On 16 August, 2020 at just after midnight Eastern (US) time, a small asteroid buzzed our planet, passing just 2,950 kilometers above the Earth's surface!
Wow. That's close. That's only a quarter of Earth's diameter! The good news is it missed, and even had it hit it wouldn't have done more than put on an amazing light show, since it's only about 3–6 meters across (so, car to stretch-limo sized). In other words it would've burned up in the atmosphere.
But it did miss, and that makes this the closest near-miss by an asteroid ever seen.
The asteroid is called 2020 QG, and was discovered only a few hours after it made its closest approach to Earth (over the Indian Ocean). That's pretty common for small rocks like this one. It was found in images taken by the Zwicky Transient Facility, a 1.3-meter telescope at the Palomar Observatory in California. This relatively modest telescope has a massive 600-megapixel camera installed on it, and takes image of so much of the sky at a time (7° on a side, so each image covers an area 250 times the size of the full Moon) that it can image the entire northern night sky over three nights.
It's designed to look for exploding stars and other events that come and go in the Universe. But closer to home it can see asteroids moving past Earth as well. Computers scan the images looking for the telltale streak of a nearby asteroid, and if they find something promising the observation gets flagged for a human to look over. About a thousand such potential visitors are found every day. Most turn out to be satellites or some such, but sometimes they're space rocks.
In this case a student in India IDed the asteroid, and the ZTF team then reported it to the International Astronomical Union Minor Planet Center, the official keepers of info about rocks in the sky, and it was rapidly confirmed by other observatories.
Given its brightness and distance the size can be estimated, and it was found to be pretty small; a bedroom is roughly 3 – 6 meters wide depending on your domestic situation. For comparison, the Chelyabinsk asteroid that burned up over Russia in 2013 was 19 meters in size, much larger than this one.
2020 QG is what we call an Apollo asteroid. These are near-Earth asteroids with orbits that on average are bigger than Earth's orbit, but also cross ours. Not all of them get close enough to Earth to pose an impact threat, but many do. On its elliptical orbit it comes just closer to the Sun than the Earth does, but goes out farther than Mars, into the asteroid belt. It takes about 2.7 years to go around the Sun once.
It doesn't look like 2020 QG will get anywhere near us for a while again, though; the next pass is in 2028 and it'll be 50 million kilometers away. That's a pretty wide margin. This time, though, it got so close that Earth's gravity bent its orbit quite a bit.
Asteroids this size pass the Earth around this close very roughly once a year. The last one we know of was in May 2020, when 2020 JJ passed about 7000 km above the Earth. The previous near-miss record holder is 2011 CQ1, which passed about 5,500 km above us.
I'll note, as I always do, that impacts big enough to do real damage are very rare. Chelyabinsk is a much more uncommon event, happening on a time scale of many decades or a century. Even then they usually burn up over the ocean (it covers 70 or more percent of Earth's surface, so the odds are higher) or over land with few or no people.
Ones like 2020 QG burn up high in the atmosphere, at best raining down tiny meteorites after they break up. But finding these asteroids is still important! We don't have good numbers for how many pass us in a given time, so the more we spot like this the better we'll understand the population of near-Earth (and potentially hazardous) asteroids. I'm all for that.
Also, it shows that we already have observatories, like the Zwicky Transient Facility, that can find them. And we'll have even more once the Vera Rubin telescope goes full time, as well as others like the space-based NEOCAM, which will not only find them but also give us a much better idea of how big they are.
Astronomers take these rocks seriously, which is why we're building better observatories to spot them. We want to learn as much as we can about the solar system, but we're human, and we have a personal stake in keep Earth from getting hit, too.