Near-Earth asteroid is actually a binary system

Discovery gives scientists opportunity to better understand binary asteroids’ formation processes and compositions.

A near-Earth asteroid discovered last year is actually a binary system of two objects that orbit one another, according to observations conducted by three of the world’s largest radio telescopes.

Asteroid 2017 YE5, found by the Morocco Oukaimeden Sky Survey on December 21, 2017, is the fourth “equal mass” binary near-Earth asteroid ever found. “Equal mass” binaries are systems of two objects almost identical in size and mass orbiting but not touching one another.

Exactly six months after 2017 YE5’s discovery, on June 21 of this year, the asteroid made its closest approach to Earth for the next 170 years, coming about 16 times the Earth-Moon distance, or 3.7 million miles (six million km) of our planet.

Because nothing was known about the asteroid’s physical properties, researchers at three separate observatories took advantage of its close approach and turned their radio telescopes to the object.

NASA’s Goldstone Solar System Radar (GSSR), located in California, found the first signs that the asteroid is really two objects, revealing two lobes.  Scientists could not tell whether the lobes were or were not attached to one another until the objects’ rotations revealed a gap between them.

After being alerted to the fact that 2017 YE5 showed signs of being a binary, scientists at the Arecibo Observatory in Puerto Rico and at the Green Bank Observatory (GBO) in West Virginia set up their radio telescopes to work together to study the object through an arrangement known as a “bi-static radio configuration.” The setup involved Arecibo transmitting a radio signal and Green Bank receiving the return signal.

The joint effort confirmed the asteroid’s status as a binary system, revealing the two objects orbit each other once every 20 to 24 hours.

Observations of the asteroid in visible light by astronomers at the Center for Solar System Studies in Rancho Cucamonga, California, confirmed the rotation data.

Radar data revealed the objects do not reflect the level of sunlight typical of rocky asteroids, meaning they have dark surfaces. The two objects show different levels of radar reflectivity, an unusual feature for binary asteroid systems. Differences in their radar reflectivity could indicate they have different density levels, surface roughness, and near-surface compositions.

According to scientists’ estimates, approximately 15 percent of near-Earth asteroids 650 feet (200 meters) or larger are binaries, but most consist of a larger object orbited by a smaller one rather than two equal-mass bodies. Another 15 percent of near-Earth asteroids this size or larger are contact binaries, in which two objects of roughly the same size touch one another while orbiting each other.

As a next step, researchers hope to determine the densities of 2017 YE5’s two components by analysis of both radar and optical observations.


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