Comet 67P Has Its Own Tiny "Churymoon"

Comet 67P/Churyumov-Gerasimenko and its tiny Churymoon snapped by Rosetta on October 21, 2015. ESA/Rosetta/MPS/OSIRIS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/J. Roger (CC BY-SA 4.0)

It’s been five years since the European Space Agency’s (ESA) Rosetta arrived at comet 67P/Churyumov-Gerasimenko, or 67P as it's easier to say, and in that time we’ve learned a lot about everyone's favorite comet shaped like a rubber duck.

We’ve photographed its surface, learned it has a water-ice cycle, seen it spout plumes of gas and dust, lost a probe on it, and found two key ingredients for life. Rosetta’s two-year mission studying 67P came to a scheduled end on September 30, 2016, with a controlled crash into the comet but that didn’t mean the discoveries stopped there.

An astrophotographer wading through Rosetta’s image archives has found something new and exciting: 67P appears to have its own tiny moon.

The tiny speck you can see below was found by Jacint Roger, an astrophotographer from Spain who regularly mines Rosetta images, processing the data and posting them as gifs on his Twitter page.

ESA/Rosetta/MPS/OSIRIS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/J. Roger (CC BY-SA 4.0)

This caught the eye of researchers who investigated the chunk of debris seen in a series of images snapped by Rosetta’s OSIRIS camera on October 21, 2015, not long after 67P, accompanied by Rosetta, reached its perihelion – its closest point to the Sun.  

In the run-up to the perihelion, the heat of our star caused the comet’s nucleus to release gases that blasted dust in spectacular jets, creating a “coma”, or gaseous envelope surrounding the object.

These new images show the 4-meter (13-foot) chunk of debris, dubbed a “Churymoon” by researchers according to ESA, which was likely ejected at that time. The images show that it spent its first 12 hours orbiting the space rock in a path that took it between 2.4 and 3.9 kilometers (1.5 and 2.4 miles) from the comet’s center.

After that, the Churymoon crossed into the part of the coma that appears too bright in images to follow its path, before popping up again on the opposite side of the coma in a position consistent with its previously observed orbit, allowing Rosetta to track its orbit until October 23, 2015.

This isn't the first chunk of debris ejected by 67P that scientists have observed since Rosetta's arrival in 2014, but they do think this is the largest detected around the comet and so it will be subject to further study.

Comet 67P continues to teach us about these small chunks of ice and rock. 67P's distinctive two-lobed shape was most likely formed by two objects in the outer regions of the Solar System colliding. About 10,000 years ago, the comet moved from the Kuiper Belt beyond the orbit of Neptune to its current location, an eccentric orbit of the Sun that brings it close to the paths of both Earth and Jupiter.

The Rosetta mission was groundbreaking in many ways. It was the first to orbit a comet, the first to land a probe on one (even if we did lose Philae in the end), and it probably hasn't given up all its secrets yet.  

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