Astronomers involved with the Catalina Sky Survey have spotted a new companion that Earth seems to have ensnared by its gravity.

The new “mini-moon” is an asteroid designated 2020 CD3 and is about 1.9 to 3.5 meters in diameter. Trajectory indicates it has been orbiting the Earth for about three years.

earth's new moon, mini-moon, 2020 CD3
The object within the circle is mini-moon called 2020 CD3. It has been captured and temporarily bound to Earth, as confirmed by the International Astronomical Union’s Minor Planet Centre at the Smithsonian Astronomical Observatory. [Image: Kacper Wiercho via Twitter]
Earth is surrounded by excess of asteroids with negligible proportion of them waiting to make an impact. Estimate suggests there are millions of them, but the actual number of near-Earth asteroids (NEAs) discovered so far is little over 22,000. That’s because they are small, and typically don’t get irradiated by the Sun enough to be noticed.

Also with many flying around, some of these space boulders do get captured by Earth’s gravity, but don’t stay long enough to become mini-moons. Either they manage to plow through the atmosphere where they get completely pulverized upon entry, or stay in a partial orbit before their velocity fling them away from Earth. So it’s just a tiny fraction of them that are cut out to be mini-moons.

Among several candidates for mini-moons, the first confirmed was an asteroid 2006 RH120 that orbited Earth between September 2006 and June 2007. It was also found by the Catalina Sky Survey. Another was 469219 Kamoʻoalewa, but it later turned out to be a quasi-satellite.

At one point, the Sun’s gravity will pick up 2020 CD3 and disengage from its orbit around Earth. Orbital simulation indicates it might happen as early as April 2020.

Mini-moons are exceptionally rare, but calculation shows there should be at least one natural satellites of one meter in diameter orbiting the Earth at any given moment.