Visit the Surface of a Comet!

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A Comet: Up-close and Personal!

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Comet 67P with an inset showing a closeup of the surface of its nucleus taken with the Philae lander. CCPetersen, based on images from ESA

Have you ever wanted to land on a comet? You can do it now, courtesy of the European Space Agency's Rosetta mission which is orbiting the oddly shaped  icy nucleus of Comet 67P/Churyumov-Gerasimenko. Thanks to this mission, you can actually find out what the comet smells like, too! 

Even better, you can watch for images from the spacecraft's Philae lander, which settled onto the comet's surface in November 2014, sent back some fascinating views and surface data, and then went into hibernation after 60 hours. On June 13, 2015, Philae awakened again and resumed its mission. 

The images coming from the orbiting Rosetta spacecraft show a gray-colored  nucleus, pockmarked with craters, with smooth areas broken by rilles and cracks. In some areas, the mission astronomers speculated that features on the crusty surface could have been made by jets of material spewing from under the icy crust.

For people who study comets, this is a rare chance to see an object that very likely dates back to the very beginning of the solar system. What secrets does it hold? What information will they find? Time will tell as they command Rosetta to orbit the nucleus as it continues in its orbit around the Sun.

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Why study a Comet?

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Rosetta mission image of Comet 67P/Churyumov-Gerasimenko. ESA/Rosetta/NAVCAM.

The chunks of ice at the hearts of comets come from the coldest, most distant parts of the solar system — the Kuiper Belt (which is the target of the New Horizons mission) and the Oort Cloud. Both of these regions contain countless numbers of chunks of ice. If they get a gravitational "shove" from a passing star or a planet's gravitational field, they often end up racing toward the inner solar system. Many comets orbit around the Sun; some don't quite get to the Sun. 

In the case of Comet 67P/Churyumov-Gerasimenko, its closest approach to the Sun is at a point somewhere between Earth and Mars. That's close enough to start heating up the ices that make up the comet, growing a "coma" (or cloud of icy particles and dust) around the nucleus. 

As the comet loses its material due to the solar heating (in a process called "sublimation"), the ice and dust scatters along the orbital path. The Rosetta mission (and Stardust some years ago) sample that material, and have shown that the ices are made of water, carbon dioxide, possibly methane or ammonia, and carbon-based dust. 

Comets date back to the earliest history of the solar system. That makes them treasure troves of information about what conditions were like when the planets were forming, some 4.5 billion years ago. By analyzing the chemical makeup of the cometary material, astronomers can tell if the comet has been heated more than once (which means it has been close to the Sun more than once), or if it has never been heated. 

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Comet Nucleus Casts Shadow on Itself

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Comet 67P/Churyumov-Gerasimenko taken from an oblique angle by the Rosetta spacecraft. European Space Agency/Rosetta Mission

It's not the first time that a comet has been approached by spacecraft. Comet 1P/Halley was studied by an armada of missions in 1986, and before that the International Cometary Explorer spacecraft took a look at 21P/Giacobini-Zinner. More recently the Stardust mission approached Comet 81P/Wild, and the Deep Space 1 probe studied 19P/Borrelly. These were all flybys or done from a distance. Rosetta is the first to go into orbit around a cometary nucleus, and its long-term studies will give incredible insights into comets and their role in solar system history.