“Geminid meteor shower defies explanation - PhysOrg” plus 1 more |
| Geminid meteor shower defies explanation - PhysOrg Posted: 07 Dec 2010 09:20 AM PST A Geminid fireball explodes over the Mojave Desert in 2009. Credit: Wally Pacholka / AstroPics.com / TWAN. The Geminid meteor shower, which peaks this year on Dec. 13th and 14th, is the most intense meteor shower of the year. It lasts for days, is rich in fireballs, and can be seen from almost any point on Earth. It's also NASA astronomer Bill Cooke's favorite meteor shower—but not for any of the reasons listed above. "The Geminids are my favorite," he explains, "because they defy explanation." "Of all the debris streams Earth passes through every year, the Geminids' is by far the most massive," says Cooke. "When we add up the amount of dust in the Geminid stream, it outweighs other streams by factors of 5 to 500." An artist's concept of an impact event on Pallas. Credit: B. E. Schmidt and S. C. Radcliffe of UCLA. This makes the Geminids the 900-lb gorilla of meteor showers. Yet 3200 Phaethon is more of a 98-lb weakling.3200 Phaethon was discovered in 1983 by NASA's IRAS satellite and promptly classified as an asteroid. What else could it be? It did not have a tail; its orbit intersected the main asteroid belt; and its colors strongly resembled that of other asteroids. Indeed, 3200 Phaethon resembles main belt asteroid Pallas so much, it might be a 5-kilometer chip off that 544 km block. "If 3200 Phaethon broke apart from asteroid Pallas, as some researchers believe, then Geminid meteoroids might be debris from the breakup," speculates Cooke. "But that doesn't agree with other things we know." Researchers have looked carefully at the orbits of Geminid meteoroids and concluded that they were ejected from 3200 Phaethon when Phaethon was close to the sun—not when it was out in the asteroid belt breaking up with Pallas. The eccentric orbit of 3200 Phaethon brings it well inside the orbit of Mercury every 1.4 years. The rocky body thus receives a regular blast of solar heating that might boil jets of dust into the Geminid stream. Could this be the answer? To test the hypothesis, researchers turned to NASA's twin STEREO spacecraft, which are designed to study solar activity. Coronagraphs onboard STEREO can detect sungrazing asteroids and comets, and in June 2009 they detected 3200 Phaethon only 15 solar diameters from the sun's surface. What happened next surprised UCLA planetary scientists David Jewitt and Jing Li, who analyzed the data. "3200 Phaethon unexpectedly brightened by a factor of two," they wrote. "The most likely explanation is that Phaethon ejected dust, perhaps in response to a break-down of surface rocks (through thermal fracture and decomposition cracking of hydrated minerals) in the intense heat of the Sun." Jewett and Li's "rock comet" hypothesis is compelling, but they point out a problem: The amount of dust 3200 Phaethon ejected during its 2009 sun-encounter added a mere 0.01% to the mass of the Geminid debris stream—not nearly enough to keep the stream replenished over time. Perhaps the rock comet was more active in the past …? "We just don't know," says Cooke. "Every new thing we learn about the Geminids seems to deepen the mystery." This month Earth will pass through the Geminid debris stream, producing as many as 120 meteors per hour over dark-sky sites. The best time to look is probably between local midnight and sunrise on Tuesday, Dec. 14th, when the Moon is low and the constellation Gemini is high overhead, spitting bright Geminids across a sparkling starry sky. Bundle up, go outside, and savor the mystery. This entry passed through the Full-Text RSS service — if this is your content and you're reading it on someone else's site, please read our FAQ page at fivefilters.org/content-only/faq.php  This posting includes an audio/video/photo media file: Download Now |
| Best meteor shower of 2010 arrives starting Dec. 13 - msnbc.com Posted: 07 Dec 2010 12:37 AM PST What should be the best meteor shower of 2010 will occur beginning on the evening of Monday, Dec. 13. Like most meteor showers, the Geminids will be at their best after midnight (early on the morning of Dec. 14), when the Earth is heading directly into the meteoroid stream. But some will be visible earlier in the night, on the evening of Dec. 13, because the meteors radiant —where they appear to originate — is nearly circumpolar, so they will stay in view above the horizon all night. This sky map shows where to look to see the Geminid meteor shower. What causes a meteor shower? Phaethon is a very odd asteroid indeed. Its orbit brings it closer to the sun than any other known asteroid, well inside Mercury's orbit. Its orbit is more like that of a comet than an asteroid, but it has never exhibited any of the features which characterize comets: no coma, no gas jets, no dust tail. When we look toward the radiant of this meteor shower, we are looking into the line of Phaethon's orbit. The meteors appear to radiate from this point in the sky, but this is an effect of perspective, much as railroad tracks appear to diverge as they get closer to us. The Geminids' radiant is, as the name implies, in the direction of the constellation Gemini, just north of the northernmost of Gemini's two brightest stars, Castor. In the early evening of Dec. 13, the radiant is low in the northeast. By 1 a.m. ET, after the date has changed to Dec. 14, the radiant is almost directly overhead. By 6.a.m., when the shower is at its peak in the Eastern time zone, the radiant is low in the west. This circumpolar pattern for the radiant means that meteors may be seen all night long, though they tend to be faster and brighter after midnight. It will also help that the moon sets close to midnight, allowing fainter meteors to be seen. Observing tips You don't need binoculars or a telescope; in fact these will prevent you from seeing the meteors by restricting your field of view. You want the wide field given by the human eyeball. It doesn't really matter much which part of the sky you watch, as the meteors can appear anywhere. Early in the evening, they often appear low on the horizon; later at night they are most frequent overhead. You will generally see more meteors if you look away from the radiant, as the meteors 90 degrees away from the radiant have the longest trails. The most important thing is to be patient. It takes a while for your eyes to adapt to darkness. Then, there are often long periods with no meteors at all, interspersed with more active times when four or five may flash by in a minute. The Geminids are a bit slower moving than their summer cousins, the Perseids, and often end with a tiny explosion — you may hear a popping sound. If you're clouded out on Dec. 13 or 14, try the night before or the night after: There still should be quite a few Geminids about. Good luck and clear skies! This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions. © 2010 Space.com. All rights reserved. More from Space.com. This entry passed through the Full-Text RSS service — if this is your content and you're reading it on someone else's site, please read our FAQ page at fivefilters.org/content-only/faq.php This posting includes an audio/video/photo media file: Download Now |
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