Monday, November 07, 2005

UFOs from Taurus!

News reports from Germany and Alaska have mentioned an abundance of "UFOs" seen in the sky in the past few weeks. Is Earth being invaded? Is it time to break out your Klingon-English dictionary in the hopes that you can save yourself with some friendly banter?

Whoa, slow down. Let's approach this thing scientifically, which means that, before invoking the Borg Cube to explain these events, we need to rule out more likely explanations. What are things that people commonly mistake for UFOs?

  • Planets: Planets, especially Venus and Mars, are often mistaken for UFOs. Right now Venus is dazzlingly bright in the west after sunset, and Mars is bright and orange in the east during the evening. When low in the sky, planets can look like they are "hovering" over the ground, and they can appear to be flashing colors due to the atmosphere. I once had a TV crew try to convince me that they had a UFO slowly landing in the Pacific on video tape. I recognized where their tape had been made, and their tape had a time stamp. A little playing around with a planetarium program showed that they had just seen Venus setting in the ocean. But, the German and Alaskan witnesses said the lights were moving in the sky, whereas the planets move much more slowly. So they didn't see planets.
  • Airplanes: Airplanes move across the sky, and when they have their landing lights on, they appear very bright. A surprising number of people don't recognize airplanes in the sky. But airplanes take several minutes to cross the sky, and the lights in Germany move across the sky in seconds.
  • Satellites: Low-earth orbiting satellites move across the sky in only a couple of minutes, and some can be very bright (the Space Station and Space Shuttle can be brighter than the brightest star in the sky). Satellites can also appear to do sudden changes in direction. This is an optical illusion, but I've seen this illusion with my own eyes many times. Satellites can also appear to flash if they are tumbling out of control or if the sun mirrors off of odd-shaped corners. However, the bright things in Germany and Alaska crossed the sky in seconds, not minutes.
  • Meteors: Meteors ("shooting stars") can be very bright (these are called "fireballs"), and they can cross the entire sky in seconds. They can also leave behind glowing trails, called "trains", which can last up to a minute or more! The descriptions of the eyewitnesses to these "UFOs" match meteors exactly. So, I think it is safe to say that space dust, and not aliens, are the cause of the ruckus.

So, why so many sightings all at once? This time of year the Earth moves through the orbit of Comet Encke. Comet Encke orbits the sun every three years, but rarely comes close to Earth. However, every time the comet comes close to the sun, little bits of dust and rock come off of it. These bits of dust and rock continue to orbit the sun, but slowly get spread over the comet's entire orbit. Every November, when the Earth passes through the comet's orbit, we sweep up some of the dust and rocks, making for a meteor shower. The meteors from Encke appear to come from the constellation Taurus, and so the meteor shower is called the "Taurids."

Normally the Taurids are a pretty weak stream, with fewer than 10 meteors per hour visible from any one spot. This year the Earth seems to be passing through a denser part of the comet's debris, so we are seeing more meteors than normal. The Taurids have a history of sometimes producing many fireballs, and this seems to be one of those years!

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