NASA has its eye on a lot of large asteroids that regularly make ‘close approaches’ to Earth.
Although a big space rock isn’t expected to hit Earth any time soon, Nasa has criteria for what would be considered a deadly asteroid.
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How big is a killer asteroid?
Thousands of space rocks are said to rip through Earth’s atmosphere almost everyday.
We don’t notice most of them because they’re small and burn up before they reach the ground.
An asteroid would need to be a certain size to break through our atmosphere’s natural defense system.
Nasa’s website explains: “Only once every few million years, an object large enough to threaten Earth’s civilization comes along.
“Impact craters on Earth, the moon and other planetary bodies are evidence of these occurrences.
“Space rocks smaller than about 25 meters (about 82 feet) will most likely burn up as they enter the Earth’s atmosphere and cause little or no damage.
“If a rocky meteoroid larger than 25 meters but smaller than one kilometer ( a little more than 1/2 mile) were to hit Earth, it would likely cause local damage to the impact area.
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“We believe anything larger than one to two kilometers (one kilometer is a little more than one-half mile) could have worldwide effects.”
For reference, a standard 65 seat American school bus is about 35 feet long.
That means an asteroid smaller than a school bus could still be deadly depending on where it hit.
However, Nasa doesn’t label all 82 feet wide asteroids as ‘potentially hazardous’.
They also need to meet other criteria.
What is a potentially hazardous asteroid?
Nasa’s Center for Near Earth Object Studies (CNEOS) explains on its website: “Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth.”
Large fast-moving space objects that come within 4.65 million miles of Earth are considered to be “potentially hazardous” by cautious space organizations.
One small change to their trajectories could spell disaster for Earth.
Thousands of near earth objects (NEOs) are tracked by scientists to monitor whether they’re on a collision course with our planet.
Nasa lists them in its NEO Earth Close Approaches table.
How big was the asteroid that killed the dinosaurs?
The asteroid that’s thought to have wiped out the dinosaurs was about six miles wide.
Manhattan in New York is about 2.3 miles wide.
In comparison, the Earth has a diameter of just over 7,917 miles.
That means the Earth only needs to be hit by a space rock much smaller than it for a complete doomsday event to occur.
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