I had just finished packing up my telescope September 8, 2017 when a giant fireball screamed across the southern sky. I saw some orange and green associated with it as it brightened up the sky. It would have been more fantastic if the Moon was not hanging around that part of the sky.
Why was it a fireball and not a typical meteor?
Well, they are the same thing. A fireball is just a larger and brighter meteor. The International Astronomical Union indicates:
"A meteor brighter than absolute visual magnitude (distance of 100 km) –4 is
also termed a bolide or a fireball."
This typically means that the meteor has to be brighter than the planets--namely the brightest--Venus. A link to the document is given below.
https://www.iau.org/static/science/scientific_bodies/commissions/f1/meteordefinitions_approved.pdf
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| This screengrab taken from a video by Rod Daroza shows the night sky in Calgary illuminated by a mysterious fireball. (@RaphaelPern/Twitter) |
This fireball was seen from all over the region. Below is a map of where some witnesses were confirmed.
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| Witness Location and Trajectory (American Meteor Society) |
The American Meteor Society has an excellent FAQ on their webpage linked below.
https://www.amsmeteors.org/fireballs/faqf/#5
As many observers witnessed different colours the following answer is provided by the AMS.
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 curiosities 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.
Also it was reported that a sonic boom was heard. This is quite possible as it was such a large fireball.
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.
Also remember the superbolide that hit Russia in 2013? It was brighter than the Sun (-26 magnitude).
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| Chelyabinsk Meteor - 2013 |
Keep your eyes on the sky, you never know what you might miss!
