Taurids – Halloween Fireballs
Meteoroids to meteorites
We call space rocks meteoroids, though most of them are more like space grains than rocks. If one hits the Earth's atmosphere at high speed it leaves behind a trail of glowing air. The trail is a meteor. Small meteoroids are vaporized, but larger ones that land on Earth are meteorites. Impact craters are evidence of very large meteorites.
We talk about empty space, but there's a lot of rocks and dust out there. Thousands of tons of it arrives every year. Most of it's due to random meteoroids, which we call sporadics. But there are also a number of meteor showers that occur when Earth travels through streams of debris.
Periodic comets leave debris in their wake, so the stream is augmented at each visit. If our orbit crosses such a stream, Earth encounters material that creates an increased number of meteors at the same time each year. Meteors in a shower all appear to originate in the same area of sky. We call it the radiant, and its location gives the shower its name. For the Taurids, the radiant is in Taurus.
But a meteor shower isn't like a rain shower. Pictures showing a skyful of meteors incorporate multiple exposures such as this one of the Orionids. The radiant is just above and to the left of Orion's Belt. [Image: Lu Shupei]
Comet Encke orbits the Sun every 3.3 years. The first person to record it was French astronomer Pierre Méchain in 1786. Others, including Caroline Herschel in 1795, reported it as a new discovery. However in 1819 German astronomer Johann Encke's calculations showed that it was the same comet reappearing every 3.3 years. The only known periodic comet was Comet Halley, recognized as such over a century before. On the same basis that Comet 1P/Halley got its name, this comet was named after Encke for discovering its periodicity. Comet Encke remains the comet with the shortest known period.
The Taurids occur as Earth travels through debris from Comet Encke, a remnant of a much larger comet that broke up some 20,000-30,000 years ago.
The matter stream
Encke's matter stream is large and spread out. It's separated into two main parts, giving the Taurids two peaks. (A meteor shower's peak is the time when we're in the densest part of the stream and the most meteors occur.) The Southern Taurids are active from September 10 to November 20, peaking around November 5. The Northern Taurids are active from October 20 to December 10, peaking around November 12. What had been one stream has diverged somewhat due to gravitational interactions, primarily with Jupiter.
A new branch of the stream – what's hiding there?
Why does Taurid activity increase in some years? A team of European astronomers investigated, using data from the European fireball network. They analyzed meteors from the very active 2015 shower, and found a new branch of the debris stream.
They also realized that the orbit of the new branch was shared with two recently-discovered asteroids, 2015 TX24 and 2005 UR. They're between 200-300 metres (650-980 ft) across, and there may well be more of these big rocks out there. Objects of that size would do considerable damage if they hit the Earth. But for now, there's no evidence of anything in this branch on a possible collision course.
However the Northern Taurids are now considered by many to be linked to asteroid 2004 TG10. This asteroid is classified as a potentially hazardous near-Earth object. It's probably either a piece of Comet Encke or of the original comet.
There is also another branch of the debris stream. It isn't much mentioned, because it produces the Beta Taurids in late June and early July. The Beta Taurid shower is a daytime one.
Yet although we can't see it, it's more interesting than you might think. Slovak astronomer Lubor Kresák made a persuasive case for the 1908 Tunguska event having been caused by a fragment of Comet Encke from the Beta Taurids. Nonetheless the debate continues about what kind of object flattened 2,000 km2 (770 mi2) of Siberian forest.
The Taurids last for three months altogether, but you won't see many on any given night. They tend to average five an hour, and during the peaks there aren't many more than that. However they can produce magnificent colorful slow-moving fireballs that leave behind smoke trails. Very occasionally, there's one so bright that observers lose their dark adaptation.
The best time to see meteors is in the early morning, an hour or so before dawn, though that's not practical for most people. You do want a dark location with a good view of the sky. To get the best view – and to be kind to your neck – find a place to lie down. A reclining lawn chair or a camping mattress is good. Meteors appear all over the sky, so you don't have to stare at the radiant. Be sure to dress warmly. The final hint: avoid lights, except red ones, or your eyes won't adapt to the dark.
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