Blurry new photos of a mountain-size, "planet killer" asteroid that recently made its closest approach to Earth in more than a century have revealed a surprising new feature of the gigantic space rock — it has a secret minimoon.
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.
