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Close encounters of the asteroid kind

(NASA/Goddard Space Flight Center/file 2012)

A previously undetected asteroid that could rival the size of a mid-sized Boston office building passed by Earth this week in a close shave that brought the object within half the distance of the moon.

The space rock — with the catchy name 2017 AG13 — was likely somewhere between 36 feet and 115 feet across, a relatively small size that makes asteroids difficult to detect when they are far away from Earth but could be very dangerous if they broke up or hit the ground near a city.

“On the rare occasions where those happen over populated areas, we need to know about them, but there are [about a million] objects that size,” said Gareth Williams, associate director of the International Astronomical Union’s Minor Planet Center.

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Close passes like the one this week are routine, according to Williams, who conducts his work at the Harvard-Smithsonian Center for Astrophysics in Cambridge.

As many as 20 per year come within the same distance, he said, and 154 are known to have come closer, though he added that 2017 AG13 is “a little on the large size” in that group.

Researchers detected the object on Sunday, and it became obvious relatively quickly it was not a threat. The asteroid sailed by Monday at a speed of about 35,000 miles per hour relative to the Earth, Williams said.

“Very quickly after we got the initial sets of observations, it was clear that it was going to make a very close approach, but not close enough that it would hit us this time around,” he said.

The objects rarely hit Earth, but when they do, the results can be dramatic.

For comparison, experts have said the 2013 meteor that caused a massive blast in the sky above Chelyabinsk, Russia, was about 65 feet across. The meteor that ignited in the Maine sky last summer, causing a large fireball, was estimated at about 5 feet across.

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Researchers have been working to get a handle on “near-Earth objects,” and a recent federal report said asteroids a little larger than the highest estimate for 2017 AG13 could cause serious problems.

The report cited the 1908 explosion of an estimated 130-foot object that felled trees across hundreds of square miles in central Russia.

“If a similar airburst event were to occur over a major metropolitan area, millions of injuries and casualties could result,” the report said.

The report said 300,000 undetected objects bigger than that size could potentially threaten the planet.


Andy Rosen can be reached at andrew.rosen@globe.com. Follow him on Twitter at @andyrosen.