|
. | . |
|
by Jonti Horner, Donna Burton, Tanya Hill Melbourne, Australia (The Conversation) Dec 12, 2014
The best meteor shower of the year should put on an impressive display this weekend - weather permitting - with the annual Geminids poised to light up the sky with bright, long meteors visible as frequently as every couple of minutes. Meteor showers occur when the Earth ploughs through trails of debris as it spins its yearly course around the sun. This weekend, the Earth will pass through the densest part of a debris trail left behind by an asteroid known as 3200 Phaethon. The resulting meteor shower, known as the Geminids, will reach its peak late on Sunday evening, December 14 in Australia (see here for times in other locations).
The best of the 'big three' January's Quadrantids and August's Perseids are only really visible to observers in the northern hemisphere. But this month's Geminids are visible in both hemispheres and so provide a spectacle to brighten Australian summer nights. One of the things that makes the Geminids so spectacular is that each meteor seems to last just a little bit longer than those in other showers. The reason is down to their unusual heritage - this is the only meteor shower known whose parent body is an asteroid, rather than a comet. Most meteor showers are made up of fragile, fluffy dust grains shed by a comet. These grains are shed as the comet nears the sun. In contrast, the Geminids are thought to be rocky fragments, eroded from the surface of the asteroid Phaethon.
We love a sun-burned asteroid That close to the sun, Phaethon's surface is baked by radiation a factor of 50 times as intense as that at the top of Earth's atmosphere (roughly 68,000 watts per square metre, against the Earth's about 1,360 watts per square metre). At its most distant, Phaethon is almost 2.5 times further from the sun than Earth, and hence receives far less radiation at that point. As a result, Phaethon's surface is continually forced to expand and contract as it experiences some of the most extreme temperature variations in the solar system. Such drastic change is thought to gradually fracture the asteroid's surface, chipping off fragments and creating a stream of debris along the asteroid's orbit. Earth passes through this debris each December, and the meteoroids it encounters are significantly denser than those of other meteor showers, thanks to their unusual origin. This allows them to survive longer as they ablate and shine brightly in the night sky.
The origin of the Geminids In the mid-20th century, US astronomer Fred Lawrence Whipple tried to solve that mystery. He used a photographic meteor survey to discover that the meteoroids in the Geminid stream moved on an extremely eccentric orbit, with a period of just 1.65 years - an orbit far smaller than those of all known comets. In 1983, the Infrared Astronomical Satellite (IRAS) discovered a fast-moving asteroid. Follow-up observations allowed the asteroid's orbit to be determined, and the result was astonishing. The asteroid followed an incredibly elongated path, plunging far closer to the sun's surface than any asteroid known at the time. More surprisingly, its orbit was striking similar to that calculated by Whipple, half a century earlier, for the Geminid meteoroids. So the mystery of the origin of the Geminids was solved; their parent discovered. The asteroid was named 3200 Phaethon after the son of Apollo, who in Greek mythology drove his father's chariot too close to the sun. Thanks to the discovery, astronomers researching meteor streams now check the orbits of newly discovered asteroids, as well as those of comets, to see whether they could one day too spawn new showers. For now, the Geminids remain the only confirmed asteroidal meteor shower.
So when and where to look? From the southern hemisphere we see the twins standing on their heads, as marked by the bright stars Castor and Pollux. The shower's radiant (the point in the sky from which the Geminids appear to radiate) is located near the bright star Castor. And the further north you are, the earlier the radiant will rise in the sky:
+ Brisbane: the radiant rises around 9pm (AEST) In the first hour or so after the radiant rises, the number of Geminids you might see will remain low - but it is well worth going outside, if you have clear skies, for a couple of reasons. First, it takes time for your eyes to fully adapt to the darkness, so allowing maybe half an hour to become fully dark-adapted is usually a good idea. Second, and perhaps more important, although the rates just after the radiant rises will be low, the meteors you might see could be spectacular. Known as "earth-grazers", they often span the sky, giving a beautiful introduction to this wonderful shower. As the night passes, the radiant will get ever higher in the sky and the number of meteors you see will climb with it. As a result, you should be able to spot a meteor every few minutes. Unfortunately, the moon will rise at around midnight local time in Australia (or 1am local time for daylight savings) at which point the fainter meteors will be washed out. But so plentiful are the Geminids, and so rich in bright meteors, that the shower should continue to put on a good show for the rest of the early hours of Monday morning. To see the best display, you should look a short way to the left or right of Gemini in the sky. The further from the radiant you look, the more spectacular the meteors you see will be. But if you look too far from the source, you will miss those flying in different directions. If you can find the constellation Orion, that's usually a great place to spot a good number of Geminids. So keep your eyes peeled on Sunday night - and you may get to see a natural display of fireworks to bring a fascinating astronomical year to a close.
Related Links The Conversation Asteroid and Comet Impact Danger To Earth - News and Science
|
|
The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service. |