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The Orionids are coming

Mark Thompson Astronomy Mark Thompson Astronomy | 13:00 UK time, Friday, 21 October 2011

Distance travelled ~ 755'224'000 km

(Mark Thompson's previous blogs for 23 Degrees include a post on the Perseids meteor shower and Jupiter's Great Red Spot. Here he explores octobers's second Meteor Shower, The Orionids.)

Halley's Comet returns to the neighbourhood of the Sun every 76 years, yet each year, around the 21/22 October it's possible to catch glimpses of this famous comet in the Orionid Meteor Shower.

halley's comet 1986

The European Space Agency's Giotto probe penetrated the coma of Halley's Comet in 1986 and took this picture of its nucleus.

There are hundreds of comets that orbit the Sun and each one is a lump of rock mixed ices. When they get closer to the Sun, the growing heat turns the ice straight into a gas in a process called sublimation. We can see this process as comets approach in the growing coma around the rocky core. As it gets closer to the Sun, the solar wind (streams of charged particles from the Sun) push against the coma producing the comet's trademark tail. It's a common misconception that the tail of a comet flows out behind it but in reality, it simply points away from the Sun regardless of its direction of travel. As the comet's journey along its orbit continues it leaves behind a trail of debris. If the Earth happens to move through or close to a comet's orbit in its 365 day journey around the Sun, then it will sweep up some of this material and we will see nature's own firework show, a meteor shower.

At around 22:00 (BST) on Friday evening (21st) the Earth is expected to pass by the densest part of the debris left by Halley and we should see the peak of the shower. However, meteor showers are difficult to predict and always tend to be best observed just after midnight so it's worth keeping an eye out in the early morning hours of the 22nd too. Unfortunately this year, the Moon is a day passed last quarter so its light may affect the number of meteors visible. Even so, there could be up to 30 meteors visible per hour. The best tip to catch a glimpse of this spectacle is to wrap up warm, get outside around 22:00 tonight just before moonrise and watch the sky. Be patient and keep an eye on the sky for the rest of the night just in case it happens later than expected. Happy hunting.


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