Astro Bob: Newfound potentially hazardous asteroid 2020 BX12 has a companion

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

A new asteroid discovered in late January made a favorable pass of Earth, allowing astronomers to bounce radio waves off it to determine its shape, size, rotation period and the fact that it has a tiny companion asteroid in orbit around it.

New radar images from the Arecibo Observatory in Puerto Rico reveal that a recently-discovered, near-Earth asteroid is really two asteroids in one! The images were created by bouncing radio waves off the asteroid on Feb. 4-5 when it passed near the Earth. By studying the returning radar echoes astronomers can deduce an asteroid’s shape, surface features, size and more. The larger or primary asteroid was discovered on January 27 by the ATLAS survey on Mauna Loa, Hawaii. Based on its size and the minimum separation of its orbit from Earth’s orbit (only 188,000 miles) astronomers classified it as a potentially hazardous asteroid (PHA).

Single images of the binary asteroid were used to make this short video sequence. The bright spot is the main asteroid’s moon or satellite.

While that means it could conceivably pass closer to the Earth than the moon and even potentially threaten the planet in the future, 2020 BX12 is not a danger at the present time. In fact, it’s currently receding from the Earth. From an early analysis of the images and data returned we know the asteroid is round, rotates once every 2.8 hours or less, and at least 540 feet (165 meters) in diameter.

Its satellite asteroid is about 230 feet across and rotates once about every two days. The distance between the two bodies is at least a fifth of a mile as observed on February 5th. From the movement of the satellite moon between the two observations it appears to be tidally locked to the larger body, presenting the same face to the bigger asteroid throughout its rotation much the same way we see only one face of the moon.


The orbit of the newly discovered binary asteroid is steeply inclined to the orbits of the planets.

The secondary appears brighter than the primary body in the radar images, which is common in radar images of binaries. Slower-rotating objects appear narrower in radar echoes because the reflected echo is less spread out compared to faster-rotating objects. This gives it greater “echo power” and makes it look more intense.

Ida and its satellite Dactyl. Asteroid 243 Ida as seen by the Galileo probe on August 28, 1993.

NASA’s Galileo spacecraft discovered the first binary asteroid in 1993 en route to Jupiter called 243 Ida . It’s 19.5 miles long and orbited by Dactyl, only 4,600 feet across. As of January 2020 , we know of 384 asteroids with 403 known companions!

Click here to learn more about how astronomers take “pictures” of asteroids with radar. If 2020 BX12 ever does collide with Earth far in the future, it’ll be a classic one-two punch.


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