A springtime comet?

Ken Tapping, March 6, 2013

In the sky this week…

  • Jupiter is high in the south after dark, moving into the west by midnight.
  • Saturn rises around 11 p.m.
  • The Moon will be New on the 11th.

It looks as though we could be getting a comet for spring! At this point we don’t know whether it will be something hard to spot in the sky, or something spectacular – or something between. However, it would be a real shame if we don’t try to see it. This comet is called Comet PanSTARRS, after the telescope that was used to discover it.

At this time of the year, most of us in Canada have only to look by the side of a city street to get an idea of what comets are made of: ice with plenty of grit, dust, trapped gas and petrochemicals all combined to form a grey-black frozen foam. Comets are lumps of this sort of stuff up to a few kilometres in size. They orbit the Sun in the outer reaches of the Solar System, where it is very cold, and they are frozen solid. However, occasionally a collision or some other encounter throws them into orbits taking them into the inner Solar System. As they fall towards the Sun, they start to warm up, and their material starts to evaporate.

Objects this small have very weak gravity, so there is not much holding them together other than ice. As they warm and the ice melts, they start to fall apart. Pressures from inside the comet speed up this process. Volatile materials evaporate to form gases and vapours, which force their way to the surface and exit as jets of gas, droplets and dust.

Comets are usually discovered when they are still well out from the Sun, and the warming process has just started. That’s why it’s so hard to predict whether any newly discovered comet will be a spectacle or something only interesting to astronomers. It depends on what particular mix of ice, gas, dust and chemicals the comet has.

As comets get closer to the Sun, the rate of evaporation speeds up enormously, with the ejected material forming a tail that can be millions of kilometres long. That’s when a tiny frozen dirty slushball may start to become really spectacular. If we get lucky, we could see a glowing tail, like a sword or scimitar blade stretching across the sky.

Comet PanSTARRS should appear in the western sky after sunset around 10-11 March, and get a little higher in the sky on following nights. It is closest to the Sun on 10 March, so it should be brightest at that point, when it is evaporating the most rapidly, however, thereafter it will moving higher, and visible against a darker sky. Look for a brighter head with a tail pointing upward, away from where the Sun is hiding below the horizon.

Binoculars are great for looking at comets, because the large objective lenses collect far more light than our eyes can. There is no need for much magnification. However, it is not safe to start searching the sky before the Sun is well below the horizon. Be patient. Don’t risk damaging your sight.

Our ancestors saw comets as harbingers of ill luck, particularly for powerful people. Now we know them to be just interesting objects in the sky, unless of course one actually hits us.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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