Comet 67P: Target of Rosetta Mission
Comet 67P/Churyumov-Gerasimenko is seen here in an image captured by the Rosetta spacecraft. The mission's Philae lander hit the surface with a big bounce, demonstrating that the comet's surface is hard.
Credit: ESA/Rosetta/NAVCAM

Comet 67P/Churyumov-Gerasimenko is a small body visited by two spacecraft in 2014: the Rosetta orbiter and the Philae lander. The European Space Agency mission — usually collectively referred to as Rosetta — was the first to do extended observations of the comet.

The mission took place while 67P's orbit took it on its closest approach to the sun. The goal of Rosetta was to investigate the comet's changes as 67P got closer. Generally speaking, solar particles and heat tend to increase activity on comets. They emit more gas and dust and, in some cases, are even visible from Earth as they get more active.

Studies of 67P are intended to teach astronomers more about how the early solar system was formed. Comets are considered leftovers of the time when the solar system was little more than a collection of rocky and icy objects, billions of years ago. Most of those objects were eventually swept up into planetary or moon formation, but there are still plenty left over.

Additionally, astronomers are hoping to better predict how comets change as they get closer to the sun. Comet brightness predictions are notoriously unpredictable. A recent example of that took place with Comet ISON, which was projected to be a prominent comet in late 2013 and early 2014. Instead, the comet broke up shortly after passing the sun in November 2013.

67P was first spotted in 1969 by two Ukrainian astronomers: Klim Churyumov and Svetlana Gerasimenko. They observed it from the Institute of Astrophysics in Alma-Ata (now named Almaty), Kazakhstan, according to the European Space Agency.

The discovery took place by pure chance, ESA added, while the astronomers were hunting down comet 32P/Comas Solà, which was discovered in the 1920s. The comet is considered “periodic,” meaning it makes regular runs into the inner solar system. Back then, astronomers would use photographic plates covered in light-sensitive emulsion. The plates were developed after each observing session; one particular one was underdeveloped due to a lack of solution.

“As Svetlana was processing one of these plates on 11 September, she realized she didn't have much solution to develop the plates, but decided to process that one anyway, before preparing a fresh solution. As a result, that plate was underdeveloped,” ESA wrote. “Svetlana almost threw the defective plate away, but she didn't. After all, a tiny splotch of light was visible at the centre, and they thought it was comet 32P/Comas Solà.”

The next month, the astronomers looked at the plates and realized that particular image did not show 32P, as it was about two degrees away. It was instead a new comet, later called 67P/Churyumov-Gerasimenko after its discoverers.

The two astronomers were still active in 2015, when Rosetta arrived at the comet. But it wasn't the first choice for mission planners. Initially, ESA was aiming for a 2003 launch to target Comet 46P/Wirtanen. A rocket failure, however, forced the mission to be postponed to 2004. Because the orbit of the initial target took it too far away for the mission to work, planners changed the target to 67P in 2003.

Rosetta took off March 2, 2004, riding an Ariane 5 rocket. To pick up speed, it did several slingshot maneuvers (three near Earth and one near Mars) before rocketing farther out in the solar system. The early stages of its mission saw it take long-distance pictures of asteroids and comets and investigate the atmospheres of Venus and Mars.

The spacecraft was put into hibernation in June 2011 and revived in January 2014 for the last few maneuvers to the comet. Rosetta safely arrived in August 2014 and deployed its lander, Philae, in November 2014.

The landing did not go as planned. Philae's harpoons did not deploy to attach it to the surface. The lander drifted for more than two hours, periodically touching the surface, before coming to rest on its side. The lander relies on solar panels to recharge, and in that zone of the comet, there was not enough sunlight to keep it active for long.

Philae did, however, have backup battery power that kept science going for a few days. As of late February 2015, the lander remained in hibernation. Planners hoped that, as the comet got closer to the sun, more sunlight would fall across Philae's landing site and it would wake up again.

With Rosetta still active around the comet, and Philae providing data during its brief burst of science on the surface, scientists got some science results from 67P. The biggest science set to date came from a series of papers published in January 2015.

Astronomers usually call comets “dirty snowballs,” but said in this case that it would be better to call 67P a “snowy dustball” because of its dirt-to-ice ratio. This demonstrates that comet composition is more complex than previously thought, said Alessandra Rotundi, the principal investigator of Rosetta's GIADA dust grain analyzer instrument, in an e-mail to at the time.

Additionally, the researchers discovered organic compounds on the comet's surface — the first time that such a find has been made on that kind of an object, said Fabrizio Capaccioni, the principal investigator of the VIRTIS instrument on Rosetta. Organics are sometimes called the building blocks of life, although their presence does not always signal life. (67P's airless environment is hostile to life as we know it.)

Rosetta is also making scientists consider how water came to Earth. While previous theories postulated that it was comets that were responsible, 67P contains a kind of water that is markedly different than what is found on Earth. This is leading some to suggest that asteroids may be primarily responsible instead.