Tonight’s episode of The Big Bang Theory begins with Howard, Raj, and Leonard camping out in anticipation of the Leonid meteor shower. True to the writers’ comic timing, the Leonid meteor shower is upon us right now. The number of meteors per minute will peak tonight (at 5:30A.M. California time, check your local listings.)
But the story really started much earlier than tonight’s opening scene in the desert….it begins November 13, 1833. Late that night, insomniac Americans were greeted with a sky filled with streaks of light. This was not just a meteor shower, but a rare event with so many meteors that it is called a “meteor storm”, so named whenever the number of meteors exceeds 1000 per hour. That night in 1833 the number of meteors exceeded 1000 per minute!
A traveling preacher, Samuel Rogers, already awake at 3am to prepare for a journey westward, gave an eyewitness account:
Some of those wandering stars seemed as large as the full moon, or nearly so, and in some cases they appeared to dash at a rapid rate across the general course of the main body of meteors, leaving in their track a bluish light, which gathered into a thin cloud not unlike a puff of smoke from a tobacco-pipe. Some of the meteors were so bright that they were visible for some time after day had fairly dawned. Imagine large snowflakes drifting over your head, so near you that you can distinguish them, one from the other, and yet so thick in the air as to almost obscure the sky; then imagine each snowflake to be a meteor, leaving behind it a tail like a little comet; these meteors of all sizes, from that of a drop of water to that of a great star, having the size of the full moon in appearance: and you may then have some faint idea of this wonderful scene.
Similar stories were reported from across the country. There was no Moon that night, yet the sky was bright enough to read by.
Theories proliferated quickly. But it was an observation that explained the phenomenon, 33 years later. In early 1866, the U.S. Civil War had just ended a few months earlier, allowing the young naval paymaster Horace Tuttle to take up a post at the U.S. Naval Observatory. There he returned quietly to his lifelong pursuit of comet hunting. He soon found a new one that passed directly through the Earth’s orbit, precisely where the Earth would be in mid-November. (Since this is an American blog, I’ve conveniently ignored the fact that Ernst Tempel, a European comet-hunter, already found it two weeks earlier.) Tuttle’s measurements showed that every 33 years, this comet, Comet 5P/Tempel-Tuttle, leaves its cold home in the asteroid belt beyond Mars, where it spends most of its time. It speeds up, passes close to the Sun and returns. But comets are basically dirty snowballs. When Comet 5p/Tempel-Tuttle approaches the Sun, the heat of the Sun frees material from its icy core, leaving behind a debris field in space.
The debris orbits the Sun in the same path as the comet, in what is called a “meteoroid swarm”. Raj tells us what happens next, “The meteors don’t get here. The earth is moving into their path.” Every year, in mid-November, we Earthlings on our “Spaceship Earth” pass right through the debris field left behind by Comet 5p/Tempel-Tuttle. The meteoroids in the debris are not stationary, they travel in their own orbit, following the comet’s trajectory. The meeting of Earth and meteoroids is a classic T-bone traffic accident:
The meteoroids are mostly tiny, like specks of sand. Only when the enter the Earth’s atmosphere, at speeds around 40 miles per second into the air do they glow and burn up. The bright light, the “meteor”, is due to the hot air and hot silicon and other metals in the meteoroid itself glowing from the heat. Note the terms here: The speck of sand is a “meteoroid”—it does not become a “meteor” until it is hot and glowing in the Earth’s atmosphere. If a small rock-like object reaches the ground, that is then called a “meteorite”. And despite what 5-year-olds might tell you, they are definitely not “falling stars”.
It does not take much air to cause the meteor to glow. When you see the meteors, they are so high that the air there is less than one part in 100,000 as dense as the air we breathe. We live in the lowest level of the atmosphere, where the densest air and weather is, called the “troposphere”. Airplanes fly at around 35,000 feet at the top of the troposphere, a bit below the stratosphere. A very high level of the atmosphere lies around 275,000 feet. This layer, the “mesosphere” is where the meteors form. Scientists give it another name though: “The Ignorosphere”. That is because it is barely studied. It is too low to fly satellites in since the friction from the small amount of air would destroy their orbits. But it is too high for flying scientific balloons, because there is not enough air to provide buoyancy. A friend of mine studies it the only way to get there, by sending up sounding rockets. But such rockets spend only about 5-10 minutes in that region before falling down, so we have precious little direct data. (My friend was not very happy during Season One when Sheldon took great offense at his sister calling him a rocket scientist.)
The Leonids storm of 1833 played a major role in our understanding that meteorites in space caused meteors. Some suspected that meteors were an atmospheric phenomenon, and doubted there were rocks or pebbles in space. When two Northern farmers claimed that they saw a meteoroid fall from their sky to their farm, Thomas Jefferson remarked:
I would rather believe that two Yankee farmers lied than to believe that rocks fall from the sky.
Yet the meteors of the 1833 storm came from a spot on the sky that moved with the stars. In fact, from the direction of the constellation Leo, hence the name “Leonids”. Since their point of origin did not stay fixed in the atmosphere, but rotated with the Earth, that showed the meteors to be initiated by objects from space. Don’t feel bad for Thomas Jefferson though, it was just one of many things he had wrong.
Astronomers predict that this year’s Leonids will put on an excellent show tonight. Try to get out of the city and see them. Camp overnight if you can. Just watch out! There will be science teachers out there.