Ken Tapping, August 4 2010

There are two astronomical events that we can depend on turning up each August. One is the annual Perseid meteor shower, and the other is, unfortunately, yet again, the “Mars Hoax”. 

The Perseid meteor shower is an astronomical event we can look forward to every August.  This spectacular display of “shooting stars” – meteors is the proper name – is produced by something very prosaic: the debris left from the gradual disintegration of a comet. In this case the slow destruction of Comet Swift-Tuttle. 

Comets are lumps of ice, dust and a range of organic chemicals. Typically they are a few kilometres across.  They have very elliptical orbits, and spend most of their time in the cold, outer reaches of the Solar System, and periodically, dive in close to the Sun and then rapidly move out again. Each time they pass close to the Sun, they lose material. Some of the ice binding the comet body together evaporates and it slowly falls apart, bit by bit. It’s this debris that gives the comet its magnificent tail, and makes it so spectacular in the sky. After a while, the comet is followed around its orbit by a stream of debris, like what is behind an overloaded dump truck driving too quickly. This debris is moving at many kilometres a second, and every August our Earth ploughs through the stream at nearly 30 kilometres a second, so a collection of very high-speed collisions is inevitable. The objects swept up by the Earth come into the atmosphere at very high speed. Friction heats them to thousands of degrees and they evaporate, leaving glowing trails as they speed across the sky.

We get a few meteors on any clear night, as particles left over from the birth of the solar system or from long-dead comets come into our atmosphere. However, when the Earth passes through one of these streams of debris, we get what is known as a meteor shower, or if we are really lucky, a meteor storm. This year we won’t get a storm, but it will still be worth spending some time sky watching. The best night should be 11-12 August, but there will be higher numbers of meteors for a few nights before and afterwards. 

To enjoy them find a place screened from street lights, preferably giving a view of the north-western sky.  A chaise longue is best, but a blanket on the ground will work too. Take an extra blanket; it can get cold even on summer evenings, especially if you are not moving. 

In August 2003, the Earth and Mars passed by one another more closely than they have in a long time. At that time Mars was close enough to look as large as the Full Moon, WHEN OBSERVED THROUGH A TELESCOPE WITH A MAGNIFICATION OF 75 TIMES. Somebody deleted the bit about the telescope and hoax e-mails have been bouncing around ever since, saying that in the coming August Mars would be so close to us that in the sky it would look as large as the Full Moon. It’s hard to come up with any scientific circumstances under which that would happen, so we will have to be satisfied with what we have, a neighbour world that is never close enough to observe easily.