This Thanksgiving, Comet ISON is heading for a close encounter with the Sun.
Comet ISON originated in the Oort Cloud, a gravitationally bound “cloud” of far-flung icy debris left over from the formation of the Solar System some 4.5 billion years ago.
The Oort cloud orbits the Sun but at the phenomenal distance of a light-year away or more. When you stop and consider that the Voyager spacecraft isn’t even a light-day away from Earth, you get an idea just how vast our Solar System is and how tenacious the Sun’s gravity is on distant objects in the Oort Cloud.
Although this is undoubtedly Comet ISON’s first encounter with the Sun, the comet’s orbit has been estimated at a staggering 400,000 years. ISON started its long, slow fall toward the Sun when Homo sapiens still shared the planet with Homo neanderthals and a bunch of other hominids!
If ISON survives its encounter with the Sun, it may eventually escape our Solar System altogether so this could be the only time this comet is ever visible from Earth.
In this animated GIF, you can see a huge coronal mass ejection (CME) coming out from the Sun (with the sun blotted out in the middle of the image to protect the camera optics). At the same time, ISON approaches from the right.
Although the sheer size of the CME dwarfs the comet (and Earth for that matter), this need not spell doom for the comet as we’re seeing a 2D image of a 3D event covering a vast region of space.
Most astronomers suspect comet ISON will break up and disintegrate while rounding the Sun. At its closest approach the comet will be just over a million kilometres away from the surface of the Sun (725,000 miles or barely three times the distance to the Moon).
ISON is called a sungrazer because it’s going to approach to within roughly 1% of the distance between Earth and the Sun. ISON is well inside the orbit of Mercury (at anywhere from 29 million to 43 million miles from the Sun) and is travelling at up to 360 kilometres per second (roughly 224 miles per second). At that speed, ISON travels the length of the Continental US in fifteen seconds, that’s over a million kilometres an hour (800,000 miles an hour)!
At its closest approach, comet ISON will be subject to temperatures around 2700 C (almost 5000F), causing the outer layer of the comet to boil away at an astonishing pace.
Rather than being a lump of solid ice, comet ISON probably has a consistency closer to packed snow or an ice-cream sundae. Being 2 kilometres wide (roughly 1.2 miles), it would make for one hell of a snow cone!
At the moment, ISON is too close to the Sun to see without damaging your eyes. If comet ISON survives Thanksgiving, December will give rise to possibly the most spectacular comet ever seen from Earth.