Meteor Showers

Meteor Showers[1]

Comets and meteors could hardly seem more different. A comet typically appears as a diffuse ball of glowing gas and dust, often with one or two majestic tails trailing from it. And a comet moves slowly relative to the background stars, often taking weeks or even months to make much headway as it travels through the inner Solar System.

A meteor, on the other hand, makes a laser-straight streak that rarely lasts more than a fraction of a second. The bright flash of light arises when a tiny dust particle gets incinerated high in Earth’s atmosphere.

Yet comets and the meteors we see in showers are linked intimately: The bits of dust that give rise to these meteors begin their journeys locked in the frozen nuclei of periodic comets.

Life in A Dirty Snowball

Comets date back to the origin of the Solar System. They formed in the outer part of the planetary realm, where temperatures were low enough that ice and rock mixed in roughly equal proportions.

Collisions among small particles were rampant, and sometimes these specks stuck together and thus grew larger. Repeated encounters eventually built up objects that became comet nuclei, with sizes ranging from a mile to dozens of miles across.

Many likely formed in the same region as the giant planets, and gravitational interactions with these massive bodies flung the comets into the distant Solar System. Some ended up in the Kuiper Belt, a disk-shaped assemblage of objects that lies just beyond Neptune’s orbit. (Pluto is the Kuiper Belt’s most famous member.) Others got thrown out to the Oort Cloud, a vast spherical shell roughly 1 light-year from the Sun that contains perhaps a trillion comets.

If a comet in the Kuiper Belt passes near another object, or a nearby star or interstellar cloud disturbs a comet in the Oort Cloud, it can be nudged onto a path that brings it into the inner Solar System. When the Sun warms the comet’s ices enough, they sublimate —turning directly from a solid into gas. Both the newly formed gas molecules and any small dust particles that had been trapped in the ice get released, forming the comet’s sometimes spectacular head and tail.

From Comet To Meteor

The solar wind soon blows away the gas. But radiation pressure pushes more gently on the dust particles, which tend to stay in the vicinity of the comet’s nucleus. If a comet makes just one trip into the inner Solar System, that’s the end of the story. But rarely, one will pass near a giant planet’s strong gravitational field and have its orbit altered.

The new path could shoot the comet out of the Solar System entirely or send it careening into the Sun. Or the comet could settle into a stable orbit with a period from a few years to many millennia. Each time this now periodic comet approaches the Sun, more gas and dust boil off. After hundreds of such encounters, the debris spreads out and creates a stream of dust particles that fills the comet’s orbit. A typical particle, called a “meteoroid” while still in space, is the size of a grain of sand but much less dense.

If the orbits of both comet and Earth intersect, you can count on a meteor shower. And because the orbits remain fixed in space, the shower occurs at the same time each year. The most famous meteor shower, August’s Perseids, derives from Comet 109P/Swift-Tuttle. Not to be outdone, the most famous comet—1P/Halley—intersects Earth’s orbit twice and its debris trail delivers two outstanding showers: May’s Eta Aquariids and October’s Orionids.

Earth crosses Comet 1P/Halley’s orbit twice each year, once in early May and again shortly after mid-October. Each time our planet plows through the comet’s debris, we get a meteor shower—May’s Eta Aquariids and October’s Orinoids.

All meteors in a shower appear to radiate from a point in the sky, even though the dust particles that create them follow parallel paths.

Burn, Baby, Burn

When Earth encounters one of these meteoroid streams, the show begins. The dust grains strike the atmosphere at high speeds. Our protective blanket of air starts to destroy an incoming particle at an altitude of approximately 70 miles even though the air density there is only one-millionth what it is at sea level.

As the particle digs deeper into the atmosphere, it encounters more molecules. Collisions generate friction, rapidly heating the grain and vaporizing its surface. Within a few tenths of a second, the intense heat destroys a typical particle. (Larger or slower-moving ones might last a second or two.) It rarely survives below an altitude of 50 miles.

The atoms ejected as the grain withers away excite nearby atmospheric molecules. The whole process creates a column of excited atoms and molecules some 12 to 20 miles long and only about 10 feet wide. The excited gases quickly return to their lowest energy states, however, releasing this excess energy in the form of visible light. We see this light as a meteor.

Geminid meteors appear to radiate from a point near the bright stars Castor and Pollux in the constellation Gemini, which lies near the upper right corner of this image

A meteor’s brightness depends largely on the incoming particle’s size and speed: Bigger and faster means brighter. Astronomers refer to any meteor brighter than Venus, around magnitude -4, as a “fireball.”

Bright meteors often leave behind a trail of ionized molecules (those that have lost one or more electrons). It can take several seconds to a few minutes for those gases to hook up with free electrons and return to their normal, neutral state. When they do, they release light that lingers in what astronomers call a “meteor train” or “persistent train,”

All the particles in a meteoroid stream orbit the Sun along parallel paths. Yet when the particles enter Earth’s atmosphere and create a meteor shower, the meteors appear to come from a point in the sky called the “radiant.” This is simply a perspective effect that makes parallel lines seem to converge in the distance. A meteor shower usually takes its name from the constellation that contains the radiant. The Perseids, for example, seem to emanate from Perseus the Hero. (See image above).

Even the biggest meteoroids that belong to a shower are only the size of a pea. None of them has ever survived its trip through Earth’s atmosphere and reached the surface. Such Earth-reaching stones, called “meteorites” derive from much larger meteoroids that follow random orbits around the Sun. They and their smaller brethren create so-called sporadic meteors, which can show up at any time of year.

[1] “The Origin of Meteor Showers,” Supplement to Astronomy (44, 10, October 2016), pp. 6-7

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