Image: Arecibo/GBO/NSF/NASA/JPL-Caltech

When is an asteroid not an asteroid? When it’s a binary pair. Turns out that an asteroid discovered late last year named ‘2017 YE5’ is actually two gravitationally bound objects in orbit around each other. But this particular duo belongs to an exceptionally rare class of near-Earth objects.

The two objects were revealed by three radar telescopes, which shoot a beam of radio waves at nearby asteroids and wait for the reflection to return to Earth. Each object is about 900 meters (3000 feet) in size and revolve around each other once every 20 to 24 hours.

Artist’s concept of what binary asteroid 2017 YE5 might look like. The two objects showed striking differences in radar reflectivity, which could indicate that they have different surface properties. Credits: NASA/JPL-Caltech

2017 YE5 came to within 3.7 million miles (6 million km) of Earth on June 21, which is about 16 times the distance of the Earth to the Moon. Using bistatic radar—a technique in which the transmitter and receiver are placed in separate locations—astronomers at NASA’s Goldstone Solar System Radar (GSSR) in California scanned the object, finding they had a pair of asteroids on their hands, not one. These observations were later confirmed by astronomers at the Green Bank Observatory (GBO) in West Virginia.

Artist’s illustration of the trajectory of asteroid 2017 YE5 through the solar system. At its closest approach to Earth, the asteroid came to within 16 times the distance between Earth and the moon. Credits: NASA/JPL-Caltech

Around 15 percent of all known asteroids larger than 200 meters (650 feet) in diameter are binaries, but 2017 YE5 is special because it’s an “equal mass” binary, in which the two objects are roughly the same mass. Their radar reflectivity patterns were wildly different, however, which suggests these objects, while similar in size and mass, feature different densities and/or compositions near the surface. So far, scientists have spotted only three other such well-matched pairs. Most binary asteroids are uneven, with one half dwarfing the other.



The observations of the asteroid pair benefited greatly by the close proximity of the asteroid’s closest approach to Earth when it flew by about 6 million kilometers (3.7 million miles) away. It will be more than 170 years before the binary asteroid comes as close again.