Asteroid Discovered Near Earth Turns Out to Have Secret Twin

Dual Asteroids
Artist representation of two asteroids found last year. The asteroids are about 3.7 million miles away, about 16 times the distance from the Earth to the Moon. NASA/JPL-Caltech

The near-Earth asteroid discovered orbiting the sun last year was hiding a twin—until now.

New sightings from various teams in June revealed something new about the asteroid called 2017 YE5. On July 12, NASA's Jet Propulsion Laboratory at the California Institute of Technology released a new finding: that the asteroid is part of a set.

On December 21, 2017, the Morocco Oukaimeden Sky Survey revealed its discovery about this asteroid, then thought to be just one. At the time, scientists didn't know of any physical properties. Then, on June 21, 2018, the asteroid passed the closest to Earth it would for at least the next 170 years, at about 3.7 million miles away, about 16 times the distance from the Earth to the Moon.

NASA's Goldstone Solar System Radar in California observed the asteroid on June 21 and 22, and it showed signs of actually being two asteroids, or a binary system. But the scientists couldn't tell if the two bodies were joined or separate until asteroids rotated; then the scientists were able to spot the gap.

Goldstone alerted scientists at Arecibo Observatory in Puerto Rico to look out for the suspected second asteroid, as they were planning to observe 2017 YE5 next. Arecibo decided to team up with West Virginia's Green Bank Observatory to study the asteroid by using a bistatic radar configuration. With this plan, Arecibo transmitted the radar signal, and Green Bank received the return signal. They were able to confirm that the asteroid was in fact two asteroids. Not only that: The scientists also discovered that every 20 to 24 hours, the asteroids revolve around each other.

Binary systems are typically between a larger asteroid and a smaller asteroid. This set is unique, because they're both about the same size, at about 3,000 feet across. When the asteroid was originally discovered, in December, scientists thought it was smaller. The new observations revealed their true size, and scientists believe they're likely made of a dark material that doesn't reflect as much sunlight as a standard asteroid, leading to the original incorrect estimate.

Despite being size twins, the asteroids probably have a different surface composition, density or surface roughness (or more than one of those factors), since their individual reflectivity differed. This could indicate that the objects started out separately and eventually fell into a mutual orbit.

The objects take 1,730 days to orbit the sun, so perhaps 4.74 years from now scientists will get to see what else is yet to be discovered about these asteroids.