An asteroid wider than the length of Vatican City is heading in Earth's direction at the beginning of March, according to NASA's asteroid tracker, and while it won't hit us, you will be able to watch it.
Designated 138971 (2001 CB21), the asteroid, passing by Earth on March 4, is estimated to be between 560 meters to 1.2 kilometers wide. For comparison, even its lowest possible size would be considerably taller than the Empire State Building in New York City, which stands at 380 meters (443 meters if going to the tip). Its maximum estimated size is larger than the entire length of the Vatican, which measures around 1.05 kilometers.
The asteroid has also been designated as potentially hazardous, meaning it is of sufficient size and flying sufficiently close to us that we should be somewhat worried.
However, it is extremely unlikely that 138971 (2001 CB21) will hit us. According to NASA, the asteroid is set to pass Earth by at a distance of around 4.9 million kilometers. While this is very close on a cosmic scale, it is still much farther than the Moon, which orbits the Earth at a distance of approximately 384,000 kilometers.
This is fortunate, as if 138971 (2001 CB21) were to hit Earth, the results would be devastating.
How bad would it be?
According to research from the Davidson Institute of Science, the educational arm of Israel's Weizmann Institute of Science, an asteroid over 140 meters in diameter would release an amount of energy at least a thousand times greater than that released by the first atomic bomb if it impacted Earth.
Something even larger – over 300 meters wide like the asteroid Apophis – could destroy an entire continent. An asteroid over a kilometer in width, like 138971 (2001 CB21), could trigger a worldwide cataclysm.
For even greater comparison, the last major asteroid impact on Earth was in 1908 over Russia in what became known as the Tunguska incident. This impact was one of the largest explosions to ever happen on Earth and was far greater than a nuclear bomb. And according to estimates, this asteroid was just 190 meters in diameter or smaller.
How can we watch it?
But while the asteroid won't hit us – indeed, NASA has already calculated that the Earth is free of risk of asteroid impacts for the next century – you just might be able to watch it.
The Virtual Telescope Project, a remote-controlled telescope service based in Italy that has several remotely operated robotic telescopes available online, will be livestreaming a visual on 138971 (2001 CB21) as it flies by the planet.
The project had aimed to do the same for the last recent asteroid flyby, the 190 meter-430 meter 455176 (1999 VF22), but it had been called off due to cloudy weather obscuring visibility.
It might also be possible to watch it for yourself at home, if you have a sufficiently large enough telescope.