| Update:
December 6, 2006 |
Look! Up in the Sky!! |
| By Pam Eastlick for THE DEEP on line |
Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.
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All of us have seen falling stars. We all used to think that these streaks of brilliant light were stars but we’ve known for hundreds of years that the stars are W-A-A-Y too far away to ‘fall’. So if they aren’t stars; what are they?
The objects we call ‘falling stars’ are in fact, rocks. |
| Fireball over Italy |
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Although I tell the schoolchildren that space is empty, in our solar system; that simply isn’t true. There is an incredible amount of solar system ‘leftovers’ out there. For the most part, this debris is dust-sized, but there are also larger rocks.
It’s estimated that the Earth strikes about 1,000 objects a day and as a general rule; these dust grains, sand grains pebbles and larger rocks don’t hit us, we run over them at 66,000 mph; Earth’s orbital speed around the Sun. These objects hit Earth’s upper atmosphere and most of them are traveling between 10 and 44 miles a second! Since the speed of your average bullet runs around 8 to 15 miles a second, most of these rocks are traveling much faster than bullets.
All of you who have ever hunted or fired a gun and then immediately picked up the bullet as soon as it lands have immediately dropped that bullet because it’s too hot to hold. It gets hot from the friction of its passage through the air. These rocks (the proper name for them after they enter Earth’s air is ‘meteors’) are traveling much faster than bullets and the smaller ones burn up entirely on the way down. A rock has to be larger than 50 feet in diameter to begin with for there to be any pieces left to hit the ground. If it’s the size of a school bus or a small house, it burns up entirely on the way down.
But Earth strikes at least 1,000 objects a day. That works out to 40 meteors an hour or almost a meteor a minute. They can’t all be so small they leave no trace. So why isn’t there a meteor show every night? Why don’t you see a falling star every time you look up?
Astronomer Bill Cooke of the Marshall Space Flight Center used a computer to make a model of the number of meteoroids (what you call these rocks before they enter Earth’s air) that share Earth’s orbit and he calculated that at least 100 of the rocks we hit every day are big enough to make a fireball. In other words, they were big enough to begin with that their burning is visible from the ground. He made this graph that shows the number of fireballs per day vs. the brightness of the fireball:
According to his calculations, more than 100 fireballs as bright as Venus (and by the end of this month, Venus will appear in our early evening sky and you’ll be able to see just how bright that is) appear somewhere over the Earth every day. Fireballs as bright as a quarter Moon happen approximately once every ten days, and fireballs as bright as a full Moon once every five or six months. So . . . the question still remains, why don’t I see a meteor most of the time when I happen to look up?
Well, let’s examine this little dilemma scientifically. The problem is not how many occur but how many are seen. There are very, very few fireballs that are bright enough to see in the daytime. So that drops the number of fireballs you can see that are as bright as Venus from 100 every day to 50 every day. Your chance of seeing one as bright as the full Moon drops to one a year.
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Then, there’s where you live. Seventy percent of Earth is uninhabited except for those ships that pass in the night. Seventy percent of the fireballs occur over open ocean and no one sees them. So that drops the number of Venus-bright fireballs to roughly 15 per 12-hour period of darkness or one an hour. So there’s a fireball as bright as Venus somewhere over the night side of Earth every hour. And of course, the night side of Earth covers a lot of territory. And then there’s the fact that most of us don’t spend those 12 hours of darkness staring at the sky. And that explains why these bright fireballs are rarely seen. Most are missed simply because no one bothers to look up.
Go outside tonight and look up. Since (unlike most of the people in the mainland US) you can go outside in your shorts and T-shirt, turn off the TV, take the kids outside, get yourself blankets or lounge chairs and lie down and look up. This weekend marks the Geminid meteor shower and I guarantee you’ll see a few bugs hit the windshield. They won’t be bright, but they’re still awesome. Connect with your kids under that vast starry sky. Even the streetlights can’t hide it all. Look up tonight, the universe in all its starry glory awaits you. And so do the meteors!
