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When scientists discovered the 2017 asteroid YE5, they first thought that they had found an ordinary NEO (near-Earth object) that crossed 16 lunar distances on June 21 2018. This close-up approach – the nearest YE5 has been in 170 years – gave us an unprecedented look at the small asteroid and allowed us to discover that it's not an asteroid at all: he is two. They revolve around a common center.
There is nothing unusual about binary asteroids; about 15 percent of all known asteroids are binary pairs. Asteroids, like planets, are also able to hold very small moons if the parent body is above a certain size, although these captures may be unstable and persist until the pair encounter a larger asteroid that disrupts their relationship. It is known that only four NEOs of equal mass exist, including 2017 YE5
The researchers discovered this binary contact by combining the resources of several terrestrial observatories. First, the Arecibo team (who were already planning to observe the asteroid) spoke to Goldstone astronomers, who had discovered the unusual properties of the pair. They then teamed up with Green Bank researchers to conduct a series of tests in which Arecibo would send the initial signal and Green Bank would return it. This bi-static radar configuration allowed telescopes to map the size and configuration of asteroids:
If you prefer to see the image in a 3D model, we have this as well:
The reason that the two objects rotate around each other in a stable configuration is because the center of gravity – the center of mass in the two body systems – is located between the two asteroids. The rule is true for larger bodies. Jupiter is only a fraction of the mass of the Sun, for example, but it is still large enough to orbit around a point outside the Sun rather than the Sun. himself. Pluto and Charon are another example of a system where the two orbits a point apart from themselves. The Earth and the Moon are in a similar configuration, although in our case the point where the Circles of the Earth-Moon system are still in the Earth itself – it is not exactly the center.
Pluto much larger than Charon, but Charon is large enough to extract Pluto's barycenter from the planet.
Another interesting finding from the observations of Arecibo / Green Bank is that the two asteroids reflect the radar differently. This suggests different compositions (or at least different surface deposits). The JPL writes:
Goldstone's images taken on June 21 also show a striking difference in the radar reflectivity of the two objects, a phenomenon that has not been observed before among more than 50 other systems. binary asteroids studied by radar since 2000. (However, most of these binary asteroids are made up of a large object and a much smaller satellite.) The reflectivity differences also appear in the images of the binary asteroids. Arecibo and suggest that the two objects may have different densities, compositions near their surfaces. Scientists estimate that among the near-Earth asteroids larger than 200 meters, nearly 15 percent are binaries with a larger object and a much smaller satellite. The equal mass binaries like 2017 YE5 are much rarer. Contact binaries, in which two objects of similar size are in contact, would constitute another 15% of the asteroids close to the Earth whose size is greater than 200 meters.
The great thing about studying the universe is that we literally discover amazing new things almost every time we turn our instruments to the sky. It has taken decades of work and constant technological iteration, but our ability to look into the sky – or into our own solar system – continues to reveal subtle wonders and little secrets, with the occasional confirmation of ancient scientific theories.
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