The mountain-size asteroid that gave Earth a close shave Monday (Jan. 26) has its own moon, new radar images of the object reveal.

Asteroid 2004 BL86 cruised within 745,000 miles (1.2 million kilometers) of Earth Monday morning — about 3 times the distance between our planet and the moon. And the space rock wasn't alone: A companion is clearly visible in an asteroid flyby movie compiled from 20 images taken Monday by NASA's Deep Space Network antenna in Goldstone, California.

The radar images "show the primary body is approximately 1,100 feet (325 meters) across and has a small moon approximately 230 feet (70 m) across," NASA officials said in a statement. "In the near-Earth population, about 16 percent of asteroids that are about 655 feet (200 m) or larger are a binary (the primary asteroid with a smaller asteroid moon orbiting it) or even triple systems (two moons)."

Radar image showing the near-Earth asteroid 2004 BL86 and its moon flying past Earth on Jan. 26, 2015.
Radar image showing the near-Earth asteroid 2004 BL86 and its moon flying past Earth on Jan. 26, 2015.
Credit: NASA/JPL-Caltech

The resolution of the new images is about 13 feet (4 m) per pixel, they added.

The size measurement for 2004 BL86 is a signficant revision; before Monday's flyby, scientists had estimated the space rock's diameter to be about 1,800 feet (550 m).

Earth was never in any danger during Monday's encounter, which scientists had known about well in advance. But a known asteroid as big as 2004 BL86 won't come so close to Earth again until 2027, when a space rock called 1999 AN10 zooms past our planet.

To date, researchers have discovered and tracked just over 12,000 near-Earth asteroids — just a tiny fraction of the total population, which is thought to number in the millions. But more than 95 percent of the potential civilization-enders out there — space rocks more than 0.6 miles (1 km) wide — have been found, and none of them pose any threat in the foreseeable future.

Asteroid 2004 BL86 was discovered in January 2004 by the Lincoln Near-Earth Asteroid Research survey in New Mexico.

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