Original subtitles

[music playing]

NARRATOR: Comets pummeled our planet eons ago,

making Earth unlivable, even as they brought the building

blocks for life.

They may be responsible for the largest mass extinction ever.

Could they hit our planet now and condemn mankind?

Today, scientists search for, track,

and study these mysterious and terrifying celestial

wanderers to reveal our past and predict our future.

A comet will hit the Earth.

The odds of impact are 100%.

Now, "Comets, Prophets of Doom."

[music playing]

Imagine this scenario, the end of a busy day in Los Angeles,

California.

As dusk gathers, the city sparkles from the glow

of a billion lights.

Cool evening breezes sweep through the boulevards

and canyons like they have a million times before.

This sleepy evening seems like any other.

But it's not.

This evening is like no other in recorded history.

A comet is passing very close to Earth,

so close that harmless bits of comet dust,

grains of rock and ice, will rain down

as storms of fiery meteors as Earth passes

through the comet's tail.

[music playing]

But Earth's gravity has also pulled pieces of this comet

apart.

Too big to burn up in our atmosphere,

these pieces of ice and rocks come hurtling down

on an unsuspecting metropolis.

[MUSIC PLAYING, METEORS CRASHING]

This scenario may seem unlikely, but the chances

of a comet grazing our planet are real, and the consequences

dire.

Scientists can't rule out that pieces of a comet

could impact the Earth.

Even worse, should one take direct aim at her,

there is a very real possibility of complete and utter

annihilation.

DAVID MORRISON: Impacts from comets

are certainly a real phenomenon.

They have happened in the past, and they will happen.

It's very rare, but it's absolutely horrendous.

It's the most extreme example of a catastrophe

of low probability, but huge magnitude.

And our civilization can not afford

to risk being hit by a large comet or asteroid.

[music playing]

NARRATOR: But what is a comet?

How is it different from an asteroid?

How is it related to shooting stars?

A comet is a small body that orbits the Sun, usually,

on kind of an eccentric orbit, so not a nearly circular orbit

like the planets.

And it's basically a ball a few miles across made

of ice and rock and other substances,

such as frozen carbon monoxide and carbon dioxide, methane,

some other things.

NARRATOR: This ball of rock and ice

is called the comet's nucleus or core.

And as the comet approaches the Sun, these ices vaporize,

and this release-- turns into gas and releases some of this

dust and dirt and stuff, making what we call the coma,

the fuzzy head of the comet, and then the really small pieces

get pushed out by the pressure of sunlight,

radiation pressure, into the dust tail.

NARRATOR: Comets are created in the deepest reaches

of the solar system.

They travel in giant elongated orbits

far away, unless some force knocks them out

of their orbit towards the Sun.

When a comet approaches the Sun, its ices and gases,

called volatiles, vaporize to produce the tail in a process

called sublimation.

Although a comet is mostly ice and dirt,

its surface is a shell of carbon that

forms as the volatiles below the surface

collect on it during the sublimation process.

It's a bit like a-- a baked Alaksa,

you take a brick of ice cream and you cover it with meringue,

throw it in the oven, and the outside layer protects

the inside layer from the warming influence of the Sun.

So it's a-- it's a very complex structure.

NARRATOR: Asteroids are other rocky bodies

that also orbit the Sun, but have few, if any, volatiles.

They orbit either in a band of asteroida;

debris called the asteroid belt, or in another more distant band

of debris called the Kuiper belt.

If you see an object and it happens to have a tail, maybe

from sublimating gases or something,

then they call it a comet.

An asteroid is something without a tail.

And that's the only distinction that observers would use.

Originally, it seemed to everyone to be clear.

Asteroids were these lumpy rocks, mostly

between Mars and Jupiter, but sometimes, you find them

elsewhere, and comets are these things that

came from further out, and when they got near the Sun,

they'd warm up, and volatiles would start to give off gases

and it'd have a tail.

So comets had tails and asteroids didn't.

However, over the years, people have certainly

been fooled a few times.

NARRATOR: For example, some comets have been found

in the outer regions of the Kuiper belt.

Curiously, Pluto, identified as a planet in 1930,

could best be described as an icy body.

In other words, Pluto fits the definition of a comet.

Pluto is the ninth planet, but it's also a comet body.

It's a collection of a vast number of icy, rocky bodies

out there orbiting beyond Neptune.

And Pluto is the largest in the cloud.

So there's currently a lot of debate

whether Pluto should be considered a planet.

NARRATOR: Blurring the line between comets and asteroids

even further, and contrary to popular belief, the phenomenon

on Earth known as a meteor shower is primarily attributed

to comets and not asteroids.

A meteor is the visual display of light in the sky

as a bit of dust and dirt burns up

upon entering our atmosphere.

BRIAN SKIFF: There's the dust material that comes off

from a comet that makes the tail that we see

in the sky for a bright comet.

If that material intersects the Earth's orbit,

particularly at night, then we would see meteors coming

from those bodies as they hit the atmosphere

and were vaporized at the top of the atmosphere.

[music playing]

Right now, there are about 30,000 to 40,000 tons

of asteroid and comet material that land

on the Earth every year.

NARRATOR: That's a lot of dust burning up in the sky

and settling to Earth.

But it's not just comet does that can hit the Earth.

Whole comets can.

In 1994, comet Shoemaker-Levy took direct aim

on planet Jupiter, producing violent impacts.

[music playing]

If it could happen there, it can happen here.

In fact, it almost certainly has.

Geologic evidence suggests large bodies, possibly comets, have

already plowed into our planet, destroying

many of Earth's creatures.

If it were to happen again today,

such a collision could send all life on Earth

toward extinction within hours, even minutes.

With literally trillions of comets in our solar system,

that threat is very real.

It's once every maybe 50 to 100 million years

that a real blockbuster comes by,

and it doesn't matter where you live.

You're dead.

NARRATOR: But comets don't even need

to impact us to incite panic.

For millennia, their mere appearance in our skies

has inspired terror and confusion.

Planet Earth, about to be recycled.

Your only chance to survive or evacuate is to leave with us.

NARRATOR: In 1997, the Heaven's Gate religious cult

saw the appearance of a new comet as something mystical,

not merely astronomical.

They believed a spaceship following

in the tail of comet Hale-Bopp would release them from Earth

and take them to heaven.

For 39 members, this belief was so

strong they committed suicide.

[music playing]

Beyond these apocalyptic stories is a dawning irony

among scientists.

The ingredients that make up these potentially

earth-shattering comets also make up our solar system

and our planet.

And they may provide the raw materials necessary for life.

In other words, the ingredients that make up comets make up us.

[music playing]

Comets play a role in life on Earth.

They play several roles.

Comets, and also, asteroids, are out of solar system materials,

which are rich in things that are important to life

that the Earth is intrinsically almost devoid of,

such as water and carbon and nitrogen.

[music playing]

NARRATOR: Comets have plowed into Earth

during its ancient past, and comet dust is raining

down on us all the time.

So what danger do these mysterious objects

pose to our immediate survival?

What's in the volatile gases that make up its core?

Are any headed towards us now?

Are we in the shadow of a lurking killer?

The best way to answer those questions is to go to a comet,

learn more about them, maybe poke one with a stick,

or chip a piece off.

Two recent NASA expeditions were conceived to do just that.

But instead of poking a comet with a stick,

NASA's engineers wanted to drive a probe traveling

six times faster than a speeding bullet right

into a comet's very core.

[music playing]

Wednesday, June 22, 2005.

This is Space Flight Operation Center at NASA's Jet Propulsion

Lab, or JPL, in Pasadena, California.

For nearly six months now, scientists and engineers

have been monitoring and adjusting

the trajectory of the Deep Impact Probe

as it closes in on its comet target

72 million miles from Earth.

The goal, to collide with comet Tempel 1,

a comet first observed in 1867.

The comet is roughly 8 miles long and 2 miles wide,

and it orbits the Sun every 5 and 1/2 years.

We're approximately 10 days from impact.

There is a charged tension in the air,

but everybody feels they're ready to go make this happen.

Deep Impact is an extremely challenging mission.

We're about to go hit a comet, which

is in a hostile environment, and there are a lot of things

we don't know about the comet.

Well, these folks are getting ready for the Deep Impact

encounter, which takes place on the evening of July 3,

so communications are being established.

We've got a roomful of scientists and engineers

who are getting their scripts and procedures ready.

NARRATOR: Designing such a precise spacecraft takes years

of careful preparation.

Work actually began on Deep Impact

in 1998, as the space craft and launcher were assembled

and tested.

The craft itself consisted of two elements, an impactor

designed to crash into the comet,

and an observer designed to separate then

photograph the impact from nearby.

Mechanisms had to be tested on the ground

before scientists were assured they

would work in the hostile environment of empty space.

The impactor is about the size of a washing machine,

and the fly by spacecraft's about the size of a small SUV.

And one weighs 800 pounds, the other 1,300 pounds.

NARRATOR: Finally, after years of assembly and testing,

the craft was loaded into a rocket

and the countdown began on the morning of January 12, 2005.

MAN (ON RADIO): 5, 4, 3, 2, 1, lift off.

[MUSIC PLAYING, ROCKET LAUNCHING]

NARRATOR: Since its launch, JPL engineers

have been monitoring the rocket as it hurtles

towards comet Tempel 1.

Computers onboard the spacecraft are linked to JPL's control

center so that engineers remain in contact with the craft.

KEYER PATEL: After we launched, we

performed a number of trajectory correction maneuvers

to get the flight system heading in the right direction.

Once we get to the desired place, which

is 24 hours before impact, we're gonna separate the two

vehicles, then the impactor on its own

is going to start heading towards the comet.

[music playing]

NARRATOR: This animation speculates

what the separation and ultimate impact might look like.

That is, if all goes well.

If it doesn't happen properly, the mission will fail.

No one can say for sure what will

happen at the moment of impact.

That's the beauty of this mission.

There's nothing subtle about running into accommodate

6 miles a second, but what we don't understand

is is it gonna be a large crater?

Is it gonna be a small crater with very little ejecta?

If we're gonna generate a large crater,

say, something the size of a football stadium,

all of the material that is ejected out of that crater

will be reflecting sunlight, and the comet

could increase in brightness by a factor of about 100.

And if it does that, it could be observable with binoculars.

NARRATOR: That assumes that these scientists can pull off

the collision between probe and comet.

A big if considering the distance and the speeds

involved.

If the impact does occur, scientists will gain insight

into the nature of comets, including their makeup

and density.

If they can drill a crater into Tempel 1,

they will peer into a time machine.

Comets hold frozen in their icy cores

the key to how our solar system formed

nearly five billion years ago.

Asteroids and comets are the leftover

building blocks of the pieces that ultimately formed

the planets.

NARRATOR: If you could go back in time

and watch the solar system form, you

would see clouds of interstellar gas and dust.

At some point, a volume of gas and dust combined.

Over millions of years, this ball of dust

gained enormous mass.

As the density increased, pressure

at the center of this sphere caused it to heat up and glow.

Finally, at a certain threshold of temperature and mass,

a nuclear reaction occurred.

The giant disk flashed over in a tremendous explosion

at the cloud center, igniting the fusion process reaction

that sustains our Sun.

A star is born.

Fast moving solar wind from this flashover rippled

through space, disrupting the arms of other swirling dust

clouds, separating them into rings

held by gravity around the young Sun.

These rings of dust and gas soon clumped together

to form the planets.

Objects that were formed closer to the Sun

tended to have more rock and less ice,

because ice can't exist close to the Sun.

NARRATOR: During and after the Sun's formation, clumps

of frozen material were thrown by the force

of the nuclear explosion into the farthest reaches

of the solar system where they have preserved a record

of our early solar system.

This is what Deep Impact hopes to learn more about.

There are two groups of these objects.

The first group is the Kuiper belt objects, asteroids

and comets orbiting just beyond Neptune.

Even farther out, nearly halfway to the nearest neighbor star,

is the second group, a diffuse shell of orbiting comets called

the Oort Cloud.

It's essentially the back of the solar system's freezer,

and there are literally trillions

of comets in this icebox.

SCOTT SANDFORD: I once sort of as a joke

wrote up a recipe to make a comet,

and one of the parts of the recipe

is you got to bake it at minus 450 degrees Fahrenheit for four

and a half billion years.

I mean, the last step basically is don't do much

with this material.

Just set it aside and store it.

NARRATOR: Sometimes in the Oort cloud,

a gravitational disruption knocks a comet

into the inner solar system.

Something similar to this happened on the young earth

about four billion years ago.

Comets and asteroids began to pummel the earth mercilessly.

It was such a violent period in Earth's history

that nothing could have survived.

Ironically, it may be the very reason we're all here.

The earth is a book whose story is

told in pages of rock, a diary, where each chapter is

a layer of time.

Each layer, millions of years of an epoch.

There are certain places where the pages of Earth's diary

are exposed for all to read.

America's desert southwest is such a place.

JAMES W. ASHLEY: Right now, we're looking at the Chi Bab

formation.

It's a system of interbedded sandstones and limestones

on the Colorado Plateau.

It dates back to about 250 to some 260 million years.

NARRATOR: Geologists come to places like this

to see millions of years compressed

into a single stratum.

JAMES W. ASHLEY: By taking a look at the rocks

and examining the different layers,

we can piece together a pretty good idea of what the Earth's

history might have been, it's climatic history,

its flora and fauna.

And geologists use the fossils to tie together different rock

units across the globe.

NARRATOR: But this diary only goes back so far.

At a certain point in the life of our planet,

a time marked by cataclysmic impacts

with comets and asteroids called the heavy bombardment period,

there are no more pages to read, no more layers to interpret.

Nothing is left from Earth's infancy.

Even rock didn't survive.

The solar system was younger then

and had more comets and asteroids orbiting the sun.

This meant there was a higher incidence of impacts

with objects, like comets.

The heat generated by impacts from this onslaught of comets

and asteroids during this heavy bombardment period

helped keep the Earth's surface molten.

The water delivered by these icy comets

was instantly turned into steam or completely evaporated.

JAMES W. ASHLEY: But whatever caused

the late heavy bombardment, it's clear

that the entire inner solar system got pounded,

including the earth.

NARRATOR: We don't know how this unfolded.

Because whatever rocks were here melted, and reformed,

and melted again.

So how do we know it happened?

DON BROWNLEE: Our whole concept of heavy bombardment on earth

did not come from geology and studying this planet.

It came from studying the moon, from studying

that little white thing up in the sky.

NARRATOR: Unlike the earth, the moon is geologically inactive.

It's crust doesn't float on a molten core of rock

or rattle with earthquakes that slowly erase and erode

its features.

DAVID MORRISON: The moon has shared the same impact history

as the earth.

And in the case of the moon, we see two kinds of impacts.

During the first 400 or 500 million years

after the moon was formed, there were just

a lot of stuff hitting it.

For the last four billion years, it's hardly buried at all.

NARRATOR: This heavy bombardment period

ended as much of the loose material in the solar system

was either gathered up by the planets through impacts

or thrown out of the solar system by gravity.

What is strange is what happened on Earth,

the geological instant all this chaos ended.

Soon after the earth cooled when comet an asteroid impacts no

longer kept the earth virtually molten, water

pooled without boiling away.

JAMES W. ASHLEY: What happened was life.

From the fossil evidence, it appears likely

that almost as soon as the earth cooled off,

we had the beginnings of life on this planet.

NARRATOR: How did life get here?

Did deadly comets also play a role in establishing life

on Earth?

If so, how?

In fact, the key to life may be the water in comets.

SCOTT SANFORD: Water is important.

But in terms of the biology, we understand here on the Earth,

it's critical.

When anyone writes a biochemical equation,

one of the things that's sort of understood

in all of those equations is that doesn't happen

because these two molecules get together in a vacuum.

It happens, because they get together in liquid water.

And in fact, water for large biochemical molecules

can play a central role in helping

get the biochemistry right.

So liquid water is absolutely critical for life

in almost any environment.

So one of the mantras of astrobiology

is to try to follow the water, that if you can find locations

where there's liquid water, there's

a much higher probability you'll also potentially find evidence

for life.

DAVID SCHLEICHER: You drop a comet out of the earth, which

is mostly ice, dirt, carbonatious compounds.

That supplies a significant part of the water

that we find on Earth today.

The estimates I've heard are that about perhaps 10% to 30%

of the water on earth came from comets.

It, therefore, turned to 30% of the water,

and you originated in comets.

I'd say that's fairly important.

NARRATOR: But comets don't just provide the water.

They may provide the very building blocks of life.

The genesis of life is beyond current scientific

understanding.

But the raw ingredients that make it up on earth are basic,

carbon, nitrogen, oxygen, and hydrogen

in various combinations, referred to as organics.

Without these, life as we know it doesn't exist.

These elements exist in comets.

Not just as elements, but in intricate combinations

called amino acids, structures that make up

the proteins in our bodies and the bodies of other living

things.

How do we know comets contain these structures?

By analyzing the light reflected by comets

as we gaze at them through telescopes.

SCOTT SANDFORD: What you see here is a whole array of vials.

Each of these vials contains a different polycyclic aromatic

hydrocarbon.

And when you turn on the lamp like this,

then you see different molecules glow different colors.

So if we measure these in the laboratory,

we can basically get a spectral thumbprint of what

these molecules look like, and then

we can search for that thumbprint with telescopes,

and find out are these molecules present in space or not.

And that's of interest.

Because we know these molecules, when they're processed

by the kinds of conditions under which planets form

can be altered in such a way as to form molecules that are

biologically very interesting.

LUKE DONES: Well, I think it says that life really

is a natural consequence of the physical and chemical

conditions in the early earth, and the solar system,

and pressler stars.

NARRATOR: Scientists still debate whether comets brought

the building blocks of life to planet earth,

but what is now widely accepted by experts

is that life was nearly wiped out on at least one occasion

from a massive impact.

Scientists initially resisted the idea

that objects outside earth could have such a catastrophic effect

on its systems.

DAVID MORRISON: One of the important discoveries

of my scientific lifetime was that the great mass extinction

of 65 million years ago that did end the dinosaurs

and a whole lot of other creatures was due to an impact.

That was a paradigm change to realize that a cosmic impact

could wreak such havoc with the Earth's biosphere.

NARRATOR: But for years, scientists

were puzzled by a thin layer of iridium rich soil,

separating the cretaceous period and ancient epoch in Earth's

history from the tertiary period.

Each of these epochs represented millions of years.

This iridium layer was found the world over.

Iridium is normally very rare on Earth,

but common in comets and asteroids.

Above this thin layer of iridium in Earth's soil,

no dinosaur fossils are found.

Because dinosaurs went extinct at the end

of the cretaceous period, but why?

Could the extinction have something

to do with the Iridium?

In the mid 1970s, a team of scientists,

including Walter and Louis Alvarez,

determined that this iridium layer was deposited

by a massive impact most likely with an asteroid that also

destroyed the dinosaurs.

This impact caused millions of tons of dirt

to fill the atmosphere, blocking out the life giving sun

and sending the earth into a long lasting deep freeze.

KEITH NOLL: It could be that the impact sets

off a chain of environmental changes

that in addition to the initial event, which kills

a lot of creatures, that these longer term events might then

result in mass extinction.

NARRATOR: This radical theory received wide acceptance

in the early 1990s when scientists, including Alan

Hildebrand, discovered an impact crater

near the town of Chicxulub on the Yucatan Peninsula

of Mexico.

It dated from the same period roughly 65 million years ago.

Now, there was a smoking gun, a crater,

but could this impact scenario have happened more than once?

DAVID MORRISON: In no case since the end

cretaceous event of 65 million years

ago has there been a clear, unambiguous identification

of an impact as the cause of a mass extinction.

NARRATOR: That hasn't stopped a team of scientists, including

Luann Becker of the University of California in Santa Barbara.

Becker thinks they may have found

the culprit for the greatest mass extinction ever, the Great

Dying.

This event, separating the permian

from the Triassic geologic eras 250 million years ago

was so catastrophic, nearly 90% of all living species

went extinct.

LUANN BECKER: It's one of the most fascinating events

for all scientists to try to explain.

Why?

Because it was almost an exterminating event.

NARRATOR: Unfortunately, the case

for the permian Triassic extinction isn't as clear.

What evidence survives is hard to interpret?

Many factors may have contributed

to the permian Triassic extinction,

including an Earth stressed by intense volcanoes.

But, Becker thinks an impact, possibly by a comet,

may have pushed creatures over the edge,

kicking up tremendous debris and cutting off the sun for years,

producing a long lasting deep freeze.

Becker is investigating a feature

off the coast of Australia called the Bedout structure,

which she believes might be an impact crater.

She needs proof before she can say definitively that this

is evidence of an impact.

LUANN BECKER: We have a pretty beat up crater,

if that's what it is, so we are facing a pretty significant

challenge to try to prove our point.

Whether we're wrong or right, it needs to be tested.

NARRATOR: In the coastal Australian town of Mackay

at a local pit mine, Becker seeks more evidence

of an impact.

So far, she has not found any telltale iridium.

But she's found something else, a rare form of carbon found

in space that isn't diamond and isn't graphite.

LUANN BECKER: It's the third form of carbon

is called buckminsterfullerene.

It's the only form of carbon this capable of trapping atoms

inside of its cage, so it's really quite unusual.

It turns out that some people who were studying stardust

actually discovered it quite accidentally,

and eventually, were able to prove that this form of carbon

actually exists.

NARRATOR: Becker is using this extraterrestrial carbon found

at the structure as evidence of an extraterrestrial impact.

LUANN BECKER: This is a tough layer.

This will be good.

We'll take this back, and take a look at it,

and see where we are.

This looks really good.

We'd like to say, yes, that is absolutely something that

does come from stars.

When we look inside of these fluorine molecules,

we find exotic gases, which we do believe

are an indicator of an extraterrestrial origin.

NARRATOR: But so far, Becker's theory has met resistance.

ALAN R. HILDEBRAND: If there was a large crater that caused

the boundary layer at the permanent Triassic

and the associated extinction, there's

no reason to think that equivalent evidence shouldn't

be there.

What I've seen of the geophysical maps,

I cannot see a crater there.

My personal opinion is I would regard the results

with caution.

DAVID MORRISON: Nobody knows what

caused the permian Triassic extinction.

It's still possible that it could have been an impact,

so I'm glad folks are out looking for a possible crater

associated with it.

But we're a long way from making a clear association

of that sort now.

NARRATOR: Scientists have had to adjust to new ways

of perceiving the world.

Just 25 years ago, the idea that comets could wreak

catastrophic changes on Earth's environment

would have been ridiculed.

It's clear that our knowledge of these strange objects

is dangerously limited.

In fact, much of our past history with comets

has been riddled with misunderstanding, confusion,

and terror.

The story of the largest mass suicide in US history

happened in the exclusive community of Rancho Santa Fe,

but it seems as though it happened

in everyone's backyard.

NARRATOR: Superstition and ignorance can still

influence our perception of comets, even in our modern age.

On March 27, 1997, 39 members of the Heaven's Gate cult,

including its leader Marshall Applewhite, also known as Doe,

committed suicide in their compound

outside San Diego, California.

Laying on their back with their hands at their sides.

It's appearing as if they had fallen asleep.

There were no visible signs of trauma on any of the bodies.

SARA SCHECHNER: With the appearance of a great comet

Hale-Bopp in 1997, members of the cult committed mass suicide

in order to release their souls from their mortal form here

and to join a spacecraft they thought

was traveling in the wake of the comet.

The students that are in front of me are of various ages.

I'm not talking about their bodies.

I'm talking about their minds.

NARRATOR: Applewhite taught his followers

that members of the cult could shed their bodies or containers

in order to advance to the next level,

hitching a ride on the spaceship, which would take

them to heaven.

Seeing the comet confirmed their faith in Doe,

and they killed themselves.

JAMES W. ASHLEY: A good scientist

has been trained to separate their preconceptions from what

they're observing and draw their conclusions based

on the evidence.

Unfortunately, the Heaven's Gate people

did something that's probably far more natural for us, which

is to make their assumptions and draw conclusions

without regarding hard evidence.

NARRATOR: We are, like our ancestors, filled with fear

and wonder when we look heavenward.

The word comet comes from the Greek phrase aster cometus,

which means long haired star.

Our ancestors knew these celestial visitors, which they

observed with their naked eyes, were

different from other objects in the sky.

SARA SCHECHNER: References to fiery swords

in the heavens, fiery torches, fiery brooms

have been interpreted as comets by later literary scholars

or artists.

So for example, in the Bible when God chases Adam and Eve

out of the garden of Eden, he sets up these cherubim,

these angel like creatures with a blazing sword

that spins every which way to prevent Adam

and Eve from coming back into the garden.

And it really seals off paradise for everybody.

NARRATOR: Many scholars link this to visions of a comet.

In a Greek myth, the son of the sun God Helios

named Phaethon begs to drive his father's golden chariot

across the sky.

But the bright horses are too strong for the inexperienced

boy, and the chariot scorches the earth

on its journey, ultimately, crashing into the water.

Is this really the story of a close encounter with a comet?

Unlike a meteor whose appearance is virtually instantaneous,

a comet is a long lasting event, sometimes visible for weeks.

Comets were seen by terrified observers as predictors

of cataclysmic events.

SARA SCHECHNER: There was a tidal wave and an earthquake

in Achaia in 373 BC.

And there was a big comet before it,

and Aristotle makes note of that.

So you get this correlation between comets

and these terrible events, which then becomes reinforced

in different ways.

NARRATOR: The Romans had a similar view of comets.

SARA SCHECHNER: There's a comet in 44 BD that

occurs at the time of Julius Caesar's assassination.

So they say, oh, the comet is there.

It's a divine sign that this event is going to occur.

It's a warning.

NARRATOR: In 729, a comet was widely

seen as a sign of the Saracen invasion of France.

In 1066, the comet now known as Halley

hailed the Anglo-Saxon defeat at the Battle of Hastings.

HAL WEAVER: Shortly before that battle, Comet Halley appeared.

Nobody knew that was Comet Halley at the time,

but King Harold's soothsayers told him

that the apparition of Comet Halley was a sign of doom.

And sure enough, of course, he lost the battle

to William of Normandy, and that was captured in the Bayeux

tapestry.

The famous Bayeux Tapestry has the picture of Comet Halley.

NARRATOR: As the Protestant Reformation of the 16th century

fractured Christianity throughout Europe,

religious leaders perceived this chaos reflected in their skies.

They condemned comets as willful stars

in the otherwise orderly heavens.

Martin Luther even called them harlot stars, because they did

not behave like other stars.

This sinister view of comets held by Luther

had evolved from a much more benevolent view held by Church

leaders a Millennium earlier.

One third century philosopher suggested that a comet may have

heralded the birth of Jesus and led the wise men to Bethlehem.

SARA SCHECHNER: This connection of a comet with the star

of Bethlehem is most prominently depicted and of truly wonderful

Fresco painted by Giotta in Padua in the Scrovegni Chapel.

In the part of the Fresco that depicts the nativity scene,

we have the star of Bethlehem depicted

as a comet with a tail.

And in fact, Giotta was interested in astronomy

and had seen a great comet in 1301,

several years before he painted this Fresco.

NARRATOR: Could the star of Bethlehem

have been an actual comet?

JAMES W. ASHLEY: The idea that a comet may have been responsible

for the star of Bethlehem is not a very popular one

among scientists.

Primarily for the reason that a bright comet in the sky

would have been recorded in secular

records all over the world at that time.

NARRATOR: In fact, the comet Giotta had seen in 1301

was the Comet Halley, the same comet that had arrived

before the fall of King Harald at the Battle of Hastings

in 1066 and that continues to appear

in the sky above our planet roughly every 76 years.

Halley is by far the most important comet in recorded

history.

It is proof of a theory that explains how the natural world

works, the foundation of our understanding of nearly

everything in physics.

Baltimore, Maryland, July 4, 2005.

The Hubble Space telescopes control center waits for news

from the NASA JPL deep impact mission, one of two NASA

comet missions.

The other, Stardust, will bring back samples of a comet's

tail six months after deep impact is scheduled

to hit comet temple one.

Now, deep impact is moments away from ramming an 800 pound probe

into comet temple one 72 million miles away.

HAL WEAVER: Tonight is the night of celestial fireworks.

You know, the deep impact spacecraft

is going to send an impactor into the nucleus comet temple

one, and the astronomers are training their telescopes

all over the world.

NARRATOR: If the probe hits the target,

Weaver will be up late analyzing the photos,

but that's a big if.

At 20,000 miles per hour, the two craft

are literally traveling six times faster than a speeding

bullet.

KEITH NOLL: The experiment we're going to do

is fire a cannon ball at one of these comets,

and the hope is that the cannon ball is going

to blast through this crust and get to the stuff that's

underneath.

And that's the stuff that we hope

is more or less unchanged since the beginning

of the solar system.

And that's very interesting.

That's what we want to get to.

NARRATOR: Because of a comet that

arrived in the 17th century, astronomers today

can calculate where any one object in the solar system

will be.

How can mathematics be used to bring

temple one and the deep impact craft together?

Gravity.

BRIAN MARSDEN: In the old days going back to Aristotle,

comets were thought to be in the atmosphere.

Because you had to have a well ordered universe with planets

at appropriate distances.

NARRATOR: Even after they realized the sun was the center

of the solar system, they still believed

planets orbited the sun in orderly paths

and in regular intervals.

But comets swooped in randomly from anywhere.

But in 1577, astronomer Tycho Brahe used triangulation

to prove this belief incorrect.

BRIAN MARSDEN: By making observations at different times

of the day and using observations

from different places, he could prove

that comet was several times farther away than the moon.

So that was certainly something.

That was really the first step.

NARRATOR: This was deeply troubling to men

who believed God fixed the stars and planets in their orbits,

and that God's heavens were orderly.

But the work of Isaac Newton and Edmund Halley

in the 17th century expanded these astronomic riddles

into universal truths.

In his monumental Principia, Newton

proposed that objects with mass exert an attractive force

on other objects with mass.

A force he called gravity.

SARA SCHECHNER: Newton and Halley worked very closely

on this.

Newton in the 1680s observes two great comets.

One of 1680 and one of 1682.

They become convinced that the comet makes a hairpin turn

around the sun and that these two comets are actually

one comet.

NARRATOR: One comet going in two directions, inbound

toward the sun and outbound back into the far reaches

of the solar system.

SARA SCHECHNER: And Newton discovers that his new theory

of universal gravitation can be used

to explain the path of this comet

just as it can be used to explain the orbit of the moon

around the Earth or the orbit of the earth around the sun.

So the comet of 1680, '81 becomes a centerpiece

of his Principia.

NARRATOR: To confirm Newton's theory, Halley pored

over historical observations of past comets.

In the process, he made a monumentous discovery.

JAMES W. ASHLEY: Edmund Halley realized that the descriptions

of comets from 1456, 1531, and 1607 actually

matched up very well with the comet of 1682.

NARRATOR: Halley was convinced that these four comets were

one, which returned at roughly 76 year intervals

or periods due to the gravitational pull of the sun.

He confidently predicted that in 1758, his comet

would return again.

JAMES W. ASHLEY: And sure enough, Halley's comet

did return in 1758 on Christmas day and was greeted

with a wide spread fanfare.

But unfortunately, Halley himself

wasn't around to see it.

He died 17 years earlier.

NARRATOR: With Halley's help, Newton

proved comets orbited by the predictable law of gravity.

Newton's laws of motion revolutionized science more

than any other discovery before or since.

Most comets circled the sun in large orbits that never cross

Earth's or any other planets path.

Some are pulled by gravity right into the sun where they burn up

completely.

And rarely, some intersect the orbits of planets,

including Earth.

Even more rarely, comets and planets

find themselves at the same place

and at the same time in an impact.

And all of this is regulated by gravity.

But these two men, Halley and Newton, products of their age,

were reluctant to stand in the light of reason.

SARA SCHECHNER: It's often said that swept away

all the old superstitious beliefs about comets.

But in point of fact, they actually embraced

the old beliefs and raised them up to a much higher level

in their science.

NARRATOR: Newton and Halley believed that God used comets

to create the world and Noah's flood,

that though comets behaved according

to certain physical laws, nonetheless,

they were tools used by God to express his desires.

But, God didn't need to throw comets at the Earth to wipe out

mankind.

Gravity allowed them to find their way all on their own.

Although we don't have specific times and dates,

we do know that over the past four billion years,

comets have been crashing into the earth.

The Deep Impact Mission may give earth the chance to hit back.

1:45 AM, July 4, 2005.

If all went well, the deep impact

probe just plowed into comet temple one.

Scientists here and all over the world

wait for the signal to travel back to earth

across 72 million miles of empty space.

HAL WEAVER: I'm pretty certain that there

will be a nice flash, a nice burst of material.

But other than that, we don't really know what to expect.

We hope that by observing exactly what happens,

we'll be able to tell something about the structure

of that commentary nucleus.

NARRATOR: This uncertainty is partly because scientists

aren't exactly sure how solid temple one's core is, if it's

frozen hard or loose and crumbling.

BRIAN MARSDEN: There's still a lot we don't know.

But when you compare it to what we knew about comets 100 years

ago, that was not very much.

There were all sorts of ideas that comets really

didn't have anything solid to them at all, that they were

just the dust particles, just the dust stream.

NARRATOR: The public, which was looking forward

to the return of Halley's comet in 1910,

received a scare when scientists reported that they suspected

comets, like Halley, contain trace ingredients,

such as arsenic and cyanogen gas, in their tails.

Earth's orbit passed particularly close to Halley

that year, prompting enterprising salesman to offer

gas masks as protection.

JAMES W. ASHLEY: There was widespread pandemonium when

people learned that there was cyanogen gas in the tail,

because they were afraid that the cyanogen gas, which

is a deadly poison, would actually contaminate the planet

and make a toxic situation for them.

But of course, there's really no way that could have happened.

The concentrations are too dilute,

and the cyanogen would have burned up in the atmosphere

anyway.

NARRATOR: It didn't stop there.

The return of Halley's comet proved

to be the sensation of the new century.

The comet was practically a celebrity

featured on the cover of leading magazines

and glorified in song.

It even became an unwitting shill

for an array of seemingly unrelated products

from shirt collars to ink wells.

SARA SCHECHNER: There were bottles of comet champagne

that were uncorked, and they had a final time partying

under the light of the comet.

NARRATOR: But the 1910 return of Halley's comet

was also bittersweet.

America's great humorist Mark Twain,

born in 1835 during the previous return of Halley,

had confidently predicted he would

die during this next return.

It will be the greatest disappointment of my life

if I don't go out with Halley's comet.

The almighty has said no doubt.

Now, here are these two unaccountable freaks.

They came in together.

They must go out together.

Halley first appeared in the sky on April 20

during the 1910 return.

Twain died at his home in Connecticut the next day.

40 years later in 1950, astronomer Fred Whipple

of the Harvard Smithsonian astrophysical observatory

in Cambridge, Massachusetts further

revolutionized our understanding of comets

when his theory of their structure

seemed to solve a dirty little secret buried

in Newton's law of gravity.

A secret that had plagued physics for 200 years.

Comets did not follow the orbits Newton prescribed.

DON BROWNLEE: They do pretty much,

but it was always embarrassing that comets did not perfectly

follow the rules of gravity.

And there was a wonder about this,

and some people even conjectured that, well, maybe the laws

of gravity don't work so far from the sun.

NARRATOR: Whipple insisted that comets weren't wispy

balls of solar fluff, but solid, massive clumps of ice

and rocks.

He memorably called them dirty snowballs.

When these solid but volatile objects flew near the warming

sun, they're gassy volatiles would expand and explode,

like jets from under the surface,

altering the trajectory of their orbits

and throwing off Newton's calculations.

DON BROWNLEE: Fred explained this by this rocket effect,

and the ice vaporizes from the comet.

It produces a little push on the comet,

and it can push it forward.

Or it can push it backwards.

NARRATOR: By explaining Newton's error,

Whipple ironically reaffirmed Newton's theory of gravity.

I would say it is really one of the most important ideas

in the whole of astronomy in the 20th century.

DAVID MORRISON: Fred Whipple had that most important insight

about the nature of comets that they

had to have solid material, that they were compact, that they

were real objects in space.

Not just a loose collection.

That allowed us to understand much better

the orbits of comments.

So in a sense, it was confirmed right from the start,

it was a theory that matched the observations

and helped predict the motions of future comets.

NARRATOR: Whipple was confident he

had solved Newton's dirty little secret, but was he right?

He and we would have to wait nearly 40 years for the answer.

Whipple spent those years establishing his credibility

within the scientific community.

He promoted space exploration and encouraged

amateur astronomers.

Whipple felt careful observations of satellites

were critical to our understanding of the atmosphere

and shape of the earth just as careful observation of comets

had led him to his insight into their structure.

Long before any satellite was launched,

Whipple organized amateur astronomers

into what he called the moon watch.

So the amateurs, in particular, the moon watch

program, was ready when the Russians did rather

unexpectedly send up Sputnik in October 1957.

So he was ready for that.

They got the orbit flight, learned

things about the figure of the earth

and the density of the atmosphere.

So that was a very great thing that he did.

At 6:17 this morning, we did obtain a successful photograph

of this first satellite in motion.

This is not a still photograph that you're going to see,

but rather one which is a motion picture.

Let's see, Dr. Whipple, if we can--

That is wonderful.

Isn't it?

It is moving.

Are we correct about 18,000 miles per hour?

That's right.

NARRATOR: For his work with Moon Watch,

Whipple received the distinguished service award

from President Kennedy in 1963.

Dr. Whipple conceived and developed an optical satellite

tracking system, which stood ready to track

the first artificial satellite launched

and has since provided valuable scientific data concerning

the nature of the Earth, its atmosphere, and outer space.

His career--

LAURA WHIPPLE: My father brought the whole family to Washington

to the White House to meet President Kennedy when

he received the award for distinguished public service.

NARRATOR: Confirmation of Whipple's dirty snowball theory

finally came in 1986 when he and the rest of the world

received proof.

Comet Halley made its first modern return voyage.

Some of us had a bit of a concern

that it would be hard to find the nucleus there, you know,

all the stuff that has come off an active comet.

I do remember Fred saying, no, don't worry about it.

The nucleus will be perfectly clear,

and indeed, it was, showing the comet in action in pretty much

the way he suggested.

NARRATOR: Unlike Edmund Halley, Fred Whipple lived

to see his theory confirmed.

19 years later, deep impact was about to expand

our understanding of comets even further.

The mission is a success.

Deep impact plowed right into the comet.

Steve, we've got a conformation.

[cheering]

[applause]

Oh, my god, look at that.

NARRATOR: Images start pouring in at Hubble's control center

and all over the world.

HAL WEAVER: The spacecraft images are spectacular.

The highest resolution images ever of a commentary

nucleus, and there's going to be, I'm sure,

a lot more questions raised and answers probably.

But just to be able to watch that thing homing in

on the nucleus, I mean, that's really spectacular,

and I guess we may never see that again.

NARRATOR: The photos are 10 times more detailed

than any previous comet photos.

The surprisingly bright and opaque plume cloud

indicates the comet is covered in a fine, talcum like powder.

So big was the plume, the companion craft

may not have gotten a good look at the crater.

About 11,000, 21,000.

Oh, that's amazing.

There's the before and after.

We got our first look at the HST images.

And in the 15 minutes between impact

and the end of our observing window,

we see a fivefold increase in the brightest pixel

at the center of our image, as well as an extension in a coma.

But we're in the process now of trying to measure that.

NARRATOR: Deep impact will provide scientists

with a wealth of information about comets.

It will be months, maybe years before the volumes of data

from this impact are analyzed.

Tomorrow, the real work begins.

Two, we have main engine start, zero, and liftoff

of the Stardust spacecraft returning a time capsule

with the elements of the formation of our solar system.

NARRATOR: Six months from the memorable 4th of July

full of celestial fireworks, deep impacts

companion missions Stardust launched in 1999

is expected to return to earth with samples from a comet's

tail.

But one thing is already clear.

Comets, like temple one, are deadly if they

cross Earth's path.

Are there any headed our way?

If the Deep Impact mission will show scientists what they can

learn from going to a comet, then Stardust

will show them what they can learn from bringing one back.

DON BROWNLEE: Comets are made out

of particles that are less than one millionth of a meter

in diameter.

And to study those in detail the mineralogical

and organic composition, you really

need tools that are much more sophisticated, and complex,

and unreliable that you can fly on spacecraft.

3, 2, we have main engine start, 0, and liftoff

of the Stardust spacecraft.

NARRATOR: Stardust is the first US space mission dedicated

solely to exploring a comet.

The mission launched on February 7, 1999,

well before Deep Impact with the goal of catching elusive grains

of material from comet [inaudible] 2

and bringing them back to Earth to be studied.

No one knows exactly what Stardust will find.

DONALD YEOMANS: I think comets have been surprising

from day one.

They were misunderstood 2,000, 3,000 years ago,

and they're still misunderstood.

Any mission of this type, there's

only one thing that's guaranteed,

and that's surprises.

NARRATOR: The most interesting part of the Stardust mission

may be the device designed to capture and store

the bits of the fuzzy, dusty headed

the comet called the coma as it encounters it in space.

SCOTT SANDFORD: The collector looks almost

like a tennis racket.

And if the strings are the frame,

then between the strings and the holes

is a material called aerogel, which has the world's lowest

density solid.

It's, in a sense, fluffy glass.

And this material has a low enough density

that when particles impact, it actually

can punch a hole into this low density material

and come to a stop without being completely destroyed.

NARRATOR: Assuming the Stardust mission proves

as successful as Deep Impact, a capsule containing

the low density fiberglass like aerogel

will touch down in the Nevada desert

on the morning of January 15, 2006.

Inside the aerogel will be bits of a comet.

The particles harvested by the Stardust mission

may be the first pristine examples of comet dust

to enter Earth's atmosphere, but bits of comet

hit the Earth every day.

In a meteor, dust falls on earth all time.

And all the meteor showers that people go out to see,

the Perseid's, the Orionid's, the Geminid's,

and all these things, these are all largely from comets,

dust from comets.

NARRATOR: Again, meteors or shooting stars

are the visible display of light in the sky as a bit of dust

and dirt burns up upon entering our atmosphere.

In other words, comet dust is always falling on us from space

as our planet intersects the debris trail left

in the wake of a comet.

DANIEL W. E. GREEN: What happens is that as a comet goes

around the sun, and it heats up and sublimates,

and it casts off material.

The material spreads out into what you might

think of as a stream of debris.

If the comet's orbit intersects the earth's orbit,

then the earth can run into that debris every year.

NARRATOR: Sometimes, comets themselves

can break into pieces.

BRIAN MARSDEN: When a comet goes very close to the sun,

it can be split tidily by the sun

by the tidal action of the sun, breaking up into many pieces.

NARRATOR: Meteorites are rocks found on Earth's surface

that have an extraterrestrial origin.

They fell from the sky.

Most meteorites are pieces of asteroids fractured as they

entered earth's atmosphere.

Many can be found in public and private collections.

DANIEL W. E. GREEN: We see meteorites hitting the earth

all the time.

You know, there are tons and tons of meteoritic debris

that come down on the earth on a regular basis.

NARRATOR: These objects occasionally

produce spectacular fireballs as they puncture the atmosphere,

flaring up brilliantly and breaking into pieces as they

fall to earth.

But until the year 2000, scientists were in agreement

not one meteorite in any collection

can be traced back to a comet.

But in the year 2000, a spectacular fireball

that flew over the Yukon territory in Northern Canada

seemed to settle the mystery.

The meteorites this fireball produced

were like no other meteorites in any collection anywhere.

They were the oldest objects anyone had ever touched.

Oh, very nice.

NARRATOR: Could they be pieces of a comet?

Beat and Jacqueline Korner run Tagish Lake Wilderness Lodge

in Tagish Yukon territory, where they lived with their 21 sled

dogs.

They have grown accustomed to bear, wolves, coyote,

and other wild creatures that surround them

in this remote corner of Canada.

But they weren't prepared for what they saw the morning

of January 18, 2000.

JACQUELINE KORNER: I just wanted to make a fire,

and I was walking toward the stove.

We had the propane light going in the cabin,

and suddenly, it got daylight in the house.

So I was looking outside the window,

and I thought just like, what is Beat doing?

BEAT KORNER: It was still very dark outside.

I could barely see just the tip of the mountains,

and then suddenly, the whole sky just lit up.

And I looked up, and I saw, first, in the beginning,

I just saw how much the landscape was enlightened.

And gradually, I saw in a high speed,

there came an object, like a flare,

coming in parallel to these mountain ridges.

JACQUELINE KORNER: Then I heard a big bang,

and then the whole house started to shake.

BEAT KORNER: Jacque came out from the lodge

and said, what happened?

And I said, well, it looks like a big meteorite just

was flying by.

And we were both very glad that we have not been hit.

NARRATOR: The fireball was seen for more than 500 miles.

ALAN R. HILDERBRAND: A week later, a local resident

was driving south on the ice of Tagish Lake

and found the meteorites.

NARRATOR: Hildebrand and colleague Peter Brown

of the University of Ontario organized an expedition

to the lake to collect the meteorite pieces and a search

for more.

They we're fortunate that this particular meteorite landed

in such a landscape.

Very nice.

NARRATOR: Even before they brought it back to their labs,

they knew it was unusual.

Let's see if we can zoom out on that.

There we go.

This is an anterior piece.

See the white inclusions?

MARGARET CAMPBELL-BROWN: Well, the first thing that we want

to know about any meteorite is how dense it is,

and we could see right away that Tagish Lake was very unusual.

Because normally, when you pick up a meteorite,

it's quite heavy for its size.

That's often how you know that you have a meteorite, not

an ordinary earth rock.

But Tagish Lake was quite a lot lighter than an ordinary rock.

It felt sort of like a charcoal briquette in your hand,

and it turns out that Tagish Lake is actually

the least dense meteorite that's ever been recovered.

Oh, Alan, beautiful.

NARRATOR: The meteorite was also exceptionally fragile,

and it appeared to have a high concentration

of organic material.

Soon, speculation grew that the meteorite

might have come from a comet.

Scientists wondered what a piece of a comet might look like.

Because we still know so little about the makeup of comets,

scientists disagree whether they can produce meteorites.

DAVID MORRISON: The question is, are there

big pieces of solid material that would make it

through the atmosphere?

Maybe something like Tagish-like meteorite

is what it would look like.

But if they only come in little, small grains,

then those won't make it through the atmosphere,

and we will never have a cometary meteorite.

KEITH NOLL: I think what you're asking is, if there are chunks

of rock, something that you might go out and find

in your yard inside of a comet, probably not.

NARRATOR: Not everyone agrees with scientists

who hold this view.

DANIEL W. E. GREEN: No, I think they're absolutely wrong.

I think that comets could very likely have very large, planet

like chunks in them.

Some comets could have a very appreciable impact.

NARRATOR: And it's not just individual fragments

that can do damage.

Certainly, whole comets can wreak incredible havoc

as a comet proved in 1994.

By the late 1990s, geologist Eugene Shoemaker

had convinced many in the scientific community

that extraterrestrial bodies had impacted earth in the past,

and there was no reason to think they wouldn't in the future.

Many felt this was highly improbable.

DAVID LEVY: Well, Gene, and Carolyn shoemaker,

and I did find something.

March the 23, 1993, the comet called Shoemaker Levy 9.

It was announced a few weeks after discovery that comet was

going to collide with Jupiter.

Not with the earth, but with Jupiter.

It was about 16 months between discovery and impact.

If we were to have found a comet that was 16 months away

from colliding with the earth, there isn't a whole lot

we could have done about it.

NARRATOR: Jupiter's gravity had torn the comet

into pieces, which all headed toward impact with the planet.

DAVID LEVY: For the first time in human history,

we were going to actually witness the impact of a comet

against a planet.

And on July the 16, 1994, all the way to July 21, 1994,

we were all transfixed as 21 fragments of this comet

collided, like a freight train, one thing after another

into the atmosphere of Jupiter.

These comet fragments were traveling at approximately 40

miles per second.

At that rate, each of these comets

would cross the United States in about a minute.

DONALD YEOMANS: That generated something

like six million megatons of equivalent energy.

That's roughly one Hiroshima type blast every second

for 13 years, so mother nature has an arsenal

that is really unparalleled.

NARRATOR: Clearly, comets collide into Earth,

because we have the impact craters to prove it.

Science tells us based on the number of asteroids and comets

in our solar system, at least 5% of Earth's impacts

must be from comets.

Which one of Earth's nearly 200 known craters

might be from a comet?

ADRIENNE FITZGERALD: We're standing here

at the rim of Upheaval Dome, which

as you look into this feature, you can see is not really

a dome.

But it's more of a crater or a hole in the ground.

Down to the bottom, it's probably

about 1,500 vertical feet.

We're up here on the top edge of it.

And across, we're looking at maybe about two miles

to the next rim and three miles for the entire structure

across.

So it's a pretty big feature, and it stands out really

clearly in the Colorado plateau.

Because it's a spot where the rocks are sticking up

at very bizarre angles, and we can see that right

in the middle here.

So something pretty dramatic happened in this spot.

NARRATOR: Gene shoemaker was one of the first to identify

Upheaval Done as an impact crater.

He and his wife Carolyn found dozens of others

all over the Earth.

When an impact takes place with a comet or an asteroid,

it's a tremendously energetic event.

The object explodes when it hits with 100 times

the force it would have if it were made of TNT,

and it explodes, whether it's a comet or an asteroid,

leaving very little, if any trace.

NARRATOR: It seems logical that a comet made up largely of ice

would melt as it entered our atmosphere.

DAVID SCHLEICHER: If it were traveling

at six miles per second, that would probably be true.

But it the hits the upper atmosphere,

and five seconds later, it's hit the ground.

There's no time to melt. There's no time to vaporize.

If you have a big enough chunk of material,

it's going to hit you.

There's no escape.

NARRATOR: After analyzing the Tagish Lake meteorite

and its trajectory, scientists were

able to say that the meteorite is not a piece of a comet.

But the Tagish Lake meteorite, the oldest, least dense,

and most unusual meteorite ever found can still enhance

our understanding of comets.

Scientists concede, if a comet chunk were to land on earth,

it might be very similar to the strange and unusual Tagish Lake

meteorite.

WILLIAM BOTTKE: The differences between asteroids and comets

are slim.

So we may just be talking semantics here.

If you have a piece of a very, very primitive body,

it may not matter if you call it an asteroid or comet.

It's still telling you something about the kinds of conditions

that existed in the solar nebula in that region of space

when planet formation was taking place.

NARRATOR: When comets plow into the earth,

the craters they make are identical to those

made by asteroids.

But there are differences between asteroids and comets.

For one thing, NASA has theories about how to move an asteroid

out of Earth's path if one were heading towards us,

but they have no plan about what to do if a comet lines us up.

Comets are too big, often miles across,

and show up too suddenly for us to make a plan.

So what's the plan?

So far, no earth killing comets have been spotted

heading in our direction.

But we could find one tomorrow, or we

could be blindsided by one without any warning, which

is why we look.

What would humans as a species do if, say, an average sized

comet was discovered in the next five years

with an orbit that would send it colliding with earth?

We'd die.

A lot of us would anyway.

It's easier to deal with asteroids.

They're usually smaller, and we'll

have tracked most of the larger ones over the next few decades.

Asteroids usually stay within the narrow band

of the asteroid belt, so we know where to look for them.

But comets can come from any direction in space

since the Oort cloud, the distant place most comets can

be found, isn't a band of debris,

but a shell around the entire solar system.

DAVID MORRISON: We do not have the technology

to defend against a comet.

We don't know how to alter its orbit.

We don't have the energy to blow it apart.

If we had warning that the comet was going to hit the earth,

we would just be trying to make the plans for how to survive

the impact.

We might evacuate the area where it was likely to hit,

but we couldn't stop it.

Comets are nature's unstoppable projectiles.

BRIAN SKIFF: Gene Shoemaker told me his hunch

once, which was that as a result of all of our efforts to find

all the near earth bodies that we'll find,

that something may be coming in to hit us.

But it will be 10,000 years downstream,

and thus, you don't do anything until 9,950 years from now.

NARRATOR: So we watch the skies, and we count the comets

and asteroids.

And we keep our fingers crossed hoping we don't find anything

headed our way, until we've got the technology to move it.

After comet Shoemaker Levy hit Jupiter,

all that watching took on a more urgent character.

The United States Congress acted to provide funds

for a space guard survey that would catalog everything

in our solar neighborhood.

That means roughly 40,000 asteroids bigger

than half a mile in diameter.

Nobody knows for sure how many comets there may be.

Probably trillions, and they're too far away to count,

unless they come into the inner solar system.

DAVID MORRISON: The purpose of the Space Guard Survey, which

is underway now, is to make sure that no comets or asteroids

sneak up on us, that we can do a complete survey of the sky,

and find these objects months or years before they hit.

Otherwise, you know, something really could sneak up.

BRIAN SKIFF: We report the observations

that we make to the Minor Planet Center

in Cambridge, Massachusetts, which

serves as a worldwide clearinghouse for these sorts

of observations.

NARRATOR: Astronomers all over the world

report new objects to Brian Marsden's Minor Planet

Center, where they are evaluated,

and cataloged, and watched.

The goal is to make sure we see anything headed

our way with plenty of warning.

The fireball that lit up the southwest United States

on the morning of January 15, 2006 was expected.

The Stardust capsule became the fastest manmade object

to re-enter Earth's atmosphere.

Recovery crews rushed to find the capsule

to see if the samples it brought back within the arrow gel

packets at the end of the collector survived reentry.

Comet expert Fred Whipple would have loved

being there for the landing.

He had watched it blast off in 1999 for its rendezvous

with comet [inaudible] 2 and wanted to see it come back.

LAURA WHIPPLE: When the Stardust mission came together,

my father was on the team, and at that time,

he was the oldest living scientist to be involved

on a team like that.

So it was very exciting, and we were all

really saddened by the fact that he wasn't

able to live to the completion of that project

and find the results.

Had he been alive, it would have been in his hundredth year.

Only lived to 97.

Didn't quite make it.

NARRATOR: Whipple died on August 30, 2004, a year

and a half shy of Stardust's return.

NASA finds that the samples caught inside the aerogel

are pristine, even better than expected and larger

than anyone anticipated.

Now, they will be sent to scientists all over the world

for further study.

Others from a new generation of scientists

will inspect the material collected

from Stardust's aerogel.

Missions, like Stardust and Deep Impact,

keep astronomers busy for years analyzing information they've

gathered, and more will likely be planned.

We'll continue to study comets.

Not only because what we learn can inform us

about our own origins and the possibility of life

throughout the universe, but also, because what we might

learn may save our species.

We won't all live to find out the answers,

but we have faith that science and reason will

be our salvation.

How ironic that future generations

may save us from the very thing that gave us life.

We are the children of comets.

Our elements are their elements, and our futures

are inextricably linked.