Meteors are better than planets

The days are shortening and our thoughts turn to dark skies. August sees one of the highlights of the astronomical calendar, the Perseid meteor shower. Each year as the Earth crosses the path of comet 109P/Swift-Tuttle our skies are decorated by many shooting stars, 10s per hour if you're somewhere dark. The night of 12/13 August looks set to see the most meteors but it's well worth looking on the couple of nights either side of this. To see a meteor every minute or two, over a couple of hours, is something memorable and thrilling, well worth sacrificing a little sleep and warmth for.

Because they're in the foreground, meteors, like planets, move against the background of the stars. You need to watch the planets over days or weeks to become aware of this whereas a meteor is over and done with in a second or two, at most. The planets are completely reliable and predictable in their apparitions whereas a single meteor, a grain of cosmic sand heated briefly to destruction, appears without warning and is never seen again.

Meteors are better than planets. Planets shine in the night sky only because they reflect light that has come in the first place from the Sun. They have no light of their own. Remove the Sun - admittedly difficult - and they would just be cold, dark orbs. Brief though its luminous life is, the brilliance of the meteor is its own. OK, that's a flippant view but I think defensible at least from the point of somebody looking at the night sky with the naked eye.

A typical, bright but not dazzling meteor is a speck of rock about the size of a grain of sand. Travelling for the first time in Earth's atmosphere, at a speed of a few 10s of kms per second, it heats up to about 1800°C, leaves a tail of brightly glowing vapour - the "meteor" that we see - and disintegrates. Even when Earth crosses the path of a comet these particles are few and far between. Could they ever be so numerous that the whole of the sky was bright?

In fact this may have been the state of affairs 65 milion years ago, immediately after the Chicxulub asteroid impact that probably killed off the dinosaurs. An asteroid 10 to 15 km across crashed into the Gulf of Mexico causing global devastation. The resulting explosion threw debris, rocks and dust, high into the atmosphere. As it fell back it heated up as individual meteors do, but involving so many more particles of all sizes that the whole of the sky would have glowed fiercely. The incandescent sky triggered forest fires all across the globe and the soot from these has been identified at the K-T boundary, the layer in the rocks corresponding to the time of this event.

Such events were common in the early history of the Earth and the other planets. The first half billion or so years of Earth history is called the Hadean eon - "hellish"! Those worlds unmodified by an atmosphere, water, etc. - like our own Moon - still show the scars in the many impact craters that dot their surfaces. Meteors and fireballs lit up the sky all the time and Chixculub type events would have occurred over and over again, albeit on a world looking very different from its present state.

Nowadays things are quiet. A meteor shower is pretty but not threatening. The vast majority of the rocky bodies flying around in the earliest times have either been incorporated into the bodies of the planets or expelled to the far edges of the solar system, to the Oort cloud. None of the known Near-Earth Asteroids or Comets poses a threat. We're living in a time of tranquility when it's been possible for us to emerge and get on with what we want to do. The biggest threat to the planet now is ourselves.

Hoping for clear skies this week!