Fuzzy new photos of a mountain-sized asteroid, a ‘planet killer’ that recently reached its closest approach to Earth in more than a century have revealed a surprising new feature of the giant space rock: it has a secret mini-moon.
Asteroid 2011 UL21 is a possibly dangerousnear-Earth object previously estimated to be anywhere from 1.1 to 2.4 miles (1.7 to 3.9 kilometers) wide, making it larger than 99% of near-Earth asteroids. At this size, it could likely wipe out an entire continent and cause devastating climate change on a global scale, making it the planet killer moniker. However, it is not predicted to hit Earth in the future.
On June 27, 2011, UL21 reached its closest point to Earth in at least 110 years when it sped past our planet at more than 58,000 mph (93,000 km/h). The giant space rock came a minimum distance of about 4.1 million miles (6.6 million kilometers) — about 17 times farther from our planet than the moon. This likely makes it one of the 10 largest asteroids to pass this close to Earth since 1900, according to astrophysicist Gianluca Masi of the Virtual Telescope Project in Italy.
Scientists at NASA’s Goldstone Deep Space Communications Complex in California monitored the historic flyby using the Goldstone Solar System Radar Telescope. The team captured grainy “bistatic” images of the giant rock by bouncing radio waves off its surface, which revealed two things: First, the space rock is likely only about 1 mile (1.6 km) wide; and second, there was a second, smaller asteroid orbiting it.
Two days later, on June 29, the same telescope also captured images of the pyramid-sized asteroid 2024 MK as it passed our planet closer than the moonThese images showed that this recently discovered space rock had been blown off course by the collision.
The new images of both asteroids were released on July 3 by NASA’s Jet Propulsion Laboratory (JPL).
Related: No, NASA Didn’t Warn of an Impending Asteroid Impact in 2038. Here’s What Really Happened.
The newly discovered mini-moon of UL21, which has not yet been named, is what researchers call a small satellite: an asteroid that orbits another larger asteroid or dwarf planet. It is not clear from the images how big the new space rock actually is, but it is likely considerably smaller than 2011 UL21.
According to JPL, the two asteroids form a so-called binary star system and are about 3 km apart.
Small satellites are the most common types of natural satellites in the solar systemand are especially common around asteroids of this size.
“About two-thirds of asteroids of this size are thought to be binary systems. Their discovery is of great importance because we can use measurements of their relative positions to estimate their orbits, masses and densities. This information gives us important clues about how they may have formed.” Lance Bennera JPL scientist who led the recent observations, said in a statement.
It’s also possible that the newly discovered moon is actually two separate small satellites orbiting close to each other, as NASA’s Lucy mission recently demonstrated is the case with asteroid Dinkinesh.
The next time we’ll get a good look at 2011 UL21 and its new companion will be in 2089, when the space rocks will arrive at a distance of 2.7 million kilometers from our planet, about two and a half times closer to us than they are today.