The greater the speed of the incoming object, the hotter its surface becomes and the faster it burns up. Such large meteors are sometimes known as fireballs.
When these large meteoroids enter Earth's atmosphere with a typical velocity of nearly 20 km/s, they produce energetic shock waves, or "sonic booms," as well as a bright sky streaks and dusty trails of discarded debris. Generally regarded as small bodies that have strayed from the asteroid belt, possibly as the result of asteroid collisions, these objects have produced most of the cratering on the surfaces of the Moon, Mercury, Venus, Mars, and some of the moons of the jovian planets. Larger meteoroidsmore than a few centimeters in diameterare usually not associated with swarms of cometary debris. For example, the Perseid shower shares the same orbit as comet 1862III, the third comet discovered in the year 1862 (also known as comet SwiftTuttle). This is how certain meteoroid swarms have come to be identified with well-known comet orbits.
**Phaeton is actually an asteroid and shows no signs of cometary activity, but its orbit matches the meteoroid paths very well.Īstronomers can use the speed and direction of a meteor's flight to compute its interplanetary trajectory. *Every 33 years, as Earth passes through the densest region of this meteoroid swarm, we see intense showers that can reach 1000 meteors per minute for brief periods of time. It can last for several days but reaches maximum every year on the morning of August 12, when upward of 50 meteors per hour can be observed. For example, the Perseid shower is seen to emanate from the constellation Perseus. Meteor showers are usually named for their radiant, the constellation from whose direction they appear to come. Table 14.1 lists some prominent meteor showers, the dates they are visible from Earth, and the comet from which they are thought to originate. Intersection occurs at the same time each year (see Figure 14.16), so the appearance of certain meteor showers is a regular and (fairly) predictable event. Earth's motion takes it across a given comet's orbit at most twice a year (depending on the precise orbit of each body). If Earth's orbit happens to intersect the orbit of such a young cluster of meteoroids, a spectacular meteor shower can result. Over the course of time, the swarm gradually disperses along the orbit, and eventually the micrometeoroids, as these small meteoroids are known, become more or less smoothly spread all the way around the parent comet's orbit. The fragments initially travel in a tightly knit group of dust or pebble-sized objects, called a meteoroid swarm, moving in nearly the same orbit as the parent comet. Each time a comet passes near the Sun, some cometary fragments dislodge from the main body. Smaller meteoroids are mainly the rocky remains of broken-up comets. Any piece of interplanetary debris that survives its fiery passage through our atmosphere and finds its way to the ground is called a meteorite. (b) An auroral display provides the background for a brighter meteor trail.īefore encountering the atmosphere, the piece of debris causing a meteor was almost certainly a meteoroid, simply because these small interplanetary fragments are far more common than either asteroids or comets. (a) A small meteor photographed against a backdrop of stars. Both are chunks of rocky interplanetary debris meteoroids are conventionally taken to be less than 100 m in diameter.įigure 14.15 A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heating the air to incandescence.
Recall from Section 6.5 that the distinction between an asteroid and a meteoroid is simply a matter of size. A meteor is a fleeting event in Earth's atmosphere, whereas a comet tail exists in deep space and can be visible in the sky for weeks or even months. Note that the brief flash that is a meteor is in no way similar to the broad, steady swath of light associated with a comet's tail. This friction heats and excites the air molecules, which then emit light as they return to their ground states, producing the characteristic bright streak shown in Figure 14.15. A meteor is a sudden streak of light in the night sky caused by friction between air molecules in Earth's atmosphere and an incoming piece of interplanetary matteran asteroid, comet, or meteoroid. On a clear night it is possible to see a few meteors"shooting stars" every hour.