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NASA says '3200 Phaethon' asteroid won't hit Earth. Here's what could happen if it did.

The 5-kilometer-wide space rock will come as close as 6.4 million miles on December 16, 2017.
Image: Asteroid Illustration
The asteroid 3200 Phaethon will whiz harmlessly by Earth on Dec. 16, 2017.Courtesy Hubble Space Telescope / ESA

A giant space rock is heading toward Earth, but there's no cause for alarm.

Calculations show there’s no chance that the three-mile-wide object — a “potentially hazardous asteroid” known as 3200 Phaethon — will hit Earth. Instead, NASA says, it will whiz harmlessly past our planet, coming as close as 10.3 million kilometers (6.4 million miles) on Dec. 16, 2017.

This will be the closest 3200 Phaethon has come since 1974. The space rock has a highly elliptical orbit around the sun, and it won’t come this close again until 2093.

Image: Phaethon Orbit
Phaethon's orbitSky and Telescope Magazine

In the world of astronomy, 6.4 million miles is pretty close. And it raises the question: How likely is it that some big, scary asteroid out there is on a collision course with our planet? The answer, astronomers say, is not very — at least during our lifetimes.

“There are no objects that have been identified that are known to be on a collision course with Earth,” Dr. Amy Mainzer, an astronomer at the Jet Propulsion Laboratory (JPL) in Pasadena, California and a noted expert on asteroids, told NBC News MACH in an email.

She said astronomers had found “most of the really big near-Earth asteroids” — objects bigger than 1 kilometer in diameter. All were determined not to pose a collision threat.

But “most” isn’t all. Mainzer’s colleague Dr. Paul Chodas, manager of the Center for Near Earth Object Studies at JPL, told MACH in an email that asteroid hunters think they’ve found about 95 percent of the big ones.

But "down at the 140-meter size, which is our next threshold of interest, we have yet to find even half of them,” Chodas said. “And the percentage of the [asteroid] population we’ve found goes down exponentially as we go to smaller and smaller sizes, down to 100 meters, 80 meters, 50 meters, etc.”

These smaller meteors can cause significant damage. The 20-meter-wide asteroid that slammed into Chelyabinsk, Russia in 2013, for example, shattered windows and injured more than 1,000 people.

And bad as that was, it’s nothing compared to the damage that would be caused by the impact of an asteroid the size of 3200 Phaethon

If a space rock that big were to smash into Earth, the online Impact Earth asteroid damage calculator indicates that — depending upon factors including its speed and angle of impact — it could create a crater 65 kilometers (40 miles) wide and 1 kilometer (0.6 mile) deep.

Image: Hyothetical Asteroid Impact Map
If a fast-moving asteroid the size of 3200 Phaethon were to collide with Earth, it could create an impact crater 40 miles wide and 0.6 mile deep. As you can see in this illustration, if the space rock were to hit New York City's Manhattan borough, the resulting crater would obliterate not just all of New York City but also several surrounding cities. Luckily, 3200 Phaethon will not hit Earth but whiz pass use harmlessly.Jiachuan Wu / NBC News

"There's no doubt that the impact of an object the size of Phaeton would be a devastating event for us," Dr. Dan Durda, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, told MACH in an email, adding that the dust and chemicals released into the air as a result of such impact could cause global cooling and "destroy the ozone layer several times over."

Lucky for us, that’s not going to happen. And NASA has a plan to save Earth from killer asteroids.