Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

comet gets near the Sun, the ice melts, and little bits of rock can work loose. This type of debris stays in roughly the same orbit as the comet for a long time. Not forever, though, because the orbit can be affected by the gravity of nearby planets, the solar wind, and even the pressure of light from the Sun. But the debris orbits are generally similar to that of the parent comet.

When the Earth plows through a ribbon of this meteoroidal debris, we see not one but many meteors. Usually it takes a few hours or nights to go all the way across the debris path, so we get what are called meteor showers — like a rain of meteors. We pass through the same debris ribbons every year at about the same time, so showers are predictable. For example, every year we pass through the orbital debris of the comet Swift Tuttle, and we see a meteor shower that peaks around August 12 or 13.

Meteor showers create an odd effect. Imagine driving a car through a tunnel that has lights all around the inside. As you pass them, the lights all seem to be streaking outward from a point ahead of you in the tunnel. It’s not real, since the lights are really all around you, but an effect of perspective. The same thing happens with meteor showers. The Earth’s orbit intersects the meteoroid stream at a certain angle, and that doesn’t change much from year to year. Like the lights in the tunnel, the meteors flash past you from all over the sky, but if you trace the path of every meteor backwards, they all point to one spot called the radiant.

This point comes from a combination of the direction the Earth is headed in space and the motion of the meteoroids themselves. The radiant is almost literally the light at the end of the tunnel.

So the meteor shower I mentioned above not only recurs in time but in space, too. Every August those meteors appear, and they seem to flash out of the sky from the direction of the constellation Perseus. Showers are named after their radiant, so this one is called the Perseids.

One of the most famous showers comes from the direction of Leo every November. The Leonids are interesting for two reasons: One is that, relative to us, the parent comet orbits the Sun backwards. That means we slam into the meteoroid stream head-on. The meteoroids’ velocity adds to ours, and we see the meteors flash across our sky particularly quickly.

The second interesting thing is that the meteoroid stream is clumpy. The comet undergoes bursts of activity every time it gets near the Sun (every 33 years or so), and this ejects lots of bits of debris. When we pass through these concentrated regions, we see not just dozens or hundreds of meteors an hour but sometimes thousands or even tens of thousands. This is called a meteor storm. The celebrated storm of 1966 had hundreds of thousands of meteors an hour, which means, had you been watching, you would have seen many meteors whizzing by every second. It must have really seemed as if the sky were falling.

So that’s why we get meteors. But why are they so bright? Almost everyone thinks it’s friction — our atmosphere heating them up, causing them to glow. Surprise! That answer is wrong.

When the meteoroid enters the upper reaches of the Earth’s atmosphere, it compresses the air in front of it. When a gas is compressed it heats up, and the high speed — perhaps as high as 100 kilometers per second — of the meteoroid violently shocks the air in its path. The air is compressed so much that it gets really hot, hot enough to melt the meteoroid. The front side of the meteoroid — the side facing this blast of heated air — begins to melt. It releases different chemicals, and it’s been found that some of these emit very bright light when heated. The meteoroid glows as its surface melts, and we see it on the ground as a luminous object flashing across the sky. The meteoroid is now glowing as a meteor.

Here I am guilty of a bit of bad astronomy myself. In the past, I’ve told people that friction with the air heats the meteoroid and, as I said above, this is the usual explanation given in books and on TV. However, it’s wrong. In reality, there is actually very little friction between the meteoroid and the air. The highly heated, compressed air stays somewhat in front of the meteoroid, in what physicists call a standoff shock. This hot air stays far enough in front of the actual surface of the rock that there is a small pocket of relatively slow-moving air directly in contact with it. The heat from the compressed air melts the meteoroid, and the slow-moving air blows off the melted parts. This is called ablation. The ablated particles from the meteoroid fall behind, leaving a long glowing trail (sometimes called a train) that can be kilometers long and can stay glowing in the sky for several minutes.

All of these processes — the huge compression of air, the heating of the surface, and the ablation of the melted outer parts — happen very high in the atmosphere, at altitudes of tens of kilometers. The energy of the meteoroid’s motion is quickly dissipated, slowing it down rapidly. The meteoroid slows to below the speed of sound, at which point the air in front is no longer greatly compressed and the meteor stops glowing. Regular friction takes over, slowing the meteoroid down to a few hundred kilometers per hour, which is really not much faster than a car might travel.

This means that it takes a few minutes for an average meteoroid to pass the rest of the way through the atmosphere to the ground. If it impacts the ground, it is called a meteorite.

This leads to yet another misconception about meteors. In practically every movie or television program I have ever seen, small meteorites hit the ground and start fires. But this isn’t the way it really happens. Meteoroids spend most of their lives in deep space and are, therefore, very cold. They’re only heated briefly when they pass through the atmosphere, and they’re not heated long enough for that warmth to reach deep inside them, especially if they are made of rock, which is a pretty decent insulator.

In fact, the hottest parts ablate away, and the several minutes it takes for the meteoroid to get to the ground let the outer parts cool even more. Plus, it’s traveling through the cold air a few kilometers off the ground. By the time it impacts, or shortly thereafter, the extremely frigid inner temperature of the meteoroid cools the outer parts very well. Not only do small meteorites not cause fires, but many are actually covered in frost when found!

Large meteorites are a different story. If it’s big enough — like a kilometer or more across — the atmosphere doesn’t slow it much. To really big ones, the atmosphere might as well not exist. They hit the ground at pretty much full speed, and their energy of motion is converted to heat. A lot of heat. Even a relatively smallish asteroid a hundred meters or so across can cause widespread damage. In 1908, a rock about that size exploded in the air over a remote, swampy region in Siberia. The Tunguska Event, as it’s now called, caused unimaginable disaster, knocking down trees for hundreds of kilometers and triggering seismographs across the planet. The event was even responsible for a bright glow in the sky visible at midnight in England, thousands of kilometers from the blast. The fires it started must have been staggering.

Understandably, such events are a cause of concern. Even little rocks — well, maybe the size of a football stadium — can have big consequences. But it does take a fair-sized rock to do that kind of damage. Little ones, and I mean really little ones, like the size of an apple or so, usually don’t do more than put on a pretty show. I remember seeing a bolide, as the brightest meteors are called, as I walked home from a friend’s house when I was a teenager. It lit up the sky, bright enough to cast shadows, and left a tremendous train behind it. I can still picture it clearly in my mind, all these years later. Sometime afterward I calculated that the meteoroid itself was probably not much bigger than a grapefruit or a small bowling ball.

But the big meteorites worry a lot of people, as well they should. Very few scientists now doubt that a large impact wiped out the dinosaurs, as well as most of the other species of animals and plants on the Earth. That impactor was probably something like 10 kilometers (6 miles) or so in diameter, and left a crater hundreds of kilometers across. The explosion may have released an unimaginable 400 million megatons of energy (compare that to the largest nuclear bomb ever built, which had a yield of about 100 megatons). It’s no surprise that some astronomers stay up nights (literally) thinking about them.

There are teams of astronomers across the world looking for potential Earth impactors. They patiently scan the sky night after night, looking for the one faint blip that moves consistently from one image to the next. They plot the orbit, project it into the future, and see if our days are numbered.

No one has found such a rock yet. But there are a lot of rocks out there…

Suppose that sometime in the near future the alarm is pulled. An asteroid as big as the Dinosaur Killer is spotted, and it will soon cross paths with us. What can we do?

Despite Hollywood’s efforts, the answer is probably not to send a bunch of wisecracking oil riggers in souped-up rocket ships to the asteroid to blow it up at the last second. That may have worked in the 1998 blockbuster Armageddon, but in real life it wouldn’t work. Even the largest bomb ever built would not disintegrate an asteroid “the size of Texas.” (Not that Armageddon was terribly accurate in anything it showed; about the only thing it got right was that there is an asteroid in it, and