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.