For a brilliant (albeit brief) period of time, Earth had two moons — our tried and true satellite with astronaut footprints and abandoned lunar buggies on its surface, and a miniature space rock pulled into our orbit.
Our (former) minimoon, known formally as 2020 CD3 (opens in new tab), was officially identified in February after it had been orbiting our planet for at least a year, astronomers estimate. This happy little surprise revealed what is most likely a small, 3-foot-wide (0.9 meters) space rock. Researchers think that gravitational forces flung the tiny space rock into our orbit, where it hung out for so long.
Related: How scientists found Earth's minimoon and why it won't stay (opens in new tab)
Now, astronomers think that our new minimoon has been flung out yet again and is back on a journey to enter an orbit around the sun. "There's no question it was still in orbit around the Earth in early February, and there's no question now that it's in orbit around the sun," Bill Gray, an astronomy-software developer, told the Atlantic (opens in new tab). 2020 CD3 likely left our orbit on March 7, Gray told the Atlantic.
The end of our days with two moons is no surprise. Being what's known as a "temporarily captured object," astronomers knew that 2020 CD3 wouldn't be staying for long. The minimoon was originally estimated to leave Earth's orbit sometime in April, so this isn't too far off.
Following the first detection in 2006, this was the second time that astronomers have identified a temporarily captured object in Earth's orbit. At first, astronomers weren't quite sure if the minimoon was really a space rock or if it might be a defunct satellite or some other space junk. But, following some observations, they are fairly certain it is a small rock, though they're not sure exactly what type of rock it might be.
2020 CD3 could be a small asteroid or even a little chunk from our own moon, broken off in an impact with another space rock and pushed into Earth's orbit. Either way, unfortunately our brief stint having the rock as a second moon won't happen again for a long time. While 2020 CD3 will come close to Earth again in March of 2044, it won't be close enough to be grabbed into our orbit, Gray reported to the Atlantic. It might eventually come back close enough to get pulled into our orbit and revisit its status as a minimoon, but that might not happen for thousands of years.
If 2020 CD3 ever orbits Earth again, it will be way past our lifetimes. However, it's likely that another small rock will someday be temporarily captured by Earth's orbit and our moon will have a rocky little companion once again.
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