The European spacecraft Rosetta has become the first ever probe to catch up with and manoeuvre alongside a comet, a landmark stage in a decade-long mission that scientists hope will help unlock some of the secrets of the solar system.
The Rosetta, launched by the European Space Agency (ESA) in 2004, will accompany comet 67P/Churyumov-Gerasimenko on its trip around the sun and land a probe on it later this year in an unprecedented move.
"We're at the comet," Sylvain Lodiot, Rosetta's flight operations manager, declared on Wednesday in a webcast from mission control in Darmstadt, Germany.
Scientists are now on a tight schedule to learn enough about the comet using data from Rosetta to safely land the spacecraft's probe on it in November.
"We know what the comet's shape is. But we haven't really measured its gravity, we don't know yet where the centre of mass is," Rosetta Flight Director Andrea Accomazzo told the Reuters news agency ahead of the rendezvous.
As it neared 67P/Churyumov-Gerasimenko earlier this year, Rosetta took pictures revealing that it is not shaped, as had been assumed, like a rugby or American football, but rather comprises two segments connected by a neck, giving it an asymmetrical shape that has been likened to a duck.
Project scientist Matt Taylor said the unexpected shape could have come from two comets that joined together, or perhaps from a single object that eroded dramatically over time, the AFP news agency reported.
Scientists hope data the probe gathers on the surface of the comet will allow them to look into a kind of astronomical time capsule that has preserved for millions of years clues about what the world looked like when our solar system was born.
It has taken the Rosetta 10 years, five months and four days to reach the comet, a roughly 3km-by-5km rock discovered in 1969.
On its way, the spacecraft circled the sun on a widening spiral course, swinging past Earth and Mars to pick up speed and adjust its trajectory.
The mission comprises several historical firsts, including the first time a spacecraft orbits a comet rather than just whizzing past to snap some fly-by pictures, and the first time a probe has landed on a comet.
Because the trip is so long and took Rosetta so far from the sun's solar rays, the spacecraft was put in a deep sleep for 31 months and woken up earlier this year. There is little flexibility in Rosetta's schedule this year.
The comet is still hurtling toward the inner Solar System at almost 55,000km per hour, and the closer it gets to the sun the more active it will become, emitting gases that can make it difficult to predict the trajectory of the Rosetta and its probe.
"We have a lot of time pressure to produce engineering models of a world that we don't know yet," said Accomazzo, who has been working on the Rosetta mission in Darmstadt since 1997.