Astrophysicists had no idea an asteroid was on the verge of crashing with Earth on Oct. 24.

After flying through Antarctica asteroid 2021 UA1 remained undetected for several hours after passing the planet’s orbit. CNET reported that the projectile came so close to Earth that it passed lower than some existing satellites, at an elevation of 1,800 miles (3,000 kilometers).

The National Aeronautics and Space Administration (NASA) defines potentially harmful asteroids as celestial objects that come within 4.6 million miles (7.5 million kilometers). To put this into context, the moon is about 238,855 miles (384,400 kilometers) from Earth.

The asteroid approached the planet’s dayside from the sun’s direction, making it comparably faint and was not discovered until almost four hours after passing the closest point. If the projectile entered the atmosphere, most of its stony body would have burnt up, CNET reported.

Near-Earth objects (NEOs) are comets and asteroids that orbit within 1.3 astronomical units (120.9 million miles or 194.5 million kilometers) of Earth. NASA classifies a harmful object as at least 460 feet (140 meters) in diameter.

NASA’s Planetary Defense Coordination Office deploys telescopes on the ground and in space to locate and monitor NEOs, follow their orbits, and identify size, shape, and composition.

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