More booming fireballs

The entire universe in blog form
March 30 2009 8:54 AM

More booming fireballs

Artist drawing of an asteroid entering Earth’s atmosphere
So it sounds like (haha) another interplanetary interloper took it upon itself to explode in our atmosphere, this time over the East coast of the US. There has been a streak (haha) of these lately, and I'm getting emails asking if that's unusual.

I do think it's a bit unusual, but not crazy or apocalyptic, or even mildly worrisome. There's a lot of junk out there in space, and it doesn't take a big piece of detritus to make a brilliant fireball. Something the size of a grapefruit will light up the night sky, and anything bigger than a beach ball will make a tremendous display even during the day. The thing to remember here is that these objects enter our atmosphere moving at least 11km/sec (7 miles/sec), and that translates into a huge amount of energy. They can have speeds up to ten times that much, too, meaning they have 100 times the energy (energy goes as velocity squared), and that energy is converted into heat and light.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


As a rock slams into the air at those speeds, it compresses the air in front of it tremendously, which is why it gets hot (it's not friction per se; compressing a gas heats it up, and we're talking major compression here). This pressure also flattens the meteoroid, and it can break up into little pieces, each of which heat up, which then break up, heat up, and then BANG! They release all their energy in an explosion. If that happens low enough in the atmosphere (though still many miles high), you can actually hear the non-Earth-shattering (sorry Marvin) but still quite loud kaboom.

That's almost certainly what happened over Virginia yesterday. People say they saw a streak in the sky, indicating this was a meteor... though I have to tell you that specifics in eyewitness reports of meteors can be misleading. One person quoted in that article said it was definitely heading down, but in reality they can't know that. As a meteor streaks across the sky, it looks like it's heading down, toward the ground, but that's just perspective. If one were to skip off our air like a rock skipping on water (and that does happen), from the ground it might still look like it was heading down even though it might be moving up, away from the ground.

Another person said it was low, but again that is probably not correct. Most likely it was low in the sky, because the person was far away and the curving Earth made it look low, like the rising Moon at first is low to the Horizon. The object coming in was probably 20 km off the ground or higher. The only way you could know for sure it was low is if it passed in front of clouds (which would be hard to tell if the fireball were really bright) or in front of trees... which would mean you're in some small amount of trouble!

But what about the frequency of these things? There have been so many lately! Are we under attack?

No, I don't think so. I think it's a mix of coincidence -- there may be a few more than usual, but it's not like these things have published schedules; sometimes there are more and sometimes fewer -- together with people being more aware of them because they've been in the news lately. It's like buying a car and suddenly seeing it everywhere when you drive. We notice what we're primed to notice.

So I'm not worried about these. Instead, I think it's great! People are getting a first-hand taste of astronomy, and in a very exciting and memorable way! I hope people don't get scared by this, and instead enjoy it. The Universe is talking to us, and people are starting to listen.



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