A blood-red comet appears in the sky. People quake in its wake.
This phenomenon, which happens in the second season of the medieval fantasy Game of Thrones, had us all wondering — can you ever actually see a red comet?
We talked to Matthew Knight, an astronomer at the Lowell Observatory in Arizona who observes comets. He gave us some answers just in time for the third season of Game of Thrones, which begins March 31.
At first blush, he said, the comet’s red color wouldn’t be possible because the strongest emissions from comets are in the blue and green regions, mostly from neutral gases such as hydroxide and cyanide.
There is a type of emission that is close to red, called “forbidden oxygen”, which occurs when atoms make a rare energy transition between states of “excitement”. But it’s very faint and short-lived, Knight wrote.
The visible light from a comet comes from a combination of reflected solar continuum (sunlight reflecting off of dust grains) and cometary emission (neutral and/or ionized molecules of gas that emit photons at a particular wavelength). The sunlight reflecting off of dust grains basically looks like sunlight and since the Sun appears yellow/white, this component cannot look red.
A small caveat is that due to the physical properties of dust grains, comet dust often actually does “redden” sunlight slightly when measured with sensitive equipment. However, this reddening is at a very low level and is not enough to cause the reflected sunlight to appear a deep red like in Game of Thrones. The strongest comet emissions in the region where human eyes can see are in the blue and green regions.
So what ingredients does a comet need to look like the one in Game of Thrones? According to Knight, it would have to meet these criteria:
- Be visible in daylight, which really only happens about once a century;
- Be close to the sun (he supposes this one is, given how straight the tail is);
- Have a “strange composition” that is different from anything we know in the solar system. The composition could be that forbidden oxygen he talked about, coming from a comet whose ices are carbon monoxide and carbon dioxide. But that would be hard, because those types of ices would not survive long when exposed to sunlight.
If we really want to think in a science fiction vein, Knight suggests that maybe the comet could be made up unpredictably:
Alternatively it could be something else entirely unknown in cometary chemistry or dust, with really weird properties causing a much stronger reddening than is normally seen. In any event, the composition would be so anomalous that this comet would almost certainly have originated in another solar system. That would make comet scientists very interested in studying it!
But don’t despair yet. Comet ISON might be bright enough for daylight viewing when it swings by Earth late in 2013. Comets are unpredictable beasts, but we’re pretty sure of one thing: no matter how bright it is, it won’t look red.