The space between the planets is practically empty -- so empty, in fact, that most laboratory vacuums have far more material in them than interplanetary space does -- but it is not entirely empty. For one thing, there is the so-called Solar wind -- pieces of hydrogen and helium atoms which are blowing away from the Sun at a few hundred miles per second (typically about 200 miles per second, but the speed can vary considerably, depending upon how "active" the Sun happens to be). Any given part of the Solar wind is extremely rarefied, with a hundred protons per cubic inch being a fairly typical density (although this also varies considerably, from time to time, and place to place). To get an idea of just how rarefied this is, consider that at the surface of the Earth, each inch of air contains approximately 500 million trillion molecules of nitrogen and oxygen, each of which weighs about 30 times as much as a proton, so that you would need almost a hundred million cubic miles of Solar wind material to have as much mass as one cubic inch of air. As a result, when we talk about the Solar wind, we are talking about something which is so close to nothing that it's hard to imagine that there's anything there, at all.
In addition to this thin gas, there are small bits of solid debris, primarily microscopic in size, but also of varying sizes ranging from sand-grain sized objects, to pea and pebble sized bits, scattered throughout the inner Solar System. About 90% of this material is debris lost by comets as they orbit the Sun (as will be made clear later on in this discussion), but a small portion of it is bits knocked off of asteroids and other bodies, in various high-speed collisions, at various times in the past. The number of such solid bits is even less than the number of Solar wind particles, with only a few grams of such materials scattered through each million cubic miles of space, but despite the tiny amount of material which is in any given region, we run into a surprisingly large amount of this debris every single day, because, with a cross-sectional area of 50 million square miles, and an orbital velocity of a million and a half miles per day, the Earth sweeps through almost a hundred trillion cubic miles of space each day. If you count objects which we might otherwise miss, but are pulled into us because of our gravity, we run into something of the order of a few dozens to a few hundreds of tons of meteoric debris each day (the term meteoric being derived from a Greek word referring to something in the air).
For that portion of the material which we encounter which is truly microscopic, there is no visible result of our sweeping it up, as the small mass which it has is easily slowed down and stopped by the thin gases in our exosphere, or outer atmosphere, more than a hundred miles above the surface of the Earth. However, bits which are sand-grain or larger in size can plunge right through the exosphere with no reduction of their speed, and run into the upper mesosphere, 50 to 100 miles above the surface of the Earth, with speeds ranging from 25 thousand miles per hour (if they happened to be moving through space with the same speed as the Earth, and were only pulled into us by our gravity) to as much as 150 thousand miles per hour (if they happened to be heading in the opposite direction in space, and slammed into our atmosphere with the maximum possible speed). At these high velocities, the passage of the objects through the atmosphere generates tremendous heat, raising the temperature of the meteoric objects, and the air through which they are passing, to temperatures of a few thousands of degrees (if the speeds are on the "slow" side) to more than a hundred thousand degrees (if the speeds are near the upper limit of possible velocities). This causes the meteoric object to vaporize, and the resultant gases from the vaporizing object, and the air through which it is passing, to glow brightly, producing a meteor -- a streak of light which, because it is only a few tens of miles away from the observer, and is moving at tens of thousands of miles per hour, seems to rapidly zoom across the sky, appearing, flaring into brightness, and then fading into nothingness, within a few seconds.
Please note that this phenomenon is in no way like the phenomenon which we call a comet. A comet is a dirty snowball, typically several miles in size, which is surrounded by a glowing cloud of gas and dust the best part of tens of thousands of miles across, and is generally observed at distances of tens of millions of miles. Even though the speeds of comets may be in the same range as the speeds of meteoric material, since their distances from us are so vast, they appear to hardly move from moment to moment, and in fact, can be seen at nearly the same place in the sky from night to night, for days or weeks on end. If you see a comet, you can call up a friend, tell them where to look for it, and give them a day, a week, or even a month to get around to doing so, depending upon how bright the comet is, and how it is moving through the sky. If, however, you spot a meteor zipping across the sky, your friend can be standing right next to you, and by the time you bring the object to her attention, and she turns to look at it, the thing can have completely faded from sight, even if it only took her a second or so to divert her attention from whatever else she happened to be observing.
"The Meteor of 1860", a painting by Frederic Church (Although theoretically in the public domain,
described as "Courtesy of Judith Filenbaum Hernstadt", who is presumably the current owner of the painting)
Although most meteors are faint streaks, some (produced by larger meteoroids) can be quite spectacular, lighting up the sky almost as if daylight, and in some cases, breaking up into a series of similarly bright objects and trails. In the image above, a painting of the great meteor of July 20, 1860, a famous meteor "procession" is shown. A meteor procession is a rare phenomenon, in which an "earth-grazing" meteoroid breaks up into a number of pieces, which can take as much as a minute and half (in the case of the 1860 meteor) to pass from horizon to horizon. This procession, which occurred around 9:45 in the evening (New York time), passed above a well-populated area in the northeastern United States, from Michigan and the Great Lakes to New York and into the Atlantic, was widely observed and reported, and was the impetus for one of the poems in Walt Whitman's "Leaves of Grass", as he was among the thousands of people who had the good fortune to observe the meteor procession. (The images below were scanned from this writer's personal copy of the Harper's Weekly issue of August 4, 1860, which had a long article about the meteor and its observers, as well as the engravings shown.)
One thing to keep in mind, when discussing meteors, is that what you are looking at is not the actual object which is passing through our atmosphere. As stated above, this will typically be, particularly for a faint meteor, which would only be visible in a dark sky, a speck of material about the size of a sand grain, and the heated air through which it is going might be a column only half an inch thick and a few miles long, sixty to ninety miles above you, in the upper mesosphere or lower thermosphere. At that distance, you couldn't possibly see the thing, even with a telescope, as an actual object. What you are seeing is the streak of light made by the gas of the vaporizing object, and the heated air through which it is passing, as it glows with a temperature of thousands, or tens of thousands of degrees. So, when you see a meteor, you are not seeing an actual object at all, but an atmospheric phenomenon produced by the rapid passage of some object through our atmosphere.
There was, of course, an actual object, out in space, prior to this passage, and to distinguish that object from the phenomenon of the meteor, or meteor trail, we call the object, when it was out in space, a meteoroid (the "-oid" ending being used quite commonly in astronomy to indicate a small object out in space -- as when talking about an "asteroid", or a "planetoid"). Sometimes, it is possible that part of the object which was passing through space happens to survive its passage through the atmosphere, and can be actually picked up and studied. In that case, because geologists often name various kinds of rocks and minerals by attaching the suffix "-ite" to them, we call the object a meteorite. So, to summarize this terminology, if there is something running through space, which could run into the Earth and produce a meteor (the streak of light produced by its rapid passage through the atmosphere), the object in space is referred to as a meteoroid, and in the unlikely event that part of it reaches the surface of the Earth and is picked up and studied, it is referred to as a meteorite. The three terms of course sound very similar, and it could be easy to get them confused, so try to remember the reason for the specific endings, "-oid" referring to an object in space, and "-ite" to a rock, to keep them straight in your mind.
Where Do Meteoroids Come From?
Just looking at a meteor doesn't tell you a lot about how it is moving, or where it came from, but it is possible to study the motion in some detail by using a specially mounted camera which has a rapidly rotating shutter passing in front of it (as often as 50 times per second), so that the meteor trail is broken up into short segments, each of which corresponds to a particular moment in time. If two or more cameras, set up in such a way, are trained on the same part of the sky, but at different locations, so that they are looking at that part of the sky from different directions, it is possible to use surveying techniques, such as triangulation, to calculate the exact motion of the meteoroid through the upper atmosphere, and from that, to calculate various data about the motion of the meteoroid which produced the meteor, and its physical characteristics. The photographs immediately below show how a meteor trail would look when photographed with, and without, a rotating shutter.
On the Left: Ordinary image of a meteor trail, taken without a rotating shutter. In this image, the camera was guided to follow the stars, so they appear as dots, while the meteor is a straight-line trail. On the Right: Image of a (different) meteor trail. In this case, the camera was NOT guided to follow the stars, so they exhibit short trails corresponding to the exposure time, while the meteor trail was broken up by the rotating shutter.
The information which is obtained by a careful analysis of the meteor trails is of several varieties:
the luminosity of the meteor trail, L,
(1) The velocity of the meteor, meaning its direction and speed, in each part of the trail.
(2) The apparent and true brightness of each segment of the trail, and the total brightness of the entire trail.
(3) The change in speed, or deceleration, caused by the pressure of the atmosphere, slowing the meteoroid.
The velocity of the meteoroid, combined with an analysis of its deceleration, and the gravitational effects of the Earth, which of course helps accelerate the object as it runs into us, can be used to calculate the true orbital motion of the object before it encountered the Earth's gravitational influence. This shows us that approximately 90% of the meteoroids have cometary orbits (very elongated orbits which come from random directions in the sky, instead of following the plane of the planetary orbits), and only 10% of them have orbits like asteroids (orbits which are in, or nearly in, the orbital plane of the Solar System, and have orbital sizes of only a few AUs).
The brightness of the trail can be used to estimate the kinetic energy, or energy of motion, of the incoming particle, presuming that all of that energy is transformed into heat and light (this may not be true for larger particles, parts of which may reach the surface of the Earth, but is certainly true for smaller objects, which are completely vaporized during their passage through the atmosphere). This kinetic energy is related to the mass and velocity of the meteoroid prior to running into our upper atmosphere, and we can calculate the mass of the object, for objects small enough to be completely vaporized, by using the following calculation:
is equal to the kinetic energy of the meteoroid, 1/2 m v2, or
L = m v2 / 2,
where the velocity, v, is determined from the segment lengths, as already discussed. Rearranging the terms, we can solve for the mass of the meteoroid as
m = 2 L / v2.
This sort of calculation tells us that the majority of meteoroids which produce faint meteors, which are only visible in a dark sky, are about the size of a small grain of sand, while brighter meteors are generally produced by pea-size or larger bits of debris. We also find, from such analysis, that there are relatively few larger meteoroids, and far greater numbers of smaller meteoroids. On the average, doubling the size of the particles, which would increase their volume and mass by about a factor of eight, decreases their numbers by about that same factor of eight.
(NOTE: MORE TO BE ADDED TO THIS PART OF THE DISCUSSION, AT A LATER DATE)
Finally, if we carefully study the deceleration (or reduction in speed) which the meteoroid suffers as a result of the pressure of the atmosphere through which it is passing, by examining any change in the segment lengths, it may be possible to estimate the size of the meteroid. As an example, look at the following diagram. In the first example, shown on the left, the lengths of the segments are approximately constant, indicating that the object just plowed through the atmosphere without any substantial effect on its speed. This implies a relatively small, dense object, for which the air resistance was small compared to its momentum and mass. The path on the right, however, in which the segments rapidly decrease in length, implies a lower density object, for which the air resistance was substantial in comparison to its momentum and mass.
A meteoroid with a high density (on left) has very little change in its motion as it vaporizes,
while a meteoroid with a low density (on right) slows down dramatically as it vaporizes.
The first object would have a relatively small size in comparison to its mass, and hence, a relatively high density, whereas the second object would have a relatively large size in comparison to its mss, and hence, a relatively low density. Rather interestingly, we find that the 90% of meteoroids which have cometary orbits almost always have very low densities, almost always less than that of water, and in many cases, as little as 1/10th the density of water, implying that they are made of ices, or even light, fluffy, nearly empty structures, such as might result from taking a mixture of ice and other debris and removing the ice, so that what is left over is mostly empty space. In contrast, the 10% of meteoroids which have orbits like asteroids have relatively high densities. About 90% of them have densities 2 to 3 times the density of water, implying that they are made of rocky materials, or something similar to that, while 10% of the asteroidal meteoroids appear to have much higher densities, as much as several times the density of water, suggesting that they are made of denser materials, such as iron.
When is the Best Time to See Meteors?
Obviously, meteors are best observed at night, because most of them are very faint (although exceptionally bright meteors can be seen even during daylight). In addition, it helps if the sky is clear, and dark (far away from city lights, haze caused by natural or human activities, on a night when the Moon is either a very thin crescent, or not up). But, in addition, it turns out that there is a substantial difference in the number of meteors that are seen before midnight, compared to the numbers seen after midnight.
The reason for this can be seen by examining the diagram below, which shows how the Earth moves about the Sun. In this view, we are looking at the Earth, and its orbit, from above the North Pole, so that our orbital and rotational motions are viewed as counter-clockwise (we can tell that the Sun is below the bottom of the diagram, from the way in which the night side of the Earth is darkened, and from the curvature of the path of the Earth, which bends around the Sun). In examining the diagram, remember that the direction that we move around the Sun (from right to left in the diagram) is the same as the direction that we rotate around our axis (counter-clockwise, as shown here).
Why more meteors are visible after midnight, than before
A and B would not be visible, because it is daylight.
C and D would have to catch up with the Earth, and would be relatively faint.
E and F would strike the Earth at high speed, and would be relatively bright.
Since the side of the Earth which faces the Sun is in daylight, and the Sun is toward the inside the of our orbit (which, as already mentioned, is on the bottom, in the diagram), we can label various parts of the Earth as representing various times of the day. On the left, where someone would be coming from the back, or night, side of the Earth, into the front, or day, side, it would be dawn. On the right, it would be dusk, and as otherwise shown, by the labeling of the graph, the part of the Earth which most directly faces the Sun corresponds to noon, and the opposite side corresponds to midnight.
Since the Earth is moving to the left in the diagram, the left-hand side of the Earth, which corresponds to places with local times between midnight and noon, is on the front side of the Earth, as far as our motion is concerned, and the right-hand side of the Earth, which corresponds to places with local times between noon and midnight, is on the back side of the Earth, relative to our motion around the Sun. Now, meteoroids can run into us, and produce meteors, no matter which part of the Earth we are on, but if they run into us on the daylight side (meteors A and B), they will be difficult or impossible to see, because of the bright light of day (although they might still be observable by radar imaging of the ionization trails left by them as they pass through the upper atmosphere). Even on the night side of the Earth, however, there are differences in the way that we see things. Meteors running into us from behind (meteors C and D) would have to catch up with us, so to speak, and would run into our atmosphere with speeds between 25 and 50 thousand miles per hour (presuming that they are going fast enough to catch up with us in the first place), but meteors running into us from in front (meteors E and F) might have their motions around the Sun (which might be as much as 85 thousand miles per hour) added to our own motion, of approximately 60 thousand miles per hour, and could hit our atmosphere with speeds of more than 150 thousand miles per hour.
In other words, between dusk and midnight, we will run into fewer meteoroids per hour, and typical impact velocities would be under 50 thousand miles an hour, giving them less energy per pound of material, and making them look somewhat fainter, whereas between midnight and dawn, we would run into more meteoroids per hour, and typical impact velocities might be well above a hundred thousand miles an hour, making them look somewhat brighter, per pound of material (since, as discussed above, the brightness of a meteor trail depends upon the kinetic energy of the meteoroid, which depends, among other things, upon the square of the meteoroid velocity). If meteoroids which hit us after midnight can run into us at 2 to 3 times the speed that meteoroids which run into us before midnight do, then, for a given size meteoroid, they would be 4 to 9 times brighter, which would make them considerably easier to see. Combined with the greater numbers of meteoroids that we would run into, anyway, this would considerably increase the number of meteors which are bright enough to easily see. As a result, on most nights, although you can see half a dozen to a dozen meteoroids per hour after midnight, you would only see one or two meteors per hour, prior to midnight. Meteor observing is, therefore, an activity best pursued in early pre-dawn hours.
Sporadic and Shower Meteors
Most nights, if you have a clear dark sky, you can see a few meteors per hour, particularly, as mentioned above, between midnight and dawn. Typically, there is no relationship between the motion of one meteor, and another one, as the meteoroids which produce them are moving through space in random directions, relative to each other, and relative to the Earth. Under these circumstances, if you make a plot of the paths followed by the meteors over a period of time, you will get a result similar to that shown in the left-hand figure, below. At one moment, you will see a meteor passing in one direction, then, a little while later, another meteor passing in a completely different direction, so that, after a while, a graph showing the motions of the various meteors displays more or less randomly oriented lines. When we see this sort of pattern, or lack of a pattern, we say that we are looking at sporadic meteors.
On many nights, however, something slightly different happens, as the Earth passes close to a stream of debris which happens to be moving through space in more or less the same direction (albeit spread out over tens or hundreds of thousands of miles of space). When this happens, the meteors that you see at various times, if plotted on a map of the sky, gradually assume a pattern like that shown in the right-hand diagram. Each of the meteor trails seems to be moving away from a particular point in the sky (indicated by a cross), called the radiant of a meteor shower, and the meteors involved are referred to as shower meteors.
Left, above: The random pattern made by sporadic meteors over a period of time.
Right, above: The organized pattern made by shower meteors over a period of time.
The cross near the center of the right-hand picture represents the radiant of the shower.
Below: The Perseid shower of 2004 -- a superposition of many short exposures taken over a period of six hours. Click the image for an animated view of Perseid meteors on August 13, 2005 (opens in new window). (Fred Bruenjes, )
A high-contrast version of superposed images taken over a period of three hours shows meteor trails streaming away from a radiant in the northeastern portion of Orion -- hence the name of the shower, the Orionids. The meteoroids which produce this shower are portions of the debris lost by Halley's Comet, during its perihelion passages. (original image © Tunc Tezel, apod061023)
Sometimes the number of meteors viewed during a meteor shower is not significantly different from the number of sporadic meteors seen on that or other nights, and the only way you can tell that you are looking at a meteor shower is by carefully plotting the motions of the meteors, and seeing that there is a pattern to their overall motions. On some nights, however, the Earth passes close to debris trails (resulting from the disintegration of comets) which have very large numbers of particles in them, and for a time ranging from a few hours to a few days the number of visible meteors may increase to several dozen or, on rare occasions, several hundreds of meteors per hour. Showers of this sort are considered to be major meteor showers, and it is nice to know when they are going to occur, and how to look for them. The table below shows information for a number of the more important meteor showers which can occur in any given year. Before discussing the tables in some detail, let's consider the two notes, indicated by (*) and (**).
The first note refers to the way in which meteor showers are named. Historically, showers have been named according to the constellation that the radiant of the shower lies inside, with the suffix -ids being added to the constellation name, to indicate that we are talking about a meteor shower. Thus, the Leonids radiate from the constellation of Leo, and the Orionids from the constellation of Orion. (The Quadrantids, which radiate from the constellation of Bo÷tes, appear an exception to this rule; but at the time that the Quadrantids were named, it had been proposed that a new constellation, Quadrans Muralis, be used to designate the northern part of the constellation of Bo÷tes. The proposal was later rejected, but in the meantime, the Quadrantids were named after it. Perhaps it would make more sense if the Quadrantids had been renamed the B÷otids, but there are other ("minor") showers associated with that constellation, so that was not done.)
The second note refers to the identification of 3200 Phaethon as the "comet" associated with the Geminid meteor shower. In general, meteor showers represent debris lost by comets as they pass close to the Sun. However, 3200 Phaethon was originally considered to be an asteroid, because its orbit lies within the inner asteroid belt, and although it has an extremely eccentric orbit (orbital eccentricity = 89%) far more typical of comets than of asteroids, it does not develop a head and tail as it approaches perihelion. It is probable that Phaethon is neither a normal asteroid, nor a normal comet, but what might be called a "dead" comet -- a comet which has lost so much of its icy material that it appears identical to an asteroid. This seems very likely because it is very dark (absorbing over 90% of the sunlight that falls on it), approaches within 13 million miles of the Sun at perihelion (less than half Mercury's perihelion distance, and closer than any other asteroid), and as a result, may reach temperatures of 1000 Kelvins (about 1400 Fahrenheit degrees) each time it passes perihelion (every 17 months), which should cause it to lose large amounts of ice, if it had any left to lose. So even if originally a comet, it would now be just a 3-mile-wide rubble pile.
The Leonid meteor shower is of particular note, as on rare occasions, it has produced meteor storms in which as many as a hundred thousand meteors per hour have been observed. The diagrams below show old woodcuts purporting to show the appearance of the Leonid showers of 1799 and 1833.
Left: A drawing of the 1799 Leonid meteor shower. Note that the trails would not actually be curved.
Right: A engraving of the 1833 Leonid meteor shower (actually drawn much later).
Why Radiants Occur
As mentioned above, during a meteor shower, the shower meteors seem to move away from a particular point in the sky, called the radiant of the shower. The direction of the radiant is determined by the combined motion of the Earth and the meteoroids, when the Earth is going past the orbit of the meteoroids. As shown in the diagram below, this is not necessarily the same as the direction of either the Earth's motion, or the meteoroids' motion, but a combination of the two motions.
In this diagram, the Earth is shown moving to the left, and the shower meteoroids as coming from up and to the left of the Earth. However, because of the Earth's motion, the meteoroids do not actually appear to be coming from that direction, but from the direction labeled "radiant". To see how this works, imagine a triangle made by moving the actual meteoroid motion to the left, so that the arrowhead representing the meteoroid motion touches the arrowhead representing the Earth's motion. Then another arrow, starting from the tail of the meteoroid arrow, and going toward the Earth, represents the direction that the meteoroids appear to be coming from.
If the swarm of meteoroids that the Earth is running through is very narrow, so that we only encounter it at a particular place in our orbit, then the radiant will be fixed in direction, according to the relative motion of the Earth and the meteoroids at the time that we encounter them. Usually, however, the meteoroid stream is spread out over several million miles, and it takes us a few days to pass through it. During that time, our motion gradually changes, as our path curves around the Sun, and so the combined motion of the Earth and the meteoroid stream gradually changes, as well, causing the exact position of the radiant to gradually move from night to night. The change is always relatively small, however, so that the meteors still appear to be coming from the same overall direction in the sky.