It's Meteor Season

August is often called, "Meteor Month," because some of the best meteor showers of the year occur in August. But why should that be? What is it about the earth's orbit that causes a flood of meteors on an annual basis? And how can they be so predictable?

Ignoring a lot of small factors, it can reasonably be said that each year the earth returns to the same point in space relative to the sun. As the earth moves around the sun, it does not move through "empty space." We may think of space as empty, but interplanetary space is strewn with gas, dust, sand, and even rocks – some of them big enough to blow our planet apart. (Fortunately, we believe we've found all of the really big "asteroids," and they're being carefully tracked. The search continues for ever smaller ones that could threaten us.)

Most of the original dust, sand, and rocks of the early solar system have been swept up by the planets over the last four billion years, leaving pretty clear sailing for Spaceship Earth. However, dust, sand, and rocks are constantly being added to the mix by asteroids crashing into each other. These result in the random meteors we can see on any given night.

Another important source of meteors, and the cause of periodic meteor showers, is comets. Comets are essentially huge dirty snowballs that orbit the sun. As comets pass the sun, their "snow" evaporates – violently – spewing tails of gas and dust behind them. Most make a single pass and escape back into deep space. Some, however, may pass too close to a planet and find themselves trapped in a "periodic orbit" that has them revisiting the sun time and again. With each pass they spew more and more of their ices into space. After a number of passes, the ices are gone, leaving the dirt and rocks free to float their separate ways.

These new "meteoroids" do not, however, stay tightly packed very long. Free to pursue their own paths, continual collisions with other dust and rocks cause minor changes in their orbits. Some particles move closer to the sun, and some a bit further away. Those that move closer slow down, and those that move farther speed up. After a surprisingly short time, the dust and rock of the comet spread out along the entire length of the comet's original orbit. If the earth intersects that orbit and passes through the comet's dust, we have a meteor shower.

It would take forever for the material in the comet's orbit to spread itself out completely evenly. Naturally, there are sparse areas and dense areas. When the earth passes through a sparse area, we see a disappointing number of meteors. But if we pass through a very dense area, we can have a "meteor outburst" or even a "meteor storm."

Normally, the number of meteors seen during a meteor shower is measured in meteors per hour. During a meteor storm they can be measured in meteors per second! I recall only one meteor storm in my lifetime, and I missed it due to clouds. I'm still bummed about that.

Most meteors are no bigger than a grain of sand, most far smaller. When a real rock enters the atmosphere, we get a "fireball" -- an extraordinarily bright (and often slow-moving) meteor. Sometimes they explode; often they enter the upper atmosphere at an oblique angle and exit the atmosphere to continue their trip through the Solar System, although rather warm and much smaller.

Every so often a large meteor plunges straight into the atmosphere and survives the fiery trip to (crash) land as a meteorite. We've actually identified pieces of rock that have traveled from Mars and the asteroid Vesta, which was recently visited by NASA's Dawn spacecraft.

Meteor showers are named for the point in the sky from which the meteors seem to be originating. If you were to map several meteors seen during a meteor shower, you would see that they seem to come from a common point (more or less). This is very much like the appearance of rain coming at you in a fast-moving car. This common point lies within a certain constellation, for which the shower is named.

The grand daddy of all meteor showers is the Perseids, which radiates from the constellation Perseus (PURR-see-us), the Greek mythic hero (which can be seen rising in the NE after midnight). As usual, the Perseid Meteor Shower peaks this year the night of August 12 and 13. Luckily, that's a Friday night/Saturday morning.

Because of some interesting interactions with Jupiter, there is reason to believe that this year's Perseid Shower may be far more exciting than in recent years. It impresses me that the professional astronomers have figured out that the Perseids are debris from comet 109P/Swift-Tuttle. It mightily impresses me that they know how Jupiter impacts the number of meteors we're likely to see this year. The August issue of Sky & Telescope magazine has a fine article about this understanding.

Another important – and inconvenient – point about meteor showers is that they are best viewed in the pre-dawn hours. This is because the highest part of the local sky is then headed directly into the path of the earth's orbit, increasing the number of collisions, and the collision speed, of the meteors, which is directly related to their brightness.

We will have to contend with a bright waning gibbous moon this year. However, it will set at 1:50 a.m., leaving those pre-dawn hours nice and dark.

So, set that alarm clock, and have those lounge chairs and blankets ready!

Almanac

At evening twilight, Mercury, Venus, and Jupiter will be dancing about each other low in the west this month, while Mars and Saturn dance around Scorpius in the south. As August progresses, watch Mars shoot east between Saturn and Antares.

Meteor season officially begins with the Southern Delta Aquariids, which peaks on the morning of July 28. It is one of several weak, but long lasting showers that precede the famous Perseids of mid-August.

Sunrise/Sunset: 6:03 a.m./7:57 p.m. (August 1st)