Leonids this Weekend
The annual Leonid meteor shower is predicted to have a surge around 11:45 p.m. EST Saturday November 18 with a rate of a couple meteors per minute. Meteors are the atmospheric phenomenon caused by the frictional heating of a meteoroid as it enters the atmosphere. Fragments that make it to the ground are called meteorites. Annual meteor showers occur when the Earth passes near the orbit of a comet. While the comet may be quite distant, comets are rather messy and tend to leave a trail of debris as they orbit the Sun. The debris is lost from the comet as the comet heats up when it approaches the Sun and its ices start to evaporate, sometimes forcefully enough to knock large chunks off the comet (see Hollywood’s visualization of this in Deep Impact (buy a copy, I’ll be happy to autograph it for you )). These bits of cometary debris continue on their merry way around the Sun, following a path very similar to that of the comet, but not exactly the same. Over the course of many orbits, the orbital path of the comet becomes relatively busy with debris so that whenever the Earth passes by the orbit it encounters the comet’s leftovers. Strong bursts in one of these meteor showers depend on the precise path of the Earth by the comet’s orbit as well as the recent history of the comet and its proximity to the Earth.
The Leonids are debris from the comet Tempel-Tuttle which takes 33 years to orbit the Sun on a highly eccentric orbit that takes it close enough to the Sun for it to be actively vaporizing for a relatively brief period each orbit. The last perihelion, or closest approach to the Sun (Tempel-Tuttle gets slightly closer to the Sun than the Earth’s average distance), was in 1998, so the Leonids in the few years after that were particularly spectacular. The shower is called the Leonids because the meteors appear to radiate from the constellation Leo. This is due simply to the direction in the sky that the Earth is moving around the Sun with a correction for the direction and speed that the meteoroids themselves are orbiting the Sun. Like driving through the rain, the meteoroids hit the windshield of the Earth’s atmosphere from the same direction. Unfortunately, at the predicted peak time, Leo is still below the horizon in the United States, so we may not see the full brunt. Nevertheless, if the sky is clear, it’s worth spending at least a few minutes looking up.