Originally posted by ahamarlin
Wouldn't a extremely hot meteor(ite) glow light orange, burning up in the earth atmosphere?
No, not necessarily.
The light that we see from an object entering the atmosphere (natural or artificial) comes from a cloud of ionized particles that surround the object. The cloud is a mixture of mostly ionized atmospheric gases, and bits of the object that have been stripped away by the process of "ablation".
It's a general misconception that meteoroids "burn up" in our atmosphere. They don't. Ablation and ionization, primaraly due to the extreme velocity, are the reason light is emmited.
The colours observed will depend on a combination of factors:
Firstly, what elements are ionized. Each has a characteristic colour/wavelength/"emission line".
Oxygen for example, when ionized emits light at green wavelengths, and it's not just a dull/pale green. It's so vivid in some cases, that it looks un natural, perhaps even prompting some to think that they saw a "UFO".
Here for example (and sorry for re-posting this pic to all who have seen it before), is a photograph I took of a green earth grazing Leonid meteor during the build up to the Leonid meteor storm of 2001.
Orion is in the bottom left of the image, to give it some scale.
One feature that destinguishes earth grazers from normal meteors is how long they are, and that is because they are grazing the edge of our atmosphere, and in some cases, going back out into space again.
Normal (non earth grazing) meteors on the other hand, are heading towards the observer and plunging into the atmosphere at a steep angle, so they ablate much faster as well as looking much shorter due to perspective.
Interestingly, although oxygen is scarce at around 100km altitude (roughly where meteors first become visible), it's makes up the greatest percentage of gas at that altitude, and is the reason why such a long meteor is all green, where as other photographs taken on the same night show meteors that start out green, going on to yellow, and then orange/red if the meteor survives long enough.
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.
Source: The American Meteor Society
Continued in the following post...
edit on 24-12-2011 by C.H.U.D. because: typo
