Jayjay

I read somewhere that hundreds of tons of material from outer space drops to Earth each day in form of meteorites. I am assuming most of these are incredibly tiny and burn completely in atmosphere. However, some of them are bound to be large enough to reach Earth at least partially solid, aren't they? Or is anything big enough not to be vaporized in the atmosphere automatically a "dinosaur killer" or at least big enough to cause a major catastrophy?

I am asking this because I heard a report about something falling out of the sky in someone's yard, making a huge loud noise and shaking the house when it hit. The thing left a crater less than 1 meter in diameter. I assume it could've been a meteorite, though I don't know enough about them to be sure. Does "space junk" for example ever fall back to Earth in this manner?

crazyfingers

Plenty of small rocks make it through the atmosphere. Fist sized items are very commonly found. Smaller than that and they would often not be noticed. OTOH larger ones are more rare.

Huginn

You can even buy them!

Jayjay

Thanks!

beausoleil

[QUOTE]Originally posted by crazyfingers
Plenty of small rocks make it through the atmosphere. Fist sized items are very commonly found. Smaller than that and they would often not be noticed. OTOH larger ones are more rare. [/QUOTE

"Very commonly found" is a bit of a stretch. There are about 10 reported falls per century from the British Isles (in the last 200 years).

You can not only buy them, you can spend your life studying them (if you're not careful!)

Friar Bellows

...he said, with a guilty look on his face.

crazyfingers

[QUOTE]Originally posted by beausoleil

Quote:

Originally posted by crazyfingers Plenty of small rocks make it through the atmosphere. Fist sized items are very commonly found. Smaller than that and they would often not be noticed. OTOH larger ones are more rare. [/QUOTE "Very commonly found" is a bit of a stretch. There are about 10 reported falls per century from the British Isles (in the last 200 years). You can not only buy them, you can spend your life studying them (if you're not careful!)

Actually, "very commonly" is a relative term.

The best place to find them of that size is Antarctica where they contrast with the snow.

And note that I wasn't talking about how often they fall but how easy they are to find when one goes looking for them. But one must look in the right place.

Jayjay

As far as I know, the people in question didn't actually find a rock, just a big hole in the ground. Is it possible (commonplace?) that a small meteorite reaches surface, but is smashed on impact?

beausoleil

The original question was about observing an actual fall.

Meteorites are found in regions of Antarctica because they are concentrated by glacial action. They fall on glaciers and are transported to regions where the glacier is eroded by wind action (the figures usually call it a katabatic wind, but I've never bothered finding out what that means!!). Then they are easy to spot against the snow because they typically have a black fusion crust. ALH84001, for instance, had been on the Earth for 13,000 years before it was discovered in Antarctica. (You can date terrestrial age from the decay of cosmogenic radionuclides like 14C.)

People also organise collecting trips to desert areas such as Western Australia, where the meteorites show up against the desert. Once again, the ones they collect have usually been lying around for a long time. Since martian meteorites hit the headlines there's been a small industry of nomads in North Africa collecting bags of rocks and selling them to dealers. Most are worth nothing but the rare types, especially martian and lunar ones are prized by collectors. The recent rapid increase in the number of known meteorites in these categories has been driven by market forces - economics really is the fundamental science

To answer Jayjay's question, it's very uncommon for a small meteorite to break up on impact with the Earth's surface. They hit the top of the atmosphere at several km per second, but are slowed by 'friction' on entry. At this point they are very vulnerable to breaking up. It's fairly common for meteorites to fragment as they come through the atmospehre and create a 'strewn field' on the surface. By the time a small meteorite makes it to the surface, however, it has been slowed to the point at which it is just falling at terminal velocity - as fast as a rock would fall thrown out of an aeroplane. So they don't usually make a crater.

I greatly mistrust people who say they see a meteorite fall - we get several a year bringing in bits of rock (usually furnace slag, but other weird stuff too) they genuinely believe they have seen fall from the sky. The only crater I ever came across was caused by a power cable shorting out underground - it was about 50 cm across, and more than a metre deep and had smoke coming out of it.

My personal favourite was the guy who brought in a rock because his dog had seen it fall. How did he know? "What's that you say, lassie?..." - one colleague at a museum saw about 5 rocks a week brought in by members of the public who thought they were meteorites. In his entire career one really was.

Boro Nut

This is a great topic of it's own, much more interesting than the original thread. Do you fancy starting it somewhere else?

I'd personally love to know what other members of the public thought they were. Of all the members of the public who brought in rocks:-
1 - What was the most common delusional identity
2 - Did any actually dress up?
3 - What was the strangest delusional identity?
4 - Do only delusional people bring rocks to museums? If not, how many rocks a week do normal people bring?
5 - How do you know one of them really was a metiorite posing as a member of the public and not just a boulder with delusions? You're friend could make a bit of money from the Daily Sport/National Enquirer if he has photos. My bet is he will say "Oh I was alone at the time and my camera wasn't working". My experience is that these "I'm a meteorite" stories usually turn out to be fake when you take time off work to investigate them.

Boro Nut