Asteroid Update

What happened?
On Friday morning, a meteorite about the size of a house exploded over a remote part of Russia. This was the largest “impact” since the 1908 explosion over Tunguska in Siberia. Then, on Friday afternoon, a large asteroid swung by Earth; it came within 17,000 miles of the surface, which put it below several satellites. And, just to cap things off, another meteorite exploded on Saturday, this time over California.

Wow. So they all came from the same place, right?
Wow indeed. But they didn’t all come from the same place; instead, these were three unrelated chunks of rock coming to visit at the same time. We know that they came from different places because they had different paths through the sky. If they had all come from the same place, then they would have appeared to come from a specific constellation (e.g., the Lyrid meteor shower which appears to come from Lyra).

Why were so many people hurt in Russia?
When the meteor sped through the sky, it put on quite a display. There was a low rumbling and bright lights and huge smoke clouds and then an intense white flash from the explosion. The flash was bright enough that it reflected off of inside surfaces and drew people to the windows to see what had happened. As they gazed outside, the air burst arrived (remember that sound travels much more slowly than light) and broke the windows, dropping shards of glass onto anyone unfortunate enough to be standing by them. About 1,500 people were hurt; thankfully, none were killed. More than $1,000,000 in damage was done by the explosion; again, it was mostly in the form of broken windows.

First Tunguska and now this. Russia must have a big target painted on it
Actually, no. Russia has had two large impacts in just over a century, but other places have had impacts or near misses. The most well-known one (in the planetology community, at least) is the Great Daylight Fireball of 1972, which screamed across the skies of North America. And then there was the Great Meteor Procession of 1913, which convinced people in several states that the end of the World had come, and the Sylacauga meteorite which hit a woman while she napped. Given the number of impacts every year, it isn’t surprising that Russia would have two of them.

A world map showing known large impact sites. There are probably thousands more that we don't know about. (Image courtesy AAA)

A world map showing known large impact sites. There are probably thousands more that we don’t know about.
(Image courtesy Earth Impact Database)

How many impacts are there every year?
The truth is nobody knows for sure. We think that somewhere between 18,000 and 84,000 meteorites arrive each year, mostly in spurts around the “meteor showers”, but we don’t have very good numbers.

Why the heck not?
There are several reasons. The first is that most of the meteorites arrive as tiny little specks of sand, about the size of a salt grain. They fly through the atmosphere in a blaze of glory and disintegrate before they hit the ground. And by most, I mean 99.9% of them.

Distribution of known asteroids (Image courtesy JPL)

Distribution of known asteroids
(Image courtesy JPL)

We do get a few larger meteorites each year. Those the size of a basketball are typically large enough that a fist-sized piece or two reaches the ground intact, and we get a few hundred of those each year. Those the size of a car are large enough to explode into a cloud of fragments similar to the Russian event; we get a couple of those each year. And those about twice the size of the Russian meteor (i.e., the size of the near miss) are large enough to make a crater when they hit the ground; this happens once ever decade or so.

The second, and more important, reason that we don’t know how many meteors hit the Earth is that we don’t know how many potential meteors there are. Those meteors come from bits of rock and ice that are broken off of asteroids and comets. And we are only now starting to understand how many asteroids are out there and how many of them come close enough to Earth to be a danger; in the asteroid biz, those are called “Potentially Hazardous Asteroids” or PHAs. We have charted some 95% of the “planet killers”, which are asteroids as big as the one in Chixulub that had a hand in killing off the dinosaurs. But we’ve only started to map the smaller asteroids.

Why?
It takes a lot of hard work to discover the orbit of an asteroid. It is something that is very small and very dark moving very quickly across a very large space. Anyone who has ever tried to swat a mosquito in a dark room will understand the problem. If mosquitoes were the size of ostriches, it would be easy to find them. Because they are the size of, well, mosquitoes, they are very hard to locate. The same is true of asteroids.

Asteroids closer than Mars (Image courtesy JPL)

Asteroids closer than Mars
(Image courtesy IAU)

And the smaller an asteroid is, the harder it is to find. Remember that an asteroid that is one meter across (the size of a manhole cover) reflects one-quarter the light of an asteroid that is two meters across. Though the recent development of automated tracking cameras (intended to detect incoming nuclear missiles) and improved software has helped us find a lot more asteroids, we’ve still got a long way to go before we know where every PHA is. Right now, we have only found about 1400 PHAs; there are still a lot of them out there that we haven’t discovered.

Will any big asteroid hit us?
There aren’t any impacts from large asteroids expected in the next century. There will be lots of near misses, but no impacts.

Honestly, this never happens (Image courtesy Scott Adams)

Honestly, this never happens
(Image courtesy Scott Adams)

Right. As if the government would tell us
Actually, the government wouldn’t have any choice; we’d be able to see it for ourselves. Any asteroid or comet large enough to cause serious damage (say 100 m or bigger) would be visible well before it hit. And it would be seen by amateur astronomers across the globe, who would then tell everyone that they knew about this really cool new object. Before you could say “conspiracy theory”, the news of the new object and its orbit would be front-line news.

Hey, you said “expected”
Right. Remember that we still don’t know where all of the asteroids are. So it is possible that we’ll get smacked around by an asteroid that we didn’t know about.

What can I do?
You can join the citizen science brigade over at Target Asteroids!. In combination with the OSIRIS-Rex mission, they are attempting to look at every asteroid in the sky in order to select a target for a NASA mission. To participate, go to:
http://osiris-rex.lpl.arizona.edu/?q=target_asteroids

3 thoughts on “Asteroid Update

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