This enormous space boulder was first spotted in 2011 as it passed by Tucson, Arizona, by the NASA-funded Catalina Sky Survey. Its estimated width is one mile. However, this time it passed close enough for radar to image it. The little moon orbiting the asteroid at a distance of roughly 1.9 miles, however, was the true surprise.
Scientists at NASA say that large asteroids like this one are frequently binary systems carrying one or more minor moons. But it’s not at all simple to find them in reality.
“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,” stated Lance Benner, principal scientist at JPL.
For thirty years, the giant 230-foot dish of the Goldstone Solar System Radar, the biggest completely steerable radar antenna in the world, has been scanning the skies from California’s Mojave Desert. Numerous missions, including the Mars rovers, Cassini on Saturn, the Hayabusa asteroid explorers, and even the recovery of the sun-watching SOHO spacecraft, have benefited from this powerhouse.
Using the same antenna, scientists from Jet Propulsion Laboratory sent radio waves towards the asteroid and collected the reflected signals. We were also able to see surface features like craters and UL21’s nearly perfectly spherical shape thanks to the high-resolution radar photos. It’s not bad, albeit grainy, for an object that traveled 4.1 million miles—or 17 times the distance to our moon—by space.
Just a few days later, on June 29, the Goldstone team managed to take a picture of another asteroid, 2024 MK, as if one cosmic photo opportunity wasn’t enough. This smaller 500-foot rock buzzed within 75% of the distance between Earth and moon, at 184,000 miles. The high-resolution photos are available in NASA’s press release.
The photos offer a close-up view of the beaten terrain of 2024 MK, which features craters, ridges, and rocks up to thirty feet in diameter. Even while it wasn’t quite as huge as 2011 UL21, this was still considered a close call.
According to NASA, these near misses aid in its research into potentially dangerous asteroids and planetary defense planning. We can forecast and get ready for potential dangers more accurately the more information we know about their orbits, spins, and physical characteristics.
“There was no risk of either near-Earth object impacting our planet, but the radar observations taken during these two close approaches will provide valuable practice for planetary defense,” the researchers stated.