But an asteroid impacting Earth isn't just the stuff of science fiction, it's also the stuff of science fact. NASA's Jet Propulsion Laboratory in California has an "Asteroid Watch" that's entire purpose is to detect and track asteroids (and comets) the agency thinks will come close to Earth.
There are at least five on the Asteroid Watch dashboard for 2022, alone, including one that is thought to be around 2.5 times the height of New York City's Empire State Building. NASA labeled this asteroid (7482) 1994 PC1 and has known about it since 1994. Two of the five asteroids flew by Earth safely (2021 YQ and 2021 YX) on Jan. 5, 2022, at distances of 1.3 and 2.4 million miles (2.1 and 3.7 million kilometers) respectively.
But not all asteroids throughout history have missed. There are obvious craters on Earth (and the moon) that prove a long history of massive objects hitting the planet. The most famous asteroid ever is the one that hit Earth 66 million years ago. The mountain-size asteroid left a crater off the coast of Mexico's Yucatán Peninsula 93 miles (146 kilometers) wide and 12 miles (19 kilometers) deep. Scientists think it created massive tsunamis and threw so much water and dust into the atmosphere that it cut off sunlight, lowered temperatures worldwide and caused the extinction of the dinosaurs.
What If an Asteroid Hit Earth Today?
Chances of an asteroid hitting are pretty small. NASA has tracked 90 percent of the near-Earth asteroids that are at least a half-mile (0.8 kilometers) wide and believes none of them has a significant chance of hitting Earth.
Still there could be some bigger asteroids out there that NASA doesn't know about. If a mile-wide asteroid hit Earth, it would strike the planet's surface at about 30,000 miles per hour (48,280 kilometers per hour). An asteroid that big traveling at that speed has the energy roughly equal to a 1 million megaton bomb.
It's difficult to imagine 1 million megatons, so let's try some smaller sizes. Let's say that an asteroid the size of a house hit Earth at 30,000 miles per hour. It would have an amount of energy roughly equal to the bomb that fell on Hiroshima — perhaps 20 kilotons. An asteroid like this would flatten reinforced concrete buildings up to half a mile from where it hit, and flatten wooden structures perhaps a mile and a half from ground zero. It would, in other words, do extensive damage to any city.
If the asteroid was as big as a 20-story building (200 feet [61 meters] on a side), it could have the amount of energy equal to the largest nuclear bombs made today — about 25 to 50 megatons. This size asteroid would flatten reinforced concrete buildings 5 miles (8 kilometers) from ground zero. It would completely destroy most major cities in the United States.
For an asteroid to wipe out most everything on Earth, it would have to be massive. Scientists estimate it would take an asteroid about 7 to 8 miles (11 to 12 kilometers) wide crashing into the Earth. Once it made impact, it would create a tremendous dust plume that would envelope the entire planet, block out the sun and raise temperatures where the asteroid made impact. Billions would die, and much of life on the planet would be destroyed. But, scientists believe some would survive. NASA scientists say it would take an asteroid 60 miles (96 kilometer) wide to totally wipe out life on Earth.
The good news is NASA's Double Asteroid Redirection Test (DART) robotic spacecraft launched into space in November 2021. Its sole mission is to slam into the small asteroid Dimorphos at 4.1 miles (6.6 kilometers) per second in an attempt to change the orbit just slightly. There's no chance Dimorphos will ever hit the Earth. NASA is using the asteroid as a safe target to test technology it might have to use in the future to alter the direction of a killer asteroid on a direct path toward Earth.
Originally Published: Dec 4, 2007