That Mountain-Sized Asteroid That Flew By Earth On Monday Has A Tiny Little Moon

At 11:19 a.m. ET, Jan. 26, Earth had a close encounter with an asteroid, called 2004 BL86.

NASA scientists knew it was coming but as the large space rock approached, they saw something they did not expect: a tiny moon.

Scientists at NASA's Deep Space Network antenna in Goldstone, California took 20 images of the asteroid as it came to within 745,000 miles from Earth - about three times farther than the distance to the Moon. Put their images together and you get this nice gif that clearly shows the asteroid's small companion moon:

Although NASA has mapped 2004 BL86's path through our solar system in detail, they were surprised to see the moon, which seems to be about 230 feet across. This is small compared to the asteroid, which measures 1100 feet across and has a large enough area to cover five football fields.

An asteroid with a moon is not entirely uncommon. In fact, scientists think that about 16% of asteroids - greater than 655 feet across - have one moon and sometimes even two.

According to NASA calculations, we won't be seeing this asteroid come by Earth again for another 200 years.

Therefore, to make the most of this close encounter, the scientists using the Deep Space Network antennas will continue firing radio waves at the asteroid until Feb. 1, according to planetary astronomer Michael Busch based at the SETI Institute.

These low-energy waves cannot penetrate the rock and are reflected off the rock and can be detected back on Earth to give us more detail about the asteroid.