Astro Bob: Hello, mini-moon! ... Earth snares a temporary new moon

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

This image of the temporary mini-moon 2020 CD3 (center) was obtained with the 8-meter Gemini North telescope on Hawaii’s Mauna Kea and combines three photos through different filters to produce the color composite. 2020 CD3 remains stationary since its motion was being tracked during the time exposure. The International Gemini Observatory / NSF’s National Optical-Infrared Astronomy Research Laboratory / AURA / G. Fedorets

Earth’s got a new moon, but it won’t be around for long. On February 15th, astronomers Kacper Wierzchos and Teddy Pruyne with the Catalina Sky Survey photographed a 20th magnitude pinprick of light in the constellation Virgo. Once its orbit was determined they realized that instead of revolving around the sun, as most asteroids do, the object, called 2020 CD3, was instead orbiting the Earth!

While still an asteroid it also fits the definition of a moon. Mini-moon might be a better description. Astronomers determine the size of an asteroid by measuring its distance versus brightness. If something’s close and bright, it’s relatively big. If close and faint, it’s usually smaller. Based on their measurements of 2020 CD3 we know that it’s really small, only 6-12 feet (2 to 3.5 meters) across. Heck, it would easily fit in my basement bedroom.

2020 CD3’s dance with Earth from 2015 to the present. This animation allows for the possibility that the asteroid may have been captured earlier. The Earth (in blue) and Moon are depicted below center. Near the end, the mini-moon goes bye-bye. Tom Ruen

Mars has two moons and so does the Earth … for now. Sadly, 2020 CD3 is only making a guest appearance. It’s expected to break free of our planet’s grasp sometime in April and return to orbiting the sun. It appears that the asteroid wandered close enough to Earth in late 2016-early 2017 to fall into its “gravity well”. Before our planet’s gravity snared it, the object likely moved slowly relative to Earth and orbited only slightly farther from the sun, both factors that assisted in its capture.


In these four 30-second-exposure discovery images of 2020 CD3 the object appears streaked because it’s relatively close to the Earth and moving quickly. Catalina Sky Survey, University of Arizona

At the time of discovery, the object was about 186,000 miles (300,000 km) away, even closer than our own moon. Because of its proximity 2020 CD3 is also classified as a potentially hazardous asteroid (PHA), a potential candidate for a future impact. No worries this time around though. Not only is it easing away from the Earth as you read this, it’s also climbing out of that gravity well and will soon be independent of our planet. Though maybe not forever.

Like a scaled-up version of catch and release, popular in recreational fishing, Earth may snare the asteroid again and set it free in the distant future. There’s even the remote possibility that 2020 CD3 slam into our planet but don’t expect the apocalypse. Objects its size typically either burn up or shatter into pieces when hurtling through the atmosphere and at worst land as small meteorites. We should be fine.

2020 CB3 discoverers Kacper Wierzchos (right) and Teddy Pruyne take a selfie with the Catalina Sky Survey 1.5-meter telescope on Mt. Lemmon, the instrument used to discover 2020 CD3. Wierzchos / Pruyne

In the early discovery phase some experts suggested that the mini-moon might be a discarded rocket stage from an old spacecraft launch. A number of these have been found orbiting the Earth, and the size was about right. But astronomers noticed that solar radiation pressure — the physical pressure exerted on objects in outer space by sunlight — appeared to not affect the motion of the object. This is what you would expect for a dense, rocky object like an asteroid not a “tin can” rocket booster.

2020 CD3 isn’t the first mini-moon by the way. That honor goes to 2006 RH120 , another 2-to-3 meter asteroid discovered in 2006 by Eric Christensen, also with the Catalina Sky Survey. Like the current mini-moon, it was captured by Earth’s gravity and pirouetted around the planet for about a year before getting the boot. Astronomers suspect Earth has latched onto many temporary moons over its lifetime. The reason we’ve only found two so far is because they’re small, faint and orbit circuitously, making them difficult to spot.


Although utterly invisible with the naked eye and most telescopes you can still use your mind’s eye to picture the newly-discovered mini-moon near the star Arcturus tonight. Stellarium

Tonight when you look up at the moon, now a day-and-a-half past first quarter (with the spectacular crater Copernicus smack on the terminator!), remember you’re standing under two moons. The temporary one is located in the constellation of Boötes the Herdsman which rises around 8:30-9 p.m. The little guy sits roughly a fist north of the brilliant orange star Arcturus.

Oh, such brief acquaintance!

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