Yes, the comet’s still there, but it’s been transformed into a blizzard of dust. Our hopes for a bright comet at Christmas appear to have gone up in smoke. Or have they?
Space-based cameras show an expanding dust cloud with two tails. According to Hermann Böhnhardt from the Max Planck Institute for Solar System Research, the tail pointing back at the sun is made of dust particles shed before the comet made its close brush with the sun Thursday, while the other is more recent dust vaporized from whatever was left of ISON’s nucleus during perihelion passage.
The solid nucleus, estimated at 1.5 mile-wide (3 km) across, once provided a steady supply of vaporizing dust and gas. It’s now crumbled to pieces, and those pieces are vaporizing away in the sun’s to create an expanding cloud that’s fading by the hour.
This morning ISON glows around magnitude 5, making a naked eye sighting increasingly unlikely in the coming days. There’s still a remote chance the comet’s engine will fire up again like Comet C/1962 C1 Seki-Lines did in April 1962. During perihelion, Seki-Lines was expected to brighten to -7 magnitude but instead it totally dropped off the map only to reappear a few days later as bright as Jupiter. Could ISON do the same?
Should ISON continue to fade as expected, it may take a fair-sized telescope to see it in the morning sky. We’ll have to wait longer to make the attempt because a dark sky will be needed to see a low-contrast object like a “cloud”. ISON will appear in a dark sky – but very low – around Dec. 7.
You might wonder if the rocks and dust in the expanding cloud have any chance of producing a meteor shower or worse, pose a threat to Earth. The answer is no. Whatever is left of the comet will continue following the same orbit it’s been following and will not come near the planet. ISON’s ghost will pass a very safe 39 million miles from Earth on Dec. 26.
I like to picture whatever chunks of ice are left over as unexploded ordinance. Maybe, just maybe, there will be a temporary re-brightening as fresh surfaces get pounded by sunlight and vaporize, but given ISON’s talent for defying predictions, we shouldn’t count on it. Still, the comet may have other tricks up its sleeve. Let’s watch and wait.