Last updated August 2, 2018 at 10:57 am
An asteroid discovered last year is actually two smaller rocks travelling through space in an intimate embrace.
Like two lovers on a lonely journey through space, astronomers have discovered of two asteroids locked in orbit around each other – the astronomical equivalent of holding hands.
However, for months astronomers had been watching the near-Earth asteroid 2017 YE5 without realising it was actually two objects, not one.
Discovered in December last year, it took three of the world’s largest radio telescopes studying a close pass in June to measure the physical properties of the asteroid. It was only then that astronomers realised the one asteroid was actually a pair, each about 900 metres in size.
This is only the fourth “equal mass” binary near-Earth asteroid ever detected, consisting of two objects nearly identical in size orbiting each other.
Investigating an astronomical love story
On June 21, the asteroid 2017 YE5 came within 6 million kilometres of Earth – its closest approach for at least the next 170 years. At its closest point, about 16 times the distance between Earth and the Moon, observations by NASA’s Goldstone Solar System Radar (GSSR) in California showed the first signs that it could be two separate objects.
Bi-static radar images of the binary asteroid 2017 YE5 from the Arecibo Observatory and the Green Bank Observatory on June 25. The observations show that the asteroid consists of two separate objects in orbit around each other. Credit: Arecibo/GBO/NSF/NASA/JPL-Caltech
The observations, announced by NASA but yet to be published, revealed two distinct lobes, but the asteroid’s orientation meant that scientists could not see if the two bodies were separate or joined. The astronomers kept watching, and were rewarded when the two objects rotated to expose a distinct gap between them.
The team at Goldstone then alerted Puerto Rico’s Arecibo Observatory and the Green Bank Observatory in the US, who teamed up to use the two observatories together like one giant array.
The Arecibo telescope transmitted a radar signal towards the asteroid, with the reflections received over 2,500km away in Green Bank. Together, they were able to confirm that 2017 YE5 consists of two separated objects.
When one becomes two
The observations between the three observatories, plus other researchers around the world, found the pair revolve around each other once every 20 to 24 hours.
Interestingly, based on their combined optical brightness, the two objects are larger than was originally thought, indicating that the two rocks do not reflect as much sunlight as a typical rocky asteroid.
Based on the new measurements, the astronomers say 2017 YE5 is about as dark as charcoal.
The images taken by Goldstone and Arecibo also revealed that the radar reflectivity of the two objects are very different, which has never been seen in other binary systems.
This, the researchers say, hints at an intriguing possibility – the two objects may differ in densities, compositions near their surfaces, or surface roughness.
In other words, they’re two quite different rocks, making it entirely possible they weren’t one rock that broke in two, nor even from the same source – an astronomical Romeo and Juliet.
They’re also different from other asteroids. Although approximately 15% of large asteroids are thought to be binary systems, they often vary in size. A binary system where the two rocks are of equal size are much rarer – until now, only three had ever been discovered near Earth.
The rare find of the two asteroid lovers provides astronomers with an opportunity to study binary systems. It could answer questions that astronomers have asked for years, including their composition and internal structure, and most intriguingly of all – how two rocks came to be caught together in an embrace.
Videos courtesy of NASA/JPL-Caltech
