Meteors provide both excitement, disappointment


By Tom Burns - Stargazing



Some of us feel as comfortable under a starry sky as we do in the comfort of our beds at home. The sky at night provides emotional and even spiritual comfort. The stars seem changeless — their positions fixed in constellations, their seasonal courses unaltered during a human lifetime or 100 human lifetimes.

And so you stand silently, almost reverently, with a few other true believers and stare at the thousand or so stars visible from your dark, dark rural hideaway.

There, bright Orion and his dogs finally set in the west. There, the Maiden and the Herdsman rise in the east. There to the south, prowls the noble and ferocious Leo, the Li …

Wait! A streak of fire lights up the night in Leo. You gasp. You point. You shout inarticulately, “There! South! Leo! Meteor!”

But by the time your companions turn their heads, the unexpected streak of fire is gone. You sigh, and they sigh as well, but they are not discouraged. They know that soon enough, the shoe will be on the other foot. One by one, they will yell, and you will be the one who turns your head with excitement and then disappointment.

On any clear, moonless night, dedicated meteor observers will see as many as five such streaks every hour if they take the opportunity to go out and look.

The experience of your thousandth meteor is nearly as startling as your first. The eternal, unchanging heavens have been split open by a rapier of flame that thrusts out of the darkness, pierces your heart, and then is gone in half a heartbeat.

We now know that the streak you saw was caused by the “fall” of a rock from space. The rock is properly called a meteoroid. The streak of light is called a meteor. In the unlikely event that it survives its fiery plunge, the rock is dubbed a meteorite.

As Earth orbits the sun, it zips along at a stunning 67,000 miles per hour. A particle no bigger than a speck of dust, a grain of sand, or a pebble may have been floating in space for billions of years, but now it’s a goner.

Its high-speed collision with Earth’s atmosphere ignites the air around it and causes the air to glow. The streak you see is not the particle at all. You see the glowing air around it.

Perhaps a trillion meteoroids slam into Earth’s atmosphere every day. Most of them are dust-sized. They produce streaks so faint that they are invisible to observers on the Earth’s surface.

Pebble-sized meteoroids are visible over an area that spans only a few tens of miles on the planet. Rarely, observers see boulder-sized meteoroids large enough to be visible over hundreds or even thousands of miles.

As obvious as it seems now, for most of human history, our forebears would have scoffed at the notion that meteors were caused by rocks falling from space.

Aristotle (384-322 BCE), the great compiler of ancient ideas, believed that the streaks were purely atmospheric phenomena. In fact, the Greek word meteōros means “high in the air.” He believed that both slower-moving comets and speedy meteors resulted from “exhalations” of hot, dry gasses from the Earth. As they rose, they ignited in the upper atmosphere. When the gasses ignited, they generated high winds.

Even in those days, people noticed that rocks seemed to fall from the sky. When he considered an apparent meteorite fall at Aegospotami in 467 BCE, Aristotle argued that it was an Earth rock that had been lifted into the sky by extremely high winds generated by a comet.

In that distinctly unscientific way, the science of meteorology was born. In his Meteorologica, Aristotle argued that meteors must be made of fire because “their appearance in any number is a sign of coming wind and drought.”

As Earth proceeds in its orbit, it sometimes passes through the trail of debris left by a passing comet. We moderns call the resulting cascade of meteors a meteor shower.

To the ancients, such events were miraculous and mysterious. If a single meteor could cause high winds, then the many streaks of light seen during a meteor shower must portend even more terrible weather.

After Aristotle, Aratus (Around 315 – 245 BCE) suggested that when many meteors seemed to come from the same direction, we should “expect a high wind coming from the same path.” When they came from many directions, you should “be on thy guard for winds from every quarter.”

The Romans took up Aristotle’s meteorological principles with enthusiasm. Pliny the Elder (23 – 79 CE) believed that when meteor-shower meteors came from a single direction, they portended “a hurricane from the same quarter.” Echoing Aristotle, Pliny believed that the hurricane could pick up many rocks and return them to Earth, a phenomenon we now call a meteorite shower.

Pliny was wrong, of course. Sometimes a larger meteoroid shatters in the upper atmosphere and sends a rain of smaller meteorites down to Earth. Hurricanes have nothing to do with it.

However, two millennia passed before anyone had the audacity to question the wisdom of Aristotle. The notion that rocks could fall from space was considered ridiculous, even after astronomers clearly understood that Earth was traveling through space as it orbited the sun.

Even Sir Isaac Newton, perhaps the greatest of all western thinkers, rejected the notion that rocks could fall from space. In 1718, he wrote that we must “empty the Heavens of Matter” so that the comets and planets could proceed unimpeded in their regular orbits.

More than half a century later, the budding science of geology began to tell the true tale of meteorites.

In the early 1790s, physicist Ernst Chladni examined an iron rock found in 1722. Weighing over 1,500 pounds, the rock did not resemble in any way the rocks that littered the nearby terrain.

Chladni concluded that high winds could not have hurled a 1,500-pound rock very far. Besides, the strange rock was composed mostly of iron. Clearly, that iron had melted. No forest fire or lightning strike could produce such a lump of almost-pure, melted iron.

In 1794, Chladni concluded that rocks such as the 1722 find could have only come from space. They must be moving at extremely high speeds through the atmosphere, which would generate friction with the air.

That friction could generate temperatures high enough to melt iron. He speculated that such meteorites might be fragments of bodies that never merged to make planets.

He was substantially correct in those assertions. However, he was roundly ridiculed for his seemingly wild speculations.

The final proof of extraterrestrial origin came with the Leonid meteor shower of 1833. People in the eastern United States saw thousands of meteors in a single predawn hour. They all seemed to radiate from a single point in the constellation Leo, the Lion, even as the Lion moved across the sky. To astronomer Denison Olmsted, the implication was clear. The particles that caused the many streaks must be coming from space.

Astronomers began to study the historical record. They discovered that the Leonids repeated every year but were spectacular every 33 or 34 years. A cloud of debris must be hovering at a specific point along Earth’s orbit. Vagaries in that orbit meant that every 33 or 34 years, Earth passed through the thickest part of the cloud.

And in 1866, the Leonids repeated their previous glory.

One piece of the puzzle had yet to be put in place — the exact origin of the debris cloud. Giovanni Schiaparelli discovered that Earth passed through the orbit of a comet called Tempel-Tuttle. As it orbits close to the sun, the comet heats up, and some of its ice turns directly to a gas, which dissipates into space.

Rocky dust and debris are released from the ice and leave a long trail along the comet’s orbital path. The debris particles don’t really fall to Earth. Earth plows into them as our planet orbits rapidly around the sun.

The meteors we see on any given moonless night are not caused by the rocky remains of a passing comet. They are Chladni’s leftovers. Many have floated serenely in space for billions of years before their encounter with Earth.

Any clear, moonless night is a good night to look for meteors.

If you have never seen one, I envy you. You will never forget your first experience.

Chances are, your first meteor will be caused by a tiny pebble, not a boulder. The streak of light you see will be faint. It will be visible for only an instant. It will be visible for only a few tens of miles around where you are standing.

At that precise moment, you may be the only one looking up. That meteor will be just for you.

I close my eyes and remember my first meteor. It disappeared in a heartbeat. But six decades later, my heart still embraces its unexpected glory. There it will remain for all the rest of my days.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.