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Small asteroid skimmed close to Earth, hours after discovery

Small Earth in center of illustration, with arrow showing an asteroid passing close to Earth inside the lunar orbit.
Asteroid 2021 RS2 geocentric flyby diagram via Minor Planet Center/ The Watchers.

Small asteroid skimmed close to Earth

Astronomers at Mount Lemmon in Arizona discovered a car-sized asteroid yesterday (September 7, 2021). Quick calculations of its trajectory showed it was due to skim past our planet just hours after its discovery. According to NASA’s Center for Near Earth Object Studies (CNEOS), the asteroid’s closest approach came at 07:28 UTC (3:28 a.m. EDT) on September 8. The asteroid now carries the designation 2021 RS2. It’s the closest space rock to pass by Earth so far in 2021. According to The Watchers website, it’s the 81st known asteroid to fly by Earth inside one lunar distance since the start of 2021.

Asteroid 2021 RS2 came just 9,532 miles (15,340 km) from Earth’s surface. Was that close? Yes, really close. Earth’s diameter is about 7,917.5 miles (12,742 km). So we can say the new-found space rock was passing slightly farther than one Earth-diameter.

The small asteroid was traveling at 39,366 miles per hour (63.353 km/h) or 17.59 kilometers per second, relative to Earth.

But … this object is small. Its size is estimated at just 11.5 feet (3.5 meters) in diameter. So it may have been taller than Earth’s tallest man, but it wasn’t big enough to put us on Earth in any danger. If 2021 RS2 had some closer to us, if it had entered Earth’s atmosphere, friction with the air would have caused this small asteroid to disintegrate. On the other hand, a space rock the size of 2021 RS2 would have caused a very impressive and bright meteor in our skies.

Why didn’t we see it sooner?

Small asteroids like this one aren’t easy to detect. They’re especially difficult to detect when located some distance from Earth. As they get closer, we’re more likely to spot them. It’s amazing we can spot them at all! A few decades ago, we wouldn’t have been able to. Bigger asteroids, on the other hand, usually reflect solar light more efficiently. So we can see them from a greater distance, and, often, weeks earlier. And the biggest known asteroids, which are hundreds of miles across, can be tracked continuously.

Astronomer K. W. Wierzchos (@WierzchosKacper on Twitter) made the discovery of 2021 RS2 on September 7, using the 60-inch (1.52-meter) telescope at Mount Lemmon, Arizona.

Small asteroid skimmed close to Earth: Graphic showing orbits of inner planets and asteroid 2021 RS2.
Orbits of inner planets and path of asteroid 2021 RS2.

Bottom line: Small asteroid 2021 RS2 skimmed less than one Earth-diameter from Earth on September 7, 2021. It passed less than 10,000 miles (about 15,000 km) away.

Via NASA: Next 5 asteroid approaches

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