About a hundred tons of meteoric material hits the Earth every day.
Now before you panic, essentially all of it burns up in our atmosphere high above the planet’s surface! The vast majority of all that is made up of very tiny bits of rock, metal, or ice sloughed off of comets or asteroids, usually the size of a grain of sand or smaller. This stuff burns up from 80 – 100 kilometers up, and makes lovely shooting stars for us to ooooh and aaaaah at.
But sometimes the object is a bit bigger. How it behaves depends on the composition, impact speed (generally several dozen kilometers per second), entry angle, and even the structure of the incoming meteoroid (some are riddled with cracks and fall apart easily while others are solid and can penetrate deeper)… but if it’s big enough at least some of it will survive to hit the ground.
In general, unless the original incoming object is more than a few meters across, it will explode due to the literally crushing pressure of its atmospheric dive, crumbling into much smaller pieces. These slow very rapidly, usually in seconds, then free-fall their way to the ground. They reach terminal velocity (a constant speed that balances the pull of gravity and the resistance of the air underneath them) quickly as well, and that depends on their size. [Note: This is such a common question that I wrote an article about it, “Why do asteroids explode high in the atmosphere?”]
The Earth is pretty big, so that daily RDA of a hundred tons of cosmic debris is spread out pretty thinly. The odds of a person or a structure getting hit are pretty long… but the chance isn’t exactly zero.
On September 18, 2015, at around 17:45 UTC, a tiny asteroid entered Earth’s atmosphere above San Carlos, Uruguay. It’s unknown how big it was, but most likely far less than a meter. It broke up as it descended, eventually creating lots of smaller pieces that fell to Earth. One of them, a chunk of rock about 9 centimeters in length, defied the odds. It slammed into a house in San Carlos, piercing the roof; a piece broke off and hit their TV (damaging the screen, because duh), while the main piece hit their bed before finally coming to rest on the floor (which it also damaged).
Wow. Happily, no one was home at the time! Even so, the odds of someone getting hit are of course even smaller; people have a far smaller cross-section than a house*. Good thing, as it was moving at about 90 meters/second (over 300 kilometers per hour). Oof. So this is a very rare meteorite indeed!
A team of meteoriticists analyzed the meteorite and published their results in a recent journal. There were a few eyewitnesses to the bolide (the bright meteor as it passed through our atmosphere) who reported it was as bright as the full Moon — and while these sorts of accounts are notoriously inaccurate, that does fit with it being something relative small (for comparison, the 19-meter Chelyabinsk asteroid impact in 2013 was as bright as the Sun).
The big chunk that hit the house is about 9 x 10 x 6 cm, and has a mass of a little over 700 grams. It’s covered in a dark fusion crust — a thin layer of burned material coating the surface, common in meteorites — and also has many regmaglypts: little scoops out of it that look like someone stuck their thumb in clay. Those form as the meteoroid tumbles as it falls; the heat and pressure carve dimples in it.
The scientists sawed off a part of the meteorite for analysis, and it’s stony, with olivine, pyroxene, and small amounts of iron and nickel. Fairly typical. It’s also brecciated, which means it’s like lots of smaller pieces ogether by other material; again that’s common. It has small spherical mineral inclusions in it, making it what we call a chondrite. Technically, it’s been classified as an LL6 chondritic breccia; meaning the chondrules aren’t well separated from the material between them (like what happens when you leave your cereal in milk for too long).
The paper, of course, has far more details if you’re interested. They also discuss the fall itself, and how often structures are hit (answer: extremely rarely).
Interestingly, the authors say this is the first confirmed meteorite impact in Uruguay. They say that another one, called Baygorria, is a hoax, a scam from certain unscrupulous people to sell cheaper meteorites for more money. The whole story is complicated, but Argentina has very strict rules about exproting meteorites, so the scammers either bought up or smuggled out a bunch of Campo del Cielos — inexpensive, not very high quality Argentinian iron meteorites — and claimed they found them just over the border in Uruguay. That way, they could sell them (they hoped) without getting in trouble with the Argentine government.
However, there is a real Baygorria; a huge 80 kilogram iron mass that was found in Uruguay in 1994. Banking on that, the scammers brought a bunch of Campos to a mineral show and tried to pass them off as Baygorria. Experts could tell the difference right away; the scammers claimed they found them in water, but there was no hint of oxidation on them, and they looked exactly like Campos.
I collect meteorites and they can be very expensive. I’m not surprised some people try to scam buyers, but bringing them to a mineral show where lots of professional, licensed dealers roam the halls is a new level of dumb. Always buy from licensed dealers!
Incidentally, my good friend Geoff Notkin is a dealer, and has a wonderful page about what to do if you think you found a meteorite. You’ll find interesting info there even if you haven’t found something. I contacted Geoff to clear up this whole Baygorria thing, and also because I couldn’t remember what meteorite hunters call ones that hit structures; I knew there was a nickname but I couldn’t recall it, and I knew he’d know. He did: They’re called hammer stones. So cool.
Bottom line for this story: Meteorite impacts on structures are incredibly rare, so this was a genuinely interesting fall with a lot of scientific wonderfulness in it. I’m glad no one was hurt, I’m glad we got to hear another interesting story from an event like this, and I’m glad that our understanding of our solar system got pushed ahead a little bit yet again thanks to the bounty of the skies.
* Meteorites are cool, but people can be jerks about them… just read about the 1954 case of Anne Hodges, who was hit by one in Sylacauga, Alabama. She was relatively unhurt, but the real tragedy was what happened after.