© Peter Ward, Barden Ridge Observatory
Comet Lemmon (C/2012 S1) is currently traveling across the sky in the southern hemisphere and has brightened to a visible magnitude of about 6.2 - just at the limit of what can be seen with the naked eye under very dark, clear skies. Peter used a 14.25″ F7.9 Ritchey-Chretien telescope to capture the image of Lemmon, set against long-exposure trails from background stars.
The green color comes from outgassed ethane surrounding the comet's nucleus interacting with radiation from the sun, causing it to glow - in very much the same way that neon signs and fluorescent lights work.
Comet Lemmon will reach perihelion - the closest point in its orbit to the sun - on March 24, when it will be at a solar distance of 0.73 AU. In the months following it may become visible to skywatchers in the northern hemisphere as well before it heads back out into the solar system on another 50-year-long trip around our home star.
At no point during this pass will Comet Lemmon come near Earth. Read more about this comet's orbital characteristics on the JPL Small-body Database here.
It's not Ethane causing the green color!
Ethane was first synthetically created in 1834 by Michael Faraday, applying electrolysis of a potassium acetate solution. He mistook the hydrocarbon product of this reaction for methane, and did not investigate it further.
Ethane is a colorless, odorless gas, and burns shiny BLUE!
But....
Boron is a chemical element with chemical symbol B and atomic number 5. Because boron is produced entirely by cosmic ray spallation and not by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax and kernite.
Chemically uncombined boron, which is classed as a metalloid, is not found naturally on Earth. (I love that statement)
And it burns a nice green color!
Duh!