Asteroid 'planet killer' reveals mysterious moon in NASA photo

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

Live Science notes that the "planet killer," officially known as 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 195 million kilometres (1.3 astronomical units). 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 come so close to Earth since 1900, as Live Science recalls.

The somewhat terrifying name of the 2011 UL21 is due to its size. Scientists previously estimated that the "planet killer" is about 1.6 to 3.2 kilometres wide, larger than 99 per cent of near-Earth asteroids. 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 Earth. Scientists, however, reassure us 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 recently, NASA discovered that the space rock is smaller than previously assumed. Moreover, it has a mysterious companion.

Thanks to a photo taken at the American space agency's Goldstone Deep Space Communications Complex in California, scientists could observe the "planet killer's" flight using the Goldstone Solar System Radar telescope. They accurately reflected radio waves off the asteroid's surface, revealing two things. First and foremost, the 2011 UL21 is likely smaller than scientists previously estimated. According to the latest findings, the "planet killer" is about 1.6 kilometres wide.

The second valuable discovery was identifying a second object next to the asteroid. 2011 UL21 moves through space accompanied by a smaller planetoid, or moon, orbiting it at a distance of about 3.2 kilometres.

"It is believed that about two-thirds of planetoids of this size are binary systems," said Lance Benner, a scientist from NASA who participated in the observations of the 2011 UL21 binary system. "Their discovery is significant 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."