Asteroid Collision
We normally think of things in space as remote—not really
affecting things around us. But there are some types of cosmic events that could really mess things up badly for us. In fossil
records, there are many instances of species going extinct—apparently
unable to cope with some change in environment. Sometimes in
Earth's history, not just one species but lots of species has died off in mass extinctions.
The latest mass extinction happened 65 million years ago, when the
Age of Reptiles ended and the Age of Mammals began. The question, “What
caused the mass extinction at the end of the dinosaur age?” made
scientists disagree and squabble for quite a while. The candidate theories for
the cause were:
a. The theory of gradual change — mass extinctions took place over
thousands or maybe millions of years, possibly due to long term climate
change. Fossil evidence indicates that prior to sixty-five million years
ago the dinosaurs were beginning to decline and many dinosaur species
had already become extinct.
b. The impact theory — set forth by Geologist Walter Alvarez from the
University of California at Berkeley. In Gubbio, Italy, he was studying a thin layer of
clay between rock layers of the Cretaceous Period (Age of Reptiles) and
the Tertiary Period (containing no dinosaurs fossils). The clay layer
contained very large amounts of the rare element iridium, which is
more common in meteorites than in the Earth’s crust. This suggested a
sudden large influx of iridium—perhaps one really huge meteor struck the Earth at
about that time. An underground crater found near Chixulub (pronounced
Chi’-shoo-loob), Mexico, was found to be about 65 million years old. Additional evidence was found in the
clay layer: it contained tiny pieces of minerals (shocked quartz) and
glassy rocks (tektites), both of which are found at nuclear test sites and large
meteor impact sites. There was also soot indicative of continent-sized
forest fires. An asteroid in the 10 kilometer diameter size range could
have caused the event.
Famous Craters
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The Chixulub Crater is buried underground, so
it cannot be directly photographed. This is a 3-D graph made with
equipment normally used to search for oil. It shows gravitational
attraction of underground structures. Photo courtesy of Virgil L.
Sharpton, Center for Advanced Space Studies, Houston, Texas, USA.
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The Barringer Meteor Crater near Winslow,
Arizona, is as deep as a 60 story building and more than a kilometer
across. It was created about 30,000 years ago by a rocky object about 30
meters in diameter traveling at 40,000 miles per hour. The impact had
an explosive energy equivalent to over a million tons of TNT. It is
1/100 the size of the crater in Chixulub, Mexico. Source: Yerkes
Observatory. |
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The Deccan Traps are extensive lava flows in India. Source: © Dr. Keith G. Cox, University of Oxford, Oxford, England.
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c. The volcanic eruption theory—A huge series of lava flows in India,
named the Deccan Traps, which covers 10,000 square kilometers and is in
some places more than 2 kilometers thick, is evidence of a period of
volcanic activity that spanned about half a million years—including the
period of the mass extinction at the end of the Cretaceous Period. The
volcanic eruption theory is in agreement with paleontologists’ original
idea that the extinction of species was gradual, or at least occurred in
several steps, over hundreds of thousands of years.
Which Theory is Right?
It may well be that we will never know for sure if either a large impact
or massive volcanism caused the death of the dinosaurs.
Will Earth be Hit by a Large Asteroid?
Eventually, it is likely. But the odds of one hitting tomorrow, or next
week, or in the next few thousand years are quite low. Of course tons of
smaller bodies (sand grain size) enter Earth’s atmosphere every day.
But the larger the body, the rarer it is.
Asteroids are not the only menace. Evidence of comets crashing into
planets was dramatically seen in 1994 when the large comet
Shoemaker-Levy 9 fragmented and created huge explosions when it struck
the planet Jupiter at more than 20 different sites. We’ll find out more
about this event at the end of Chapter 3.
It would behoove us to seek advance warning of such an approaching body.
An asteroid could come in quickly, but it may be technically possible
to deflect an asteroid from an Earth impact course if there is enough
advance warning.
The Search for Near Earth Asteroids (NEOs)
Asteroids must be discovered and their orbits tracked. At this point in
time North American Aerospace Defense Command (NORAD) has a limited
number of people monitoring the skies for asteroids. There is also a
project called Space Guard which is an international network of
telescopes and people working together to discover and track asteroids,
with the idea of providing lots of advance warning if an asteroid is
found to be on a collision course with Earth, so that an effort could
be made to divert it. In the 1990s NASA carried out the Spaceguard
Survey to find NEOs greater than 140 meters in diameter. This
program was budgeted at $4.1 million per year for FY 2006 through FY
2012. Most recent findings can be found at the NASA Near Earth Object Program website (http://neo.jpl.nasa.gov)
In chapter 7, we'll learn more about asteroids, as well as other bodies
in the solar system.
Supernova
It is in the darkness before dawn on July 4, 1054 A.D. Chinese
astronomers are greatly excited by the appearance of a “guest star”—a
new star in the sky, never seen before. As the Earth spins, half a day
later on July 5, it is predawn in Navajo lands of what is now northern
Arizona. The Anasazi, the “ancient ones” of this region, have spotted
that same new star and are equally excited. The legacy of that “guest
star” continues to this day.
Each culture records the event in its own way. Anasazi etch a rock
drawing—a petroglyph—on a rock overhang, depicting a circle next to a
crescent Moon as it appeared to them that morning. Petroglyphs in Navaho
Canyon and White Mesa, Arizona, as well as in the Chaco Canyon National
Park of New Mexico are lasting records left by those Anasazi
skywatchers. Mimbres Indian ceramic art from New Mexico also apparently
depicted the supernova.
Japanese and Chinese astronomers record that the “guest star” shone at
least as bright as the maximum brightness of planet Venus. Some said it
was about four times brighter than Venus, which was nearly a hundred
times brighter than the brightest stars in the sky (aside from the Sun).
It was visible in broad daylight for 23 days, and in the night sky it
shone for 653 days to the naked eye. Its location was in a part of the
sky that we now call the constellation Taurus.
Six hundred and seventy seven years later, in 1731, a nebulous cloud was
spotted in the constellation Taurus by John Bevis through his
telescope. Charles Messier observed the nebula again through a telescope
on August 28, 1758, and first thought it was a comet. He recorded it in
a catalog he was compiling to help him distinguish comets which he
eagerly sought to discover from “comet impostors” which stayed always in
the same places in the sky. In 1844, the 3rd Earl of Rosse, at Birr
Castle in Ireland, examined and made a detailed drawing of the nebula
using his giant 6 foot diameter, 56 foot long telescope. The drawing of
the nebula resembled a "Crab" which has been the name of the nebula ever
since.
Image of M1, the Crab Nebula, the aftermath of a supernova explosion. From the Grasslands Observatory near Tucson, Arizona.
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The Crab Nebula, or M1 in Charles Messier’s catalog of “comet
impostors,” is in the exact same spot as the recorded position of the
“guest star” of 1054. But it wasn’t until 1928 that Edwin Hubble
measured the rate of expansion of the Crab nebula which led him to
conclude that it had been expanding for about 900 years. The connection
with the “guest star” of 1054 was clear. The “guest star” was actually a
supernova—exploding star— and the Crab nebula, the supernova remnant,
consists of the material ejected in the supernova explosion which has
been spread over a volume approximately 10 light years in diameter and
is still expanding at the very high velocity of about 1,800 km/sec.
A supernova is an unimaginably powerful explosive event. It occurs when a
very large star, over eight times the Sun's mass, has burned most of
its nuclear fuel. When such a star's central fires go out, its core
collapses releasing a huge amount of gravitational energy. A blast wave
ejects the star's outer layers into space. The Crab nebula supernova
remnant is incredibly bright, even though it looks quite dim at our
distance from it—6,300 light-years. If we were to observe it from much
closer, we would find
that it puts out 1000 times as much visible light
as our Sun. In 1948, the Crab nebula was found to be an even stronger
source of radio wave radiation, and in 1964, observations with a
high-altitude rocket showed that the energy emitted in X-rays by the
Crab nebula is about 100 times more than that emitted in visual light.
Taking into account all energies of radiation, the Crab nebula is
putting out over 100,000 times more energy than our Sun.
Another indication of the violence of a supernova explosion is that in
some cases all that is left is a neutron star, which is an extremely
dense object. It is denser than an atomic nucleus, concentrating more
than the entire mass of the Sun in a volume only 30 kilometers (20
miles) across. On November 9, 1968, a pulsating radio source was
discovered in the Crab nebula by astronomers using a 300-meter radio
telescope in Arecibo, Puerto Rico. It pulses about 30 times per second!
It’s that neutron star (pulsar) rotating 30 times per second and
putting out 100,000 times more energy than our Sun.
If the Crab nebula pulsar is pumping out that much energy now, over 900
years after the supernova explosion, how much energy did the supernova
release at its peak? At its brightest, a supernova can put out the
energy of 10 billion suns!
Could Our Sun Become a Supernova?
Not a chance! Our Sun is not large enough to become a supernova of any
sort. Besides, it will take another 5 billion years before our Sun's
supply of hydrogen is depleted. At that time it will begin its dying
process and eventually become a white dwarf with a surrounding shell of
material much like the Ring Nebula (M57) in the constellation of Lyra!
Can a Nearby Supernova Affect Us on Earth?
Yes, if it’s near enough, within several dozen light-years. The intense
influx of radiation could kill all life on Earth, sterilizing the
planet. However, the odds of this happening are extremely small,
especially in the short life-span of us humans. The most important
effects of more distant supernovae are extreme excitement among
astronomers who study supernovae to understand cosmology—the history and
fate of the Universe.
We will be learning a great deal more about supernova in Chapter 5.
The Sun Engulfs the Earth
If you have read the book or seen the movie “The Time Machine” by H.G.
Wells, you may know of the eerie landscape that the Time Traveller
(Alexander Hartdegen) saw near the end of his time travels. The passage
here is from Chapter 11 of H.G. Wells’s The Time Machine as serialized
in the New Review, cut from the book, but later published as a short
story, “The Grey Man.”
“I
have already told you of the sickness and confusion that comes with
time travelling. ...when I brought myself to look at the dials again I
was amazed to find where I had arrived. One dial records days, another
thousands of days, another millions of days, and another thousands of
millions. ...the thousands hand was sweeping round as fast as the
seconds hand of a watch—into futurity.
"...I stopped. ...The
time was midday, the orange sun, shorn of its effulgence, brooding near
the meridian in a sky of drabby grey.
"...I rose to my feet,
and stared at this grotesque monster. I can only describe it by
comparing it to a centipede. It stood about three feet high, and had a
long segmented body, perhaps thirty feet long, with curiously
overlapping greenish-black plates. It seemed to crawl upon a multitude
of feet, looping its body as it advanced. Its blunt round head with a
polygonal arrangement of black eye spots, carried two flexible,
writhing, horn-like antennae. It was coming along, I should judge, at a
pace of about eight or ten miles an hour, and it left me little time for
thinking.
"...When I gained the machine the monster was scarce
fifty yards away. It was certainly not a vertebrated animal.
"...But I did not care for a nearer view.
"...As I drove on, a
peculiar change crept over the appearance of things. The unwonted
greyness grew lighter; then—though I was travelling with prodigious
velocity—the blinking succession of day and night, which was usually
indicative of a slower pace, returned, and grew more and more marked.
This puzzled me very much at first. The alternations of night and day
grew slower and slower, and so did the passage of the sun across the
sky, until they seemed to stretch through centuries.
"At last a
steady twilight brooded over the earth, a twilight only broken now and
then when a comet glared across the darkling sky. The band of light that
had indicated the sun had long since disappeared; for the sun had
ceased to set—it simply rose and fell in the west, and grew ever broader
and more red. All trace of the moon had vanished. The circling of the
stars, growing slower and slower, had given place to creeping points of
light.
"At last, some time before I stopped, the sun, red and
very large, halted motionless upon the horizon, a vast dome glowing with
a dull heat, and now and then suffering a momentary extinction. At one
time it had for a little while glowed more brilliantly again, but it
speedily reverted to its sullen red-heat. I perceived by this slowing
down of its rising and setting that the work of the tidal drag was done.
The earth had come to rest with one face to the sun, even as in our own
time the moon faces the earth.”
Wells described how our Sun might appear not after thousands of years,
or even millions of years, but after a few billion years. Current
theories for progression of the Sun in its lifetime predict that when
the nuclear furnace at its core runs out of fuel, it will begin a stage
of expansion and surface cooling that will make it huge and its surface
red—a red giant star. The surface could swell up big enough to engulf
the current orbit of Earth. Even if the gravitational pull of the Sun
will have weakened by then from loss of mass, so that Earth migrates to a
larger orbit, Earth still will get hot enough for the oceans to
evaporate to space, and our biosphere will be destroyed.
The good news is the enormous time span before this particular fate
occurs. Humans may not even be recognizable as humans by then, in terms
of species adaptations.
Understanding, Helplessness, and Empowerment
Understanding three ways that life on Earth could be threatened by cosmic events can invoke
various reactions.
- In the case of the threat of the Sun in its red
giant stage, we are pretty much helpless for now, but not too many
people are worrying a whole lot about what will
be happening in 5 billion years.
- In the case of a nearby supernova
explosion, we are also pretty much helpless, since protecting the Earth
from a huge influx of planet-wide radiation is a bit much to contemplate
in practical terms. However, we can learn about what the probabilities
are for such a scenario, and for the time being, the likelihood of that
kind of disaster is exceedingly low, almost vanishingly so.
-
The likelihood of Earth being hit by a large body, such as an asteroid,
is much, much higher than our being saturated by radiation from a
supernova. It is nearly inevitable—only a matter of time, though we do
not know if it will be today, tomorrow, or in many millions of years.
But in this scenario, we certainly are not helpless. If we are able to
detect a body that is hurtling towards Earth with enough advance
warning, there are a number of strategies proposed to avert disaster.
It's tempting to try blowing the thing up with nuclear weapons, a
typical video game-style mentality, that unfortunately, at best, would
create a number of smaller bodies that would still continue on their
trajectories and impact Earth with devastating effect. Other ideas are
mostly different ways of nudging the asteroid to deflect it into a path
that will not strike Earth.
This book is devoted to better understanding various astronomical ideas,
many of which relate directly or indirectly with the challenge of early
detection of "near Earth asteroids," as well as other intriguing
aspects of our "changing cosmos."
