Hello everyone, I’m Darrell Heath with UA Little Rock’s College of Arts, Letters, and Sciences.
Welcome to The Night Sky.
It’s a bit magical being outside on a clear, dark night and looking up at all those glittering stars.
It's made all the more magical when from out of nowhere there appears a sudden streak of light
that flashes for a brief moment and then is gone.
Some folks call these transient flashes of light either “shooting” or “falling”
stars and, as kids, we all made wishes whenever we saw one.
But of course, stars don’t fall out of the sky, what we are really seeing is the result
of sand grain or pebble-sized bits of interplanetary debris slamming into the Earth’s atmosphere,
usually at altitudes of around 50 to 75 miles.
But how can something so tiny become so incredibly bright?
First off, a little bit of terminology is needed.
Out in space, we call these bits of interplanetary rubble, “meteoroids” and they can range
in size from dust grain-sized fluff to objects that are about a meter in diameter, any bigger
than that, we call it an asteroid.
They are composed of either rock, metal, or ice, or some combination thereof.
All of this stuff orbits around the Sun while cruising along at speeds of about 26 miles per second.
However, once a meteoroid gets caught within the Earth’s gravity well, it gets greatly
accelerated and by the time it enters the atmosphere, they can be travelling at speeds
ranging from 25,000 mph to 160,000 mph.
At these velocities, meteoroids have a tremendous amount of kinetic energy, which is the energy
obtained from their motion.
Once an object slams into the Earth’s atmosphere at such speeds, it’s almost as though it
has hit a brick wall and the meteoroid violently compresses a column of air out ahead of it.
The gases in the air become flash-heated to thousands of degrees for a second or two and
it is the glowing, superheated air that you and I see as a streak of light across the
night sky.
More often than not, the meteoroid is vaporized, and the glowing streak of light is now called
a “meteor”, which refers more to the phenomena of the glowing column of air than it does
to a physical object such as a meteoroid.
It’s not so much the mass of these tiny meteoroids being converted into energy that’s
responsible for these bright displays, it’s actually their high velocities imparting energy
into a thin slice of the atmosphere, making it so hot, that the air literally glows.
It’s cool to think that a tiny sand grain or pebble-sized bit of space debris has created
a brilliant meteor streak that’s usually less than 3 feet-wide yet several miles long.
The average meteor that we see is made of icy dust particles shed from the body of a
comet, these burn up long before they get to the ground but, on occasion, larger meteoroids
made of rock and/or metal, do penetrate our atmosphere and portions of them can survive
their descent.
If they hit the ground, we call them “meteorites”.
And Arkansas is no stranger to these visitors from outer space, to date, there have been
15 meteorites found within the state, 9 of them are from un-witnessed falls while another
6 were observed falls.
The most famous of these occurred a little after 4:00 AM during the early morning hours
of February 17th, 1930 in Paragould, AR.
One or more large rocks entered the Earth’s atmosphere and blazed a bright trail across
the sky over Indiana, Illinois, and Missouri before finally, under the stresses of heat
and pressure, exploded in the atmosphere.
Before the explosion was heard, the sky lit up like daylight, moments later, there was
the sound of 3 loud sonic booms, indicating that the object had broken up into at least 3 pieces.
Then, when the fragments hit the ground, there was a thunderous quake of the earth.
By this time, residents were wide awake, there are reports of livestock stampeding, and most
people thought that an airplane had crashed.
About a month later, residents found the largest piece of the rock responsible for all this chaos.
The largest surviving fragment weighed over 800 pounds and had created a hole 9 feet deep,
it took 5 men, a team of horses, and 3 hours of work to unearth it.
To this day, it is the second largest meteorite ever to be seen by witnesses to strike the
earth and then recovered, it’s now on display at UA Fayetteville.
The smaller piece was found within hours of the impact by a local farmer, the object weighed
about 80 pounds and had created a hole some 2 feet deep, it now resides at the Smithsonian.
The possible third piece has never been found.
Generally, there are more faint meteors than there are bright ones.
The fainter ones are made by smaller meteoroids while the brighter ones are made by larger material.
On occasion, you might see a meteor shooting across the sky that is brighter than the planet
Venus, the third brightest object in our night sky outside of the Sun and the Moon.
We call these exceptionally bright meteors, “fireballs”.
Rather than being the tiny bits of interplanetary fluff, fireballs are meteoroids that range
in size from a small pebble to a basketball, to something over a meter across.
Vivid colors, sonic booms, and smoke trails have all been reported by witnesses to these
spectacular meteors.
So where does all this meteoric material come from?
About 5% of the meteoroid population is asteroid rubble produced from the collision of large
space rocks.
The other 95% of meteoroid material comes from comets.
The primary difference between asteroids and comets is their compositions.
Asteroids are mostly stony and metallic while comets are mainly made of ice (including water
ice as well as frozen gases such as carbon dioxide, carbon monoxide, and methane) and
are generally low in silicates.
All of these ice and rock objects formed early on in the history of the solar system, with
asteroids forming closer to the Sun where temperatures were too high for ice to exist
while the comets formed out on the fringes of the solar system, within the deep freeze
you might say, where volatiles could remain as solid ice.
Comets usually hang around on the outskirts of the solar system while going around the
Sun in large, fairly circular orbits.
Every now and again, one of them gets kicked out of its normal orbit and finds itself hurtling
inwards, towards the Sun, in an orbit that is now shaped like a highly elongated ellipse.
When it gets about as far as 300 million miles from the Sun, the ice and frozen gases begin
to vaporize, producing long trails of debris composed of particles that range in size from
dust grains to those that are about 1 cm in diameter.
Even asteroids might behave in a similar way if they contain enough volatile material to
produce outgassing when heated by the Sun or if they get so close that the heat makes
them fall apart.
The trail of debris left behind in the wake of a comet stretches out along the orbital
path of the parent body for millions of miles.
Over time, due to interactions with the solar wind and the gravitational tugs of the Sun
and planets, the dust trail will spread out to form a meteoroid “stream”, which can
persist for thousands of years.
If the Earth’s orbit intersects or overlaps the stream at some point, then we get a meteor
shower every year whenever our planet plows through the debris.
But if you go outside on any dark, moonless night, away from human-made light pollution,
you may see a handful of meteors every hour over the course of the entire night.
We call these “sporadic meteors” and they probably represent bits of debris from long
lost comets whose centuries old meteor streams are now so widely dispersed that they can
hardly be called streams any longer.
Meteor showers however come from known comets and asteroids whose orbits and meteor streams
are fairly well defined.
To meet the definition of a meteor shower, the stream must contain enough material within
it so that, under optimal observing conditions, we would see an average hourly rate of at
least 10 meteors per hour.
I think when most people hear the words “meteor shower” they visualize a steady torrent
of meteor streaks throughout the night.
In reality, that 10 to 20 meteors per hour, is the average rate for many meteor showers.
However, some showers are capable of producing as many as 60 to a 100 or more meteors per hour.
On very rare occasions, instead of a shower, we get a storm.
The Leonid Meteor Shower, which occurs in mid-November, normally produces a rather tame
15-20 meteors per hour on a good night.
But rather than a faint meow, roughly every 33 years or so, the lion decides to roar.
The most spectacular display in recorded history occurred in November of 1833 when an estimated
200,000 meteors per hour were seen to fall like rain from the sky for about 4 hours.
The spectacle was so intense that it gave the illusion of the planet Earth suddenly
zooming into space.
Witnesses to these kinds of events have reported feeling the need to drop to the ground and
hold on for dear life.
There have been other storms from the Leonids over the years, most notably on November 18th, 1966,
when more than 140,000 meteors per hour were seen from some locations in the American west.
Historical accounts from here in Arkansas indicate that folks were seeing at least 400
meteors per hour.
Then again, in 1999, 2001, and 2002 the Leonids continued to put on spectacular displays of
over 3,000 meteors per hour.
At this point I know what you are thinking: “I want to see this!”.
Well, so do I, but predicting these kinds of things isn’t easy, but by using advanced
computer modeling of the meteor streams, the next expected Leonid Meteor Storm won’t
occur until the year 2034, when the Earth will again pass through a particularly dense
stream of material.
Meteors can be seen in any part of the sky but, during a meteor shower, if you were to
trace back their lines of motion across the sky, you would see that there is a specific
area within a given constellation from which they all appear to have originated.
This point upon the sky is known as the shower’s “radiant”,
and most meteor showers are named for the
constellation in which their radiant is located.
For example, August’s Perseid Meteor Shower has its radiant within the constellation of
Perseus while December’s Geminids has its radiant in Gemini.
In some cases, a shower gets its name from a star that appears close to the radiant point.
May’s Eta Aquariid Meteor Shower gets its name because the radiant is situated close
to the star Eta Aquarii, found within the constellation of Aquarius.
There’s no real physical link between the constellation and the meteor shower,
it’s an optical illusion.
Just as the parallel lines of a railroad track appear to converge off in the distance, so
to do all the meteor streaks appear to converge upon a point in the sky.
Remember, a meteor shower is what happens when the Earth is moving through a meteoroid
stream left behind in the wake of a passing comet.
The particles that make up the stream are all moving in the same general direction as
the parent body that produced them.
So, what the radiant is actually showing us is the pathway of the comet we have to thank
for the meteor shower we are observing.
It's easy to take the viewpoint that meteors, meteor showers, and meteorites are nothing
more than curiosities, transient events that have no real significance.
But that would be greatly underestimating their potential.
For example, we know that there is always a risk that a large asteroid or comet could
collide with the Earth, causing either localized destruction to full scale extinction level events.
But there is another thing to consider.
Asteroids and comets could, early on in the Earth’s history, have very well either delivered
some of the key ingredients needed for life to begin, or perhaps, have even brought life
itself to the young planet.
The results of a study of two meteorites that landed on Earth in 1998 was published in early
2018 by an international team of scientists at the Department of Energy’s Lawrence Berkeley
National Laboratory in the journal Science Advances.
The team discovered salt crystals in both meteorites and the crystals contained water,
amino acids, hydrocarbons, and other organic materials.
These crystals, with their pre-packaged/pre-biotic material could be viewed as potential delivery
vessels for kickstarting life on a planet where conditions are just right.
Is that what happened here on Earth billions of years ago?
Do we owe our existence to some kind of cosmic express delivery system in the form of asteroids
and comets?
We may never really know, but it’s something to think about the next time you look up in
awe and wonder at the ephemeral streak of a meteor across the night sky.