Meteor Showers

A named meteor shower appears just about every month. The months February, March, June, and September seem to be left out. Why is that?

Cometary paths and timing.

Most meteor showers are related to comets that have crossed paths with our Earth. As a comet’s path brings it closer to the Sun the Sun’s heat and ion wind combine to make that comet start outgassing. Its carbon dioxide ice starts to sublimate as temperatures rise above the freezing point of CO2. Carbon dioxide is unique in that it transitions from solid (frozen) to gas without a liquid transition, especially in the partial vacuum of space. This outgassing produces pressure that causes debris to fly off the comet’s surface. It doesn’t take much – comets have little gravity, so a slight nudge sends stuff flying. This debris forms a haze (coma) around the comet’s body (nucleus), obscuring it.

Solar ion wind pressure pushes at the haze of debris, sending some of it away from the comet and a trail of cometary material (tail) forms along the comet’s orbital path.  It’s mostly dust, ice, and small pebbles. When a comet comes close enough to Earth, the hazy cometary head (coma) and the trail of debris (its tail) become visible.

Comets are such a treat to observe when they are bright.

Some comets have an orbit that takes them around the Sun in a matter of a few to dozens of years. If their orbit is less than 200 years, they are called “short period” comets. More than 200 years and they are “long period” comets. Their period mostly depends on interactions with planets and the Sun.

Most comets appear to originate in the outermost parts of our solar system; the Kuiper Belt, containing larger bodies like Pluto, outside the orbit of Neptune, or the really far-out spherical haze of ice, debris and comets called the Oort Cloud.  Just about every year the inner solar system is visited by a new comet. Some will orbit around the Sun and become short period comets. Some will get shot back to the Kuiper Belt or Oort Cloud and might return in tens to hundreds of thousands of years. That’s a long period!

With all these comets spewing stuff, Earth is bound to cross into a comet’s tail, and we do. We’re always running into cometary debris but certain times of the year present higher concentrations. These are the tails of comets. Depending on what part of the tail Earth slams into, we get a so-so, good, or great meteor shower.

Tonight’s Perseid meteor shower might be great. It has the potential and has been great before. Timing and patience is key. Get that chaise lounge you used last time to check out the summer triangle. Starting at 11pm tonight, look to the northeast. Queen Cassiopeia, the misshapen W (an asterism) on its side is a guide. Perseus, from where the Perseid meteor shower appears to originate, is just below. Use the graphics from Sky & Telescope magazine I included.

Fingers crossed.