One Of The Asteroids That Zipped Past Earth Is So Big It Has Its Own Moon, NASA Has Images

Scientists with NASA’s Jet Propulsion Laboratory (JPL) tracked two near-Earth asteroids recently, one with a moon of its own, and the second having only been detected 13 days prior to its closest approach to Earth. The data collected during the radar observations will provide valuable practice for planetary defense, as well as given greater insight into their sizes, orbits, rotation, and more.

Asteroid 2024 MK recently made headlines due to it only being detected two weeks prior to its closest approach to Earth. The asteroid, roughly the size of the Great Pyramid Giza, reminded everyone of how such a large object can still escape detection. Not to be outdone, however, asteroid 2011 UL21 provided a surprise for scientists and astronomers of its own. For the first time, it was determined the nearly mile-wide object was not alone, and in fact had a moonlet orbiting it at about a distance of 1.9 miles.

JPL used the Deep Space Network’s 230-foot-wide Goldstone Solar System Radar, also known as Deep Space Station 14, to track the two cosmic objects. According to a blog post by JPL, scientists transmitted radio waves to asteroid 2011 UL21, and received the reflected signals by the same antenna. The images of the asteroid and its moonlet, seen above, show the asteroid during its June 27, 2024 close approach to Earth from about 4 million miles away. Highlighted inside white circles are asteroid 2011 UL21 and its little friend.

“It is thought that about two-thirds of asteroids of this size are binary systems, and their discovery is particularly important because we can use measurements of their relative positions to estimate their mutual orbits, masses, and densities, which provide key information about how they may have formed,” explained Lance Benner, principal scientist at JPL who helped lead the observations.

Asteroid 2024 MK made its closest approach to Earth two days after 2011 UL21, coming within 184,000 miles from Earth. To put how close that is into perspective, it is slightly more than three-quarters of the distance between the Moon and Earth. In the radar images (see image at top), the 500-foot-wide asteroid appeared to be elongated and angular. JPL scientists also captured the observations of 2024 MK using the Deep Space Station 14 to transmit radio waves to the asteroid. However, they used Goldstone’s 114-foot Deep Space 13 antenna to receive the signal that bounced off the asteroid and back to Earth.

The data collected during the radar observations of 2011 UL21 and 2024 MK could prove invaluable in future attempts to detect celestial bodies making close approaches to Earth before it is too late, as well as providing scientists with more information about how these large objects interact with Earth’s gravity as they fly by. According to Benner, “This was an extraordinary opportunity to investigate the physical properties and obtain detailed images of a near-Earth asteroid.”