February 18, 2020 2

Impact Earth [ARABIC]

Impact Earth [ARABIC]

Millions of asteroids and comets
lurk among the planets – left over bits and pieces from
the solar system’s formation four and a half billion years ago. Asteroids and comets once delivered
raw materials to a young, growing Earth. Now they may be the most
attractive places near Earth for mining the minerals, water, and oxygen needed to sustain colonies on other worlds. I’m Tom Jones, a planetary scientist,
and a four time shuttle astronaut. I will be your guide as we
explore asteroids and comets – friends and foes, and discover the role the
have played in our past and how they could affect our
future in space and here on Earth. Each day, Millions of tiny meteors
burn up in Earth’s atmosphere. Those as small as a grain of
sand look like shooting stars as they streak across the sky. Of the hundred tons of space rocks
and pebbles swept up by Earth daily, only a few pieces are large enough to survive their fiery descent
and strike its surface. Just over 10,000 years ago, the last Ice Age was releasing its frigid grip on the North American plains. The first humans had recently arrived, traveling over a land bridge
exposed by the ice. These Native Americans of the Great Plains may have witnessed a spectacular meteorite fall. As the fireworks ended, hundreds
of meteorites fell to the ground. Native Americans and then farmers collected these strange rocks from the sky and used them as a source of iron. Now meteorite hunters are returning to
the wheat fields of southwestern Kansas to search for larger meteorites, buried at least a meter under the ground. In October 2006, a research team
arrived to locate and excavate one of these buried meteorites – a rock that had not been seen or touched since impact over 10,000 years ago. To find a meteorite under the ground, the team used ground penetrating radar, which could detect an object below the surface. For days the team pulled the radar unit
over grid patterns marked on the ground – above places where a magnetometer
had identified buried metal, watching for reflections at the correct depth and then creating a 3D image of the buried object. The radar image allowed the team
to rule out discarded farm equipment. Finally the diggers began to excavate
a pit around the probable meteorite that the ground penetrating radar had identified. Soil layers in the pit indicate that this fall happened in the Pleistocene epoch, fixing the probable impact date over 10,000 years ago. Comparing the radar image
with the real meteorite shows how accurately radar
can depict a buried rock. The meteorite’s composition reflects it origin inside
an asteroid that broke apart millions of years ago. Most meteorites are made of stone or iron. In contrast, this meteorite is a mixture, a rare Pallasite, composed of olivine crystals
in a nickel-iron matrix. Such a rock came from the boundary between the core and the mantle
of a large asteroid, fragmented long ago in a violent collision. Meteorites like this one
provide valuable information about the composition and history of the asteroids in our solar system. The news media were fascinated with the idea of imaging a buried
meteorite before digging it up. With the meteorite’s dramatic formation as a remnant of an asteroid collision and with the possibility of using
this technique in the future to explore below the surface of Mars. The European Space Agency
has proposed installing ground penetrating radar on a robotic rover to map the Martian subsurface for drilling and to reveal the location of meteorites buried under the Martian terrain. We live in a dangerous cosmic neighborhood. Impacts still shape the
surfaces of planets and moons. Most of our Moon’s craters were created in the first half billion years
of the Moon’s history, ending with a cataclysmic heavy bombardment almost four billion years ago. On the Moon’s southern highlands and on most of its far side, craters overlap craters so thickly that the original crust
is almost completely obscured. Looking down from the
International Space Station, we see small space rocks burn
up in Earth’s atmosphere. In the distant past, Earth,
like the Moon, was hit hard, but crustal motions and weathering
by wind and water have erased the evidence of most impacts. Still geologists have identified over
180 impact scars around the globe. Many are located through satellite imagery and photographed by astronauts orbiting on
the International Space Station. In the early 1960s, Eugene Shoemaker, a geologist and astronomer, examined the kilometer-wide
Barringer Crater near Winslow Arizona. The desert climate has preserved
this crater’s sharp outline, allowing us to compare it with
similar craters on the Moon. Perhaps the most famous
impact crater on Earth lies below the village of Chicxulub, on the northwestern tip of
Mexico’s Yucatan Peninsula. The Chicxulub crater is buried
under layers of marine limestone with an arc of sink holes on the surface
marking the crater’s circular rim far below. Its central depression,
buried rim, and outer rings match impact features on the Moon. We can imagine the impact that
created this buried crater. Sixty-five million years ago,
a 10 kilometer-wide asteroid blazed through Earth’s atmosphere
and struck a shallow sea. The asteroid became an intensely hot fireball. But Earth’s atmosphere had little effect on the velocity of this enormous
flying mountain of rock. Impact with the ocean floor created a crater over 150 kilometers wide and a giant tsunami. Millions of tons of dust from the sea floor were hurled into the atmosphere. Global darkness followed, killing vegetation, while acid rain poisoned the upper oceans. Eighty percent of the planet’s living species, including all non-flying dinosaurs, were wiped out. The search for evidence of impact cratering now extends to the Sahara Desert where shifting sands have buried the past, leaving only hints of ancient cratering events. We can imagine meteoroids
falling toward the desert: some burning up in the atmosphere
and others reaching the surface. Gradually desert sand has filled the craters. Only tools like air and space borne radar can look below exposed crater rims to see the buried features of an impact crater. A recent extraterrestrial encounter produced no craters at its impact site in the remote Tunguska region of central Siberia. On the morning of June 30, 1908, a 40-meter-wide asteroid fragment entered Earth’s atmosphere traveling at a speed of over
50,000 kilometers per hour. During its quick plunge, the space rock heated the surrounding air to four times the temperature
of the Sun’s surface. Just after 7 a.m. local time, the few startled inhabitants
observed a brilliant white fireball. At a height of 8 kilometers, pressure and heat caused the space rock
to fragment and annihilate itself, producing a firestorm and releasing energy equivalent to hundreds
of Hiroshima atomic bombs. Eighty million trees were blown down – a catastrophe powerful enough
to destroy a modern city. Earth’s history of impacts show
that we still face the potential for global devastation from space. If an asteroid larger than a couple
of kilometers across struck the Earth, the explosion could throw
enough dust into the atmosphere to shut down agriculture for a year or more, destroying natural ecosystems
and possibly leading to a collapse of modern civilization. NASA has funded several survey teams
to find objects wider than a kilometer that could impact the Earth. Each evening, observers from Massachusetts to Arizona and Hawaii search the sky for asteroids
on paths that cross Earth’s orbit. None of the potential civilization
killers found thus far are on a collision course with Earth. Amateur astronomers also volunteer
their time and equipment to search for new asteroids, working at facilities like
Houston’s George Observatory. Asteroid hunters photograph sections
of the sky through large telescopes taking images of the same
starfield about 15 minutes apart. They compare the photos as
they look for an object that has moved against
the background starfield. The orbits of most asteroids lie
in a region called the asteroid belt, between the paths of Mars and Jupiter. Like the rest of the solar system,
the asteroid belt is almost empty, with millions of asteroids
spread over the entire area. The total mass of these asteroids is much less than the mass of Earth’s Moon. Collisions and the gravitational
tugs of nearby planets can nudge asteroids out of the asteroid belt and perhaps send them sunward. In such a collision, a large asteroid
might shatter into many smaller asteroids. These impacts create the meteoroids that become meteors in Earth’s atmosphere and meteorites if they survive
to reach Earth’s surface. Astronomers are now tracking
an asteroid named Apophis that will soon come very close to Earth. Apophis is a stony asteroid 270 meters wide. On Friday, April 13, 2029, Apophis will come within
33,000 km of the Earth – reaching a lower altitude than
the geostationary satellites monitoring the weather and
carrying television signals. The impact of an asteroid the size of Apophis could wipe out a city or cause a devastating tsunami. The Earth can expect an impact of this size as often as once every 50,000 years on average. Apophis serves to warn us that dangerous asteroids are close by and that it is only a matter of time until we find one on
a collision course with Earth. We have launched robotic
spacecraft to study asteroids up close. In 1991, the Galileo spacecraft imaged Gaspra and in 1993 it approached Ida and discovered that this asteroid
has a tiny moon called Dactyl. In 1997 the NEAR-Shoemaker spacecraft flew past the dark asteroid Mathilde,
over 50 kilometers wide – twice the size of Ida and
four times as large as Gaspra. In 2000, the NEAR-Shoemaker spacecraft went
into orbit around the asteroid Eros, the first discovered Near-Earth Asteroid. In 2001 it landed on
the asteroid’s irregular surface. The Hayabusa spacecraft visited
the asteroid Itokawa which looks more like a loose pile of rubble than a solid rock. In 2005 Hayabusa actually
touched down on the asteroid trying to collect samples. Data from these encounters help scientists
design ways to deflect an asteroid like Eros or Itokawa that could someday hit Earth. Suggestions range from lasers and solar sails to a nuclear blast at close range. A kinetic impactor could hit the asteroid and nudge it forward or backward along its orbit. A gravity tug with its small,
but persistent, gravitational attraction could gradually pull a threatening
asteroid from its impact trajectory. Space agencies may test these methods
on nearby asteroids in the near future. Comets also threaten Earth. On July 23rd, 1995, Alan Hale and Thomas Bopp became
the discoverers of Comet Hale Bopp – the most widely observed comet in history. As Comet Hale Bopp approached the Sun, its nucleus and atmosphere continued to brighten, with a blue gas tail pointing
straight away from the Sun and a yellowish dust tail
curving away toward its orbital path. Comet Hale Bopp last visited the inner
solar system over 4,200 years ago, during ancient Egypt’s golden age. In 1996, Jupiter altered the comet’s orbit and it will return again in about 2,400 years. Like asteroids, comets are time capsules that hold clues about the history
of the solar system. Formed around four and a half billion years ago, they are made of ice and dust: primitive debris from the solar system’s
most distant and coldest regions. Comets like Hale Bopp have spent most of their lives
in deep freeze, beyond Neptune. Other short-period comets follow paths
that remain inside Neptune’s orbit and bring them back into view
on a regular schedule. Comet Tempel 1 is a good example
of a short period comet. With each return, the Sun heats up
the comet’s dirty snowball-like nucleus, causing it to shed material
into its gossamer tail. The Deep Impact spacecraft reached Comet Tempel 1 on July 4th, 2005. The larger “flyby” spacecraft
carried a small “impactor”, which it released into the comet’s
path for a planned collision. To observe the impact, the flyby spacecraft
maneuvered to a new orbit that passed just 500 kilometers
from the comet. From this mission we discovered
that Comet Tempel 1 is a fragile icy dirtball
covered with powdery dust, and that the ice deep inside its nucleus may be unchanged from the
early days of the solar system. In the future, we may have
to use the same technique to change the orbit of a comet or asteroid, preventing a collision with Earth. At Mt. Palomar in March 1993, astronomers Gene and
Carolyn Shoemaker and David Levy discovered a very unusual
comet orbiting Jupiter. Comet Shoemaker Levy 9 had come
too close to Jupiter in 1992 and the giant planet had torn it apart, leaving comet fragments arranged like pearls
on a string along the comet’s orbital path. In 1994 the Hubble Space Telescope
resolved Comet Shoemaker-Levy 9 into a train of 21 icy fragments stretching over three times the
distance between the Earth and Moon. The fragments were on course for
an impact with the giant planet in July. These impacts would occur
on Jupiter’s night side, out of view from Earth, but so close to the day-night terminator that each impact site would
soon rotate into view. The Shoemakers and David Levy stared at
the huge spots their comet made in the atmosphere of this giant world. The fate of Comet Shoemaker-Levy 9 was a graphic reminder that the impact process is alive
and dangerous in our solar system. Hour after hour the impacts continued, producing a band of dark
blotches in Jupiter’s cloud tops and Earth-size soot rings that
marked the cloud tops for weeks. These bruises gradually dissipated
in the planet’s atmosphere. Comet Shoemaker Levy 9’s
longest-lasting effect is its demonstration that our Earth
orbits in a cosmic shooting gallery. Comets are friend and foe. Comet impacts probably brought water
and organic material to the early Earth and perhaps ice to the Moon’s poles. Unlike asteroids, comets come
from distant regions that we cannot search from Earth. After its discovery, we would
have at most a few years to assess the threat and prepare
for a comet encounter. To understand the hazard,
let’s see what would happen if a comet the size of
Shoemaker Levy 9 hit Earth. Earth does not have Jupiter’s
thick atmospheric cushion so a 2-kilometer wide comet would rush quickly through the atmosphere creating an enormous glowing fireball. Let’s suppose that the comet crashes
into the shallow Gulf of Mexico just south of Houston, Texas – releasing a million megatons of energy on impact. The impact would gouge out a crater about 30 kilometers wide and generate a shock wave that almost instantaneously converts
the energy of the impact into heat and vaporizes the comet. The same pulse melts the surrounding rock and produces a rapidly expanding fireball. The blast raises a gigantic
tsunami that races inland. A decompression wave follows, hurling molten and shattered debris
away from the impact site. Wood and concrete buildings topple. Glass windows shatter. Bridges collapse and cars
become flying projectiles. Debris reaches the stratosphere and blocks sunlight for months. From the lunar surface, astronauts see Earth as a
fragile and vulnerable world. Earth supports a delicate biosphere, with life forms that adapt
poorly to dramatic changes. Even if humans do survive the actual impact, the loss of agriculture and
the complex structures of civilization would put human society at risk. Because we have so much to lose, we must step up our search
for comets and asteroids. We can find and track them, even use our space technology
to shift their orbits. What we still lack, and must find, is the will to act together to
ensure our survival.

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