Comet ISON, as imaged by the Hubble telescope in May.
Image from NASA
During the formation of our solar system — in the outer regions where it is cold enough to permit ices of water, methane, ammonia and carbon dioxide to exist — the comets were born.
Comets are collections of these ices mixed with small rocky particles. Fred Whipple, a Harvard astronomer, called them “dirty snowballs.”
For more information go to the NASA observing site for ISON.
Because of gravitational interactions with giant planets, millions of these objects were flung into highly elliptical orbits that took them as far from the sun as one-fifth the distance to the nearest star. They are still orbiting the sun, but one orbit could take millions or even billions of years to complete.
Objects in elliptical orbits travel at different orbital speeds — very fast when they are near the sun and very slow when they are farthest from the sun. This means that they spend most of their time far from the sun.
In the 1950s Jan Oort hypothesized that the solar system was surrounded by a reservoir of these comets, which we now call the Oort cloud. Because the cloud is so far away it is not yet possible to detect these objects directly, but there may be as many as 5 trillion of them.
One of these comets, C/2012 S1 (ISON), is approaching the sun this fall, maybe for the first time. Some media have billed ISON as “the comet of the century.” This is because it is a sungrazer. On Nov. 28, when it is closest to the sun, it will fly through the sun’s atmosphere just a little more than 720,000 miles from the solar surface.
The brightness of a comet depends on its distance from the sun. As the comet approaches, it heats up and the ices begin to vaporize. The closer it comes to the sun, the higher the temperature and the more gases are released. As these gases boil away, they release the small, rocky dust particles, which are then blown away from the comet by the solar wind to form its tail.
ISON is just now crossing the frost line, the distance from the sun at which an object’s water ice begins to boil off into space. At this point the water locked in the 3-mile-wide nucleus should begin to sublimate — change from a solid directly to a gas. This process creates jets of gas and dust that should cause the comet to brighten even more.
However, because sungrazers pass so close to the sun, the temperatures get so high that the nucleus of the comet can completely disintegrate and the comet disappears. This is a possible fate for ISON.
One of the determining factors is the size of the nucleus, which can be estimated indirectly by its brightness. As of Aug. 26, it was at right about 13th magnitude (astronomers measure brightness in units of magnitude, under which the smaller the unit number is, the brighter the object is).
If the next measurements of its brightness are dimmer than 13th magnitude, then astronomers believe it has a fair chance of completely disintegrating before, during or just after it rounds the sun. If it is brighter than 13th magnitude, they believe it will stay intact as it passes the sun.
If ISON does stay intact, it could still facture into several pieces. If this happens, it could release large amounts of dust, which could produce a spectacular long and bright tail.
If the comet survives its trip around the sun, it will appear in the morning sky in December. On Dec. 1 it will be just above the eastern horizon 30 minutes before sunrise, with a tail about 10 to 15 degrees long. The tail should grow longer each morning, while the head of the comet becomes less distinct.
The view of the comet, if we get one, will peak between Dec. 10-14. During this time, look for the comet just before dawn, after the moon sets. It will slowly climb higher in the eastern sky, getting dimmer each day until it fades from view.
So this may or may not be the “comet of the century,” but it sure could be a great show.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at email@example.com.