Our solar system is littered with trillions of small pieces of ice and rock. The rocky objects are called meteoroids. The icy objects are called comets. Observing meteors and comets is one of the most pleasant aspects of stargazing. Often, you don’t need a telescope or binoculars to see these leftover pieces of the early solar system.
Meteors and Meteor Showers
When the Earth runs into a meteoroid, it falls through the atmosphere where it grows hot and burns up in a few seconds, igniting a sudden trail of light across the sky. Such burning bits of rock and ice are called meteors. Look up on any night and you might see 3 or 4 meteors each hour shoot randomly across the sky.
Most meteors burn up before they hit the ground. The fainter meteors are the size of sand grains. Brighter ones range from the size of a pea to a golf ball, and very bright meteors might be the size of a softball. Very rarely, large meteors, perhaps the size of a basketball or a little larger, and especially if they are made of iron and nickel, will burn through the atmosphere and hit the ground, or explode before they hit the ground. When it hits the ground, it’s called a meteorite.
Sometimes, on fixed dates throughout the year, the Earth passes through the path of a comet which has left behind small clouds of icy pellet-sized debris as it orbits the Sun. When this happens we’re treated to a meteor shower during which you may see dozens or even hundreds of meteors each hour. Perhaps the finest meteor shower each year occurs around August 12 when the Earth passes through the path of Comet Swift-Tuttle. This shower is called the Perseids, since the trails of meteors all trace their paths back to a point in the constellation Perseus (see image above).
There are dozens of conspicuous meteor showers during the year. Some of the best are listed below along with dates when the most meteors are visible. The name of each shower refers to the constellation back to which the meteors trace their apparent paths. You don’t need to see each constellation to see the meteors. They can appear anywhere in the sky.
- Lyrids, April 21-22
- Perseids, August 11-12
- Orionids, October 21-22
- Leonids, November 16-17
- Geminids, December 13-14
Watching a meteor shower is one of life’s under-appreciated pleasures. You don’t need binoculars or a telescope. You just need to lay back and look up at the sky. Depending on the shower and on the time of night, you might see a meteor every 5 or 10 minutes. Or you might see one or more meteors every minute. You will see more in dark sky away from the light-pollution of the city, and look in the part of the sky away from a bright Moon if it’s out. Because the Earth usually turns into streams of meteors after midnight, the most meteor showers show more action between midnight and dawn.
Comets: Visitors from the Outer Solar System
Comets are small, dark, icy bodies that are likely remnants of the formation of the solar system. Most comets lie far from the Sun, frozen and dark in a halo of trillions of comets far past the orbit of Neptune. Sometimes a comet is sent plunging towards the inner solar system by a little gravitational push from another comet or a passing dust cloud. As the comet nears the Sun, it heats up and forms a halo of gaseous material called a coma, and in some cases, a tail which is pushed away from the Sun by the solar wind, a stream of charged particles ejected from the Sun. The central massive part of a comet is called the nucleus. It’s usually just a few kilometers across. It consists of a mixture of icy material such as ammonia, water, carbon dioxide, and methane, along with traces of rock, dust, and carbon-based debris left over from the formation of the solar system. Comets are often described, more or less accurately, as dirty snowballs.
As seen from Earth, comets are far less common than meteors. There might be a half-dozen comets visible each year, though they are usually faint enough to require a telescope to see well. Every 10 years, on average, a comet grows bright enough to become easily visible with the naked eye. Comet Hale-Bopp in 1997, and Comet McNaught in 2007 were examples.