No one can deny that it was an epic trip. The European Space Agency's comet-chasing Rosetta spacecraft has arrived at its quarry, after launching more than a decade ago and travelling 6.4 billion kilometers through the Solar System.

That makes it the first spacecraft to rendez-vous with a comet, and takes the mission a step closer to its next, more ambitious goal of performing the first ever soft landing on a comet. Speaking from mission control, Matt Taylor, ESA's Rosetta project scientist, called the space mission “the sexiest there’s ever been.”

Rosetta is now within 100 kilometers of its target, 67P/Churyumov–Gerasimenko, which was recently discovered to be shaped like a rubber duck. After a six-minute thruster burn, at 11.29am local time this morning (August 6), scientists at ESA’s Operations Centre, in DarmstadtGermany, confirmed that Rosetta had moved into the same orbit around the sun as the comet.

Rosetta is now moving at a walking pace relative to 67P – though both are hurtling through space at 15 kilometers per second.

Unlike NASA’s Deep Impact and Stardust craft, and ESA’s own Giotto mission, which flew by their target comets at high speed, Rosetta will now settle in for the long haul, taking a ring-side seat as 67P approaches the Sun, and eventually swings around it, next August.

In the coming weeks, Rosetta will hover slightly ahead of the 4km-wide comet, and gradually spiral inwards, moving in arcs on its Sun-side that form the edges of ever-smaller triangles. This will eventually take it as close as 10 kilometers to the comet: only from below 30km will the comet’s weak gravitational field be able to keep the probe in orbit.

Chaotic city
Speaking from the control room, Andrea Accom­azzo, Rosetta’s spacecraft-operations manager, said the environment for controlling the spacecraft had changed from zooming along a highway, to entering a “chaotic city”.

Scientists have now entered an intensive three-month phase which will involve learning to manoeuvre the comet at close quarters. They will also take baseline measurements, before 67P becomes more active, spewing out gas and dust as it approaches the sun.

From a distance, Rosetta’s instruments have already detected water in the ejected gas and measured the comet's temperature – finding it to be around 20 degrees warmer than they would expect from a purely icy body. “It fits instead with having a dusty, dark surface with ice underneath,” says Mark McCaughrean, senior scientific adviser at the ESA directorate of science and robotic exploration.

Comets fascinate scientists because they are primitive objects, little changed since the beginning of the Solar System, 4.6 billion years ago. Comets are also thought to have brought water and possibly the building blocks of life to Earth.

Another big task for Rosetta will be to look for a place to set down its lander, Philae. Although Rosetta can study the comet from afar, including “sniffing” ejected gas to figure out its composition, there are some tasks - such as studying the chirality of any organic molecules it finds and glimpsing inside the comet with a radar - that only Philae can do.

Crazy stakes
Recent pictures of the comet revealed it to be shaped a bit like a rubber duck or boot with a ragged surface, nothing like “boring, flat billiard balls” previous missions have flown by, says McCaughrean. “We’ve won the jackpot in terms of crazy stakes. There’s a lot that can be learned just from looking at it, but the risk will be landing there,” he says.

Once Philae is released from Rosetta, its descent is uncontrolled, meaning the most precise landing area engineers can determine is between 500 and 800 meters long, and 300 meters wide. From the pictures that have emerged so far, there seem to be few spaces on the comet that are suitably devoid of cliffs or other obstacles such as house-sized boulders, says McCaughrean.

On its way to 67P/Churyumov–Gerasimenko, the comet-chasing spacecraft has passed three times around Earth and once round Mars, making gravity slingshots that took it out into the depths of the solar system before entering hibernation and rewakening this January.

Rosetta will now follow the comet until August next year, when it reaches its closest point to the Sun, and stick with it until at least the following December. McCaughrean compares the journey to a piece of music. “In January, it was the first movement in a symphony. We’ve had a quiet movement in the middle and today, this is the crescendo, the opening chords of the final movement. The mission starts this morning," he adds.

This article is reproduced with permission and was first published on August 6, 2014.