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Space is full of material left over from the creation of the Solar System, and the Earth is continually encountering this debris, which enters our atmosphere at very high velocities- many kilometers per second. At such speeds, the air in front of a meteoroid is rapidly compressed, producing enough heat to ablate its surface and usually burn it up completely while still many kilometers high. As it burns, it generates a bright streak across the sky- a meteor, or shooting star. From a location with dark skies, you will normally see several meteors per hour as sand-sized particles burn up.
Occasionally, larger particles encounter the Earth's atmosphere, producing much more spectacular light shows. These very bright meteors are called fireballs (the International Astronomical Union defines a fireball as a meteor brighter than magnitude -4, about the same as Venus.) Another term that is sometimes used is bolide, especially for fireballs that explode. However, this term is not formally defined, and should not be used when referring to fireballs. Adding to possible confusion, scientists who study impact structures use the term bolide for a meteoroid that forms a crater.
The image above shows the fireball produced on 9 October 1992 by the Peekskill meteorite on its way to a collision with a parked car! (Technically, it did not become a meteorite until it actually reached the ground, but I think the usage is fair here.)
- Very bright fireballs are caused by objects ranging in size from a few inches to a few feet.
- They encounter the atmosphere at speeds up to 70,000 mph.
- They stop burning when they are 10 to 20 miles high.
- If you see a fireball reach the horizon, it is over 300 miles away.
- If you see it drop behind a hill or trees, it is probably more than 50 miles away.
- It can take more than five minutes for meteorites to reach the ground after the meteor burns out.
- If meteorites do land, they won't be hot and they cannot start fires.
- Usually, not more than one or two percent of the mass of a fireball will survive to the ground.
- Any colors you see are probably caused by ionized atmospheric gasses, and not by material in the meteoroid. Green, for example, is produced by ionized oxygen (meteoroids do not contain significant amounts of copper.)
- Fireballs are most common around sunset. That's because this is the time that the Earth is receding from the sky above (think of it as looking out the Earth's rear window.) As a result, meteoroids are trying to catch up with us, and enter the atmosphere fairly slowly, enabling them to survive to lower altitudes where they can interact more impressively with the denser air.
- Fireballs are as common in the day as at night, but usually go unnoticed.
Seeing a great fireball is an unforgettable, once in a lifetime experience (unless you happen to live in Colorado and may have had several once in a lifetime experiences lately.) If you should be lucky enough to see one, and can keep your wits about you, your observation could be scientifically valuable. First, take note of the time. Next, look around for references such as telephone poles, chimneys, trees, or peaks. Carefully note where you first saw the meteor, and where you last saw it. What is important to note for both of these points is the altitude, which is measured in degrees from the horizon (90° is straight overhead), and the azimuth, which is measured in degrees clockwise from north. You also need to note your exact location, from a map or GPS receiver. Think about the colors you saw, whether pieces were breaking off, and whether there was a smoke trail or glowing air behind the meteor. If you heard any sounds, how long after you saw the meteor did they happen?
You can estimate the altitude by placing your hands together as shown here, and counting upwards from the horizon, hand over hand. Each hand width is about 10 degrees. So if you first saw the meteor about five hands above the horizon, its starting altitude would be 50 degrees. You can also measure the altitude accurately with a protractor and a weighted piece of string.
Estimate the azimuth by figuring out where north is from a map or by using a compass. Enter the exact angle in degrees from your compass, or use the diagram here to estimate the value.
With good reports from several locations, it is possible to estimate the orbit of the original meteoroid, and determine something about its parent body. If you see a fireball from Colorado or the surrounding states, I would like to hear about it. I'm part of a team from the Denver Museum of Nature and Science that actively investigates these events, both in the interest of determining the original orbit and also of recovering any possible meteorites that might be produced. You can report fireballs here.
The network has recorded thousands of fireballs, and some of the most unusual have been investigated in detail, to determine the parent body orbit, the ground path, and the possible location of meteorites. This is a list of all the fireballs that have been studied. Notable events are flagged with diamonds (♦♦♦).