Every year, on the night of August 12-13 the Earth passes through a stream of debris left behind by a comet. The Earth and debris are moving in different directions at many kilometres a second, so this debris hits our atmosphere at very high speed. The friction vaporizes it, leaving glowing trails in the sky, which are often wrongly referred to as “shooting stars.” “Meteors” is a more precise term. The Earth is always running into pieces of orbiting grit and dust left over from the formation of the Solar System, so we can see meteors most dark, clear nights. However, when we pass through one of these debris streams, we can see many more meteors per hour.
Comets are lumps of ice, dust, grit and petrochemicals up to a few kilometres across. There are many of these objects orbiting the Sun out in the cold, dark outer reaches of the Solar System. However, periodically an impact, close encounter with something or some other process deflects one onto a path taking it into the inner Solar System. The object is now doomed.
As it approaches the Sun it starts to warm up, and the volatile chemicals and the ice holding it all together start to evaporate. Since such a small object has only weak gravity, once the glue goes there is not much else holding the object together, and it starts to lose material. This gets pushed out by the solar wind and the pressure of solar radiation into glowing tails which get bigger as the Sun gets closer and the heat builds up.
The chance of seeing a dark, cold lump of dirty ice is negligible, but once a tail starts to develop, the object becomes increasingly visible. It is now a comet. The presence of glowing tails, which get bigger and brighter as the comet gets closer to the Sun makes it increasingly likely to be spotted by satellite telescopes, observatories or the many amateur astronomers who spend hours searching for new objects in the sky. If we’re lucky we might get to see a real spectacle, like Comet Hale-Bopp, which visited us a few years ago.
Unless the comet passes so close to the Sun that it is completely evaporated, its orbit takes it back into the outer reaches of the Solar System again. However its years are numbered, because it is destined to keep making visits to the inner Solar System, providing a spectacle for us to enjoy but losing a bit more of its substance each time. After a few passes close to the Sun its orbit becomes filled with debris released by the comet. This is the situation with Comet Swift-Tuttle. An additional factor is that the Earth ploughs through this stream every August, producing a meteor shower called the Perseids. This is usually one of the best meteor events of the year.
Our observatory is located at a really dark site, and for some years we have been having a public star party to enjoy the Perseids. In addition to meteor spotting, there are telescopes brought out by local amateur astronomers for looking at planets and other objects in the sky, and we lay on some astronomy talks and a demonstration or two.
We will be doing that this year. Of course Mother Nature has pitched us a curve ball this year. We will have the Moon to contend with. It will be just past full and its brightness will obliterate the fainter meteors. However, it also means the Moon can be a telescope target. There are two bright planets, Mars and Saturn, to look at too, in the southern and southwestern sky. Saturn is possibly the most beautiful object in the Solar System and a must-see. Actually, there is another way to detect meteors, which is unaffected by the Moon, cloud, or even whether it is day or night. It uses radio waves; we will demonstrate.
Mars and Saturn are still well placed to enjoy. Saturn, a moderately bright object in the south, is magnificent, even through small telescopes. Venus and Mercury lie low in the predawn twilight. The Moon will be new on the 26th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.