Explainer
Earth’s course through space dust produces a remarkable spectacle in the night sky
Although seemingly empty, the space between the planets of our Solar System is teeming with vast numbers of meteoroids, small pieces of rock and dust largely originating from comets and asteroids but also, to a lesser extent, from the terrestrial planets and rocky satellites. A small number even originate from outside our Solar System: so-called interstellar dust. Meteors are produced when these particles enter Earth’s atmosphere at extremely high speeds (ranging from around 11km to 72km per second) causing them to burn up and leave a bright momentary streak across the sky.
Earth encounters approximately 40,000 tonnes of extraterrestrial dust every year. Although this may sound like a lot, on a typical night it means you might see just a few meteors an hour streaking randomly across the sky. These are called sporadic meteors. At certain times of the year these numbers can increase to around 100 meteors an hour in events called meteor showers, as Earth ploughs through denser streams of particles on its orbit around the Sun.
Of all meteor showers that occur today, one of the oldest known is the Lyrids; observational records for it date back to 687 BC. In these ancient times, meteor showers were interpreted as portents and then later as phenomena of the upper atmosphere. Their extraterrestrial nature was not realised until the idea of an Earth-centred Universe lost favour and astronomers became intrigued with meteors. This was spurred on by the occurrence of spectacular meteor showers in the 19th and 20th centuries, as well as the findings from studies of meteorites
– fragments of larger meteors that survive atmospheric entry and reach Earth’s surface.
We now know that the debris streams which produce meteors are left behind by comets during their passage through the inner Solar System. The ice that binds the rocky and dusty constituents of comets is heated by the Sun and turns to vapour, flowing
outwards from the nucleus and carrying these grains with it. These grains create a trail of particles that follows approximately the orbit of the parent comet.
Points of view
While Earth’s orbit around the Sun is roughly circular, comets whose paths cross the inner Solar System have orbits that are typically highly elliptical and inclined to the ecliptic. This means the paths of Earth and cometary debris can, albeit rarely, intersect. In such cases, a meteor shower will be observed annually when Earth reaches this point in its orbit. As Earth hits these trails, meteors viewed from the surface appear to radiate from specific points in the sky. These meteor showers are given names relating to the constellations that are nearest these radiant points. The Leonid meteor shower, for example, appears to originate from the constellation of Leo.
Type the name of any meteor shower into an internet search engine and you’ll find details of when to see it and what the theoretical peak number of meteors per hour will be. These predictions of meteor shower timings and intensities are generated by using a combination of past observational data from radar and optical telescopes and computer modelling. Observations provide details of the activity profile (how the number of meteors changes as Earth barrels through the stream), while computer models allow us to simulate and study the release and subsequent movement of particles from particular comets.
Meteor showers are observed annually, and some have been recorded for millennia, but for how long will they continue? The answer is entwined in the fate of the parent comet. Ultimately, the meteor shower will begin to diminish when the comet is no longer able to top up the debris stream. The comet could disintegrate, it could be gravitationally perturbed by a planet into another orbit, or even simply become dormant after losing all of its volatile ices. So, while well-known and well-loved meteor showers like this month’s Perseids exist, take the time to step outside, look up and try to catch sight of a few.
► For more details about the Perseid meteor shower see pages 26, 42 and 68