The huge asteroid that made a close pass by Earth on Monday has a small moon in tow, radar images released by NASA show.

Asteroid 2004 BL86 flew about 1.2 million km (745,000 miles) from Earth, about three times farther than the moon, with closest approach coming at 11:19 a.m. ET (1619 GMT) on Monday.

While it posed no threat to Earth, the flyby did provide astronomers an opportunity for some close-up studies without having to launch and operate a robotic probe.

Radar images taken by NASA's Deep Space Network antenna in Goldstone, California, show the 325 meter (1,100-foot) wide asteroid has a small moon in orbit, NASA said.

The asteroid is about 300 metres across - here's how its size compares to the CN Tower and a cruise ship. (Earl Cabuhat/CBC)

The moon is about 70 metres (230 feet) in diameter, NASA said.

About 16 per cent of asteroids that are about 200 metres (655 feet) or larger are a binary - with a primary asteroid and a smaller asteroid moon - or even triple systems, with a primary body and two moons, NASA said.

Made of basalt

Related images taken with an infrared telescope in Hawaii show the asteroid is primarily basalt, with a composition similar to lava flows found in Hawaii.

The asteroid that flew past Earth this week likely broke off from Vesta, a protoplanet orbiting in the asteroid belt between Mars and Jupiter. This image of Vesta was captured by the Dawn spacecraft.

Asteroids of this type are believed to be pieces of Vesta, a large protoplanet circling in the main asteroid belt between Mars and Jupiter, that were blasted into space by impacts, NASA said.

Monday's flyby was the closest Asteroid 2004 BL86 will come to Earth for at least the next 200 years.