Fireball Physics and Facts
Frank Sanders’ Quick Guide to Fireballs:
Everything you really need to know about fireballs.
(See the text below for my Quick Guide to Fireballs. Or, for the brave of heart, download my PDF, Fireball Physics and Meteorite Falls for the complete but gory mathematical details.)
For the story of how we used my knowledge to locate three Colorado meteorites from a fireball event a few years ago, click here: The Amazing Story of Meteorite Recovery from a Colorado Fireball.
For a detailed physical description of fireballs (with the math included), download the 43-page PDF version of my work here...Fireball physics and meteorite falls.
The document describes in detail the physics of fireballs and where to find meteorites after fireballs. I authored this back in 2000 for the Denver Museum of Natural History (later Nature and Science) Fireball and Meteorite Workshop. It starts with a set of detailed physical and mathematical derivations that describe the physics of fireballs. The document then moves on to provide analytical tools for finding the places where meteorite strewnfields can be expected to be found after fireball events.
For guidance on how to mathematically analyze fireball reports, download my document on how to analyze fireball reports.
We used my fireball physics paper and my fireball report analysis paper to find meteorites from a Colorado fireball event. Click here to read the story of how we recovered three meteorites from a Colorado fireball.
Click All-sky camera talk for a Powerpoint presentation I created that discusses all-sky cameras and what you can do with an all-sky camera network.
To see my original pitch for the Colorado All-Sky Fireball Camera Network that the museum did finally build, click here: Colorado all-sky fireball camera network proposal.
Want to learn fireball basics quickly and easily? Read my Quick Guide to Fireballs (much briefer than my PDF document, and with no math), below:
Frank Sanders’ Quick Guide to Fireballs
Introduction
I became interested in the phenomenon of fireballs (mega-meteors, technically called bolides) back in the mid-1990s. I worked for a number of years with the then-Curator of Geology at the Museum of Natural History (now the Museum of Nature and Science) in Denver on the problem of locating meteorite falls from eyewitness reports of fireballs. In the course of this work, I did two things: I made my own, independent study of why fireballs happen, and of how to capture fireball trajectory information for the purpose of locating likely locations of meteorites on the ground.
Fireballs, Meteorites and Related Terminology
Here is the proper terminology for meteorites, fireballs and so forth: A meteor (from the Greek meteos = atmosphere) is the visible streak of light in the sky that is generated by the passage of a space rock, called a meteoroid, into the Earth’s atmosphere. The bright light is an effect of a process called ablation, described below. If a meteor is especially bright and long-lived it is called a fireball or bolide. If one or more pieces of a meteoroid manage to survive the fireball event and reach the Earth’s surface then it (or they) become one or more meteorites. The passage of a fireball meteoroid to one or more meteorites on the ground is called a fall or a fall event. The area where a group of meteorites are found on the ground from a single fall event is a strewnfield.
What Causes a Meteor
Here is the cause of meteors and fireballs. (A much more detailed version of my explanation being contained in my fireball physics and meteorite falls downloadable PDF document.) When a space rock (that is, a meteoroid) enters the Earth’s upper atmosphere from deep space, where it has presumably been orbiting the sun for eons, it is traveling at about 25,000 mph (41,000 km/hr). Friction with the Earth’s upper atmosphere causes the surface of the rock to heat up beginning at an altitude of 60 miles above the Earth’s surface. As the surface heats, it glows incandescently (like metal heated in a forge) and begins to lose flakes of its surface material. As the original surface ablates into the atmosphere, a new, fresh surface is exposed from beneath the original surface. This new surface layer likewise is heated and ablated. The ablation process continues, like peeling layers from an onion (in very small flakes rather than in sheets, however), until the original meteoroid ablates down to nothing and vanishes.
The glowing surface and the hot, ablating flakes are seen by people on the ground as a meteor. The meteor leaves a trail of flaked-off ablation debris behind it. If the rock is small, on the order of a few kilograms (usually less), it will quickly ablate away to nothing and the meteor trail will vanish. Most meteors are caused by space rocks that are only a few grams in mass. They flare and burn to nothing at extremely high altitudes, on the order of 40 miles high. (By comparison, commercial airliners typically fly at 6 or 7 miles altitude while specialized, high-performance U-2 or TR-1 reconnaissance aircraft might be found twice as high, at 15 miles altitude. Satellites orbit at altitudes of 100 miles or more.)
What Causes a Fireball
But now consider what happens when a much larger rock, on the order of ten kilograms or more, enters the Earth’s atmosphere. Its surface heats up and glows incandescently and it begins to lose material to ablation just as the smaller meteor-generating rocks do. But a large rock falling into the atmosphere has enough material to survive much longer than a smaller rock. During its relatively long flight life, it continues to drop deeper and deeper into the Earth’s atmosphere. The denser lower atmosphere intensifies the frictional heating effect. The surface glows brilliantly and the ablating material flares with tremendous brightness. This is a fireball. A fireball is nothing more than a space rock (meteoroid) that is so large that it survives long enough to plummet into the dense lower atmosphere where the frictional ablation effect enhances its brightness to an awesome intensity.
Re-entering manned spacecraft, ballistic missile re-entry vehicles (RVs) and space junk from decayed orbits can and often do produce fireballs. The only difference is that their initial entry speed is only about 17,500 mph (28,000 km/hr), the speed of a body in a low Earth orbit, instead of the 25,000 mph (41,000 km/hr) that is typical of meteoroids arriving from deep space. (25,000 mph is the escape velocity from Earth--the speed that an object needs to climb away from the Earth and finally decelerate to zero velocity under the influence of Earth’s gravity when that object is infinitely far away from the Earth. By symmetry, it can be seen that 25,000 mph is also the speed that a deep space object, such as a meteoroid, will have if it falls to the Earth under the influence of Earth’s gravity from an infinitely large initial distance away from the Earth. Actual meteoroid entry speeds vary, but 25,000 mph is the sort of order-of-magnitude value that physicists use to estimate the behavior of a physical phenomenon when precise data are lacking.)
Length of Fireball Flights
Meteoroids enter the Earth’s atmosphere at dip angles that range from shallow (nearly parallel to the Earth’s surface) to straight down (aimed straight for the Earth’s center). The initial entry angle depends upon the geometry of the meteoroid’s orbit relative to the Earth’s surface at the moment of entry. All other things (such as atmospheric entry speed and meteoroid size) being equal, a fireball that is flying nearly horizontally, parallel to the Earth’s surface, will last longer than a fireball that is plunging almost vertically toward the Earth’s surface. This is because a fireball that is plunging steeply will decelerate much faster than a fireball on a shallow-angle trajectory. Rarely, a fireball on an extremely shallow trajectory that is exactly parallel to the Earth’s surface can skip back out of the Earth’s atmosphere and return to its orbit around the Sun. Some fireballs that are very large (that is, the rocks at their cores are large) and are flying on shallow trajectories have been observed for tens of seconds to perhaps nearly a minute. But most fireballs last only a few seconds.
Fireball Visual Effects
Fireballs are, by definition, intensely bright. There is no hard-and-fast cut-off for the brightness. (To paraphrase former Justice Powell of the U.S. Supreme Court: “I can’t define pornography, but I think I know it when I see it.”) Fireballs are often described as leaving a trail of sparks in the sky. These “sparks” are bright pieces of ablation debris. Fireballs usually flicker as they shoot across the sky. The flickering indicates that the ablation rate varies in time, and might be related to rotation of the meteoroid as it flies. Fireballs can produce brilliant colors, including greens and reds. These colors are presumably related to the composition and temperature of the fireball surface that is ablating.
Fireballs produce smoky-looking debris trails from ablation that can linger in the sky for some time. If there is enough moonlight, the debris trail may be visible to observers on the ground at night. Daylight fireballs have been filmed with spectacular trails of ablation debris in their wake.
Sometimes the meteoroid at the center of the fireball breaks apart under the dynamic stress of atmospheric pressure. Observers on the ground will see this break-up. The fireball will terminate as a visual effect shortly after such a break-up, because the smaller resulting fragments have a relatively high surface-area-to-mass ratio that slows them down very rapidly.
Fireballs are regularly observed by strategic early warning satellites that look down at the Earth’s surface and which are designed to find (and track) the hot exhaust plumes of ballistic missiles. These include the United States Defense Support Program (DSP) satellites. In fact, fireballs provide nice targets of opportunity for the satellites to observe.
Fireball Acoustics
Because fireballs are initially traveling at supersonic speed, they generate an atmospheric bow-shock wave that is heard by people on the ground as a sonic boom. The boom will not be heard by people who are located along the line of flight of the fireball, but people who are off to the side of the flight-line (located at right angles to the line of flight, that is) will likely hear the boom.
Occasionally witnesses who are very close to fireballs report hearing a sizzling sound, like bacon frying, during a fireball event. We do not know what causes this effect. It has been speculated that it could be a true acoustic effect, or an electromagnetic effect inside the human auditory system, or a sound produced by local objects (as pieces of a metal fence) that are forced to vibrate by electromagnetic energy (basically radio energy) radiating from the fireball.
Sensitive acoustic sensors can hear fireball sounds at distances of tens to hundreds of miles. During the Cold War the United States government developed sensitive low-frequency acoustic (infrasound) arrays that were used to listen to the sounds of above-ground atomic bomb tests in northern China at that country’s Lop Nor desert test facility. The sounds of the Lop Nor tests could be (and were) heard from prototype versions of the arrays at Boulder, Colorado. Eventually these arrays were deployed at multiple locations in the Unites States so as to not only hear the sounds of the bomb tests but to also allow the locations of the tests to be triangulated. Even after the major powers renounced above-ground atomic tests in the Partial Nuclear Test Ban Treaty (PNTBT) the acoustic sensor arrays were preserved so as to be able to identify and triangulate the locations of illicit above-ground atomic bombs that countries might try to test secretly. An example would the covert atomic test that someone (Israel and/or South Africa?) fired in the Indian Ocean near Prince Edward Island in 1979.
Infrasound acoustic arrays are now operated at a variety of locations across the USA by both Los Alamos National Laboratory (LANL) for the identification of illicit bomb tests and by the National Oceanic and Atmospheric Administration (NOAA) as research tools. NOAA arrays in Colorado can pick up the sounds of avalanches in the Rocky Mountains, the sounds of tornados on the Great Plains, the sounds of re-entering space vehicles hundreds of miles away (including space shuttles), and the sounds of fireballs at distances of tens to hundreds of miles. Infrasound acoustic data on fireballs are useful because they can be used to triangulate the locations of fireball flight paths and the exact times that fireballs have entered the atmosphere.
Fireballs as Multiple-Object Events
Infrasound acoustic array data indicate a strange aspect of fireballs: Commonly fireballs are not single-object events but rather are associated with other objects--many fireballs seem to be parts of multiple-object events. Frequently, infrasound arrays not only pick up the signature of a fireball that was seen by eyewitnesses, but also the signature of yet another object (and very rarely a third object) that entered the atmosphere nearby at nearly the same time. The sound from the second object may be picked up a hundred miles away from the first one. The paired objects are not ordinarily noted by eyewitnesses. Perhaps many meteoroids entering the Earth’s atmosphere may have been multiple-object specimens when they were in space, and that the larger member of a pair may form a fireball while the smaller object may be too small to make a fireball but nevertheless produces a detectable acoustic signal.
Read the related meteorite recovery page on this web site regarding the Elbert, Colorado fireball of 1998 for a case in which a visible fireball was apparently associated with a second object that entered the Earth’s atmosphere at the same time a hundred miles away.
The End of a Fireball Flight
A fireball flight ends for either of two reasons: Either it finally ablates down to nothing, or else the core rock survives long enough to finally decelerate to such a low velocity that friction no longer causes enough heating to make it glow, and its ablation ceases. There are documented cases in which eyewitnesses who were quite close to a fireball saw very faintly glowing embers of rock flying through the sky well after the bright part of the fireball event had ceased.
The point in the sky where the meteoroid ceases to shine brightly and stops ablating is the point of retardation. The rock’s forward speed from this point onward is initially sub-sonic (at or less than 650 mph). The meteoroid’s forward velocity component decreases very rapidly after that, until the amount of motion in the forward direction is nearly zero. Meanwhile the vertical component of motion reaches terminal velocity. Terminal velocity is the speed at which the upward-pointing air resistance just matches the downward-pointing component of an object’s weight. For a human being it is about 200 mph if falling flat and 400 mph if falling head-first or feet-first; for a meteoroid with a higher ratio of mass to cross section it might be 400 mph. This terminal fall phase of flight lasts from a few seconds to a few minutes before the rock hits the ground. How long it lasts depends upon the altitude of the point of retardation. Typical point of retardation altitudes will be between 10 to 20 miles high.
Fireballs and Meteorite Falls
Every meteorite on the Earth’s surface must have generated a fireball during its atmospheric flight. There are very rare cases in which people have found falls after having seen both the fireball up to the point of retardation and then some weakly glowing remnants during the first part of the the terminal plunge. However, it should be emphasized that even if a weak glow is seen immediately after the point of retardation, that glow ceases in a second or two. Thereafter the fragments are quite dark as they make their final plunge.
Because the forward progress of the meteoroid ceases soon after the point of retardation, the surviving fragment or fragments that reach the Earth will do so at a location that is only slightly forward of the place that was directly below the point of retardation. So a person who sees a fireball flare-out directly above their head could possibly be very close to the eventual fall location. This situation is documented as having occurred, albeit very rarely.
The Black Stone that is kept in the famous Meccan Cube (Ka’bah or Kaaba depending upon how the Arabic letters are transliterated) and which was placed in that structure’s predecessor shrine at an indeterminate date before the late-sixth and early seventh-century CE advent of Islam in Arabia, is commonly thought to be a meteorite based on descriptions from people who have examined it closely. If it is a meteorite, then one could speculate that it might have originally been collected precisely because it originated in a witnessed fall. If it was seen literally falling from the heavens it would have been recognized as something that needed to be collected and preserved.
Strewnfields
Meteorite falls usually consist of elliptically-shaped areas on the ground where multiple pieces of the fireball rock (which often shatters at or near the point of retardation) have hit the ground. Such areas are called strewnfields. The long axis of the ellipse is parallel to the direction of flight of the fireball. A strewnfield may contain a few, or dozens, or even hundreds of meteorite fragments. Most meteorites are not recognized as such because they look like ordinary rocks. Nickel-iron meteorites are distinctive but they rust to nothing in a relatively short period of time, geologically speaking. Antarctic ice fields, where meteorites are concentrated in certain places by the action of ice movement, are exposed by wind activity, and then show up clearly against the white ice background, are the only places where meteorites can be collected in something like their true fall distribution as regards type (iron, stony and all of the sub-categories of stony).
All-Sky Fireball Cameras
All-sky cameras are wide-field-of-view ground-based devices that stare upward. They continuously record the appearance of the sky. An all-sky camera typically consists of a wide-angle camera that is mounted to look downward at a convex mirror which in turn reflects a view of the entire sky into the camera. Back in the day of the old Prairie Network, all-sky cameras used film and had to be serviced on a regular basis. Nowadays the cameras are high-sensitivity black-and-white CCD devices that can record and transmit their data continuously without any need for regular service. All-sky cameras are ideal tools for monitoring the sky for meteors and fireballs.
Years ago I designed an all-sky camera of my own. I demonstrated it and pitched the idea of building a Colorado-based sky camera network to the staff of the Denver Museum of Nature and Science (previously the Denver Museum of Natural History). My idea was somewhat unique in that I proposed that the cameras in the network should be built and operated by Colorado high school students. It took some persuasion on my part, but eventually the museum implemented my idea. Today the DMNS All-Sky Camera Network has stations spread across the state at schools. It has been terrific to see my idea implemented by the museum!
The big trick with a sky camera network is to use it to gather quantitative data on fireballs. The data can be used to find likely meteorite fall locations as well as the orbital parameters that fireball objects had before they entered the Earth’s atmosphere. Here’s how it works:
First, each camera’s data allows us to form a plane that is defined by a line and a point that is not on the line (a la Euclid’s definition of a plane). The line is the flightline of the fireball and the point that is not on the line is the camera location. Since the intersection of two planes is a line, the overlapping coverage of two all-sky cameras forms the intersection of two planes and thereby defines the line of flight of the fireball in space.
So far so good. Now, with the the fireball flightline defined in space, the angular rate of change of the fireball in each camera’s frame-by-frame footage can be converted into the true velocity of the fireball on its flightline.
OK, now, with the fireball flightline and velocity known (along with, hopefully, the location of the point of retardation where the fireball slowed significantly), the likely location of the resulting meteorite fall (if any exists) can be estimated to within a couple of square miles. This tells people where to look for meteorites.
Lastly, with the flightline orientation known and the speed known and the time of the atmospheric entry known (from the data-time log for the cameras), it is possible to compute the original orbit that the fireball rock was following around the Sun, before it encountered the Earth. This means that, if a meteorite can be found from a fireball that has been captured by an all-sky camera network, the object can be connected to its original orbit around the Sun. Since orbits can give clues as to regions of origin, this allows the recovered meteorite to be connected to its source region in the solar system.
A few years ago, when I pitched the sky camera idea to the museum, I went the extra mile and wrote software in National Instruments Labview to compute meteoroid orbits from sky camera data. I was very happy when I finished the software and verified that it worked properly by comparing its results to the published orbital parameters of meteoroids that had been captured by existing sky camera networks.
Popular Myths and Fallacies about Fireballs and Meteorites
Myth 1: “I saw a fireball and it was only about a mile away.”
Fact: You cannot tell how far away a light is in the night sky. If you don’t believe me, consider whether you can tell how far away a bright star or planet might be just by looking at it. I think you must admit that you can’t. People mistakenly think that they know how far away a light is at night based on the light’s brightness. Since fireballs are super-bright, witnesses always think that fireballs are super-close to them. The reality is that witnesses often think that a fireball was only a mile away when in fact the fireball was much further away, often more like 30 to 100 miles or even more. I can testify personally to a case (a typical case, I might add) in which we interviewed an eyewitness to a fireball event who thought that she had “definitely” seen the fireball plunge into a vacant lot behind her house. When we subsequently triangulated the fireball from multiple eyewitness accounts across Colorado we found that the fireball was nearly 100 miles from her location!
By the way, if you see a fireball burn out directly overhead, then it was pretty close, probably a mere 10 miles above you.
Myth 2: Most fireballs are produced by space rocks (meteoroids).
Fact: Many (although by no means all) fireballs are produced by re-entering space junk. This can be demonstrated by cases in which NORAD satellite and radar data have been used to track decaying space junk and satellites right down to the stage at which the junk sank into the atmosphere and generated widespread eyewitness reports of fireballs.
Myth 3: Fresh-fallen meteorites and space junk are red-hot when they land.
Fact: Fresh-fallen meteorites and space junk are quite cold to the touch even if they are found immediately after their fall. This is attested by eyewitness testimony from rare cases in which bona fide meteorite falls have been found within minutes or even seconds of the fall. The result seems counter-intuitive given the high temperature of the meteoroid surface during re-entry. How can its temperature be cold when it lands? The answer is in the word “surface.” Although the meteoroid surface is heated to several thousand degrees Fahrenheit, it also flakes off continually as it is heated. The ablation process carries heated material away from the meteoroid at the same rate that heat is being generated by friction. The net effect of atmospheric entry on the temperature of the main body of the meteoroid is therefore zero. On top of that, the meteoroid may take a minute or more to fall to Earth after the ablation episode, and that allows additional cooling to occur even if some slight excess of heat had managed to accumulate in the meteoroid body during the ablation episode. The result is that a fresh-fallen meteorite on the Earth’s surface is still at about the same temperature it had when it was in space--which is quite cold.
Whenever I read a news account of a fresh-fallen meteorite in which an eyewitness claims that the alleged meteorite was “hot” or “red-hot” or “glowing,” or that some object that the meteorite allegedly hit (such as a dog-house or the spot on the ground where the meteorite landed) was “scorched,” I know that the respondent must either be mistaken or lying. Fresh-fallen meteorites are stone cold. They won’t scorch or burn anything.
Myth 4: Meteorites are sometimes radioactive.
Fact: Meteorites are either composed of rock or else a nickel-iron alloy. Neither type is ever radioactive. Fresh-fallen meteorites do contain a very small trace of a weakly radioactive radionuclide, sodium-22 (Na-22) immediately after they have fallen. The Na-22 is created while the parent meteoroid is in outer space. But the Na-22 radioactivity in fresh-fallen meteorites is so weak that it can only be measured with special laboratory equipment. The background radiation from native Earth rocks is substantially higher. Even the barely measurable Na-22 radioactivity rapidly decays down to nothing, within weeks or months of the fall. It is scientifically important to get a suspectedly fresh-fallen meteorite to a lab immediately after it falls so that this residual Na-22 level can be measured before it fades to nothing; the result will prove that the meteorite is a fresh fall. It can be used to precisely date the calendar day of the fall and thus can possibly be correlated with a fireball report with certainty. There is absolutely no health danger in picking up any meteoritic material, even if it has just landed.
The author pitched the idea of a school-based network of all-sky cameras to the Denver Museum in 2001. Today the museum has implemented his concept with a state-wide network in Colorado.
My 3-d model of all-sky camera fireball coverage showing how the flight path is determined from the intersection of two planes formed by the fields-of-view of two all-sky cameras.
An actual fireball witness-data diagram. Eyewitness reports of a fireball are plotted on a map to try to determine the place where the fireball went dark, and where a meteorite might be found nearby.
A simulated picture of a fireball. Meteorite fragments might be found in the vicinity of the place where the fireball blinks out. Eyewitness accounts and all-sky cameras can be used to determine this zone.
Canon City meteorite thin section studied and photographed by the author
Fireball meteoroid geometry diagram developed by the author
Meteoroid orbit picture developed by the author for a Colorado fireball
