All About ISON

By Andrew Cooper

Professional and amateur astronomers alike are getting ready for comet C/2012 S1 ISON, possibly the highlight of a year that has already seen several good comets.

Comet ISON was discovered in September 2012 by Vitali Nevski and Artyom Novichonok as part of the International Scientific Optical Network project, a group of telescopes dedicated to discovering and tracking solar system objects. The comet quickly attracted attention from the astronomy community.

When astronomers first calculated the comet’s orbit they encountered some surprises.

First, orbital calculations showed that this comet will pass extremely close to the Sun. On November 28th the comet will pass perihelion, its closest approach to the Sun, at a mere 724,000 miles above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day in Hawaiʻi.

The second surprise also came from examining the orbit. The comet has a nearly hyperbolic path, suggesting that this was a new comet, one that had never visited the inner solar system. For much of the age of our solar system, 4.5 billion years, this comet has orbited in the Oort Cloud, a spherical cloud of icy planetesimals that define the limit of our solar system.

The early detection of this comet was a huge boon to astronomers. The discovery of a sun-grazing comet more than a year before it passes us by offers a unique opportunity. The comet represents a sample of the primordial solar system, a leftover building block of the planets. Orbiting far from the Sun, this object is unaltered by passage through the inner solar system. As the comet passes close to the Sun this material will sublimate and become visible to the many instruments directed at the comet. A concerted campaign has been organized to observe the comet with numerous spacecraft and telescopes, including the Keck Observatory, to learn as much as possible from this opportunity.

Aside from the potential death-defying dive into our Sun, comet ISON appears to be a fairly ordinary comet. Astronomers often use the term “dirty snowball” to describe comets, a fairly accurate description of theses icy bodies. A mix of rock, dust and various types of ice form the comet’s nucleus, estimated to be a typical, 1km (0.6miles).

As it approaches the Sun, there is a fair chance ISON may come apart. Comets form in the microgravity environment of the early Solar System. In addition to being dirty, comets are also poorly packed snowballs, liable to fall apart. Subjected to solar heating and gravitational stress, many comets do crumble, fragmenting into many smaller parts.

For the next few weeks, as the comet nears the Sun, it can be seen in the dawn sky. While brightening rapidly, the comet is currently a bit too faint to see without optical aid. This will change as the comet brightens. How bright is still an open question, the predictions for comets are always somewhat uncertain. If the comet does disintegrate it can fade from view or flare to brilliance.

In any case it is safe to say this comet will be bright enough to see without optical aid – always a treat. A dark sky, away from city lights, will dramatically enhance the view. A little optical aid will also be helpful. Given the size of a comet and tail, a low power instruments that will work best, and one of the most rewarding ways to observe the comet is a decent pair of binoculars.

For sky watchers in HawaiʻI, the comet will remain a dawn object requiring setting your alarm clock for the wee hours. It is currently well up in the sky at sunrise, getting a little lower and brighter each day as the comet approaches rendezvous with the Sun.

Check out www.keckobservatory.org for a recent Astronomy Talk we did with the head of NASA’s Comet ISON Observational Campaign, as well a recording of an actual Observing Run of ISON with some of the leading astronomers in the world.

Watch the sky!