A tiny, 4-meter fragment dubbed Churyumoon has been spotted orbiting Comet 67P / Churyumov-Gerasimenko, which the Rosetta mission visited from 2014 to 2016.

A small fragment (circled) in a temporary orbit around Comet 67/P.

Is this the smallest moon discovery yet? An image sleuth spotted something interesting recently in images taken by the European Space Agency's (ESA) Rosetta mission: a small moonlet orbiting Comet 67P/Churyumov-Gerasimenko.

Spanish astrophotographer Jacint Roger Perez made the discovery while sifting through and processing images taken by Rosetta on October 21, 2015. The images came from Rosetta's Optical Spectroscopic and Infrared Remote Imaging System (OSIRIS) camera. Roger Perez posted a brief animation of the comet and its moonlet on his Twitter account, alerting ESA researchers to the object's existence. ESA's Julia Marín-Yaseli de la Parra has since dubbed the moonlet "Churyumoon."

The comet had just passed perihelion two months prior as it continued on its 6.4-year orbit around the Sun, and the Rosetta spacecraft was keeping pace with it at 400 kilometers distant. The icy nucleus of the comet was very active at that point, spewing jets of material off into space. After the comet ejected the SUV-sized fragment, it spent its first hours coming as close as 2.4 kilometers and as far as 3.4 kilometers from the comet's center. The object was briefly lost in the glare of the coma, which appears bright in OSIRIS images, before it reappeared on the opposite side, confirming its orbit around the comet. The object remained visible until October 23, 2015.

An animated image sequence shows "Churyumoon" in orbit around Comet 67/P.
ESA / Rosetta / MPS / OSIRIS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / J. Roger

Visiting a Comet

Rosetta's comet
Rosetta's last images of its final resting place on Comet 67/P, with context.
ESA / Rosetta / MPS for OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Rosetta launched on March 2, 2004, atop an Ariane 5 rocket from the Kourou Space Center in French Guiana and arrived at Comet 67/P after a decade-long journey on August 6, 2014. The mission then studied the comet up close for more than two years, releasing the Philae lander to examine its surface and following the comet's activity through perihelion. The comet may seem bright, but its surface's reflectivity, or albedo, is actually lower than the surface of Earth's Moon. Rosetta finished up its mission on September 30, 2016, coming to rest on the surface of the comet.


A follow-up mission to the comet, named CAESAR (the Comet Astrobiology Exploration Sample Return mission), was in the running for selection by NASA's New Frontiers program, but the Dragonfly mission to Saturn's moon Titan was selected instead, earlier this year.

DIY Sleuthing

Amateur image sleuths have turned up interesting finds in the past while culling through spacecraft archives: Ted Stryk noticed the shadow cast by Neptune's tiny moon Despina in Voyager 2 images, taken during the probe's 1989 flyby past the planet. More recently, an astute observer noticed the shadow of Phobos slipping across the Martian sky in images taken by the Curiosity rover during a post-sunset eclipse. Pro-am collaboration really ramped up with NASA's Juno mission: Online volunteers routinely process and post stunning images taken during perijove passes. And volunteers also routinely spot sungrazing comets by looking at images taken by the joint ESA/NASA Solar Heliospheric Observatory (SOHO).

It's worth sifting through the treasure troves of online data and images, all freely available to the public courtesy of NASA and the ESA. You never know what you might find.


Image of Curt


August 23, 2019 at 9:37 pm

The late Dr. Tom Van Flandern is proven correct once again. Go back to 1981 and read "Do Comets have Satellites?" published in the Bulletin of the American Astronomical Society, Vol. 13. Van Flandern long advocated that satellites of asteroids and comets exist, but that they would be found to be the rule rather than the exception. For this kind of thinking he was pilloried for years, until Ida's satellite Dactyl was discovered. Been an avalanche of satellites of asteroids and satellites discovered. Van Flandern also explained Comet Shoemaker-Levy 9 as a cometary satellite system that was disrupted by Jupiter's gravitational "sphere of influence."

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