Lion's GROWL

This picture shows a bright Leonid meteor that was seen in November 1998. The light from this meteor lit up some clouds, creating this beautiful photo opportunity!  Click on image for full size (70 Kb) Image courtesy of Lorenzo Lovato of Imola, Italy.

The Leonid meteor shower is one of several major meteor showers that occur on roughly the same date each year. The Leonids typically "peak" (are at their greatest level of activity) in mid to late November. The Leonid shower's name is derived from the fact that its meteors appear to fan out from a point in sky, called the shower's "radiant", which lies within the constellation Leo.

In 2009, the Leonid shower is expected to peak on the night of Tuesday, November 17th. Since there will be a New Moon around this time, this year should be a good year to spot some Leonids. A bright Moon makes it hard to see meteors.

As is the case with most meteor showers, it will be possible to see Leonid meteors for several days before and after the peak on the 17th. In fact, it is sometimes possible to see some Leonids as early as November 14th and to continue to spot a few up to November 21st. At this shower's peak observers may be able to see as many as 100 "shooting stars" per hour under dark skies (away from city lights). However, the Leonids, as compared to other meteor showers, present a highly variable celestial show. The number of meteors visible at the shower's peak can change quite a bit from year to year in an unpredictable fashion.

The Leonids are actually bits of dust that have been shed over the centuries by a comet named comet 55P/Tempel-Tuttle! The dust, which spews forth from the comet's nucleus each time it passes near the Sun, gradually spreads out over the entire orbit of the comet. If Earth's orbit happens to cross the comet's orbit, the swarm of debris scattered along the comet's orbit is visible to us as a meteor shower. Since the Earth crosses the comet's orbit at the same time each year, each meteor shower is predictably visible at the same time of year, year after year.

Most meteors in a shower are quite small, about the size of a grain of sand. Leonids are among the fastest-moving meteors. These meteors typically strike our atmosphere while traveling at speeds around 72 kilometers per second (about 161,000 mph). Because of their high speeds, Leonids typically leave long, glowing trails in the sky.

This picture shows several meteors during a meteor shower. These meteors were seen in 1995, and are part of a lesser-known meteor shower called the Alpha-Monocerotid meteor shower.  Click on image for full size (34 Kb) Image courtesy S. Molau and P. Jenniskens, NASA Ames Research Center.

A meteor shower is an astronomical event during which many meteors can be seen in a short period of time. Most meteor showers have a peak activity period that lasts between several hours and a couple of days. During this peak period, observers may spot between about 10 to more than 100 meteors per hour in the case of the major meteor showers. On rare occasions, during an extraordinarily intense meteor shower, observers might see as many as thousands of meteors per hour. There are about eight major meteor showers each year, and many more minor showers that produce smaller numbers of meteors.

The meteors from each meteor shower seem to fan outward from a specific spot in the sky. This spot in the sky is called the shower's "radiant". The name of each shower is derived from the location of its radiant. For example, the radiant of the Orionid meteor shower is in the constellation Orion, while the radiant of the Geminid meteor shower is in the constellation Gemini.

Astronomers have long known that each meteor shower occurs around the same date each year, year after year. Why is this so? The many small meteoroid particles that produce meteor showers are actually the cast-off debris from comets! As comets occasionally swing close to the Sun in certain parts of their orbits, the icy materials that make up the bulk of the comets are heated and melted. They escape into space, trailing behind the comets as the spectacular tails that we sometimes see from Earth. Although comets are mostly made of ice, they also contain bits of dust and small rocks embedded within the ice. As the ice melts, the rocks and dust also are freed and float off into space. Gradually, this cloud of debris drifts away from the comet itself, but continues to orbit the Sun along roughly the same path as the comet. Over many years, the debris spreads out along the orbit of the comet. When Earth, traveling along its orbit, crosses a comet's orbit (though not necessarily anywhere near the comet itself), the cloud of debris slams into our atmosphere at high speeds, producing a meteor shower! For example, the Orionid meteor shower, which happens each October, is generated by Earth's passage through the debris stream strewn along the orbit of the most famous comet of all - Halley's Comet!

Most meteoroid particles that generate the meteors in a shower are quite small, ranging in size from a grain of sand to a pea-sized pebble. However, the particles are traveling extremely fast relative to Earth when they streak into our atmosphere. Typical speeds range between about 20 and 70 km per second (45 thousand to 157 thousand miles per hour)! The incredible pressure meteoroids experience when they collide with Earth's atmosphere shatters them, transferring energy to atoms and molecules in the atmosphere, which then release the energy by glowing. This glow produces the bright trails of light in the sky we see as meteors.

Although the peak activity period for each meteor shower lasts a short time, it is often possible to see at least a few meteors associated with a given shower for several days before and after the peak. How should you look for meteors when there is a shower? Find a comfortable place to sit or lie down outside; lounge chairs that let you recline for viewing the sky are ideal. You don't need to look directly at a shower's radiant to see meteors; though the meteors will seem to fan out from the radiant, they will be visible all over the sky. It is best to be sure that the constellation in which the shower's radiant lies is in the sky; take a look at our sky maps to help you with this. Since many meteors are faint, it is best to get away from lights if you want to see many meteors. Some years a shower's peak happens near the time of a full moon; those are not the best years to view that shower. The best time for viewing meteors is between midnight and dawn, but meteors can be seen at any time of night if you don't want to stay up that late!

This bright meteor, seen lighting up some clouds, was part of the Leonid Meteor Shower in November 1998.  Click on image for full size (70 Kb) Courtesy of Lorenzo Lovato of Imola, Italy

Meteors are streaks of light, usually lasting just a few seconds, which people occasionally see in the night sky. They are sometimes called "shooting stars" or "falling stars", though they are not stars at all. Meteors are caused by the entry of small pieces of rock, dust, or metal from space into the atmosphere at extremely high speeds. These particles, called "meteoroids" when they are floating around in space (think of very small asteroids), are traveling at incredible speeds of tens of kilometers per second (tens of thousands of miles per hour) when they streak into the atmosphere. The incredible pressure meteoroids experience when they collide with Earth's atmosphere shatters them, transferring energy to atoms and molecules in the atmosphere, which then release the energy by glowing. This glow produces the bright trails of light in the sky we see as meteors.

Most meteoroid particles are quite small, ranging in size from a grain of sand to a pea-sized pebble. Almost all of them disintegrate in the atmosphere long before reaching the ground. Very rarely, a larger meteoroid actually survives to strike the ground, creating a meteor crater in a huge explosion. This explosion often vaporizes whatever solid material is left of the meteoroid after its fiery flight through the atmosphere. Sometimes, however, pieces of the meteoroid survive and are found in the crater or nearby. These chunks of rock or metal are called meteorites.

Meteors are not the same thing as comets. Meteors appear briefly as they streak through the sky. Comets are much larger objects that are actually still out in space. Comets can form tails, and though they do change position from night to night, they don't move fast enough for the eye to notice; they seem to hang in place in the sky. There is a connection, though, between some comets and some meteors. Several times each year Earth passes across the orbit of a comet, where dust and small bits of rock from the comet have been left behind. When this happens we can see many meteors in a single night; sometimes as many as 100 or more per hour! These events are called meteor showers.

Especially bright meteors are called fireballs. Some fireballs are so bright that they can be seen in the daytime. It would be possible to see meteors above any planet that has an atmosphere. A camera on the Mars Exploration Rover Spirit captured a picture of meteor in the sky above Mars in 2004!

How can you remember whether something is a meteor, a meteoroid, or a meteorite? Here's how I do it! When they are out in space, like asteroids, they are called meteoroids. When they are streaking through the atmosphere as bright flashes of light, we call them meteors - which reminds me of meteorology, which is the science concerned with weather and the atmosphere. [Meteorology is notthe science of meteors!] When they reach the ground, we call them meteorites - which reminds me of the stalactites and stalagmitesthat are found under the ground in caves. I hope that helps you remember too!