Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)

Part III. Skies at Night Are Big and Bright

Chapter 15. Meteors, Meteoroids, and Meteorites, Oh My!: The Impact of Meteors and Asteroids

n December 4, 2000, at roughly 5:00 P.M., something fell out of the sky and landed in David and Donna Ayoub's backyard in Salisbury, New Hampshire. Witnesses say the object was moving rapidly and glowing hot. When it landed, it set two small fires a couple of meters apart on the Ayoubs' property. The couple quickly ran outside to put them out.

The event certainly brought a lot of attention to the town. At first, it was a small story in the news section of the local newspaper, the Concord Monitor. However, the story was quickly picked up by an e-mail list sent out to astronomers interested in asteroid, meteor, and comet impacts. Soon the Ayoubs were receiving phone calls and were welcoming news media from all around the world. Everyone wanted to hear about what they saw, and most people assumed it was a meteorite impact.

I was skeptical when I heard the story the next day. I decided to look into this myself, so I phoned several of the witnesses. These people were sincere, and really wanted to know what had happened. After listening to them I believe that something truly did fall from the sky and set two fires. However, I don't think it was a meteorite, whatever it was.

Why don't I believe it was a meteorite? Well, that's a tale of bad astronomy.

I've always felt sorry for small meteors.

A given meteoroid may spend billions of years orbiting the Sun, perhaps first as part of a magnificent comet or an asteroid. Finally, after countless times around the Sun, its path intersects the Earth. It closes in on the Earth at a velocity that can be as high as 100 kilometers (60 miles) per second. Upon contact with our atmosphere, the tremendous speed is converted to heat, and, unless the meteoroid is too big (say, bigger than a breadbox), that heat vaporizes the tiny rock.

From our vantage point on the Earth's surface, the meteoroid generates a bright streak that may or may not be seen by human eyes. After all those billions of years, the life of that small rock is over in a few seconds, and no one might even see it.

But its story doesn't end there. When I am asked to name the most common example of bad astronomy, I almost always answer: meteors. Nearly everyone who is capable has seen a meteor flashing across the sky, yet, ironically, most people don't understand them at all.

Worse, even the naming of the phenomenon gets confused. Some people call them "shooting stars," but of course they aren't really stars. In chapter 3, "Idiom's Delight," I go over the three names describing the various stages of the rock: The solid part is called a meteoroid both while out in space and passing through our atmosphere, the glow of the meteoroid as it passes through the atmosphere is called a meteor, and it's a meteorite when (or if) it hits the ground.

But giving them names doesn't help much. We need to know what's going on during those stages.

A meteoroid starts out life as part of a bigger body, usually as either a comet or an asteroid. Asteroids can collide with each other, violently flinging out material or, in a worst-case scenario, shattering the parent body completely. Either way, you get debris going off rapidly in all directions. That debris can take on new orbits, where it might eventually cross paths with the Earth. When that happens, we might see a single bright meteor flash across the sky. Since the bits of meteoroid may be coming from any random direction in space, we see them come from any random point in the sky, traveling in a random direction. We call these sporadic meteors.

Cometary meteors are different. Comets are about the same size as asteroids but have a different composition. Instead of being mostly rock or metal, comets are more like frozen snowballs; rocks (from pebble size to kilometers across) held together by frozen material like water, ammonia, and other ices. When a 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 meteoroidthe 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 asteroids do indeed exist.) In the same year, the movie Deep Impact depicted a comet getting shattered by a bomb shortly before it entered the Earth's atmosphere. That's even worse! Instead of a single impact yielding an explosion of billions of megatons, you'd get a billion impacts each exploding with a yield of many megatons. In his fascinating book, Rain of Iron and Ice (New York, Helix Books, 1996), University of Arizona planetologist John Lewis calculates that breaking up a moderately sized asteroid can actually increase the devastation by a factor of four to ten. You'd spread the disaster out over a much larger area of the Earth, causing more damage.

If we cannot blow it up, then what? Of course, the best option is for it to miss us in the first place, so we'd have to shove it aside. The orbit of an asteroid can be altered by applying a force to it. If enough time is available, like decades, the amount of force can be small. A larger force is needed if time is short.

There are several plans for pushing such rocks out of the way. One is to land rockets on the surface and erect a giant solar sail. The sail, made of very thin Mylar with an area of hundreds of square kilometers, would catch the solar wind and also react to the minute pressure of sunlight. It would impart a gentle but constant force, moving the rock into a safer trajectory.

Another plan is more blunt: attach rockets to the asteroid and use them to push it. This has the engineering difficulty of just how you'd strap boosters to a rock in the first place.

Ironically, Hollywood came close to another good plan. Instead of blowing the rock up, we use nuclear weapons to heat the asteroid. Again, in Rain of Iron and Ice, Lewis finds that a small nuclear explosion (he implies a yield of about 100 kilotons) would suffice. Exploded a few kilometers above the surface, the intense heat of the explosion would vaporize material off the surface of the asteroid. This material would expand outward, and, like a rocket, push the asteroid in the other direction. Lewis mentions that this has two benefits: it prevents the impact, and also removes a nuclear weapon from the Earth. This is the favored method of all the people who have studied it.

All of these methods have a subtle assumption attached, that we understand the structure of asteroids and comets. In reality, we don't. Asteroids come in many flavors; some are iron, some stony. Others appear to be no more than loose piles of rubble, barely held together by their own gravity. Without knowing even the most basic information about asteroids, we are literally shooting in the dark.

As with most problems, our best weapon is science itself. We need to study asteroids and comets, and study them up close, so that we can better understand how to divert them when the time comes. On February 14, 2000, the NASA probe Near Earth Asteroid Rendezvous entered orbit around the asteroid Eros. The amount learned from the mission is astounding, such as the surface structures and mineral composition of the asteroid. More probes are planned, some of which are ambitious enough to actually land on asteroids and determine their internal structure. We may yet learn how to handle dangerous ones when the time comes.

There is an interesting corollary to all this. If we can learn how to divert an asteroid instead of merely blowing it up, that means we can steer it. It may be possible to put a dangerous asteroid into a safe orbit around the Earth. From there we could actually set up mining operations. Based on spectroscopic observations of meteorites and asteroids, Lewis estimates that an asteroid 500 meters across would be worth about $4 trillion in cobalt, nickel, iron, and platinum. The metal is pure and in its raw form, making mining relatively easy, and the profit from such a venture would be more than enough to pay off any initial investment. And that's a small asteroid. Bigger ones abound.

Science fiction author Larry Niven once commented that the reason the dinosaurs became extinct is that they didn't have a space program. We do, and if we have enough ambition and enough reach, we can turn these potential weapons of extinction into a literal gold mine for humanity.

Until then, we don't have too many options. Maybe we can divert the big one when the time comes, but for now all we can do is imagine what an impact might be like. Unfortunately, movies have had their own impact. Anytime an unexplained phenomenon involves something falling from the sky, meteors are usually blamed.

Which brings us back to the Ayoubs, still searching for a meteorite in their backyard in Salisbury, New Hampshire. Initially, this night visitor sure did sound like the usual description of a meteorite. But my knowledge of their behavior was telling me otherwise. As I said, meteorites won't cause fires unless they are very big. But other things didn't add up, either. The path was described as an arc, while a meteor's trajectory would have been straight down. Also, no meteorite was ever found, despite a dedicated search. I mentioned to the property owner that meteorites can be sold for quite a bit of money, so he had strong incentive to find it. I never heard of anyone finding anything.

In the end, these events usually have some mundane, terrestrial cause. I would bet money that it was someone setting off fireworks in the thick woods near the Ayoubs' house. This is a guess on my part, and it may be wrong. We may never know what started those fires, but we know what it wasn't. We can blame Hollywood for our mistaken understanding of meteorites, but we can't blame everything else on the poor things themselves.