Space race: EU craft catches up with comet after 10 years
It has taken more than a decade, but the European Space Agency's craft Rosetta has finally caught up with the comet it has been chasing since it left Earth in 2004.
Rosetta, which was woken from deep space hibernation in January, has spent the past seven months catching up with 67P/Churyumov-Gerasimenko.
In November, it will be close enough to harpoon the frozen ball of dirty ice and attempt to make a soft landing.
Comets are the primitive building blocks of the Solar System, left over from a planet-building time when the Sun was just a disc of spinning dust and gas. Made of ice, dust and small rocky particles, it is likely they delivered the first water to Earth and may have even seeded the planet with the essentials for life.
Cometary dust brought back to Earth by NASA's 1999 stardust mission contained glycine, an amino acid that is a basic building block of life.
The comet and spaceship are now 400 million kilometres from Earth, about halfway between the orbits of Jupiter and Mars, rushing towards the inner Solar System at nearly 55,000kph.
"After 10 years, five months and four days travelling towards our destination, looping around the Sun five times and clocking up 6.4bn km, we are delighted to announce finally 'we are here'," said Jean-Jacques Dordain, the director general of the European Space Agency.
"Europe's Rosetta is now the first spacecraft in history to rendezvous with a comet, a major highlight in exploring our origins. The discoveries can begin."
Rosetta was launched on March 2, 2004, from Kourou, French Guiana. The comet is moving far faster than speeds that could ever be achieved by a spacecraft leaving Earth. So the craft has spent the intervening time using the gravitational pull of the Earth and Mars to act as a sling shot, allowing it to accelerate.
When it reached the critical speed in July 2011, the Rosetta was put into deep-space hibernation for the coldest, most distant leg of the journey as it travelled some 800 million kilometres from the Sun, as the comet headed towards the outer Solar System. Scientists extended the craft's solar arms to catch the Sun's rays and placed it in a slow spin to maintain stability. The only devices left running were its computer and several heaters.
Yesterday saw the last of a series of 10 rendezvous manoeuvres that began in May to adjust Rosetta's speed and trajectory gradually to match those of the comet.
"If any of these manoeuvres had failed, the mission would have been ruined, and the spacecraft would simply have flown past the comet.
"Today's achievement is a result of a huge international endeavour spanning several decades," said Alvaro Gimenez, ESA's director of science and robotic exploration.
"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."
The comet began to reveal its personality while Rosetta was on its approach. When the craft was 9,000km away, photographs taken by the Rosetta began to show that the nucleus comprised two distinct segments joined by a "neck", giving it a duck-like appearance.
"Our first clear views of the comet have given us plenty to think about," says Matt Taylor, ESA's Rosetta project scientist.
"Is this double-lobed structure built from two separate comets that came together in the Solar System's history, or is it one comet that has eroded dramatically and asymmetrically over time? Rosetta, by design, is in the best place to study one of these unique objects.
Yesterday, Rosetta was just 80km from the comet's surface, but it will edge closer still. As many as five possible landing sites will be identified by late August, before the primary site is identified in mid-September.