Sky Matters with Niall Smith

IN ANCIENT lore it was said that a shooting star was a “sign of a soul going to heaven”. Borne out of a lack of understanding of what causes these random streaks of light in the night sky, it was perhaps not surprising and indeed, to this day, one continues to hear people use the phrase. The actual origin of shooting stars is now reasonably well understood, but there is more than one source for this phenomenon and new research has suggested some interesting results, of which more in a moment.

The first source of meteors is small dust particles spewed out in the tails of comets. As Earth passes through them at speeds ranging from 40,000km/h to an incredible 260,000km/h, the friction between the dust particles and the Earth’s atmosphere results in instant vaporisation, unless the dust particle is larger than a grain of sand, in which case it can take a couple of seconds before vaporisation is complete. In November, we will have the opportunity to see shooting stars caused by the Earth passing through the tail of comet 2P Encke on the night of November 11/12th (the so-called Taurid meteor shower) and through the tail of 55P Temple-Tuttle on the night of November 17/18th (the so-called Leonid meteor shower).

The Taurids shower is very gentle, which is a euphemism for not being very spectacular, while the Leonids are less predictable and can sometimes put on a fantastic display.

In both cases, the best way to observe is by finding a dark-ish location and just looking at the sky. It’s best to put away the mobile phones and let your eyes adapt to the dark for about 10-20 minutes; in our modern world, this can sound like an eternity, but it’s not!

The second source of meteors is dust which simply never became part of a planet when the solar system was initially being built, some 4.5bn years ago. Most of this dust is shepherded and contained between the orbits of Mars and Jupiter in a zone called the asteroid belt. Larger objects in the asteroid belt, from metres to kilometres in diameter, are commonly referred to as asteroids, but most “asteroids” are a few millimetres across.

LARGER asteroids in the kilometre-scale range have generally been considered the poor cousins of comets, because they don’t have spectacular tails. For a long time, they were considered to be relatively unimportant inert rocks that had little to do with life on Earth. Indeed, they generally get blamed for one of the largest mass-extinctions ever, when a kilometre-sized asteroid set in train a sequence of events that killed off the dinosaurs.

However, recent research has come to the surprising conclusion that asteroids can contain large quantities of the type of water we find on Earth, whereas the water on comets is somewhat different.

This is an incredibly important discovery, because Earth was too hot in the early days of its formation to hold onto any water it would have had. Any water would have been superheated and ejected into space.

As Earth cooled, water must have been brought back somehow, which is obvious from the amount of water we see on our planet today. It now appears the water came from asteroids crashing into the Earth, not comets.

So, since all life on Earth requires water for its existence, this leads us to the surprising conclusion that you are reading this article because of, in part, the existence of “boring” asteroids rather than “spectacular” comets. (There may be a more general life lesson in this observation!)

So, when you next see a shooting star, don’t think of it as the end of a life, but as a reminder of the role played by “dust” in our existence.