Spacecraft catches up to comet
Scientists are celebrating after a European spacecraft launched 10 years ago finally caught up with a fast-moving comet more than 250 million miles (402 million km) away.
The Rosetta probe completed a journey that took it four billion miles (6.4 billion km) across the asteroid belt and more than five times the Earth's distance from the Sun.
Its destination was comet 67P/Churyumov-Gerasimenko, which is a misshapen lump of ice and dust swinging in a wide circuit round the Sun at around 34,175 mph (55,000 kmh).
At 10am UK time, a critical six minute 26 second thruster burn set the probe into a temporary triangular holding pattern 62 miles above the comet.
From this position, measurements will be taken of the object's mass and gravity before Rosetta is placed in a fixed orbit around the object, edging closer until it is just 18.6 miles (30 kilometres) above the surface.
In November Rosetta will deploy a small robotic craft, Philae, which will make the first controlled landing on a comet before sending back images and data on 67P/Churyumov-Gerasimenko's composition.
As it arrived, Rosetta had to slow down to match the comet's speed to within walking pace.
A signal confirming the insertion manoeuvre was beamed back to Earth, taking 22 minutes 29 seconds to cross the void from the comet's current location between the orbits of Jupiter and Mars.
Shown live in a European Space Agency television broadcast from the control centre in Darmstadt, Germany, operations manager Sylvain Lodiot was heard to shout: "We're at the comet."
Muted applause followed - but none of the ecstatic whoops and cheers that accompany such moments in American space missions.
The scientists know they have a lot of tough challenges ahead as the mission enters a new phase, but were delighted with how well it had gone so far.
Rosetta mission director Andrea Accomazzo said: "It's like driving on a motorway at high speed. What we're doing today is like entering a chaotic town with lots of traffic."
One of the first tasks will be to search for a suitable landing site for Philae, which must be relatively free of holes, stones and craters, and away from areas where jets of gas erupt from the comet's interior.
For the next 17 months, Rosetta, a box-like structure just under 10 feet (3m) long with two wing-like solar panels, will remain close to the comet as it heads towards the Sun and heats up, throwing out increasing amounts of gas and dust.
At perihelion - the point at which it is closest to the Sun - Rosetta and the comet will be 1.3 times further from the Sun than the Earth, a distance of 118 million miles.
Speaking at the operations centre, project scientist Matt Taylor said: "Rosetta is the sexiest space mission there has ever been.
"Once we've got the rendezvous done we're going to have a ringside seat through perihelion and as (Rosetta) starts to move away from the Sun. It's going to be an awesome ride."
During its journey through space the probe looped around the Sun five times and picked up speed from three "gravity-assist" swing-bys of Earth and one of Mars.
To conserve power in the cold outer regions of the Solar System, it spent more than two years in "hibernation" with most of its systems shut down.
British scientists and engineers developed a mini-laboratory for Philae which will analyse grain samples drilled out from the comet and answer key questions about its origins.
Professor Richard Holdaway, director of RAL Space at the Rutherford Appleton Laboratory in Oxford, said: "This is an historic and hugely exciting moment for the Rosetta Mission. After 10 years of travel through space from Earth, Rosetta finally arrives at the comet and will land a unique shoe-box sized chemistry set designed and built by RAL Space and The Open University.
"We are very proud of our involvement and eagerly anticipate receiving the first results."
Information from the mission is expected to help scientists understand the origin of comets, the Solar System, and possibly life.
Comets are known to contain complex organic molecules rich in carbon, hydrogen, oxygen and nitrogen - the basic elements that make up the essential ingredients for life, nucleic and amino acids. Many scientists believe comets may have helped to plant seeds of life on Earth.
Rosetta is named after the Rosetta Stone, a slab of basalt containing inscriptions that helped archaeologists decipher ancient Egyptian hieroglyphs.
The lander Philae is named after an island in the Nile where an inscribed obelisk was discovered in 1815. Comparing the hieroglyphs on the Rosetta Stone and Philae obelisk led to greater understanding of the ancient Egyptian writing system.
Jean-Jacques Dordain, director general of the European Space Agency, said: "We are delighted to announce finally 'we are here'.
"Rosetta is now the first spacecraft in history to rendezvous with a comet, a major highlight in exploring our origins. Discoveries can start."
ESA's director of science and robotic exploration Alvaro Gimenez added: " We have come an extraordinarily long way since the mission concept was first discussed in the late 1970s and approved in 1993, and now we are ready to open a treasure chest of scientific discovery that is destined to rewrite the textbooks on comets for even more decades to come."
As it approached the comet, Rosetta's cameras sent back images that highlighted the unpredictable nature of the comet's activity.
Between late April and early June the comet's coma - the extended envelope of gas and dust surrounding the object - became rapidly brighter before fading again.
Over the same period, measurements suggested that the comet was emitting water vapour into space at a rate of about 0.3 litres per second.
Temperature readings suggested that the comet's surface was mostly dark and dusty rather than clean and icy.
As Rosetta closed in on the comet, its cameras showed the true shape of 67P/Churyumov-Gerasimenko in stunning detail.
The object turned out to have two distinct lobes joined by a "neck", drawing comparisons with a rubber duck.
Scientists believe it may have formed from a pair of comets colliding and fusing together. Alternatively it might have been sculpted by erosion over time.
As many as five possible landing sites will be identified by late August, before the primary location is identified in mid-September.
Philae is currently scheduled to touch down on the comet's surface on November 11, anchoring itself in place with harpoons and ice screws.
"Arriving at the comet is really only just the beginning of an even bigger adventure, with greater challenges still to come as we learn how to operate in this unchartered environment, start to orbit and, eventually, land," said Mr Lodiot.
Rosetta emerged from hibernation at 18:18 GMT on January 20, 5.6 million miles (nine million kilometres) from the comet.
After sending the spacecraft its "wake-up call" mission controllers reactivated the orbiter's 11 science instruments and Philae's 10 instruments.
Between May 7 and August 6, 10 correction manoeuvres were made to reduce Rosetta's velocity relative to the comet from 775 metres per second to one metre per second.