Starwatch: Celestial fireworks

Affelia Wibisono Royal Observatory, Greenwich on the annual display of the Perseids meteor shower, and on the origin of meteor showers in debris left behind by comets
Graphic: Finbarr Sheehy

Get ready to watch nature’s very own fireworks display as the Perseids meteor shower rolls into town. The shower can be seen every year between 17 July and 24 August, with the peak falling this year on the morning of 13 August. Conditions for viewing the shower are favourable as the moon – a natural source of light pollution - diminishes into a new moon on 14 August.

Try to get to an area with few or no artificial lights and let your eyes adjust to the dark. Hunting for meteors, like the rest of astronomy, is a waiting game, so it’s best to bring a comfy chair to sit on and to wrap up warm, as you could be outside for a while. If luck is on your side, you may see around 80 meteors per hour.

The meteors can be seen in all parts of the sky, so it’s good to be in a wide open space where you can scan the night sky with your eyes – binoculars or telescopes are not needed. But if you trace the paths that the meteors take, they seem to originate from a point in the constellation of Perseus, which gives this particular meteor shower its name. Don’t wait until peak viewing time on 13 August, as the meteor shower will build up in the week and days before.

Meteor showers occur when the earth ploughs into the debris left behind by comets. In the case of the Perseids, the fragments are from comet 109P/Swift-Tuttle.

Comets can be thought of as dirty snowballs; they’re mostly made up of a slurry of different ices, organic material and dust. They contain the primordial material of the early solar system that wasn’t used to form the planets and the sun, and could help explain how the solar system formed. They’re usually found in the same region of the solar system as Pluto, called the Kuiper Belt, beyond which lies the Oort Cloud – a bubble of trillions of objects of frozen material at the edge of our solar system.

As a comet heads into the inner solar system, the heat from the sun vaporises the ice and the comet starts to form a coma, or fuzzy atmosphere around the solid nucleus. The comet might also form a tail of dust and a second tail of gas. The closer the comet gets to the sun, the more active it gets, as more and more ice sublimes.

By chance, 13 August also marks the perihelion of 67P/Churyumov-Gerasimenko, the comet that is being studied by scientists working on the historic Rosetta-Philae mission. The Rosetta orbiter arrived at the comet in August last year and Philae bounced to the surface in November 2014. Little Philae’s instruments have already sniffed out compounds that play a part in the production of the ingredients for life, but scientists can’t yet confirm if they were brought to earth by comets like 67P.

However, how the coma and tail develop before, during and after perihelion is one of the main investigations for scientists. Hopefully we’ll find out a lot more before the mission ends in September 2016.

Affelia Wibisono (Royal Observatory, Greenwich)

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