Last week, Earth was rewarded with a rare event: not one, but two large asteroids passed within shouting distance.
Neither 2024 MK nor 2011 UL21, as the asteroids are called, came close enough to pose a threat, but both were within range of radar imaging systems, so NASA took some nice snapshots to mark the occasion.
These are more than mere souvenirs from asteroid flybys. Scientists can study the images to understand the properties of the rocks found near Earth – information that can help us develop strategies for future asteroids that may one day threaten our planet.
The small corner of the solar system where Earth is located is mostly empty, but not completely empty. Occasionally a comet or asteroid sails by on its own orbit around the sun.
The vast majority of these will not pose a problem. But anything that passes within a certain distance of Earth or exceeds a certain brightness is considered potentially dangerous.
Because even if their current path is fine, something unexpected could happen, such as a collision with another object that puts them on a collision course with Earth. That’s not likely, but it’s not impossible either.
Both the 2024 MK and the 2011 UL21 fell into the “potentially dangerous” category. Fortunately for us, no unforeseen situation diverted them from their course in our direction.
2011 UL21 flew past Earth on June 27 at a distance of 6.6 million kilometers (4.1 million miles), about 17 times the distance between Earth and the Moon.
Then, less than two days later, 2024 MK appeared. On June 29, it flew by at a minimum distance of 295,000 kilometers (184,000 miles). That’s much closer, about three-quarters the distance between Earth and the Moon.
Imaging such objects is not easy, even when they are relatively close and classified as “large” asteroids. Overall, they are still quite small and not very bright.
That’s why NASA uses a large radar telescope to send radio waves into space and receive the return signal, which scientists can use to create images.
Because 2024 MK was much closer—an asteroid only passes this close in the sky every few decades or so—we were able to capture much more detailed images.
NASA used one telescope to send the radio waves and a second to receive them, producing images of 2024 MK that reveal not only the asteroid’s shape, but also bumps, dents, boulders and ridges.
It is about 150 meters in diameter and has an elongated shape with many flat surfaces. It also wobbles as it moves through space.
It was only discovered on June 16th and its orbit has been altered by Earth’s gravity, so the observations allow scientists to figure out what 2024 MK will do in the future. They’ve shown that it will stay safely out of our way for the foreseeable future. Phew.
“This was an extraordinary opportunity to study the physical properties of a near-Earth asteroid and obtain detailed images,” says astronomer Lance Benner of NASA’s Jet Propulsion Laboratory.
2011 UL21 did not provide such detailed images from its much greater distance, but it still contained a small surprise. There, astronomers discovered a tiny little moon in an orbit of about 3 kilometers next to the 1.5-kilometer-wide asteroid.
In fact, we are seeing this more and more often with large asteroids.
Last year, the asteroid Dinkinesh, an object in the asteroid belt visited by NASA’s Lucy probe, was found to have a small moon. And NASA’s famous Double Asteroid Redirection Test, in which a spacecraft collided with an asteroid, was carried out on Dimorphos, the smaller of a pair of double asteroids.
We’re discovering more binary asteroids because our imaging capabilities are improving. This is great news for planetary defense and our understanding of solar system evolution.
“It is assumed that about two-thirds of all asteroids of this size are binary star systems,” says Benner. “Their discovery is particularly important because by measuring their relative positions we can estimate their orbits, masses and densities, which in turn provides important insights into how they might have formed.”
And they’re just so damn cute. Hey, little buddy. Come by anytime.