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How scientists found Earth's new minimoon and why it won't stay here forever

A weird "minimoon" found circling Earth likely won't be there long.

The scientists who discovered the object on Feb. 15 estimate that, because of the instability of its path through space, the minimoon will likely leave Earth's orbit sometime in April.

Astronomers know little about this minimoon — so little, in fact, that they can't even say if it's an artificial object, such as a dead satellite. However, they say, it's most likely a small asteroid. And although the object poses no danger to Earth, it does show how changeable our neighborhood is.

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The international Gemini Observatory/NSF’s National Optical-Infrared Astronomy Research Laboratory/AURA/G. Fedorets (Image credit: The international Gemini Observatory/NSF’s National Optical-Infrared Astronomy Research Laboratory/AURA/G. Fedorets)

Information from the International Astronomical Union's Minor Planet Center, which keeps observation records for all known asteroids, shows that the orbit of the minimoon doesn't match the precise orbit of any known human-made object, Kacper Wierzchos, a senior research specialist for the Catalina Sky Survey and co-discoverer of the minimoon, told The object's orbit didn't display any perturbations resulting from solar radiation pressure coming from the sun; such wiggles are common for human-made satellites in Earth's orbit.

But Wierzchos said he doesn't want to assume the minimoon is an asteroid quite yet. "The possibility it is artificial still exists, so I am trying to be cautious with every statement," he said. "I'd hate it to be artificial after [everyone is] making a fuss [about the discovery]."

The NASA-funded Catalina program at the University of Arizona is an automated survey that scans the sky for fast-moving objects. On discovery night, Wierzchos and Catalina research specialist Theodore Pruyne happened to be at the telescope, Wierzchos said. On a good night, the survey can study around 40 near-Earth asteroids, which are typically objects astronomers already know about. But on Feb. 15, something in Catalina's observations looked a little funny and didn't match anything known by astronomers. The duo submitted the discovery to the Minor Planet Center, and other astronomers soon confirmed the find.

Data from the Catalina Sky Survey shows the recently discovered minimoon currently orbiting Earth. (Image credit: K. Wierzchos/T. Pruyne/University of Arizona/Catalina Sky Survey)

The newly identified object, now known as 2020 CD3, was very faint when it was discovered, at only about magnitude 20. (The lower the magnitude, the brighter the object.) That faintness stretches the capabilities of Catalina and is beyond what most amateurs can see in their telescopes. (Since then, the object has faded to magnitude 23, making it visible only to the largest professional telescopes.) 

In the nights after the discovery, Wierzchos and his collaborators kept following the object to try to determine its orbit. Their calculations showed that, most likely, the object was circling the sun and Earth's gravity snatched it into our planet's orbit sometime in 2017.

How did the minimoon go unnoticed for years? First, the sky is vast, and telescopes have limited time dedicated to searching for asteroids, Wierzchos said. He also cited the minimoon's faintness and highly variable orbit.

2020 CD3 has a "chaotic" orbit, he said, because it is pulled between the gravity of the moon and the gravity of Earth. Its distance to Earth varies between the equivalent of 0.2 and 4.5 Earth-moon distances (The average distance to the moon is roughly 239,000 miles, or 384,000 kilometers.) When Wierzchos last observed the minimoon, on Wednesday (Feb. 26), it was roughly 2.5 lunar distances away, he said.

Because the object's distance to Earth varies, so does its orbital period, or the time it takes the minimoon to circle Earth. Wierzchos said the object's orbital period is difficult to measure precisely, but it seems to be about a month.

Astronomers have observed 2020 CD3 only about six or seven times so far, so they don't have enough information yet to derive a "light curve," which shows the variation in an object's brightness. If they can get that data, Wierzchos said, it may help astronomers determine what kind of asteroid it is (if it is, indeed, an asteroid), how quickly it rotates and how big it is. 

Assuming the object is a common type of asteroid called a carbonaceous asteroid, Wierzchos said, the minimoon is probably about the size of a car. Ideally, he hopes other telescopes will be able to observe the object before it drifts away. Telescopes he would like to see participate include Puerto Rico's Arecibo Observatory (which has radar optimized to get the shape and size of nearby asteroids) or large optical telescopes, such as the Keck Observatory in Hawaii.

Wierzchos is scouring past data from the Catalina Sky Survey to see if 2020 CD3 showed up in past imagery and went unnoticed. He hasn't found such observations yet, but he said it's possible that Catalina or other asteroid surveyors, such as the Asteroid Terrestrial-impact Last Alert System, have that information in their archives.

Wierzchos hopes Catalina will be able to see 2020 CD3 again before it fades. But astronomers are racing against the gravitational tug-of-war between Earth and our traditional moon. Before long, they know, the minimoon will drift out of sight, bound for new adventures. 

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook

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Elizabeth Howell

Elizabeth Howell, Ph.D., is a contributing writer for since 2012. As a proud Trekkie and Canadian, she tackles topics like spaceflight, diversity, science fiction, astronomy and gaming to help others explore the universe. Elizabeth's on-site reporting includes two human spaceflight launches from Kazakhstan, and embedded reporting from a simulated Mars mission in Utah. She holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, and a Bachelor of Journalism from Canada's Carleton University. Her latest book, NASA Leadership Moments, is co-written with astronaut Dave Williams. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday.