It has turned out to be a unique kind of double space rock -- a "binary asteroid" with twin components of almost equal size that lazily revolve around a common axis as they pursue a long track around the Sun, astronomers said Thursday.

Scanning by the world's most powerful radar, a 300-metre (1,000-feet) dish at Arecibo in Puerto Rico, has revealed that the two rocks that make up 1937 UB (Hermes) take up to 21 hours to complete this revolution, according to the US Lowell Observatory in Arizona.

"This re-sighting of Hermes is the Holy Grail of near-Earth asteroid discovery," the observatory's Edward Bowell said.

Hermes created a stir when it flew by close to the Earth in October 1937 at a distance of less than a million kilometers (650,000 miles), just 60 percent further than the distance of the Earth to the Moon.

It then swiftly disappeared from view, breeding fears that one year, its unknown orbit could bring it perilously close to the Earth.

The body is about 900 metres (1,000 yards) long, which is certainly big enough to cause massive destruction if it collided with Earth, but calculations show that its track does not present any danger for the foreseeable future.

It will be at its closest to the Earth on November 4, when it will be 6.4 million kilometers (four million miles) away, reflecting enough light from the Sun to make it visible for amateurs using backyard telescopes.

Hermes is divided into two rocks of almost equal size, the observatory said.

That is unusual. About 10 other large binary asteroids have so been been scanned in detail by radio, and all of them show a large "mother" rock escorting a "baby".

According to calculations by NASA's Jet Propulsion Laboratory (JPL) in the United States, Hermes' orbit around the Sun is "very chaotic" because it is tugged by gravitational pull from Earth and Venus whenever it flies close to those planets.

It takes roughly two years to swing around our star.

According to modelling by JPL's Steven Chesley and Paul Chodas, Hermes passed even closer to Earth in 1942, but was never spotted.

However, the closest it will get to our planet over the next century is eight lunar distances -- ie eight times the distances between the Earth and Moon.

Hermes is named after the messenger of the Greek gods. The rediscovery was made on the morning of October 15 by Lowell Observatory astronomer Brian Skiff.

Asteroids are speculated to be the rubble left over from the making of the Solar System -- space rocks that orbit the Sun, although sometimes along long and highly elliptical orbits.

SPACE.WIRE