Comets are changeable by nature. When far from the sun, they’re frozen, inert bodies that look like points of light in large telescopes. No warm, fuzzy outside, no tail. But once a comet’s orbit takes it “downtown” to the inner solar system, heat from the sun vaporizes dust-laced ices to form a hazy atmosphere around the comet called a coma. Comas can grow up to 60,000 miles or more across or nearly as big as Jupiter. While impressive in size and appearance, they’re extremely rarified and possess little mass.
Being made of nearly nothing, makes it easy for the sun to fashion them into tails by the sun. Radiation pressure – literally the pressure of sunlight – pushes back dust inside the coma to form a yellow-hued dust tail.
Ultraviolet light from the sun ionizes or electrifies atoms and molecules inside the comet’s temporary atmosphere. One of the most common gases found in comets is carbon monoxide. Yep, the same stuff that comes out of your car’s tailpipe. As the sun’s magnetic field washes across the solar system like so many waves rippling a pond, it sweeps ionized carbon monoxide molecules out of the coma to form a second, blue-colored ion tail.
Ion tails always point directly away from the sun much like a wind vane, while dust tails tend to follow the curve of the comet’s orbit. Depending on where the comet is in relation to Earth, tails can appear long and narrow, short and spiky, fan-like or even hide for a time behind the coma.
Current bright telescopic comet C/2012 K5 has displayed both types of tails during its travels across the evening sky the past week. Because the comet plunged from directly above the plane of the planets to below it around the time of its closest approach to Earth (Dec 31, 2012), our perspective on it has been changing daily.
I’ve seen comets’ tails morph this way and that, but few as quickly as C/2012 K5’s twists and turns. You’ve seen a few pictures of the comet in this blog already, but today I thought I’d put them all together to give you a perspective on the changeable nature of this icy wonder.
Sadly, the comet is fading but it remains visible in 7×50 or 10×50 binoculars from a dark sky; through an 8-inch or larger telescope, the coma – along with it bright central nucleus – opens up into beautiful, broad dust tail fanning to the northeast. I’ve included an updated chart to help you find it.
Tomorrow I’ll have some news on another comet, C/2012 S1 ISON, which is expected to put on a spectacular show later this fall.