The large asteroid is 540 feet in diameter, making it nearly as wide as the Washington Monument is tall. It spins on an axis, as does Earth, though it completes its swift twist in under three hours. Its moon is less than half its size, just 230 feet wide. The moon spins much slower, completing a rotation every 50 hours or so.

The orbiting moon is “tidally locked” to the large asteroid, Zambrano-Marin said — if you were to stand on the surface of asteroid 2020 BX12, you would always see the same face of its moon.

Astronomers detected 2020 BX12 from Hawaii’s Mauna Loa Observatory as part of the NASA-funded asteroid warning system called ATLAS. In January, the Hawaii telescope sensed sunlight reflecting off the asteroid, but that instrument was unable to distinguish more than a single fast-moving blot against a background of stars.

Further inspection from the Puerto Rico telescope revealed that what appeared to be one space rock was two. Because radio telescopes can produce their own signals, Zambrano-Marin said, they “have the power to illuminate both targets.”

These binary asteroids traveled as close as 188,000 miles to Earth’s trajectory. “While this means it could conceivably come closer to the Earth than the Moon, 2020 BX12 poses no danger at this time and is currently receding from Earth,” per the National Astronomy and Ionosphere Center, which operates the Arecibo station.

“The Arecibo radar is the most prolific discovery instrument for binary asteroids,” said Jean-Luc Margot, a planetary scientist at the University of California at Los Angeles. “About 60 binary and triple asteroids have been detected with radar to date.”

Moons orbit roughly 1 in 6 large asteroids, according to an estimate Margot and his co-authors published in the journal Science. At that rate, binary systems among asteroids “are five times more common than twins in the human population,” Margot said.

These binary systems can form when a single asteroid spins so rapidly it begins to shed mass. “An imperfect analogy is an accelerating merry-go-round,” he said. “There is a speed above which a rider would be ejected.” When that occurs in space, the thrown-off chunks continue to circle the parent asteroid.

Though nobody knew it then, the asteroids cruised near Earth in 1931, too. The duo will return close to our planet in 2101.

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