Astronomers say they can now spot previously unknown Earth-bound asteroids early enough to evacuate potential impact sites.
2019 MO is only four metres across, but was detected barreling towards the Earth when it was 500,000km away - about 30 percent further than the moon - according to the University of Hawaii's Institute for Astronomy, which operates space telescopes on the remote state's Big Island and Maui.
Initial analysis suggested a collision with Earth was possible, and when astronomers discovered a second telescope had picked it up, they determined it was likely.
Twelve hours later on June 22, something slammed into the atmosphere just south of San Juan, Puerto Rico - matching where the astronomers had predicted it would hit.
2019 MO harmlessly exploded in the atmosphere with a force of five kilotons, about a third the size of the bomb that was dropped on Hiroshima in World War II, space news site EarthSky reported. The impact was picked up by weather satellites and an infrasound monitoring station in Bermuda.
LiveScience reported it's just the fourth time an asteroid has been detected before entering the Earth's atmosphere, but previous impacts have happened within a few hours of discovery.
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The Institute for Astronomy says its telescopes have now proven capable of detecting objects as small as 4m across with a half a day's notice.
"It will find larger objects, like the one that exploded over Chelyabinsk, Russia in 2013, a few days before they impact," the institute said in a statement.
"That asteroid was about 20m across, or the size of a house. For the first time in history, astronomers can provide sufficient warning to move people away from the impact site."
Objects the size of 2019 MO hit the Earth about twice a year - so it shouldn't be too long before regular warnings start coming.