Finding a life partner, someone to spend your days with while you circle the sun, is hard. Now scientists have discovered a rare perfect match: Asteroid 2017 YE5, originally thought to be solo, is actually a double asteroid. Two asteroids, each approximately 800 meters (almost half a mile) in diameter, are orbiting each other about once a day, as they orbit the sun together. Scientists do not know how long the two have been together.
“An asteroid that has as a companion, another chunk that’s just as big. Two of equal size. Those we don’t have very many of,” said Anne Virkki, the group lead for the Planetary Radar Research Group at the Arecibo Observatory in Puerto Rico. The asteroid, originally discovered by the MOSS observatory in Morocco in December 2017, was thought to be a single asteroid. Last month, Virkki and her team, working parallel with a team at NASA's Goldstone Observatory in California, independently confirmed that it was a matched set.
There are only four known double asteroids of equal size, according to Lindley Johnson, NASA’s planetary defense officer. Far more common than 2017 YE5 are double asteroids that consist of one large “primary” asteroid and one smaller asteroid that orbits the larger one. Double asteroids, or binary asteroids, usually form after an impact breaks off a small piece of one asteroid. This piece then gives the primary, original asteroid a “moonlet,” according to Marina Brozović, a radar scientist at Jet Propulsion Labs in California.
The radar observations for 2017 YE5 were scheduled during Brozović's shift. “It’s wonderful because you know with that first track you’re looking into the unknown, and it’s always an opportunity for discovery. As the radar images are coming down live, you are seeing it for the first time,” Brozović said. “When we look at this object, it’s so interesting to us, it kind of generates more questions than answers.”
One leading theory about this rare double rock is that it started out as conjoined twins. Large asteroids can be more like cemented rubble piles than solid rock. Brozović said that a spinning object like this can become elongated, like a peanut. It can then spin rapidly enough to separate, forming two roughly equal asteroids.
Alternatively, though less likely, the asteroids could have started out as two separate asteroids with similar orbits and locations, slowly getting closer and closer until they joined orbits. Starting out apart, the asteroids could eventually have become “gravitationally bound,” and begun to swing around each other.
“It looks like they were doing a dance in space,” Johnson said, “and then the question came up is it a tango or cha-cha.” In grainy radar images, the asteroids twirl around each other, looking like something between an ultrasound and a Bigfoot sighting.
The Planetary Defense Coordination Office at NASA finds, tracks and investigates space objects that come close to Earth. “It’s not the highest priority thing we do at all, but the day might come it’s the most important thing we do,” Johnson said. According to its website, Planetary Defense Coordination is responsible for “ensuring the early detection of potentially hazardous objects,” such as asteroids or comets that come close to Earth. 2017 YE5's closest approach to Earth would be 3.7 million miles away. “We knew it was not an impact concern,” Johnson said.
“Part of our job is to find these [space objects] and know about them. But we don’t get worried. Most of what we’re talking about is less than 10 meters [about 30 feet] in size. Earth’s atmosphere will take care of those,” Johnson said. The work is part of a congressionally directed objective to track near-Earth objects, called the Nearest Object Preparedness Plan. So far they have found 8,000 of 25,000 estimated target objects that are within about 4 million miles of Earth and larger than 100 feet across.
“Asteroids are debris left over from the solar system formation, so they are about 4.6 billion years old,” Brozović said. By studying them, “you're understanding the solar system past.” NASA investigates not only the trajectory, size and location of near-Earth objects, but also their composition. “We believe that the building blocks of life exist with these objects,” Johnson said.
“I know they’re rocks but they’re awesome rocks,” Brozović said. The asteroids have been described as “dark as charcoal,” although in the infinite blackness of space, she said, charcoal actually looks bright.
What are the chances of an asteroid being an equally sized duo? Currently, Planetary Defense is tracking more than 18,000 objects near Earth of all sizes. Of those, 15 percent are confirmed binary, but only four are binary and equally sized. So they're really rare. And the odds of two equally sized asteroids joining after billions of years of solo space travel is extremely low (far lower even than your supposed odds of finding a mate, which a dating site has calculated to be 1 in 560).