On August 16, 2020, just after midnight Eastern (US) time, a small asteroid alerted our planet and traveled just 2,950 kilometers above the earth’s surface!
Wow. That is close. That’s only a quarter of the earth’s diameter! The good news is that it’s missing, and even if it had hit, it would have done no more than produce a great light show, since it’s only about 3–6 meters left (i.e. car to stretch limo size). In other words, it would have burned into the atmosphere.
But it the missed, and that makes this the closest thing to miss by an asteroid ever seen.
The asteroid is called 2020 QG, and was only discovered a few hours after it made its closest approach to Earth (across the Indian Ocean). That’s pretty much just for small rocks like this. 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 in it, and takes pictures of as much of the sky at a time (7 ° on one side, so that each image occupies an area 250 times the size of the full moon) that it can image the entire northern sky over three nights.
It is designed to look at exploding stars and other events that come and go in the Universe. But closer to home, it can also see asteroids moving past Earth. Computers scan the images in search of the vertical line of a nearby asteroid, and when they find some promise, the observation is flagged for a human to look over. About a thousand such potential visitors are found every day. Most it turns out that satellites are like some of these, but sometimes they are space rocks.
In this case, a student in India identified 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 quickly confirmed by other observers.
Given the brightness and distance, the size can be estimated, and it was found to be small; a bedroom is roughly 3 – 6 meters wide, depending on your household situation. By comparison, the Chelyabinsk asteroid that burned over Russia in 2013 was 19 meters in size, much larger than this.
2020 QG is what we call an Apollo asteroid. These are asteroids near the Earth with orbits that are on average larger than the Earth’s Earth, but also cross us. Not all come close to the earth to form an impact threat, but many do. In its elliptical orbit, it is closer to the Sun than the Earth is, but it extends beyond Mars, in the asteroid belt. It takes about 2.7 years to get around the sun once.
However, it does not look like 2020 QG will be back with us for a while; the next pass is in 2028 and it will be 50 million kilometers away. That’s a pretty wide margin. This time, however, it came so close that the earth’s gravity bent its orbit quite a bit.
Asteroids of this size pass through the Earth roughly once a year. The last we know of was in May 2020, when 2020 JJ traveled about 7000 km above the earth. The previous record holder for near-miss is 2011 CQ1, which ran about 5,500 km above us.
I will find, as I always do, that influences large enough to do real damage are very rare. Chelyabinsk is a much more unusual event, happening on a time scale of many decades than a century. Even then, they normally burn over the ocean (it occupies 70 or more percent of the earth’s surface, so the chances are higher) than over land with few or no people.
One like 2020 QG burns high in the atmosphere, and rains at best small meteorites after they break. But finding these asteroids is still important! We do not have good numbers for how many pass us by in a given time, so the more we see this, the better we understand the population of near-Earth (and potentially dangerous) asteroids. I’m all for that.
Also, it shows that we already have observers, such as the Zwicky Transient Facility, that to be able to fine se. And we will have even more when the Vera Rubin telescope goes full time, as will others like the space-based NEOCAM, who 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 are building better observatories to view them. We want to learn as much as possible about the solar system, but we are human, and we have a personal stake in not touching Earth either.
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