frequently asked questions

Almost every time a meteor or meteorite-related story comes up in the media a few reporters and article responses spread common misconceptions about meteorites.  There are a few things I’d like to set straight.

Meteorites are not dangerous.

Page 2, The Adventures Of Superman #479
Meteorites are not radioactive.  They are not poisonous.  They are not made of any elements that you wouldn’t find in a rock on Earth.  What makes a meteorite special is how it formed, and the fact that it hasn’t changed since then.  That’s what makes them so interesting to scientists and folks trying to understand what was going on in the early solar system.  Meteorites are made of the same stuff as Earth rocks, but they contain different structures that formed in different environments.  Which kind of makes sense.  Earth is made up of a collection of meteorites from ~4.5 billion years ago and onwards.  While Earth has been a very active place since then (plate tectonics, volcanoes, erosion, etc.), meteorites have mostly been in ‘cold storage.’

Most of the time, meteorites don’t hit the ground at thousands of miles per hour.

Image courtesy of Leigh Blair

This is what happened when a ~2 ounce meteorite ‘slammed’ into a San Francisco Bay Area household in October 2012.  How did they notice it?  The homeowner saw a news story about a local fireball several days later and thought back to a funny noise she’d heard on her roof that night.  It almost passed unnoticed.

Long story short, the meteoroids that produce meteorites enter the atmosphere at velocities between ~10 and ~35 kilometers per second.  The average space shuttle entry velocity was ~7.7 kilometers per second.  So, these rocks are moving fast — much faster than the space shuttle — and if they hit anything at those speeds, they would cause a lot of damage.

But, meteors get slowed down by the atmosphere, fragment, and, for the most part, never even make it to the ground.  The few solid pieces of stuff that reach the Earth’s surface have usually been slowed down to terminal velocity, between ~200 and ~700 mph.

Here’s a gif of the Chelyabinsk bolide as an example:

chelyabinskWhat you see above is a ~20 meter-diameter asteroid passing through the atmosphere with enough speed that friction and ram pressure are acting strongly on the rock.  The surface of the rock and surrounding atmosphere are being heated to incandescence (~5,000+ K), and the outer portion of the meteorite is being vaporized.  The energy that’s being used to do this is kinetic energy, so, as this happens, the solid portion of the bolide is slowing down — from ~17.5 km/sec to as little as 1-2 km/sec or less.

The bright flash is the point at which a major fragmentation event occurs; fragments have been breaking off all along the bolide’s path, but, at that point, there’s so much pressure acting on the rock that it shatters, creating thousands of specimens that are all moving just as quickly and ablating.  They’re mostly small; those small pieces are slowed rapidly by wind resistance, and diminish rapidly in brightness.  Some larger fragments aren’t slowed as quickly.  They are visible continuing along the path of the fireball, ablating well after the main fragmentation event.  One of these larger specimens was later fished out of Lake Chebarkul:

Andrey Yarantsev/Science/AAASAndrey Yarantsev/Science/AAAS

You might remember this particular event from the news in early 2013.  A massive fireball was spotted above Russia, and hundreds of people were subsequently injured when the shockwave reached the ground and shattered their windows.  All of the people who had gone to their windows to look at the spectacle were showered with broken glass.  Fortunately, no one died.

So, they do travel around in space at thousands of miles per hour, but even meteorites that weigh as much as a few tonnes get slowed by the atmosphere and usually fall at ~terminal velocity.  Most meteorites that have struck buildings have never even made it through the floor and into the basement, with one or two exceptions.  Here’s a fine example: the Ellerslie meteorite, which fell through the roof of the Archer family residence in Auckland, New Zealand.  The (fairly large) stone punched a hole through their ceiling, but didn’t even make it through their sofa.  Photos are public, Reuters, and the Auckland Museum (6), respectively.

Some finders have reported that freshly-fallen meteorites have formed a frost immediately after falling.  This might seem odd, but near-Earth space is a chilly -243°F (-153°C) in the shade.*  And meteorites ablate so quickly that the heat of atmospheric entry doesn’t have a chance to penetrate more than a few millimeters in stony meteorites and a few centimeters into irons.

Other finders have reported that freshly recovered meteorites have been warm.  It is hard to tell what is actually true, and it might differ with bolide fragmentation history, meteorite size, and/or meteorite type.  The debate is unresolved.

DSCN1837
A Sutter’s Mill individual with a broken side. This stone experience stable flight; the right side of the stone was the front while it was coming through the atmosphere. Note the reddish heat-altered rim on that side. It’s about a millimeter thick.

Putting a meteorite through the atmosphere is kind of like putting an ice cube in an oven.  The outside melts and evaporates away, but if it survives the trip, it’s unchanged on the inside.  Mostly.  The heat does penetrate a little, as you can see above.

There is one exception to all of this.   If the meteorite is big enough to punch through the atmosphere without breaking up or being slowed down, we wind up with a different story:

ESP_034285_1835_RED.NOMAP
A new ~30 meter crater formed on Mars between 2010 and 2012. We know when it formed because it appeared between different sets of images taken of the same area. Ejecta was thrown up to 15 km from the crater!


This sort of thing happened a few years ago, in Peru
.  If it does happen, you’ll know about it: there will be a crater in the ground.

So, the next time you see a news story about a child who has been burned or cut by a falling meteorite, think about it.  That small piece of gravel couldn’t possibly have done him or her any harm.  The same goes for that person who found a glowing 10-pound rock in a huge smoking hole in the woods.  Meteorites that size just don’t do that.

I’ve found a meteorite!  What now?

GarfieldMeteorite

How do you know it’s a meteorite?

Thousands of people send thousands of rocks to qualified scientists around the globe each year, and precious few of them turn out to be actual rocks from space.  There are a lot of iron-bearing rocks on Earth that are attracted to a magnet, and there are even more achondrite look-alikes that won’t stick to a magnet.  If you do happen to be ridiculously lucky, and that rock you have is actually a meteorite (or you think it might be), please get in contact with someone at a lab capable of analyzing and submitting the stone formally.  A sample is required to be donated by the Meteoritical Society; at least 20 grams or 20% of the stone, whichever is less.  That’s important; it ensures that some of the meteorite will be available for future research.

In the meantime, I have made a page that might help to show people what to look for.  Not everything you might find will look like the rocks on this page, but it’s a good place to start:

What do meteorites look like? 

If you think you might have a meteorite on your hands, please send us an email.  We’re always curious about new finds, and it’s easy to confirm or rule out just about any sample with a few clear images.

 

And, if you have any suggestions for future questions to be featured on this page, please let me know!

 

*In direct sunlight at 1 AU, the ~ambient temperature is +253°F (+153°C).  But the density of these high-energy photons is so low that they don’t impart much heat to bodies that they hit.  The thickness of the layer of rock or ice they heat is much less than a meter.  In other words, the ‘nighttime/shade’ temperature is usually a better estimate of the inner temperature of a small body in our solar system.