NASA snaps images of an asteroid large enough to have a moon of its own

What just happened? NASA struck gold late last month when it snapped a hefty asteroid cruising by Earth, giving us an up-close look at the rock–and its mini-moon tagging along. The images, captured by the Goldstone radar dish, show “2011 UL21” in great detail as it made one of the closest asteroid approaches of the year on June 27.

Initially discovered in 2011 by the NASA-funded Catalina Sky Survey in Tucson as it passed by, this giant space boulder is estimated to be around a mile wide. But this time it flew past close enough to be imaged by radar. The real surprise, though, was the tiny moon orbiting the asteroid at a distance of about 1.9 miles.

According to NASA scientists, it’s pretty common for big asteroids like this to be binary systems with one or more little moons in tow. But actually spotting them is far from easy.

“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,” said Lance Benner, principal scientist at JPL (Masthead: asteroid 243 Ida and its moon).

The Goldstone Solar System Radar’s massive 230-foot dish, the world’s largest fully steerable radar antenna, has scanned the heavens for three decades from California’s Mojave Desert. This powerhouse has supported numerous missions like Mars rovers, Saturn’s Cassini, asteroid explorers Hayabusa, and even recovered the sun-watching SOHO probe.

Jet Propulsion Laboratory scientists used the same antenna to transmit radio waves to the asteroid and receive the reflected signals. The high-resolution radar imagery also gave us a better look at UL21’s almost perfectly spherical shape and surface features like craters. Although grainy, it’s not bad for an object that passed within 4.1 million miles, or 17 times the distance to our moon.

As if one cosmic photo op wasn’t enough, the Goldstone team also captured another asteroid, 2024 MK, just days later, on June 29. This smaller 500-foot rock buzzed much closer, at 184,000 miles, within 75 percent of the Earth-moon distance. You can check out the full-res images in NASA’s press release.

The images provide a detailed look at 2024 MK’s battered surface, including craters, ridges, and boulders as big as 30 feet across. While nowhere near as massive as 2011 UL21, this still qualified as a relatively close shave.

NASA says these close encounters help it study potentially hazardous asteroids and prepare for planetary defense. The more data we have on their orbits, spins, and physical makeup, the better we can predict and prepare for future threats.

“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 team noted.

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