NASA has captured images of two asteroids that made a close pass around Earth last week. 

The space agency used the Goldstone Solar System Radar in California to transmit radio waves to the asteroids and analyze the reflected echoes. 

The first and larger asteroid, 2011 UL21, came within 4.1 million miles of Earth, or about 17 times the distance to the Moon. NASA captured faint images of the space rock, which is nearly one mile wide. The results reveal that 2011 UL21 has a spherical shape and a small “moonlet” orbiting it.

“This is the first time it has come close enough to Earth to be imaged by radar,” NASA said of 2011 UL21, which was first detected about 13 years ago. 

According to NASA, it’s actually common for asteroids the size of 2011 UL21 to have their own moon. In this case, the moonlet is orbiting at a distance of about 1.9 miles from the parent body. 

The second asteroid, 2024 MK, came far closer to our planet at 184,000 miles, or about three quarters the distance between the Earth and the Moon. Using a combination of radar systems, NASA was able to take a clearer image of the asteroid, which is about 500 feet wide.  

“Close approaches of near-Earth objects the size of 2024 MK are relatively rare, occurring about every couple of decades, on average, so the JPL team sought to gather as much data about the object as possible,” NASA says. 

Neither asteroid had or will have a remote chance of hitting the Earth in the foreseeable future. Still, 2011 UL21 is large enough that if it did, the collision could cause serious damage. An impact from 2024 MK could destroy a city.

To protect the planet from such asteroids, NASA in 2022 conducted a test demonstrating it has the capability to knock incoming asteroids into a new orbit.