There is always someone on call, watching the ATLAS telescopes.

The ATLAS survey, or Asteroid Terrestrial-impact Last Alert System, is a system of specialized instruments that sleuth the skies for any space rocks that might be whizzing near Earth. They're on the hunt for unknown, potentially threatening asteroids that pose a threat to our planet. And one of the ATLAS telescopes was in prime position to film NASA intentionally slam a spacecraft into an asteroid on Sept. 26. You can see the unprecedented footage below.

NASA's mission, called the Double Asteroid Redirection Test, or DART, is humanity's first-ever attempt to purposefully move an asteroid. The rocky target, Dimorphos, is not a threatto Earth, but the mission is an experiment to see how civilization could alter the path of a menacing asteroid, should one ever be on a collision course with our planet. 

The aim wasn't to destroy Dimorphos, which is currently some 6.8 million miles from Earth. Rather, the intention is to just smack the stadium-sized asteroid with a spacecraft the size of a vending machine. Scientists hope to just slightly nudge the asteroid, and ultimately prove the ability to alter an asteroid's trajectory. The ATLAS footage shows some pulverized rock ejecting into space after the collision, which was expected.

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Unlike some telescopes in NASA's planetary defense network, ATLAS — created by the University of Hawaii and funded by NASA — doesn't peer into the deep solar system (particularly the asteroid belt between Mars and Jupiter) for new space rocks. Rather, ATLAS acts like a "floodlight" for anything orbiting relatively close-by, like the asteroid Dimorphos.

"When smaller objects come to Earth and whizz by, that's when we see them," Larry Denneau, one of the researchers who heads the ATLAS survey, told Mashable earlier this year. The ATLAS telescopes — two in Hawaii, one in Chile, and one in South Africa — can scan the entire sky each night.

"We're looking everywhere all the time to find anything close to us," Denneau added.

The ATLAS telescopes can can spot something around 65 feet across a few days out; a 300 or so foot-wide rock can be detected weeks out. ATLAS has spotted small, fortunately harmless rocks that indeed did hit Earth. For example, in June 2019 the survey detected the 13-foot-wide asteroid 2019 MO. Just twelve hours later it exploded in the sky near Puerto Rico.

These telescopes can provide warning if a space rock surprises us, like the unexpected football-field-sized asteroid that swung just 40,000 miles from Earth in 2019. But overall, the goal in asteroid detection is to know if a sizeable space rock will impact Earth many years or decades in advance. That will give NASA and other space agencies the ability to deflect it away, by slightly changing its trajectory. Over the course of years, just a small initial change adds up to a significant trajectory shift.

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So far, astronomers estimate they've found just 40 percent of the rocks 460 feet across (commonly referenced as 140 meters) or larger in our solar system neighborhood, meaning that they pass within 30 million miles of us. These are still relatively large, menacing objects.

"There are a lot of those out there waiting to be discovered," Denneau explained. "One-hundred and forty meters is take-out-a-large-city size."

Fortunately, astronomers like Denneau are finding about 500 such space rocks annually. Crucially there are no known asteroids on a collision course with Earth for the next century.

But in the decades, or centuries, ahead, it's possible astronomers may find a threatening space rock. And we might send a spacecraft — or a fleet of spaceraft — to slam into that asteroid.