On any given night, the sky is alive with light --- the Moon, planets, and the stars. If you stare at the sky long enough, you will see a "shooting star" --- a brief streak of light, blazing a trail across the sky. In reality, these bursts of cosmic brilliance are not stars at all -- they are meteors. Meteors are the fiery death knell of small bits of detritus burning up in the atmosphere of the Earth.
When the solar system first formed, it was a vast morass of gas and small bits of rocky dust. Most of that gas collapsed to form the Sun, and some went into the planets as well. Over time most of the left over bits of dust were captured, pulled inexorably to the planets and moons of the solar system by 4.5 billion years of gravitational tugging. But just like vacuuming your house, not everything gets sucked up! There are always little bits left here and there, drifting in the cosmic living room of the Sun and its planets. Every now and then the Earth runs headlong into one of these left-over bits, and it burns up in our atmosphere. Why do they burn up? Because the Earth is traveling along in its orbit at 67,000 miles per hour! All told, astrophysicists estimate that the Earth collects roughly 110 tons per day of meteoritic material.
If you wait long enough on any night of the year, you'll see a meteor or two in the sky, but on certain nights each you, you can see MANY more! The Earth experiences annual "meteor showers" on the same days every year, when tens to hundreds of meteors can be seen in an hour. What causes a meteor shower? When the solar system was forming the Sun and planets, the outermost reaches were also forming new bodies, called comets. Much smaller than a planet or a moon, a comet is made up of small amounts of rocky rubble and dust, covered by large deposits of ices -- water ice, dry ice (carbon dioxide), and methane and ammonia ices.
Every now and then, a comet falls in from the dark wilderness of the solar system toward the Sun. As it approaches the inner solar system, where we live, the Sun heats the comet up, and the comet begins to boil off and evaporate large quantities of ice and dust. From Earth, we see this as its tail. All told, a comet's tail can reach lengths of hundreds of millions of miles --- larger than the spacing between the worlds. All of that junk dropped off in a comet tail remains in orbit around the Sun --- small bits of gas and dust, waiting to be swept up by a passing planet. If the Earth happens to pass through an area once covered by a comet's tail, we experience a meteor shower (here is a long list of them).
The strongest meteor showers are those where a comet periodically revisits the Sun and "refreshes" the debris stream left by its tail. The most well known are the Leonids in November, and the Perseids in the second week of August. The Perseid meteor shower is parented by the comet Swift-Tuttle, which revisits the Sun once very 133 years; it last visited the Sun in 1992, and will next return in 2126.
If you'd like to watch a meteor shower, here are some good tips:
- Find a comfortable chair; a reclining lawn chair is excellent.
- Use your naked eye to see meteors; binoculars are good to have for seeing other objects in the sky, but meteors move too fast to watch this way.
- Dress warmly, even in summer! You can get pretty cold sitting still staring at the sky!
- Conventional wisdom is to face the direction the shower comes from. For the Perseids, this is to the NE for Northern Hemisphere observers.
- The shower peaks when the part of the Earth under your feet is moving toward the meteor stream (the constellation Perseus in this case). This happens in the early morning hours, just after midnight.
- Enjoy seeing the rest of the night sky while waiting to see meteors!
Don't miss these additional viewing tips from NASA.
This year (2014) the Perseids are expected to be most numerous on the night of August 12 and morning of August 13. Viewing will be hampered by the almost full Moon, but you should still see some bright ones! Happy meteor watching!
Join us at Cantigny Park on Tuesday, August 12 for our Perseid Meteor Star Party. This annual event features fun activities for the whole family, including live music, telescope viewing and stargazing, hands-on activities, and lectures by Adler astronomers. Purchase tickets today!
Written by Shane Larson, research associate professor, CIERA at Northwestern University, and Adler astronomer.