Friday, December 13, 2013

The Geminid Meteor Shower

We may be in the grip of some Arctic weather here in the US right now, but if you can stand to venture outside then the skies have a treat in store this week, in the shape of the Geminid meteor shower. Hyped as the best meteor shower of any given year, the Geminids reaches its peak this weekend (13/14 December).

So what is a meteor?

They’re also known colloquially as ‘shooting stars,’ but have nothing to do with stars. A lot of things in astronomy that are basically the same are given different names depending on how and where we see them. Space debris is no different. A small, solid body moving within the Solar System is known as a meteoroid. If that meteoroid happens to cross paths with the Earth, it burns up in the atmosphere, creating the distinctive streak that we know as a meteor. In exceptional cases, a large rock might not entirely burn up, and survives intact to hit the ground. The solid remains that hit the Earth’s surface are known as meteorites.

Space is full of debris. A quick look at the crater-covered moon is a good indicator of what the Earth might look like if we didn’t have the atmosphere to burn up most of what might impact us, and erosion on the ground to smooth over the damage caused by those that do. On any clear night, if you watch a patch of sky for long enough, chances are you’ll see a meteor.

What makes meteor showers different to these random occurrences is that they’re highly concentrated – a lot of meteors all originating in the same place – and that they occur regularly, at the same time each year. This is because the Earth is moving through space as it orbits the Sun, carving out the same path every year, and so at the same time each year we hit the same particularly intense patches of debris.

Most of these debris patches have been left by comets. Comets have been observed for as long as astronomical observations have been recorded, and often viewed as divine messengers or omens. One of the earliest recorded sightings was in China in 240BC. The same comet was also recorded by the Babylonians and in medieval Europe, and is even featured on the Bayeux Tapestry. It wasn’t until 1705 that Edmund Halley realised these sightings were of the same object: it now bears his name, Halley’s Comet, and is due to pass the Earth again in 2061.

Comets are small bodies made up of ice, rock and dust, thought to originate in the outer reaches of the Solar System. Some of these ‘dirty snowballs’ are pushed towards the centre of the Solar System, where they enter into highly elliptical orbits that see them pass close to the Sun before shooting off back to the outer reaches of the Solar System. As they approach the center of the Solar System, radiation from the Sun causes some of the material in the comet to melt and vaporize; this gives rise to the characteristic tail. Consequently, the tail always points away from the Sun.

Forging paths through the entire length of the Solar System is dangerous work, and not all comets survive the journey intact. In 1994, Comet Shoemaker-Levy 9 collided spectacularly with Jupiter. More recently, you may have seen Comet ISON in the news as it ventured into the inner Solar System. ISON made its closest approach to the Sun (called perihelion) on November 28th 2013, but is believed to have disintegrated as it whipped around the Sun.

Those that do survive their journeys, however, are not good at cleaning up after themselves. There are still debris trails from several comets that have crossed the path of the Earth’s orbit in the past, and each time the Earth reaches that point in its orbit – once per year – we collide with this debris, which burns up in the atmosphere to produce the streaks of light we call meteors. Comet Halley mentioned above actually intersects the Earth’s orbit twice, and its trail is believed to give rise to both the Eta Aquarids in May and the Orionids in late October.

The Geminids are unusual for a meteor shower in that the origin is not actually a comet, but an asteroid known as 3200 Phaethon. The asteroid is on an unusual orbit that brings it closer to the Sun than Mercury, and it sheds enough material to generate the most intense meteor shower of the year. The video below from NASA Science Casts explains more about the origin of the Geminids.


Meteors can be seen all over the sky, but most will appear to originate at a single point, known as the radiant. For the Geminids, this radiant is in the constellation Gemini (which gives the shower its name), close to the star Castor. This effect is caused by the fact that the Earth is moving into the debris; this is the same effect used to demonstrate spaceships moving at faster-than-light speeds in science fiction.

The Eastern sky as seen from Austin, Texas at 9pm on Friday December 13th 2013.
The Geminid meteors appear to radiate from the constellation Gemini, near to the
star Castor. Gemini can be most easily located by finding Orion with its distinctive belt.
A little way over Orion's left shoulder (the red star Betelgeuse) are the two bright
stars Castor and Pollux. (Image credit: Stellarium)
The Geminid meteor shower will peak on Friday and Saturday nights (December 13th – 14th), but meteors can be seen for a few days either side. The best thing about meteor showers is that no equipment is required (save something to keep you warm) – just pick a dark location and lie back so that you can see as much of the sky as possible.
The Geminids regularly peak in intensity around mid-December and seem to have increased in strength in the past years. This year, astronomers expect 120-160 meteors per hour during the peak, which would be early in the morning on Dec. 14. However, the moon is close to  full during the peak so only the brightest meteors will be easy to spot. For the truly dedicated, the best time to watch is an hour before dawn, when the moon will have set leaving the sky much darker. Reports say that we can still expect around 50 per hour under the best observing conditions (clear skies, no light pollution, etc.) and we may even be able make out their different colors (mainly white and yellow and a few being blue, green or red).

And if you’re not brave enough to venture out into the cold, you can even watch online. Now that’s astronomy for the 21st century.

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