What would happen if a comet hit the Earth?
Well, there would be one heck of a big explosion! All the gas and other comet materials would quickly be heated to incredible temperatures and that would produce a huge, fast-moving pressure wave that we think of as an explosion.
On June 30th 1908 at 7:17AM (local time) just such an explosion occurred above Tunguska, Siberia. The explosion was so powerful that people 60 kilometers away were knocked to the ground. Around the world barometers (instruments used to measure air pressure) detected the shock wave as it traveled several times around the Earth. The heat from the explosion set the forest on fire.
This event occurred in a sparsely populated area and it appears that no one was killed. It wasn't until the 1930s that scientists finally reached the site. No crater and no fragments of meteorite were found. What they did find was that all the trees had been burned (no surprise) and these trees were knocked over in a pattern that you would expect if a bomb had exploded above them. Based upon the extent of the damage, scientists estimate the explosion at about 40 megatons (a thousand "Hiroshimas").
All the evidence is consistent with the idea that a comet caused the explosion over Tunguska. A comet would break up easily in the atmosphere, its water and gases would quickly dissipate and the explosion would have distributed the dust and small rocks as a gentle "rain".
Tunguska was "hit" by a comet with a pretty small nucleus. If the Earth were to collide with the nucleus of even a fairly large comet the impact explosion and its aftereffects could wipe out most life on Earth. (But it would make for an exciting movie! )
What would happen if the Earth passed through a comet's tail?
If we passed through the wispy tail of a comet we would have spectacular meteor showers.
Tiny particles of dust and rock, the leftover comet debris (which
is itself leftovers from the formation of the Solar System) is constantly falling to Earth but most of them go unseen because
the material is so small. The larger ones, however, produce "shooting
stars".
While in space these bodies are called meteoroids but they can enter the Earth's atmosphere
at high speed (72 kilometers per second) becoming meteors. Meteors produce a great deal of heat by friction with the air. This heat is so intense that it ionizes the atoms and
molecules in the air around the meteor causing the air to glow
by incandescent light. [This "incandescent light" is the
same "hot light" produced in an incandescent light bulb.]
Note: it is a common misconception that the glow is the glowing
of the rock. It is not! The glow of a shooting star is the glow
of the air molecules, made incandescent by the heat of friction
caused by the falling meteor. However, small bits of the meteor flake off during its fall and these are heated to such high temperatures that they, too, give off a little glow. But that extra glow (caused by the bits of meteor) is insignificant compared to the over all light from a meteor (caused by the air). However (again!) the spectrum of these meteor bits can be observed, collected and "subtracted" (using computers) from the spectrum caused by the incandescent air . The remaining spectrum can be used to reveal the composition of the meteor without ever actually collecting it! (You learned all about spectroscopy in an earlier lesson.)
A meteoroid becomes a meteor when it's heated to incandescence by the atmosphere - about 115 kilometers above the Earth's surface for an average meteoroid. That average meteoroid, now an average meteor, will usually burn out at an altitude of about 70 kilometer as it breaks up into tiny particles called micrometeors. Most of these are simply vaporized into a fine dust but a few survive all the way to the Earth's surface. By definition, micrometeorites are less than one millimeter in diameter. About 100,000 tons of material fall on the Earth each year and most of that arrives as micrometeors. Their small size allows micrometeors to quickly radiate the heat generated by friction so they do not undergo incandescence, so they produce little if any light because they produce so little heat.
I'm not sure about these definitions.
It's easy to become confused. Micro just refers to the small size.
Space contains meteoroids, our atmosphere contains meteors and the remains of a meteor that survives to the Earth's surface is called a meteorite. [Read that sentence again to be sure you understand the definitions.]
On an average night you should be able to see about ten naked-eye meteors per hour. However, if the Earth passes through a comet's tail or the debris ring left by a long dead comet, the number of meteors seen is greatly increased.
The chance of seeing a meteor is also increased after midnight.
This image shows a view of Earth as seen above its North Pole. The Earth moves counterclockwise in its orbit (from left to right in this image) and also rotates counterclockwise. Notice that sunrise is to the right, in the same direction as the Earth's orbit but sunset is on the other side of the Earth. Before midnight the observer will be on the side of the Earth opposite its path and the only meteors seen will be those that have spiraled into the Earth's gravity well and have been swept in from behind the Earth's orbit.
After local midnight the Earth has rotated and it presents
its sky to the forward facing part of Earth's orbit so meteoroids
strike it directly. If you understand why bugs get splattered
on the a car's front instead of its back, you'll understand why
more meteoroids hit the "after midnight" side of the Earth
than the "before midnight" side of the Earth.
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As the Earth enters a field of meteoroids they will seem to enter the Earth's atmosphere from a certain part of the sky called the radiant. This is really a matter of perspective because the meteors are not really coming from a single point in space. Instead the Earth intercepts the meteors in a series of parallel paths that give the effect as having come from a single point in the sky. This point is like the one you see when looking at the parallel lines of a road's edge or a railway's rails as they appear to meet in the distance.
The Zenith Hourly Rate (ZHR) is the number of naked-eye meteors seen under perfect conditions (no clouds or moonlight) with the radiant overhead (at the zenith). This value is really a hypothetical number because it is unusual for the radiant to actually be directly overhead. [Or, for that matter, to have perfect conditions!] The number of meteors actually seen per hour is less than the ZHR of a meteor shower so the ZHR should be used as a value for comparison from one shower to another.
Although there are occasional sporadic meteor showers, most are very predictable because they occur when the path of the Earth crosses the orbit of a comet.
You should keep clear in your mind that meteoroids are the cometary
debris in space but meteors are cometary debris in our atmosphere and meteorites are cometary debris on Earth.
Speaking of comets, now is a good time to introduce another word to your vocabulary - asteroid.
What's an asteroid?
An asteroid is a big meteoroid. Astronomers like to use the word asteroid for an object that is too small to be a planet but too big to be ignored! Also, asteroids tend to orbit the Sun near the ecliptic and in fairly circular orbits, so an asteroid's orbit is not like that of most comets.
We tend to think of asteroids as being made of a more substantial material than comets. Indeed, many asteroids are not "dirty snowballs" - they are gigantic bullets! This isn't a great definition and it's fair to argue that the nucleus of a comet could be mistaken for an asteroid especially when far from the Sun and in an orbit that a planet could have.
However, taken together all these properties of an asteroid make them more like a tiny planet than like a comet. Indeed, asteroids are often called minor planets.
The Solar System is full of asteroids!
Between the orbits of Mars and Jupiter is a large belt of leftovers
called the Asteroid Belt.
Jupiter's gravity dominates this part
of the Solar System and it does not allow planets to form here.
Mass in this part of space accumulates (from accretion) up to
a certain size before Jupiter's gravity pulls it apart as Jupiter
passes by. Therefore the Asteroid Belt is a large orbit full of minor planets or asteroids.
Most asteroids are in pretty stable orbits with little eccentricity. (Notice I said "most"!)
Jupiter's gravity tends to herd asteroids into
groups so there are gaps in the Asteroid Belt. These are called Kirkwood gaps after the American mathematician, Daniel Kirkwood, who predicted them. When Jupiter swings pass a Kirkwood region, any asteroid (or anything) there will be tugged into a different orbit, leaving a gap.
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The other areas of the Belt are thick with asteroids. Some astronomers refer to these asteroid groups as "families".
Of course the Asteroid Belt is much closer to the Sun than the Kuiper Belt (or Oort Cloud) so any water or gas that may have been there at one time has now burned away. Asteroids are rocks that are far too small, with far too little gravity, to hold onto an atmosphere. By analyzing the light reflected from asteroids, astronomers have concluded that an asteroid's composition (or at least its surface composition) varies with its location. The asteroids closest to Mars are high in silicates and designated S-type while those on the Jupiter side of the Asteroid Belt are mostly made of carbon and called C-type. Scattered throughout this Belt are asteroids made of various metals and these are designated M-type. About 3/4 of the asteroids in the Belt are C-type.
Thousands of these minor planets have been discovered. Because they are so small they do not take on the spherical appearance of a planet. [A planet's shape is caused by the planet's own gravity working on it. Asteroids don't have enough mass to do that.] Asteroids are often described as "potato-shaped". The largest,
Cerus, is over 900 kilometers in diameter and along with Vesta
(575 kilometers) and Pallas (580 kilometers) accounts for over
half the mass in the Asteroid Belt. It is likely that asteroids
come in all smaller sizes. Some, like Cerus, are chunks of carbon (C-type), others
are mostly quartz (S-type), some are almost pure olivine (a mineral) and
many asteroids are rich in metals (M-type). Some have dark surfaces while
others are very bright. Indeed Vesta is so bright that it is sometimes
visible to the naked eye.
One day space explorers will mine the riches of the Asteroid Belt.
Most asteroids are confined to the Asteroid Belt but some have
been moved into other orbits. Jupiter and the Sun play
a gravitational tug-of-war that places some asteroids into the
same orbit as Jupiter but 60 degrees ahead or behind it.
These places are points of gravitational stability and the asteroids occupying them are called Trojans.
Some asteroids have highly eccentric orbits like a comet and astronomers divide them
into three groups.
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Asteroids that survive (even partially) their plunge through the
atmosphere are properly called meteorites.
Only 2500 meteorites have been collected and they are divided into three types based upon composition:
1. irons (which contain nickel as well as iron)
2. stones (full of carbon and volatile materials like water and gases that are trapped inside)
3. stony-irons (an intermediate)
Like many astronomy teachers, I try to make
a clear distinction between comets and asteroids but things are
not always so clear cut. For example, the asteroid Chiron has
an orbit that causes it to spend most of its time out by Uranus,
but at perihelion is develops a coma so it might be classified
as a comet. However it has a huge "nucleus" (150 kilometers)
and astronomers think that something that big should be called
an asteroid. However, some of the objects detected in the Kuiper
Belt are even bigger but they are considered comets! Weird. Also,
some comets may have big rock chunks inside them. How should
we know? Has anyone ever been inside to look? So, maybe debris
from some comets could make it to the Earth's surface and be called
a meteorite without ever being an asteroid.
If you find these definitions confusing, you are in good company!
If meteoroids become meteorites when they reach Earth, why don't asteroids become "asterites"?
Hmmm, good question. I don't know. I think we've just got a system that calls them all "meteorites" regardless of their true origin. And an asteroid is called a "meteor" as it passes through the atmosphere - not an "aster"!
By the way, not all meteorites are the products of asteroids or comets. A few rare ones are chunks of planets or even the Moon! You see, sometimes an asteroid will slam into a planet or moon with enough force as to cause small bits of that world to be hurled into space. Some of those bits will wander through space until they are captured by the gravity of another world - the Earth, for example.
Several meteorites have isotopic signatures that indicate they came from Mars! Long before we understood isotopic signatures or the idea that these particular rocks had come from Mars, geologists and astronomers were intrigued by these strange meteorites. They have a different composition and are much younger than "normal" meteorites. Until recently, only eight of these strange meteorites had been found in one of three locations - Sherrgotty (India), Nakhla (Egypt) and Chassigny (France), so they are often called SNC meteorites.
In the past few decades SNC meteorites have been found in Antarctica. (But the name SNC sticks. We haven't renamed them "SNCAs".) Indeed, Antarctica is a great place to find all kinds of meteorites (they stand out real well on the white, ice surface) and less of a bother to visit than outer space (but only slightly so ).
Your education would not be complete without knowing something about tektites. These unusual objects are found in scattered but specific areas of the Earth (particularly Australasia and Czechoslovakia) and are completely unrelated to the geological conditions in which they are found. It's as if they just fell out of the sky - if you catch my drift. Tektites are small black, greenish or yellowish glassy (silicaceous) objects aerodynamically shaped as if they have been melted while passing through the atmosphere. Their isotopic signatures indicate that they are from the Earth. It is very likely that they are the result of an asteroid impact with the Earth that hurled bits of Earth rock into space. However, instead of escaping the Earth's pull, these bits were dragged back and returned to Earth as meteorites! This scenario (story) would explain why their composition is like that of Earth while also explaining why they have the characteristics of having been melted by the atmosphere.
Tektites are a bit controversial. Some astronomers and geologists think that at least some tektites are from the Moon. You will recall, from your lesson about the formation of the Moon, that the Moon probably came from a piece of the Earth's mantle (the outer portion of the Earth). Lunar rocks have the same composition and isotopic signatures as Earth rocks so any Moon rocks that came to Earth as meteorites would be indistinguishable from the "native" Earth rocks. Most tektites have evidence of having been melted and aerodynamically shaped TWICE. That would imply an Earth origin. They melted once on the way up and once again on the way down. However, some tektites appear to only have been melted and aerodynamically shaped only once. That implies and extraterrestrial origin, from an airless world, and the Moon seems an excellent candidate!
It's weird to think that bits of the Moon and Mars have traveled here.
Yes, and very exciting.
The details of these interplanetary transportations (as they are called) can be very complicated. These scenarios are like a detective story requiring chemistry, geology and astronomy to provide the clues. Computer simulations, based upon well-understood mechanisms in physics, show that it's possible for Mars and Moon rocks to be blasted away by asteroid impacts and bits of them can make their way to Earth. Those same computer models show that it's possible that Earth rocks might get to other worlds the same way - although it's easier for rocks to be blown off the surface of Mars or the Moon than the Earth because of the Earth's stronger gravity and thick atmosphere. Nonetheless, there has been some speculation recently that Earthling bacteria may have been transported to Mars, the Moon and beyond by this mechanism. Similarly, maybe our world was seeded with extraterrestrial bacteria!
In 1871, in a lecture at a meeting of the British Association in Edinburgh, the physicist William Kelvin speculated that interplanetary transportation of rocks may have seed the Earth with life. Although he was a well-respected scientist, his speculation was ignore. In the early part of the 20th century this idea was revived by the chemist Svante A Arrhenius, but he, too, didn't attract many supporters. The idea that life originated elsewhere and then traveled here is called panspermia and it has been treated as voodoo science for a long time. (I thought it was pretty far fetched when I first heard it. ) I think that one reason panspermia has a bad reputation is because it sounds too much like creationism to some folks. Another reason might be that panspermia simply moves the "origin of life" to a more complicated level. Most importantly, however, panspermia is hard to prove or disprove - at least so far!
Recent advances in our understanding of bacteria show that some of these micro-organisms, if hidden deep inside a rock, could survive the proposed impact, withstand the conditions of space for long periods of time and survive the descent to the Earth. But that's not proof that it occurred. Panspermia might one day be found to be true. Or not.
So, when you see a meteor streaking through the sky, enjoy its beauty because it may be carrying micro-invaders destined to take over the Earth!
Or maybe those tiny organisms arrived long ago and we've learned to accept them because we evolved from them.
If the panspermia theory is correct we, and all life on Earth, are descendants of space traveling microbes!
Cool! When is the best time to see these space invaders?
August!
If you want, you can continue on to the next lesson where I will teach you exactly where to find the best meteor display of the year as well as show you how to identify a lot of the new constellations and stars in the late summer sky.