When scientists discovered the asteroid 2017 YE5, they first thought they’d found an ordinary NEO (near-Earth object) that passed within 16 lunar distances on June 21, 2018. This close approach — the closest YE5 has been in 170 years — gave us an unprecedented look at the small asteroid and allowed us to discover it isn’t one asteroid at all: It’s two. They rotate around a common center.
There’s nothing unusual about binary pair asteroids; about 15 percent of all known asteroids are binary pairs. Asteroids, like planets, are also capable of retaining very small moons if the parent body is above a certain size, though these captures may be unstable and persist only until the pair encounter a larger asteroid that perturbs their relationship. Only four equal mass NEOs are known to exist, including 2017 YE5.
Researchers discovered this contact binary by combining the resources of multiple Earth observatories. First, the team at Arecibo (already planning to observe the asteroid) spoke to astronomers at Goldstone, who had picked up on the pair’s unusual properties. They then teamed up with researchers at Green Bank to run a series of tests in which Arecibo would send the initial signal and Green Bank would return it. This bi-static radar configuration allowed the telescopes to map the asteroids size and configuration:
If you prefer to see the image in a 3D model, we’ve got that as well:
The reason the two objects rotate around each other in a stable configuration is because the barycenter — the center of mass within the two-body system — is located in between the two asteroids. The rule holds true for larger bodies. Jupiter is just a fraction of the mass of the Sun, for example, but it’s still large enough to orbit a point outside the Sun rather than the Sun itself. Pluto and Charon are another example of a system where both orbit a point outside themselves. The Earth and Moon are in a similar configuration, though in our case the point the Earth-Moon system circles is still within the Earth itself — it just isn’t at the exact center.
Pluto is much larger than Charon, but Charon is large enough to pull Pluto’s barycenter out from the planet.
Another interesting finding from the Arecibo / Green Bank observations is that the two asteroids reflect radar differently. This hints at different compositions (or at least different surface deposits). The JPL writes:
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. (However, the majority of those binary asteroids consist of one large object and a much smaller satellite.) 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.
Scientists estimate that among near-Earth asteroids larger than 650 feet (200 meters) in size, about 15 percent are binaries with one larger object and a much smaller satellite. Equal-mass binaries like 2017 YE5 are much rarer. Contact binaries, in which two similarly sized objects are in contact, are thought to make up another 15 percent of near-Earth asteroids larger than 650 feet (200 meters) in size.
The great thing about studying the universe is that we literally find out incredible new things almost every time we turn our instruments skyward. It’s taken decades of work and constant technological iteration, but our ability to peer into the heavens — or into our own solar system — continues to reveal subtle wonders and small secrets, along with the occasional confirmation of long-standing scientific theories.