MarkGeldmeier's Blog

There were numerous reports of a bright light in the sky this evening across the metro area and in fact much of Eastern Missouri and West Central Illinois. From the reports, it sounds like a "Fireball"...which is an especially bright meteor...was what graced our skies. I'm pasting below some questions and answers about Fireballs - I will also post a collection of the viewer reports as a separate posting/

*** Below is courtesy of the American Meteor Society website ***

1. What is a fireball? What is the difference between a fireball and a bolide?

A fireball is another term for a very bright meteor, generally brighter than magnitude -3 or -4, which is about the same magnitude of the planet Venus in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation.

If you happen to see one of these memorable events, we would ask that you report it to the American Meteor Society, remembering as many details as possible. This will include things such as brightness, length across the sky, color, and duration (how long did it last), it is most helpful of the observer will mentally note the beginning and end points of the fireball with regard to background star constellations, or compass direction and angular elevation above the horizon.

The table below will aid observers in gaging the brightness of fireballs:


Object magnitude
----------------------------
Polaris +2.1
Vega +0.14
Sirius -1.6
Bright Jupiter -2.5
Bright Mars -2.8
Bright Venus -4.4
1st Quarter Moon -10.4
Full Moon -12.6
Sun -26.7

2. How frequently do fireballs occur?

Several thousand meteors of fireball magnitude occur in the Earth's atmosphere each day. The vast majority of these, however, occur over the oceans and uninhabited regions, and a good many are masked by daylight. Those that occur at night also stand little chance of being detected due to the relatively low numbers of persons out to notice them.

Additionally, the brighter the fireball, the more rare is the event. As a general thumbrule, there are only about 1/3 as many fireballs present for each successively brighter magnitude class, following an exponential decrease. Experienced observers can expect to see only about 1 fireball of magnitude -6 or better for every 200 hours of meteor observing, while a fireball of magnitude -4 can be expected about once every 20 hours or so.

3. Can you see fireballs in daylight, and will a fireball leave a trail?

Yes, but the meteor must be brighter than about magnitude -6 to be noticed in a portion of the sky away from the sun, and must be even brighter when it occurs closer to the sun.

Fireballs can develop two types of trails behind them: trains and smoke trails. A train is a glowing trail of ionized and excited air molecules left behind after the passage of the meteor. Most trains last only a few seconds, but on rare occasions a train may last up to several minutes. A train of this duration can often be seen to change shape over time as it is blown by upper atmospheric winds. Trains generally occur very high in the meteoric region of the atmosphere, generally greater than 80 km (65 miles) altitude, and are most often associated with fast meteors. Fireball trains are often visible at night, and very rarely by day.

The second type of trail is called a smoke trail, and is more often seen in daylight fireballs than at night. Generally occurring below 80 km of altitude, smoke trails are a non-luminous trail of particulate stripped away during the ablation process. These appear similar to contrails left behind by aircraft, and can have either a light or dark appearance.

4. I saw a very bright meteor. Did anyone else see it, and to whom should I report it?

The American Meteor Society (AMS) collects fireball reports from throughout North America, the Caribbean, and the Pacific islands for use by our organization and other meteor organizations. Persons who have seen a bright meteor event are encouraged to report their sighting to us. If multiple sightings of a single event can be grouped together, it is sometimes possible to determine the actual trajectory of the object in question.

The easiest way to report a fireball to us is to utilize our on-line form, located at our Internet Web site. This site is located at http//www.amsmeteors.org/fireballs.html.

Information on reporting fireballs is also provided by the International Meteor Organization Fireball Data Center (FIDAC). It can be located on the web at: http://www.imo.net/fireball/index.html.

5. Can fireballs appear in different colors?

Vivid colors are more often reported by fireball observers because the brightness is great enough to fall well within the range of human color vision. These must be treated with some caution, however, because of well-known effects associated with the persistence of vision. Reported colors range across the spectrum, from red to bright blue, and (rarely) violet. The dominant composition of a meteoroid can play an important part in the observed colors of a fireball, with certain elements displaying signature colors when vaporized. For example, sodium produces a bright yellow color, nickel shows as green, and magnesium as blue-white. The velocity of the meteor also plays an important role, since a higher level of kinetic energy will intensify certain colors compared to others. Among fainter objects, it seems to be reported that slow meteors are red or orange, while fast meteors frequently have a blue color, but for fireballs the situation seems more complex than that, but perhaps only because of the curiousities of color vision as mentioned above.

The difficulties of specifying meteor color arise because meteor light is dominated by an emission, rather than a continuous, spectrum. The majority of light from a fireball radiates from a compact cloud of material immediately surrounding the meteoroid or closely trailing it. 95% of this cloud consists of atoms from the surrounding atmosphere; the balance consists of atoms of vaporized elements from the meteoroid itself. These excited particles will emit light at wavelengths characteristic for each element. The most common emission lines observed in the visual portion of the spectrum from ablated material in the fireball head originate from iron (Fe), magnesium (Mg), and sodium (Na). Silicon (Si) may be under-represented due to incomplete dissociation of SiO2 molecules. Manganese (Mn), Chromium (Cr), Copper (Cu) have been observed in fireball spectra, along with rarer elements. The refractory elements Aluminum (Al), Calcium (Ca), and Titanium (Ti) tend to be incompletely vaporized and thus also under-represented in fireball spectra.

6. Can a fireball create a sound? Will the sound occur right away, as you watch the fireball, or is their some delay?

There are two reported types of sounds generated by very bright fireballs, both of which are quite rare. These are sonic booms, and electrophonic sounds.

If a very bright fireball, usually greater than magnitude -8, penetrates to the stratosphere, below an altitude of about 50 km (30 miles), and explodes as a bolide, there is a chance that sonic booms may be heard on the ground below. This is more likely if the bolide occurs at an altitude angle of about 45 degrees or so for the observer, and is less likely if the bolide occurs overhead (although still possible) or near the horizon. Because sound travels quite slowly, at only about 20 km per minute, it will generally be 1.5 to 4 minutes after the visual explosion before any sonic boom can be heard. Observers who witness such spectacular events are encouraged to listen for a full 5 minutes after the fireball for potential sonic booms.

Another form of sound frequently reported with bright fireballs is "electrophonic" sound, which occurs coincidentally with the visible fireball. The reported sounds range from hissing static, to sizzling, to popping sounds. Often, the witness of such sounds is located near some metal object when the fireball occurs. Additionally, those with a large amount of hair seem to have a better chance of hearing these sounds. Electrophonic sounds have never been validated scientifically, and their origin is unknown. Currently, the most popular theory is the potential emission of VLF radio waves by the fireball, although this has yet to be verified.

7. How bright does a meteor have to be before there is a chance of it reaching the ground as a meteorite?

Generally speaking, a fireball must be greater than about magnitude -8 to -10 in order to potentially produce a meteorite fall. Two important additional requirements are that (1) the parent meteoroid must be of asteroidal origin, composed of sufficiently sturdy material for the trip through the atmosphere, and (2) the meteoroid must enter the atmosphere as a relatively slow meteor. Meteoroids of asteroid origin make up only a small percentage (about 5%) of the overall meteoroid population, which is primarily cometary in nature.

Photographic fireball studies have indicated that a fireball must usually still be generating visible light below the 20 km (12 mile) altitude level in order to have a good probability of producing a meteorite fall. Very bright meteors of magnitude -15 or better have been studied which produced no potential meteorites, especially those having a cometary origin.

8. Can a meteorite dropping fireball be observed all the way to impact with the ground?

No. At some point, usually between 15 to 20 km (9-12 miles or 48,000-63,000 feet) altitude, the meteoroid remnants will decelerate to the point that the ablation process stops, and visible light is no longer generated. This occurs at a speed of about 2-4 km/sec (4500-9000 mph).

From that point onward, the stones will rapidly decelerate further until they are falling at their terminal velocity, which will generally be somewhere between 0.1 and 0.2 km/sec (200 mph to 400 mph). Moving at these rapid speeds, the meteorite(s) will be essentially invisible during this final "dark flight" portion of their fall.

9. Are meteorites "glowing" hot when they reach the ground?

Probably not. The ablation process, which occurs over the majority of the meteorite's path, is a very efficient heat removal method, and was effectively copied for use during the early manned space flights for re-entry into the atmosphere. During the final free-fall portion of their flight, meteorites undergo very little frictional heating, and probably reach the ground at only slightly above ambient temperature.

For the obvious reason, however, exact data on meteorite impact temperatures is rather scarce and prone to hearsay. Therefore, we are only able to give you an educated guess based upon our current knowledge of these events.