What to Know About 2021 PJ1, the Asteroid Speeding Past Earth at 20,600mph

NASA has announced the discovery of the 1000th near-Earth asteroid to be detected by planetary radar.

The space agency announced the news on September 3, stating that the space rock had been given the name 2021 PJ1.

"Near-Earth" is a relative term. For astronomers it's close, but compared to the everyday distances we're used to on Earth, the asteroid is still hugely far away. 2021 PJ1 passed our planet at a distance of more than 1 million miles, which is more than four times as far away as the moon is.

It's also traveling at breakneck speed. 2021 PJ1 is thought to be moving at around 9.25 kilometers every second, or more than 20,600 miles per hour.

The space rock is somewhere between 65 and 100 feet wide. Due to its size and distance, astronomers haven't deemed it a threat to Earth. But it's noteworthy as the 1000th near-Earth asteroid NASA has spotted via radar observation since 1968, when the first radar observation of the asteroid 1566 Icarus was performed.

This detection method means that NASA researchers fired radio waves at the object and then detected them when they returned. By studying these returning waves, scientists could determine how far away 2021 PJ1 is, its motion around the sun, and how fast it was traveling.

They used the powerful 230-foot Deep Space Station 14 antenna at the Deep Space Network's Goldstone Deep Space Complex near Barstow, California, to carry out the measurements.

The space rock passes close to Earth's orbit during its regular journey around the sun, during which it also passes close to the orbit of Venus and then out past the orbit of Mars for a time, according to its orbital diagram.

While 2021 PJ1 is the 1000th near-Earth asteroid to be detected via planetary radar, it is far from the 1000th asteroid that we know about.

There's more than one way to spot a space rock, and another method used by astronomers is to use an optical telescope instead. These scan the skies looking for movement, and when they spot a moving speck, additional observations, including by the amateur astronomer community, help confirm its path through space and other characteristics.

Altogether, NASA's Center for Near-Earth Object Studies (CNEOS) notes that there are more than 26,000 near-Earth asteroids that we know about. Just under 1,000 of these are thought to be around 1 kilometer in size (around 0.6 mile).

Detecting near-Earth asteroids is useful for planetary defense, and scientists are also working on ways to deflect any that come too close for comfort.

A stock image shows an artist's impression of an asteroid in space. Scientists can spot asteroids from Earth using radar or optical telescopes. Alexandr_Zharikov/Getty