An asteroid barely missed Earth last week, and no one knew it was coming

In this artist's illustration, an asteroid passes close to Earth.
In this artist's illustration, an asteroid passes close to Earth. (Image credit: Science Photo Library/Andrzej Wojcicki/Getty Images)

An asteroid about the size of a refrigerator shot past Earth last week, and astronomers didn't know the object existed until hours after it was gone. 

It was a close call (from a cosmic perspective); the space rock's trajectory on Oct. 24 carried it over Antarctica (opens in new tab) within 1,800 miles (3,000 kilometers) of Earth — closer than some satellites — making it the third-closest asteroid to approach the planet without actually hitting it, CNET reported (opens in new tab).

Scientists were unaware of the object, dubbed Asteroid 2021 UA1, because it approached Earth's daytime side from the direction of the sun, so the comparatively dim and small visitor went undetected until about 4 hours after passing by at its closest point, according to CNET.

Related: Top 10 ways to destroy Earth (opens in new tab)

But with a diameter of just 6.6 feet (2 meters), UA1 was too small to pose a threat. Even if it had struck Earth, most of its rocky body would have burned away in the atmosphere before it could hit the ground, CNET reported.

Comets and asteroids (opens in new tab) that orbit within our cosmic neighborhood, approaching Earth within 1.3 astronomical units (120.9 million miles, or 194.5 million kilometers) are known as near-Earth objects (NEOs), according to NASA's Center for Near-Earth Object Studies (opens in new tab) (CNEOS). NASA uses telescopes on the ground and in space to find and monitor NEOs; to track their orbits and identify their size, shape and composition; and to pinpoint potentially hazardous objects, managing these efforts through the agency's Planetary Defense Coordination Office (opens in new tab)

For an object to be considered dangerous, it has to measure at least 460 feet (140 m) in diameter, NASA says. UA1 may not have been big enough to threaten the planet, but what about bigger asteroids that might be headed our way? NASA is also investigating defensive technologies for protecting Earth from possible collisions with larger space rocks, through deflection. 

The Double Asteroid Redirection Test (DART), scheduled to launch Nov. 24, will test a method for diverting asteroids by hitting them with high-speed remote-controlled spaceships, NASA representatives said in a statement (opens in new tab). Scientists will send the DART spacecraft hurtling into the near-Earth binary asteroid Didymos, which is shaped like a spinning top and has two bodies; the bigger one measures about 2,600 feet (780 m) in diameter, and its smaller moonlet measures around 520 feet (160 m) in diameter. 

Schematic of the DART mission shows the impact on the moonlet of asteroid Didymos. After the impact, observations from Earth-based optical telescopes and planetary radar will measure changes in the moonlet's orbit about the parent body. (Image credit: NASA/Johns Hopkins Applied Physics Lab)
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Didymos came closest to Earth in 2003, skimming it at a distance of approximately 4.5  million miles (7.18 million km), but it typically circles the sun just outside Earth's orbital path, according to NASA. While Didymos doesn't threaten Earth, it's just about the right size to test if the collision can nudge a hazardous NEO enough to divert it from a collision course with Earth, according to the statement. 

Still, to divert an asteroid, NASA would need to detect it before it hit Earth. That's why another mission, NEO Surveyor, is developing an infrared space telescope that could improve the chances of spying sneaky asteroids such as UA1 that approach from behind the sun, according to the University of Arizona's Lunar and Planetary Laboratory (opens in new tab), which is collaborating with NASA on the project.

To date, NASA has identified approximately 27,000 NEOs, of which about 9,800 measure at least 459 feet (140 m) in diameter and 890 measure 0.6 mile (1 km) in diameter, according to the CNEOS (opens in new tab)

While UA1 may have been a relative pip-squeak, other asteroids zooming by Earth on Nov. 2 are significantly bigger, according to NASA Jet Propulsion Laboratory's Asteroid Watch (opens in new tab). Those five space rocks range in diameter from 56 feet (17 m), or about the length  of a house, to an airplane-size 170 feet (52 m). 

Luckily, none of these space rocks will come within 515,000 miles (829,000 km) of Earth, NASA says.

Originally published on Live Science.

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Mindy Weisberger is a senior writer for Live Science covering general science topics, especially those relating to brains, bodies, and behaviors in humans and other animals — living and extinct. Mindy studied filmmaking at Columbia University; her videos about dinosaurs, biodiversity, human origins, evolution, and astrophysics appear in the American Museum of Natural History, on YouTube, and in museums and science centers worldwide. Follow Mindy on Twitter.