So long, Rosetta: End is near for comet orbiter

Rosetta is about to take its final bow.

On September 30, the comet orbiter will wrap up its nearly 26-month visit to comet 67P/Churyumov-Gerasimenko by touching down on the surface and then shutting down. Before sending its last signal to Earth, Rosetta will snap pictures and gather data all the way to the end, collecting some of the most detailed looks ever at a comet.

“Every time you look at a body and increase the resolution … it’s another world,” says Jessica Sunshine, a planetary scientist at the University of Maryland in College Park. “It’s going to be very interesting to see what this place looks like.”

After more than 10 years in space, Rosetta arrived at 67P on August 6, 2014 (SN: 9/6/14, p. 8). About three months later, a lander named Philae detached from the orbiter and dropped to the comet’s surface. It was a rough landing: Philae bounced twice and nicked a ridge before coming to rest on its side in the shadow of a cliff. With insufficient sunlight to charge its battery, Philae went to sleep about 60 hours later, though not before getting some pictures of its new home.

Unlike Philae, the orbiter was never designed to land on the comet. Despite meeting the ground at a walking pace of about 3 kilometers per hour, Rosetta’s landing — described as a “controlled impact” by mission scientists — will probably snap pieces off of the spacecraft.

“Feelings are mixed,” says mission lead Matt Taylor of the European Space Research and Technology Center in Noordwijk, the Netherlands. “Sadness that this is over, but great joy on what [we’ve] achieved.”

Before bidding the spacecraft adieu, here’s a look back at five mission highlights.

1.  One surprise came right away when researchers got their first good gander at the comet. Described as resembling a rubber duck, comet 67P has two distinct lobes. Some planetary scientists suspect that 67P was once two comets that smooshed together (SN: 10/31/15, p. 17).
    
2.  Comets are not just big balls of ice, as once thought. Towering cliffs, dusty dunes, shadowy pits — the landscape on 67P is a hodgepodge of terrains, some scarred by erosion, others blanketed under seasonal flows of fine dust (SN: 2/21/15, p. 6). Comets “are much more dynamic than a lot of surfaces in the solar system,” Sunshine says.
    
3. Water on 67P is unlike Earth’s, suggesting that comets provided little help in bringing H₂O to our planet (SN: 1/10/15, p. 8). The ratio of deuterium (a heavy form of hydrogen) to hydrogen in 67P’s water is roughly three times that on Earth. Comets as a whole, however, exhibit a large range in this ratio, implying comets have diverse origins.
    
4. Comet 67P carts around a cocktail of chemicals that includes oxygen and noble gases, both indicators of a birthplace that was cold and far from the sun (SN: 11/28/15, p. 6). Organic molecules are prevalent as well. While asteroids probably delivered the bulk of Earth’s water, “comets do have complex organics and could have brought those to Earth and provided the seeds of life,” Taylor says.
    
5. The interior of the comet is quite porous, which suggests that 67P was assembled gently 4.6 billion years ago (SN: 8/22/15, p. 13). That means the comet’s building blocks weren’t altered by forceful collisions, which supports the long-standing idea that comets are time capsules that preserve samples from the solar system’s formative years.

Comet science doesn’t end with Rosetta. Ground-based telescopes will continue to study them from afar, and next year NASA will consider proposals for flying a spacecraft to a comet, plucking a piece off the surface and bringing it back to Earth.

“As for Rosetta data, there is loads of it,” Taylor says. “There is decades of work to do. So Rosetta isn’t over — it’s just begun.”