Earlier this year, highly sensitive telescopes on Earth discovered a dishwasher-sized asteroid that had been temporarily circling the Earth-moon system. Essentially, the object, known as 2020 CD3, was a natural satellite, or minimoon, if only briefly. In a new study, researchers have confirmed that 2020 CD3 is indeed a space rock rather than some man-made contraption, making it only the second minimoon detected thus far.

Rarely detected by not rarely present

Not all asteroids whizz past Earth — some may get trapped in our planet’s orbit before escaping back into deep space. Astronomers say that by studying these temporary ‘minimoons’, it is possible to answer important mysteries surrounding asteroids. The challenge, however, is monitoring these fast-moving, tiny objects.

In 2006, astronomers found the first and, at the time, only known natural satellite other than the moon. The temporarily-captured orbiter (TCO) called 2006 RH120 measured only 2-3 meters across and orbited the planet for 13 months before it broke free of Earth’s gravity – only to be immediately recaptured into orbit around the sun. This TCO is so small that, initially, NASA thought it was the third stage Saturn S-IVB booster from Apollo 12. However, later observations determined that it was, in fact, a captured asteroid.

Since then, there had been other minimoon candidates, but they all proved false positives, which on closer examination turned out to be upper stage rocket boosters from Apollo-era rockets or Chinese Chang’e missions — but that’s not the case for 2020 CD3.

In a new study, an international team of 23 astronomers performed calculations on data from the Catalina Sky Survey, showing the asteroid’s area-to-mass ratio and low luminosity were indicative of a silicate object.

“We compare the surface area to the mass. For rocket boosters, which are hollow, the surface to mass ratio is much higher,” Grigori Fedorets, a research fellow at Queen’s University Belfast, told Universe Today.

According to Fedorets, 2020 CD3 is only 1-2 meters in diameter, which should disintegrate in the planet’s atmosphere were it to ever enter a collision course with Earth. That might never happen, though. When the astronomers reversed the path of the mini-asteroid, they found the object had been circling the Earth-moon system since 2018. After 2.7 years as a natural satellite of Earth, the tiny object — which is believed to be a fragment of a much larger asteroid originating from the main asteroid belt between Mars and Jupiter — resumed its journey around the sun.

 “According to our simulations, an average minimoon would only be captured for about nine months…it’s a longer capture time than we expected,” Fedorets told Universe Today.

CD3 is only the second natural minimoon that astronomers have discovered so far. But, when you think about it, it’s actually remarkable we’ve come this far with sensing technology that we can now detect and track boulder-sized objects up to two million kilometers away from Earth. Naturally, there are many other minimoons, or temporarily captured objects, currently orbiting Earth. According to astronomers, there should be “a steady state population with about 1 to 2-meter diameter captured objects at any time, with the number of captured meteoroids increasing exponentially for smaller sizes.”

More minimoons ought to be identified once the upcoming Vera Rubin Observatory becomes operational in 2021. And since these objects are believed to be older than Earth, studying them could offer new insights into the evolution of the solar system.

The findings were reported in the Astronomical Journal.