An asteroid that whizzed within 4.8 million kilometres of Earth last week turned out to be two objects in a binary system, according to new observations.
The automated Asteroid Terrestrial-impact Last Alert System (ATLAS) on Mauna Loa in Hawaii discovered 2020 BX12 on January 27. It immediately determined that the asteroid was a potentially hazardous one. Now, that doesn’t mean that it was truly hazardous to life on Earth, just that it’s relatively large and its orbit will one day take it especially close to Earth. But when scientists followed up by pointing the Arecibo Observatory’s radar telescope at the rock, they realised it had a small companion orbiting it.
Just days after coming back online after a month-long shutdown due to the Puerto Rican earthquakes, Arecibo captured images of 2020 BX12 on February 4 and 5. The main asteroid is at least 165 metres across, rotating once every 2.8 hours. Its companion is 70 metres across and appears to orbit the larger rock approximately every 45 to 50 hours (though it could potentially be much quicker), according to a post on the Arecibo Planetary Radar Science Group website.
Arecibo is a radio telescope, but it detects asteroids using a radar, beaming microwave and radio waves at targets and creating images from the reflected light. Asteroids rotate, causing doppler shifts to the radar signal. The images (above) of 2020 BX12 show the smaller asteroid as brighter because it rotates slower. You can read more about radar-imaging asteroids here.
Binary asteroids aren’t so rare; one estimate finds that as many as 16 per cent of asteroids larger than 200 metres might have a companion. But it’s an exciting find, especially for an asteroid that will now live on in NASA’s Centre for Near Earth Objects database.