Asteroid that acts like a comet spotted by citizen science

Do you know the difference between an asteroid and a comet? A new astronomical object is blurring the line between the two.

Citizen scientists involved in the Active Asteroids project have spotted an asteroid that displays a comet-like tail of dust or gas, but only some of the time.

Asteroids are small, rocky objects mostly found in the asteroid belt – a region in our solar system located between the orbit of Mars and Jupiter – that tend to have short, circular orbits.

Comets, on the other hand, are composed of frozen gases, dust, and rock – kind of like a dirty snowball. They also tend to have very elongated orbits, coming from a region called the Oort Cloud beyond the orbit of Pluto. As they come closer to the sun they heat up and develop a bright atmosphere, or coma, that can form into a long, bright tail.

Cosmos first covered the Active Asteroids project in October 2021. It’s a citizen science project finding astronomical objects known as active asteroids – objects have asteroid-like orbits but show signs of comet-like activity, such as tails.

Quasi-Hildas are related to these active asteroids. They orbit between the outer edge of the asteroid belt, but within Jupiter’s orbit. Only about 300 have been identified so far, but of those only about 15 have been found to display comet-like activity.

Now, the project has described a quasi-Hilda, 2009 DQ118, with 2 periods of comet-like activity, in a new study in The Astrophysical Journal Letters.

Active Asteroids uses publicly available images of known asteroids and other small solar system bodies from the Dark Energy Camera (DECam) archive. These images are then examined by citizen scientists on Zooniverse and classified as either active or inactive for comet-like activity.

Figure from the paper showing four images of the active asteroid
Images of 2009 DQ118 (green dashed arrows) displaying a cometary tail (white arrows). Frames (a) and (b) are from the first activity epoch and resulted from the Active Asteroids citizen scientist project and archival search. Frame (c) is an APO follow-up image showing faint signs of activity resulting in the tentative discovery of the second epoch of activity. Frame (d) is a stack of Magellan follow-up observations confirming the discovery of the second activity epoch. Credit: Oldroyd et al/The Astrophysical Journal Letters. DOI 10.3847/2041-8213/acfcbc

Citizen scientists identified a tail on 2009 DQ118, so researchers carried out follow-up observations and determined that the object was active in periods where it was closest to the Sun during March 2016 and again in April 2023.

This is probably due to the Sun’s heat warming up solid forms of volatile compounds such as water, carbon dioxide, and ammonia on its surface. This causes the solids to be converted straight into gases in a process called sublimation.

“Comet-like activity on traditionally non-cometary bodies, such as asteroids, has revealed the presence of a previously unrecognised reservoir of volatile ices in our solar system,” the researchers write in their paper.

“The distribution of this material throughout the solar system is poorly understood, and further study may shed light on pathways for delivery of these volatiles to Earth. Additionally, volatile ices on small solar system bodies may provide crucial resource reservoirs for future space exploration.”

Active Asteroids was launched in August 2021. With more than 6,000 volunteers to date, the project has resulted in 7 scientific publications so far.

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