You may not have noticed, but the Earth had a close shave on Monday as an asteroid with its own moon made a cosmic near miss. As it passed within 3.1 times the distance from the Earth to the Moon, NASA's 230-foot-wide (70-meter) Deep Space Network (DSN) captured radar images of asteroid 2004 BL86 and its tiny satellite.
Asteroid 2004 BL86 was discovered in 2004 by the Lincoln Near-Earth Asteroid Research (LINEAR) survey in White Sands, New Mexico. First seen in telescope observations, its irregular brightness indicated that it had a moon, though it was too small to be seen directly. According to NASA, 16 percent of asteroids bigger than 655 ft (200 m) are binaries.
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On January 26, at 8:19 am PST, the 1,100 ft (325 m) object came within 745,000 mi (1.2 million km) of the Earth during in an encounter that won't be repeated for another 200 years. As BL86 passed, radar images were taken by the DSN antenna at Goldstone, California. This was actually a bit of a bonus because the radar's main purpose was to gather precise measurements of the asteroid's velocity to better predict its orbit. However, with a resolution of 13 ft (4 m) per pixel, Goldstone was able to build up 20 images into a movie showing BL86's tiny moon circling it.
The space agency says that Monday's encounter is a rare event and that another known asteroid of similar size won't visit the Earth until asteroid 1999 AN10 makes a flyby in 2027.
The video below shows BL86 and its moon.