In late June 2024, two small asteroids passed relatively close to Earth. One was 2011 UL21, a 1.5-kilometer wide rock that passed about 6.6 million km away on June 27. Two days later the much smaller 2024 MK zipped by just under 300,000 km from us, closer than the Moon!

When asteroids get this close it’s possible to use radio telescopes to ping them with radar. These radio waves hit the asteroid and are reflected back to Earth, which are received by radio telescopes (sometimes the one that sent them, and sometimes others). By carefully measuring the amount of time it takes for the pings to get there and back, we can get the distance to the asteroid extremely accurately. Also, the shape and rotation of the asteroid can be determined as well, since that changes how the radar waves are reflected back to Earth (that’s a wee bit more complicated, but my friend Emily Lakdawalla wrote about how that works for The Planetary Society).

Astronomers used NASA’s Goldstone radar system to look at both asteroids as they passed. Here’s what they saw for UL21:

Along the top you can see seven images of the asteroid, each taken about 15 minutes apart. The image is faint because UL21 was very far away when these were taken, so the radar waves were spread out more. The shape changes slightly because the asteroid is rotating. The surprise, though, is along the bottom: an asteroid moon! That tiny dot is a smaller rock orbiting 3 km away. Quite a few small asteroids are actually binary systems, so in reality this isn’t a surprise. They may form when two asteroids collide, and the impact dislodges a chunk or creates a debris cloud that coalesces into a smaller moon.

The other asteroid, 2024 MK, got closer, so the images of it are brighter and easier to see:

The asteroid spins roughly once every half hour, and the images show it to be irregular and even a bit angular, with no obvious craters (which isn’t surprising; it’s pretty small) and some odd flat areas in various spots (there’s also another mosaic of hundreds of images of it taken a minute apart that shows the rotation better, as well as an animated GIF). Some boulders can be seen as well.

Images like these serve multiple purposes. They help refine the orbit more precisely, which is critical for predicting future passes… or potential impacts. For example, 2024 Mk’s orbit was changed by Earth’s gravity, reducing its orbital period by about 24 days (it takes 3.3 years to orbit the Sun). They also tell us about how the asteroids behave physically, which can tell us about their composition (solid or rubble piles) and history.

Happily, neither of these space rocks are an immediate threat to Earth. But that won’t always be the case, so it makes me feel better knowing we’re keeping an eye out for any possible hazardous asteroids, and taking their measure when we can.

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