False color APOD!
Comet Hyakutake in true color. Photo: Fred Espenak.
Comet Hale-Bopp in true color. Photo: Fred Espenak.
True color comet pictures will show that the comet has a greenish coma ("head"), a yellow-white, diffuse dust tail and a streaky blue gas tail. As for Comet Hyakutake, it was remarkably dominated by its blue gas tail, and we never saw a yellow dust tail developing. As for Comet Hale-Bopp, its greenish coma was swamped by its bright yellowish dust tail.
is seen to be "all blue". Such comets don't exist. If we are seeing the coma of the comet, it should be greenish, and if we are seeing the dust tail, it should be yellow-white. The reason why a comet's coma is greenish, as well as the reason why a comet's gas tail is blue, has to do with the way certain certain chemical compounds (such as carbon monoxide) are ionized by the Sun. The dust tail, by contrast, is made up of larger particles that are shed by the comet and illuminated by the Sun.
Comet Holmes in true color. Photo: Tony Cook.
Comet Borisov in false color. NASA, ESA, D. Jewitt (UCLA).
Compare Comet Borisov with Comet Holmes. Comet Holmes had a very bright, egg-white dusty coma. A faint green glow surrounded the coma. A faint blue gas tail is seen to be disconnected from the comet in Tony Cook's picture.
As for Comet Borisov... what can I say? Hubble imaged it. Let me guess... Maybe through a 606 nm filter? Or even an 814 nm filter? The picture of Comet Borisov and a background galaxy has probably been imaged through two filters, but Hubble won't tell us which ones. Or maybe even the comet-galaxy picture is a single filter image, and the image has just been processed to show the comet and the galaxy as differently colored.
Edit: It is the fact that we are not told what filter was used to image Comet Borisov that makes me most irritated.