NASA reveals "planet killer" asteroid and its mysterious moon

NASA published new images of an asteroid that flew past Earth last month, as reported on the Live Science portal. The asteroid, referred to as a "planet killer," is the size of a mountain, and next to it, there is a mysterious and unexpected companion.

Live Science reminds us that the "planet killer," actually asteroid 2011 UL21, is a potentially dangerous object moving close to Earth. It orbits the Sun once every three years and approaches our star at a distance of 1.3 astronomical units (AU), about 195 million kilometres. The space rock was closest to Earth in 2011, about 6.6 million kilometres away. This means that 2011 UL21 is one of the 10 largest asteroids to have passed so close to Earth since 1900, as Live Science recalls.

The rather terrifying name of 2011 UL21 is due to its size. Scientists previously estimated that the "planet killer" has a width of about 1-2 kilometres, making it larger than 99 per cent of asteroids near Earth. Such large dimensions allow asteroid 2011 UL21 to effectively destroy an entire continent upon impact and, as Live Science explains, cause catastrophic climate changes across the entire Earth. Scientists, however, reassure that the "planet killer" is not expected to hit Earth in the future.

In theory, 2011 UL21 does not surprise scientists. However, as it turned out, in recent days, NASA discovered that the space rock is smaller than previously assumed. Moreover, a mysterious object accompanies it.

Thanks to a photo taken at the American space agency's Goldstone Deep Space Communications complex in California and observing the "planet killer's" flight using the Goldstone Solar System Radar telescope, scientists were able to accurately reflect radio waves from the asteroid's surface. This revealed two things. First and foremost, 2011 UL21 is likely smaller than scientists previously estimated. According to the latest findings, the "planet killer" has a width of 1 kilometre.

The second valuable discovery was the identification of a second object next to the asteroid. As it turned out, 2011 UL21 moves through space accompanied by a second object – a smaller planetoid, or moon, orbiting it at a distance of about 3 kilometres.

– It is believed that about two-thirds of planetoids 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, providing key information about how they may have formed – said Lance Benner, a scientist from NASA who participated in the observations of the 2011 UL21 binary system.