Wednesday, April 29, 2009

April Showers (or the Lyrids)

Whenever a comet makes its nearest approach to the sun, some pieces break off from its nucleus. The larger fragments take up orbits near the parent comet, but some fall behind, so that the comet’s path is eventually filled with these tiny micrometeoroids. Periodically, the earth’s orbit intersects with a cluster of such micrometeoroids, resulting in a meteor shower as the fragments burn up in our upper atmosphere.
Meteor showers associated with certain comets occur with high regularity. They are known by the constellation from which their streaks appear to radiate. The following table lists the most common and prominent showers. The shower names are genitive forms of the constellation name; for example, the Perseid shower comes from the direction of the constellation Perseus, the Lyrids from Lyra. The dates listed are those of maximum expected activity, and you can judge the intensity of the shower by the estimated hourly count. The table also lists the parent comet, when known.
You can detect meteor showers on your FM radio or even on unused VHF television frequencies. But if it’s clear outside, we suggest that you take your radio outside, and as you listen for distant radio stations to pop up, look up at the skies and watch as well. It might be hard to believe that most of those streaks of light are following meteoroids no larger than a pea. But be thankful that they are!

Meteors, Meteoroids, and Meteorites

Meteors are commonly called shooting stars, although they have nothing to do with stars at all. A meteor is a streak of light in the sky resulting from the ionization of a narrow channel in the Earth’s upper atmosphere. The heat generated by friction with air molecules ionizes a pathway behind the piece of debris.
While smaller meteoroids (often called micrometeoroids) are typically the rocky fragments left over from a broken-up comet, the meteor phenomenon is very different from a comet. A meteor sighting is a momentary event. The meteor streaks across a part of the sky. As we have seen, a comet does not streak rapidly and may, in fact, be visible for many months because of its great distance from the earth. A meteor is an atmospheric event, whereas a comet is typically many A.U. distant from the earth.
Meteor is the term for the sight of the streak of light caused by a meteoroid—which is the term for the actual rocky object that enters the atmosphere. Most meteoroids are completely burned up in our atmosphere, but a few do get through to strike the earth. Any fragments recovered are called meteorites.
While most of the meteors we see are caused by small meteoroids associated with comet fragments (about the size of a pea), larger meteoroids, more than an inch or so, are probably asteroid fragments that have strayed from their orbit in the asteroid belt. Such fragments enter the earth’s atmosphere at supersonic speeds of several miles per second and often generate sonic booms. If you see a very bright meteor—the brightness of the planet Venus or even brighter—it is one of these so-called fireballs. It is estimated that about 100 tons of meteoric material fall on the earth each day.

A-Hunting We Will Go

Visitations by major comets, such as Comet Hyakutake in 1966 and Hale-Bopp in 1997, are newsworthy events. Turn on the television or read a newspaper, and you’ll be told where to look and when. But most comets don’t make the front pages. For the latest comet news, check out the NASA comet home page at Sky and Telescope magazine also publishes comet information Of course, you don’t have to limit yourself to looking for comets whose presence or approach is already known. You can head out with your trusty telescope and hunt for new ones.
Comet hunting can be done with or without a telescope, but a good telescope greatly increases your chances of finding a new comet. Remember that telescopes catch more light than our eyes, and most comets are discovered as a tiny, wispy smudge. The coma will not appear much different from a star, but you should see a gradual, not sharply defined, tail attached to it. The tail may be a short, broad wedge or a long ion streamer.
The following tips will increase your chances of finding a comet:
  • Set up your telescope in a rural area, away from city lights. Choose a moonless night so that the skies are as dark as possible. You will be looking for a faint object.
  • According to David H. Levy, just before dawn, two days before or five days after the new moon, is an ideal time to search.
  • Comets can be seen in any part of the sky, but they are brightest when they approach within 90 degrees of the sun. You might concentrate on this part of the sky. That is, at sunset you could look from directly overhead to the western horizon.
  • Gradually and methodically sweep the sky with your telescope. Stake out perhaps 40 degrees of sky and sweep in one direction (either from east to west or west to east).
  • Remember one thing. Discovering a comet requires you to see something unusual or different in the sky. For this reason, you would do well to spend time becoming familiar with the sky, the constellations, and your telescope, so that you will be better able to recognize when something is not quite right.