa’s Deep Impact mission smashed a projectile into comet Tempel 1 to study debris from the blast.
Philae, in contrast, will float gently down at walking pace.
During the descent it will be beyond the control of scientists on Earth, relying on automated systems and careful pre-programming.
As the dishwasher-sized robot makes contact with the surface it will pin itself down with two harpoons and ice screws attached to each of its three legs. Without these anchors there would be a risk of the probe bouncing away into space in the extremely low gravity.
Rosetta flight director Andrea Accomazzo said: “We have to be a bit lucky. If the lander lands in the proximity of a boulder there is nothing we can do.”
The chosen landing site, named Agilka after an island in the Nile, was the least hazardous of five possible candidates. But the whole of the comet is covered in deep pits, towering cliffs and peaks, craters and boulders – some the size of houses.
Philae will be landing on the smaller of two lobes that make up the comet, giving it the appearance of a “rubber duck”.
The robot is equipped with cameras, a suite of 10 instruments, and a drill that can bore out samples to a depth of 23cm.
One British-led instrument, Ptolemy, will be used to analyse the composition of samples in the craft’s on-board laboratory.
Philae is designed to collect data on the surface of the comet for just two and a half days, but the Rosetta orbiter will remain with the comet as it flies past the Sun and heats up, approaching as close as 118 million miles.
The landing is the culmination of a mission that began with the launch of Rosetta 10 years ago. The spacecraft finally caught up with the fast-moving comet in August after an epic four billion-mile journey that took it across the asteroid belt.
Information from the mission is expected to shed new light on the origin of the Solar System, the Earth and possibly even life.
Planetary scientist Professor Stanley Cowley, from the University of Leicester, who worked on the planning stages of Rosetta, said: “Comet impacts are thought to have been one of the principal means by which water was delivered to the early Earth around 3.6 billion years ago, possibly contributing half the water in our oceans.
“Furthermore, the comet material is also known to contain simple organic molecules which may also have seeded Earth with the material from which life emerged.”
British astronomer Dr Daniel Brown, from Nottingham Trent University, said: “Although we have landed on planets, moons and asteroids, it has never been attempted for a nucleus of a comet – and with good reason. These objects have a very low gravity, are loosely composed of ice, dust and rocks, and are very irregular in shape. They are temperamental in their behaviour and notoriously difficult to predict.
“Apart from the amazing scientific results, the sheer challenge and ambition of such a mission is outstanding and illustrates how our space exploration of the Solar System has become more advanced and successful. It gives us much to hope for in future missions.”