Ison's surface is very dark – darker than asphalt – pockmarked and dusty with ice beneath the surface. It's a small body, a few tens of miles across, with a tiny pull of gravity. If you stood upon it you could leap 20 miles into space taking over a week to come down again, watching as the comet rotated beneath you. You could walk to the equator, kneel down and gather up handfuls of comet material to make snowballs, throw them in a direction against the comet's spin and watch them hang motionless in front of you. But it will not remain quiet on Comet Ison for the Sun's heat will bring it to life.Hopefully, no cults commit suicide when these comets get close. That mass suicide (with castrations) was pretty weird.
By the end of summer it will become visible in small telescopes and binoculars. By October it will pass close to Mars and things will begin to stir. The surface will shift as the ice responds to the thermal shock, cracks will appear in the crust, tiny puffs of gas will rise from it as it is warmed. The comet's tail is forming.
Slowly at first but with increasing vigour, as it passes the orbit of Earth, the gas and dust geysers will gather force. The space around the comet becomes brilliant as the ice below the surface turns into gas and erupts, reflecting the light of the Sun. Now Ison is surrounded by a cloud of gas called the coma, hundreds of thousands of miles from side to side. The comet's rotation curves these jets into space as they trail into spirals behind it. As they move out the gas trails are stopped and blown backwards by the Solar Wind.
By late November it will be visible to the unaided eye just after dark in the same direction as the setting Sun. Its tail could stretch like a searchlight into the sky above the horizon. Then it will swing rapidly around the Sun, passing within two million miles of it, far closer than any planet ever does, to emerge visible in the evening sky heading northward towards the pole star. It could be an "unaided eye" object for months. When it is close in its approach to the Sun it could become intensely brilliant but at that stage it would be difficult and dangerous to see without special instrumentation as it would be only a degree from the sun.
Remarkably Ison might not be the only spectacular comet visible next year. Another comet, called 2014 L4 (PanSTARRS), was discovered last year and in March and April it could also be a magnificent object in the evening sky. 2013 could be the year of the great comets.
Thursday, December 27, 2012
The Giant Comet of 2013?
The Independent, via Ritholtz: