Asteroid 2011 UL21 is a potentially hazardous , near-Earth object that was previously estimated to be somewhere between 1.1 and 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, earning 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 zipped past our planet at more than 58,000 mph (93,000 km/h). The giant space rock reached a minimum distance of around 4.1 million miles (6.6 million kilometers) — around 17 times further 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 snapped grainy "bistatic" images of the gigantic rock by bouncing radio waves off its surface, which revealed two things: First, the space rock is probably only around 1 mile (1.6 km) wide; and second, there was a second, smaller asteroid orbiting it.
These bistatic images show 2011 UL21 (top) and its unnamed minimoon (bottom) at five different points during its recent flyby. (Image credit: NASA/JPL-Caltech)
2011 UL21's newly discovered minimoon, which has not been named, is what researchers call a small-body satellite — an asteroid that orbits another larger asteroid or dwarf planet. From the images it is unclear how large the new space rock really is but it is likely significantly smaller than 2011 UL21.
The two asteroids form what is known as a binary system, and are separated by a distance of around 1.9 miles (3 km), according to JPL.
"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," Lance Benner , a JPL scientist who led the recent observations, said in a statement.
It's also possible that the newly discovered moonlet is actually two separate small-body satellites in close proximity, as NASA's Lucy mission recently proved is the case with asteroid Dinkinesh .
The next time we will get a good look at 2011 UL21 and its newfound companion will be in 2089, when the space rocks will get to within 1.7 million miles (2.7 million km) of our planet — about two and a half times closer to us than its current approach.
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