Our Dangerous Cosmic Neighborhood
;Asteroid ships DONE
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 astronaught. I will be
your guide as we explore asteroids and comets -- friends and foes,
and discover the role they have played in our past and how they
could affect our future in space and here on Earth.
Featuring Astronaut Tom Jones
Part 1 – Meteorite Hunting
A. Meteor NA DONE
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.
B. Brenham today
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.
Image in silhouette
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.
Overlay of video
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 broke apart
millions of years ago.
Collage of close ups
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.
Collage of headlines
Followed by video
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.
Part 2: Asteroid Impact
A. Fly past the Moon
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
B. Zoom in on Earth w/
Space station in orbit
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.
Look through viewing port
At Meteor Crater
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.
Look through viewing port
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. It’s central depression, buried rim,
and outer rings match impact features on the moon.
C. Circle wipe to animation 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.
D. Back in ISS, zoom in on
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.
Circle wipe to animation
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 born
radar can look below exposed crater rims to see the buried features
of an impact crater.
Back in ISS, zoom in on
A recent extraterrestrial encounter produced no craters at its impact
site in the remote Tunguska region of central Siberia.
E. Tunguska DONE
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
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 center.
Part 3: Asteroid Search
A. Walking on George path
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.
Reveal the research dome
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.
Slow zoom to asteroid belt
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.
B. Asteroid impact in belt
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.
C. Apophis slow approach
Astronomers are now tracking an asteroid named Apophis that will
soon come very close to Earth. Apophis is a stony asteroid 270
Apophis pass near Earth
On Friday, April 13, 2029 it will come within 33,000 km of the
Earth – reaching a lower altitude than the geostationary satellites
monitoring the weather and carrying 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.
Collage of asteroids filling
the dome (to scale)
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.
Images of mitigation plans
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.
Part 4: Comets Up Close
HB & cactus
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.
HB & Egypt
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
Comet fly out
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
B. Tempel 1
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 dust-like powder and that the ice deep
inside its nucleus may be unchanged from the early days of the
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.
C. Mt Palomar
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
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.
Sight at STI plus impacts
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.
Part 5: Comet Collision
Storyboard 1 and 2:
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.
Storyboard 3 and 4: 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.