Rare double asteroid discovered: NASA

An asteroid discovered last year is actually two objects, each about 900 metres in size and orbiting each other, new observations by three of the world's largest radio telescopes have revealed.

Near-Earth asteroid 2017 YE5 was discovered with observations provided by the Morocco Oukaimeden Sky Survey in December last year, but no details about the asteroid's physical properties were known until the end of June, NASA said in a statement.

This is only the fourth "equal mass" binary near-Earth asteroid ever detected, consisting of two objects nearly identical in size, orbiting each other.

The new observations provide the most detailed images ever obtained of this type of binary asteroid, according to the US space agency.

On June 21, the asteroid 2017 YE5 made its closest approach to Earth for at least the next 170 years, coming to within six million kilometers of Earth, or about 16 times the distance between Earth and the Moon.

On June 21 and 22, observations by NASA's Goldstone Solar System Radar (GSSR) in California showed the first signs that 2017 YE5 could be a binary system.

The observations revealed two distinct lobes, but the asteroid's orientation was such that scientists could not see if the two bodies were separate or joined.

Eventually, the two objects rotated to expose a distinct gap between them.

Scientists at the Arecibo Observatory in Puerto Rico teamed up with researchers at the Green Bank Observatory (GBO) in West Virginia 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, NASA said.

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.

Scientists estimate that among near-Earth asteroids larger than 200 metres in size, about 15 per cent are binaries with one larger object and a much smaller satellite. Equal-mass binaries like 2017 YE5 are much rarer.