New observations by NASA’s Goldstone Solar System Radar in California, Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia have revealed that the recently-discovered near-Earth asteroid 2017 YE5 is actually two objects — each about 3,000 feet (900 m) in size — orbiting each other. 2017 YE5 is only the fourth binary near-Earth asteroid ever observed in which the two bodies are roughly the same size, and not touching.
A bi-static radar image of the binary asteroid 2017 YE5 from the Arecibo Observatory and the Green Bank Observatory on June 25, 2018. Image credit: Arecibo / GBO / NSF / NASA / JPL-Caltech.
The near-Earth asteroid 2017 YE5 was discovered on December 21, 2017 by astronomers with the Morocco Oukaimeden Sky Survey, but no details about the asteroid’s physical properties were known until the end of June 2018.
On June 21, 2017 YE5 made its closest approach to Earth for at least the next 170 years, coming to within 3.7 million miles (6 million km) of Earth, or about 16 times the distance between Earth and the Moon.
On June 21 and 22, observations by the Goldstone Solar System Radar showed the first signs that the asteroid could be a binary system.
The observations revealed two distinct lobes, but the asteroid’s orientation was such that astronomers could not see if the two bodies were separate or joined.
Eventually, the two objects rotated to expose a distinct gap between them.
Arecibo Observatory astronomers had already planned to observe 2017 YE5, and they were alerted by their colleagues at Goldstone of the asteroid’s unique properties.
On June 24, they teamed up with researchers at the Green Bank Observatory and used the two observatories together in a bi-static radar configuration — in which Arecibo transmits the radar signal and Green Bank receives the return signal.
Together, the observatories were able to confirm that 2017 YE5 consists of two separated objects.
The new observations obtained between June 21 and 26 indicate that the two objects revolve around each other once every 20 to 24 hours.
Radar imaging shows that the two objects are larger than their combined optical brightness originally suggested, indicating that the two rocks do not reflect as much sunlight as a typical rocky asteroid. 2017 YE5 is likely as dark as charcoal.
The Goldstone images taken on June 21 also show a striking difference in the radar reflectivity of the two objects, a phenomenon not seen previously among more than 50 other binary asteroid systems studied by radar since 2000.
The reflectivity differences also appear in the Arecibo images and hint that the two objects may have different densities, compositions near their surfaces, or different surface roughnesses.
