ISON shows how unpredictable comets can be

bobmcdonald-190.jpgBy Bob McDonald, Quirks & Quarks

Skywatchers were briefly disappointed this week, as Comet ISON plunged towards the sun at more than a million km/h, disappeared around the far side, and seemed to vanish. Now, the latest images from a space telescope show that at least part of the comet has survived, continuing the pattern of this comet's unpredictable behaviour.

The latest images from the Solar and Heliospheric Observatory (SOHO) show a streak of material flying away from the sun along the comet's projected path. Whether this is an actual piece of the comet's nucleus or just a flying cloud of leftover debris will be determined over the next few days.

The big lesson here is that comets are very unpredictable and should never be labeled "comet of the century," until after their show is over.

When ISON was discovered, in September 2012, by Russian observers Vitali Nevski and Artyom Novichonok, it was very bright, considering its distance out beyond Jupiter. The thinking was that since comets tend to get brighter as they approach the sun, this one could be the brightest of the century and even become a modern "star of the East" during the Christmas holiday.
But predicting how bright a comet will become is risky business, because the icy nucleus, only a few kilometres across, is hidden inside the gas and dust that forms the comet's long tail. So, if you can't see that surface in telescopes, it's hard to predict how the comet will behave.

Comets are a mixture of ice and rock, so if there is a lot of ice, it will evaporate more quickly, producing a long tail. But if the ice is covered by a lot of dirt, the comet will be less bright and have a dimmer tail. That nucleus is also tumbling end over end, so different sides are being presented to the sun, which can make its activity vary from day to day. ISON started out active and bright, but then its brightness flat-lined for much of its journey, making the "comet of the century" predictions less popular. Some even believed it might follow in the footsteps of Comet Kohoutek in 1973, which failed to produce the once-in-a-century spectacular sky show that had been anticipated.

Scientific interest in ISON is also high because this is its first and only journey towards the sun from the distant Oort Cloud, well out beyond Pluto. That makes it extremely old - a pristine relic of the original cloud that formed our entire solar system, four and a half billion years ago.

ISON is a sun grazer, which are comets that come about as close to the solar disc as possible, without actually hitting it. Most don't survive the passage. ISON streamed by the sun at a distance of only 1.6 million km from the surface. That may sound like a long distance, but it's less than the diameter of the sun itself. Blasted by intense radiation and charged particles, the surface of the comet heated up beyond 2,000 C, making it literally "a snowball in hell." Imagine the view from the surface of the comet at closest approach, with the broiling sun's disc filling the entire sky, the ground bursting with violent gas eruptions, flinging rock and debris upwards as the comet comes apart.

Solar astronomers will use the comet as a probe into the sun, looking at how the two objects interacted with each other, to get more information on the activity close to the sun's surface, which is very difficult to study from Earth.

The trail of material that now remains may actually be a bonus for astronomers because it came from inside the comet, which means it is undisturbed and could provide more information about our own past. It could also become brighter again, perhaps visible in our morning sky, but this writer is not going to predict that.

The unpredictability of comets is what makes them so interesting. Not only is it difficult to forecast their behavior, we never know when the next one will arrive. Whenever the next cosmic visitor does appear, let's leave predictions out of the equation and just enjoy the show.