NARRATOR: In the beginning,
there was darkness. And then bang, giving birth to
an endless expanding existence of time, space and matter. Every day, new discoveries
are unlocking the mysterious, the mind-blowing, the deadly
secrets of a place we call the universe. Chart a daring course with
comets, the cosmic time travelers of the skies. These mysterious objects
have been agents of change. They've been agents
of destruction. NARRATOR: Discover why these
celestial bodies carry secrets from our past, and cast
doom over our future. If it's getting
into our onramp, we need to worry about it. There's very little we
can do, other than sending a giant bomb toward them
and trying to deflect them. NARRATOR: Brace yourself
as we rendezvous with the frozen intruders from
the great beyond on an epic journey to ride the comet. [main theme] They appear out of
nowhere, gliding slowly across the nighttime sky, and
then disappearing from sight. Throughout human
history, ancient cultures believed these celestial
intruders carried special powers from the gods,
mystical messengers of life and death. Scientists like to think
that comets are fairly mysterious objects, because
we know that they're very old. They're like frozen
time capsules from when our solar
system was first formed. DON YEOMANS: Comets
are a bit of show-offs. They come into the night sky
unannounced, unpredicted. In the Middle Ages, they were
thought to be fireballs thrown at a sinful Earth from the
right hand of an avenging God. And so they've always been
feared or seen as mysterious. NARRATOR: We're
going to hitch a ride on a bold, uncharted journey
through the secret life of our solar system's comets. Our daring quest begins
4.5 billion years ago as our futuristic spacecraft
time travels back to the birth of our solar system. We zig-zag through a
maelstrom of debris to catch our first glimpse of
comets, the wayward rocks that didn't meld into planets. Comets are basically dirty
snowballs or icy dirt balls. They consist of rocks and
dust and dirt held together by ices-- water ice, ammonia, methane,
carbon dioxide, dry ice. That's sort of
like the glue that holds all these little pebbles
and particles of dust together. NARRATOR: The heart of
these cosmic icebergs is the nucleus, or
central core, which averages in size
from a half mile wide to as big as a modern city. One of the things we've
learned by flying close by and observing the
nuclei of comets is that they're these strange
sort of compacted conglomerates of ice and rock. They're not uniform. They're not perfectly mixed. They're quite chunky
and oddly-shaped. DON YEOMANS: The
nuclei of comets are very fragile entities. If you had a chunk, you could
probably break it easily in your hands. NARRATOR: Up close, comets look
like bland chunks of charcoal, rather than
super-sized snowballs. They don't even remotely
resemble the glowing orbs we see in our pitch-black skies. AMY MAINZER: We think that
most comets are extremely dark on their surfaces. That the surfaces are
covered with something kind of like barbecue soot-- this really dark black material
that makes them very difficult to see. NARRATOR: While their
asteroid cousins share similar properties,
such as rock and metal, scientists suspect that
comets were born farther out, near the gas giant planets,
where it's much colder. So they possess more ices
and volatile elements. About 3.8 billion years ago,
many of these icy bodies were whisked away to the Kuiper
belt, a frigid neighborhood and just beyond the
outermost planet Neptune. Icy objects in the Kuiper
belt probably formed closer in toward the Sun, where
the giant planets are. But then gravitational
interactions with those giant planets
made those icy objects migrate outward. NARRATOR: Our
spacecraft navigates through this dark
disk-like region, which is almost 2 billion miles
wide and is estimated to hold over 6 billion comets. We eventually touch down on
the surface of one of these icy boulders. AMY MAINZER: Landing on a
comet would be a pretty tricky affair. So you can kind of
imagine dirty snow that's covered with exhaust. And if you step on it in places,
you can punch through to places where there's more fresh
material underneath it. NARRATOR: The Kuiper
belt objects appear to have fairly stable orbits. So they won't be that
much fun to ride. We lift off in search of a comet
that will cut a more exciting path through our solar system. We navigate farther out to
a sparsely populated region called the scattered disk,
which is home to icy bodies with long elliptical orbits. The slightest disturbance can
send these unstable objects careening into the inner solar
system, where they become known as short-period comets, those
which orbit the sun every 20 to 200 years. The scattered disk region
is thought to be the primary source of the
short-period comets. They are kicked into
the inner solar system either by interacting
with Neptune, or possibly passing the stars. NARRATOR: Our
spacecraft witnesses Neptune's gravitational
influence at work, dislodging a comet out
of the scattered disk. This comet would be too
dangerous to land on. So we'll track alongside it
on its transformational flight through our solar system. PETER SCHULTZ: So what
would happen if it was on a journey toward the Sun? Well, it would start off
looking like a dark asteroid. Nothing much is
happening It doesn't have an atmosphere around it. LAURA DANLY: As
you start to travel toward the inner solar system,
get up to Jupiter, or even closer to the Sun, the comet
would start to become active. AMY MAINZER: The
warmth from the sun starts to heat up the comet. And all the frozen material
that's trapped on the inside starts to convert from
ice into gaseous material. And so these gas
jets can actually punch through the crust of
the comet and form jets. As these jets flow
away from the surface, they can start to pull
dust and rocks with them. And this forms a kind of hazy
atmosphere around the comet's nucleus. We call this the coma. NARRATOR: As it zooms by
Earth, the coma of a comet can grow to be larger than
the diameter of our sun. Solar radiation eventually
pushes this cosmic halo behind the nucleus,
forming two bright tails-- one of dust and
the other of gas. LAURA DANLY: There will be
a tail growing and growing and growing. And you would eventually
become engulfed in just a really rocky, icy,
particle-filled environment. It would be very difficult
on your spacecraft because it would be like
flying through a blizzard. NARRATOR: Some tails can
stretch for 100 million miles, more than the distance
between the Sun and the Earth as it travels towards
our home star. It's one of the most spectacular
spectacles of our solar system. Today, we're going to see
what happens when we take a comet that's been in the
cold, dark, icy outer reaches of the solar system. And we're going to send it
inward towards its perihelion, or the point of closest
approach to the sun. Now, dry ice is
frozen carbon dioxide. And this is a very common
constituent of comets. Let's see what happens. When we take this
piece of dry ice, and we drop it into
the warm water, the dry ice starts to sublimate. That means it converts directly
from a solid, frozen state into a gaseous state. And as the water starts to warm
up certain parts of the dry ice first, you can start to see that
little jets kind of form on it. And this forms a hazy
atmosphere called a coma. You can see that the
little chips of dry ice are constantly changing
as they sublimate away. Sometimes they speed up. Sometimes they slow down. So if this was a comet,
you can get the idea that, as the comet approaches
the Sun and the volatiles start to vaporize,
this can actually change the comet's rotational
state, and even its orbit. NARRATOR: Comets are brightest
and fastest when they're closest to the Sun. The searing heat and immense
gravity of our mother star can accelerate a comet's speed
to nearly 2 million miles per hour. The speed increases a lot,
and it whips around the Sun. So it goes zoom, like that-- like being propelled at the
bottom of a giant roller coaster. A roller coaster starts
out high very slowly. Picks up speed due to gravity. Goes zooming past the bottom,
and then slows down again on its outward journey. Eventually, as our comet
leaves the vicinity of the Sun, it's going to travel back
out into the far reaches of the solar system. Everything's going
to quiet down. All the gas and dust is going
to settle back onto its surface. And eventually, it'll cruise
away into the quiet dark. NARRATOR: Back in the cold
region of the scattered disk, a comet transforms back into a
dull black lump of rock and ice until it makes its journey
around the sun once again. Halley, the most famous
short period comet, was first recorded in 240
BC, and has passed by Earth every 76 years. Estimated to be the
size of Manhattan, this super-star comet
is sometimes so bright, it has been seen
in broad daylight. Throughout history, the
appearance of Halley has inspired awe and dread
among earthly observers. DON YEOMANS: In 1910, it was
feared because spectroscopy had just been introduced. And they had determined that
comets contain cyanogen, which is a odorless and poisonous gas. And so astronomers
predicted that there'd be problems because of
this poisonous gas sweeping the Earth. And so there were actually
comet pills manufactured. Life insurance policies on
the comet were taken out. NARRATOR: While comets
have been thought to be extraterrestrial
harbingers of doom, astronomers have long wondered
if these glowing rocks contain precious information
about our past. By the 21st century,
NASA was ready to ignite a bold offensive strike. One that would hopefully
expose one of the holy grails of planetary science. Could these
interplanetary vagabonds have been the cosmic
transporters of life? [ambient music playing] So far, we've traveled with
comets, from the frozen hinterlands of the Kuiper
belt and scattered disk to an exhilarating
orbit around the Sun. We've watched these dark bodies
come alive as they formed brilliant comas and tails. But scientists are still
puzzled about what's really inside these cosmic interlopers. PETER SCHULTZ: Man, I wish
I knew what the interior of a comet looked like. I would love to be able to look
inside the nucleus of a comet to see if it's clumpy,
if it's uniform. Is there crystalline ice? Is there glassy ice? NARRATOR: Our protective
spacecraft time travels back to 2005, and meets up with
NASA's Deep Impact space probe on a comet hunting mission. It zeros in on a
short-period comet named Tempel 1 with an
aggressive plan of attack. We hover at a safe distance
as Deep Impact launches an 800-pound projectile
into the passing comet. Debris sprays out for thousands
of miles as the impactor excavates a crater, and sends
tons of blinding gas, rocks, and dust into space. For the first time in
history, scientists are able to peer inside the
icy, muddy interior of a comet. AMY MAINZER: Right
after the impact, you could see silicate grains
fluorescing and glowing. This tells us that comets
are very primitive material, and that they pretty much do
preserve in pristine state the materials that we
find in the youngest star-forming systems. So indeed, they really
are time capsules that let us look back
on our own solar system at the very beginning. NARRATOR: While Deep Impact
captures a rare glimpse of a comet's interior, it is
unable to snap clear images of the crater,
which was obscured by an enormous plume of debris. In February 2011,
another spacecraft named Stardust-NExT
catches up to Tempel 1 as it completes its five-year
orbit around the Sun. To everyone's surprise,
the impact crater is much smaller than
had been expected from the historic smash-up that
occurred five years earlier. PETER SCHULTZ:
Amazing, but the crater looked like it was small. So this now creates this issue. Was the crater always small? We know the crater
isn't what we expected. So we're going to do
some experiments to see if we can explain why the
crater looks like it does. [inaudible] light's coming on. Can you move up a bit? At NASA's Ames
Vertical Gun Range, Stardust NExT co-investigator
Pete Schultz is conducting high-velocity impact experiments
to find out why Deep Impact produced such a tiny crater. He will use a massive 30
caliber high-velocity gas gun to fire tiny projectile beads
at various targets located inside a vacuum chamber. PETER SCHULTZ: We're
trying to simulate what the nature of the surface
of comet 9P/Tempel 1 is like. So we're putting in perlite,
which is this low-density material that you'll
find in gardens. So we're adding in
hollow microspheres. And this also resembled what
the nature of the comet is like. The purpose of the experiment
is to see what happens to the crater after formation. Does it stay there? Does it collapse? With this experiment, we
should be able to find out. OK, I'm out of the tank. Lock and load. NARRATOR: High-speed cameras
mounted around the vacuum chamber will
document the impact. PETER SCHULTZ: We've
got this one covered. We just have to hit the target. OK. Let's shoot this puppy. JP, are you ready? NARRATOR: As the crew anxiously
awaits in the control room, Pete mixes science with
a little superstition. PETER SCHULTZ:
There's always a risk. Sometimes you miss. Sometimes you hit. [explosions] Whoa. Oh, sweet. The whole thing sort of
collapses in on itself. It's getting smaller now. Yeah, it was bigger. Now it's getting smaller. So what we see here
is that there's a lot of dust and
ejecta sent upwards. And that blocked the view. We couldn't see
the crater forming. So what started off being
a nice-looking crater, it just doesn't stay there. It just simply heals itself. So what may have
happened for 9P/Tempel 1 nucleus is that the
crater healed itself. The nucleus healed itself
from the scar created by us with Deep Impact. Let's see what we did. So the results were fantastic. We got to see just
what we planned to. There may be more to the story. So we're going to try a
different experiment as well. NARRATOR: Pete has a hunch
there's a missing piece to the Deep Impact mission. He now wants to see if the space
probe created a different type of crater on Tempel 1 due to the
makeup of the comet's nucleus. So this time, we're
going to put a denser layer below that perlite. So we want to find
out whether or not we'll get a different
type of crater if we have two different
types of material. One is very soft on top, and
one is denser on the bottom. NARRATOR: Pete and his crew
return to the control room to see what happens to the
new target of perlite poured over a heavier layer of sand. PETER SCHULTZ: Oh! Now, that formed a big
crater in the perlite. Boy, look at that. Oh, man. That is gorgeous. OK, the projectile went deep. And now the stuff is
coming back out that hole, while on the surface,
it's excavating debris. Pow. OK. NARRATOR: The double-layered
target created not one, but two craters. A small crater inside a
larger, shallower crater. This could be what
happened for 9P/Tempel 1, because it went so
deep into the nucleus. And then it just
simply collapses away. And we're left with just
this very, very faint rim on the outside, with a
small pit in the center. NARRATOR: These
impact experiments yield surprising clues
about the interior of Tempel 1 and perhaps all comets. PETER SCHULTZ: Now we know
in that comets have history. We see layers. I don't know if these
layers go all the way through the nucleus, or
are they only in one part? NARRATOR: Unraveling the
secrets of what's inside comets will help scientists understand
what causes them to exhibit some really bizarre behavior. Our spaceship now shadows
NASA's EPOXI mission as it encounters Hartley 2,
a fast-spinning comet that tumbles through space like
a hyperactive cosmic peanut. AMY MAINZER: Hartley
2 is a smaller comet. It's only about a
mile or so across. And so it's kind of a
surprise that it's so active. It's putting off huge amounts of
CO2, and actually cyanide gas. And the question is, are all
small comets active like this, or only a few? And what makes them so active? I've got a peanut-shaped
plastic bottle here that's supposed to
represent comet Hartley 2. It's full of dry
ice and warm water. And as you can see, as the warm
water starts to make dry ice sublimate, it shoots
out these jets through the holes that we've
got poked in the bottle. If I drop it into the water,
instead of holding it fixed in space, you can see
that the jets actually start pushing the bottle
around, causing the comet's rotational state to change. This is very similar to
what's going on on the surface and comet Hartley 2. The CO2 jets are actually
changing the rotational state of the comet, causing it
to speed up and slow down. NARRATOR: When
tracking Hartley 2, our spacecraft gets caught
in a cosmic blizzard as the spastic comet
burps and belches out a trail of frozen snowballs that
extend for millions of miles. PETER SCHULTZ: One of the
big surprises about Hartley 2 is that it was surrounded by
this posse of mini comets. This is simply ices
that are coming off about the size of a snowball,
all the way up to a basketball. NARRATOR: Scientists suspect
the snowball-sized debris rains back down on Hartley 2,
producing its unusual landscape of craters and towering spires. MICHAEL MISCHNA:
Because these comets have very little
gravity, most of the gas has blown out into space. But a very small
fraction can actually re-deposit on the surface,
generating very smooth textures and other locations generating
this very bumpy or spired-like texture. NARRATOR: From Hartley
2, we blast off to the extreme outer
edge of our solar system, a vast eerie place barely
within the gravitational grasp of our sun, and
impossible to see even with modern telescopes. Here, we come upon the largest,
and perhaps most elusive, icy bodies in our
galactic neighborhood. [dramatic music playing] Our journey through the
fascinating world of comets has provided us a ringside
seat to some of the greatest cosmic shows observed
from Earth and space. We now change course, and travel
over 50,000 times the distance between Earth and the
Sun to the outermost edge of our solar system. We arrive at the Oort cloud,
an even larger fraternity of comets-- perhaps over a trillion of them. If we'd actually
gotten a spaceship, and tried to go out and
visit their Oort cloud, this would have
been a long journey. There's billions and
billions of these objects. That's just a lot of
space in between them. NARRATOR: Like the Kuiper
belt and scattered disk, the Oort cloud objects may
have formed closer to the sun. But about 800 million years
after the solar system was formed, the gravitational
influence of the gas giant planets flung these comets
out to the frigid edge of our solar system. Some of these
objects were flung into very highly elliptical
orbits out into the Oort cloud. Passing stars could
circularize those orbits, making somewhat stable
orbits for them. NARRATOR: Most of
the Oort cloud bodies have been in frozen
hibernation since the birth of our solar system. They only become
long-period comets when they get sucked
into orbits that carry them inward towards
the planets and the Sun. Scientists have never
actually captured an image of the Oort cloud. But they have good reason
to believe it's there. Even though scientists can't
directly observe the Oort cloud, because it's so far away
and so faint, what they are able to do is infer
its existence, because we can look at where all
these long period comets come from on the sky. And they seem to come from
all different directions. There doesn't really seem to be
a preferred direction for them. This suggests that the
Oort cloud, if it exists, is probably roughly spherical. NARRATOR: One of
the big mysteries is what knocks these
icy bodies off course and sends them cruising by
our neighborhood of Earth. LAURA DANLY: The comets
in the Oort cloud are very susceptible
to gravitational pulls from other things
outside the solar system. One thing is passing stars. If a star happens to
come close to our sun, it can scatter comets
from the outer Oort cloud. Icy objects in the
Oort cloud can also be dislodged by gravitational
perturbations that occur when the solar system
goes through the plane of our galaxy. So the solar system is basically
orbiting around the center of our galaxy, but it's also
oscillating up and down. And when it goes
through the plane, then gravitational interactions
can perturb objects from the Oort cloud into
the inner solar system. NARRATOR: Some
long-period comets take up to about
30 million years to complete one round
trip around the sun. Our spacecraft tracks
a long-period comet for hundreds of
thousands of miles until we approach planet Earth. As its icy tails unfurl,
a layer of dust and ice zips through our
planet's thin atmosphere, leaving visible trails
known as a meteor shower. AMY MAINZER: I like to think
of comets as sort of the pig pens of the solar system,
because as they orbit around the sun, they leave a big
messy trail of debris. But sometimes, the Earth's
orbit can intersect some of these dust bands. And when it does, we can
sometimes see meteor showers. And those beautiful
bright streaks of light that you see
coming through the night sky are actually
produced by particles that are most often no
bigger than a sand grain. NARRATOR: While long and
short-period comets make up the bulk of comets that
have been observed, new evidence suggests that
not all icy bodies come from the sub-zero suburbs
of our solar system. Some have secretly taken up
residence in a much warmer neighborhood. Scientists have now discovered
there are comets masquerading as asteroids in the asteroid
belt, located between Jupiter and Mars. The only reason that we've
discovered these main belt comets is because occasionally,
the warming rays of the sun get into the interior,
vaporize the ices. And then so the comet just
burst forth for a while. And then they go back to being
inactive objects or asteroids. Scientists used to think that
there were strong distinctions between asteroids and comets,
and that they were two totally different types of objects. But what we're finding nowadays
is there are some asteroids that have comet-like
properties, and there are some comets that eventually
kind of look like asteroids. NARRATOR: Some scientists
think main belt comets may have delivered water to early
Earth, and the materials to create life. We blast back to January 2004,
and follow NASA's Stardust spacecraft on an
unprecedented mission to collect pure
comet dust in space. Just beyond the planet
Mars, we meet up with a comet named Wild 2. Upon approaching
its enormous coma, Stardust flips open a
paddle-shaped collector tray, filled with a durable foam-like
substance called aerogel. Cometary particles, no
bigger than specks of dust, fly into the aerogel at six
times the speed of a rifle bullet. The mission's task is to
preserve the precious particles without damaging
or altering them. But this is no easy feat,
as we show here on Earth. We're here at a firing range
with Sergeant Connett Brewer, a former army paratrooper, who's
going to demonstrate for us what happens when
this birdshot strikes a large block of modeling clay. This is an excellent analogy
for what happened when cometary particles impacted
the aerogel on the Stardust spacecraft. Except there, the particles
were moving about 50 times faster than this birdshot. Ready to give it a go? Yes I am. Glenn, put on your eye
protection and your earplugs. What we're using for
this today is a 12 gauge model 1100 Remington shotgun at
a distance of about 35 yards. Basically, I'm just going
to load a total good shot shell into the shotgun. And we're hot. [gunshot] Wow. It really did some
damage to that clay. Do we want to go take a look? CONNETT BREWER:
Yeah, we're clear. All right. Wow, look at the damage it
did to the-- to the clay. Yeah. Each pellet created
one of these holes. So what do you say
we cut this open and try to find some pieces?
- Sounds good. All right. So you see the trail here
that's formed inside the clay is going to lead you ultimately
to where the birdshot pellets are located. You can imagine how
difficult it would have been and how time-consuming
for the Stardust team to locate these microscopic
pieces of cometary material inside the aerogel. NARRATOR: Upon Stardust's
return to Earth, scientists recovered over
10,000 cometary fragments from the aerogel. Chemical analysis revealed
that the particles contained the organic compound glycine
a fundamental building block of life that had been
preserved in ice for over 4 billion years. ALEX FILIPPENKO: The discovery
of organic compounds in comets suggests that organic compounds
can form pretty easily. Now, it doesn't mean that there
was ever anything truly alive in those comets. But at least the
building blocks of life could have been built in comets. NARRATOR: Comets appear to
hold invaluable information about the origin of our solar
system, and perhaps life itself. But astronomers have discovered
that these icy bodies are not immortal. Now state-of-the-art
satellites have captured images of a select group of comets
that will end their life in a suicidal death dives. While our journey
with the comets has felt like an endless
cosmic roller coaster ride, these icy objects will
not orbit our sun forever. Most will make the voyage
for several thousand years before evaporating
into specks of dust. Even massive comets like Halley
only have 150,000 years left. That's because every second,
a comet loses tons of ice. Comets gradually
wither away or fade away, because every time they
pass close to the Sun, they lose some of their ices. They evaporate away. Eventually, there is
very little ice left. And so the comet coma
and tail doesn't form. It just looks like an asteroid. Or it may even break
apart into a whole bunch of little objects, because the
icy glue is no longer there, or because tidal effects
actually break it apart. NARRATOR: But not all comets
quietly fade into the sunset. We are now in hot pursuit
of a group of comets called sun grazers that
live fast and die young. Inside our spacecraft, we
feel the heat as the comets' extremely elongated orbits
bring us very close to the sun-- within a few hundred
thousand miles. As we enter this
danger zone, we see that some of these
kamikaze comets occasionally plunge
right into our home star, creating a ferocious spectacle. Sun grazer comets start out
their life as normal comets. They probably live
either in the Oort cloud, or they're short-period comets. But they've had an unlucky
encounter with another planet-- probably Jupiter. If they're really, really
lucky, they might just escape, and be able to get
away with one close passage by the Sun. But if they're not
lucky, they just get swallowed up
whole by the Sun. NARRATOR: The Solar and
Heliospherical Observatory, or SoHO, is a space satellite
that has observed 2,000 comets on suicidal orbits. The most famous sun grazers
are the Kreutz family, which originated from one
giant comet that broke up into many smaller pieces. It's a kind of cool thing
to think of a comet that's lived in the outer solar
system, just flying all the way and smashing into the sun. But they do. We see them all the time. NARRATOR: And some comets
do more than self-destruct. They become the messengers
of mass destruction. It's estimated that a large
comet may have struck Earth roughly every 40 million years,
based on the amount of craters still visible on our planet. [explosive rumbling] A comet may have even been
responsible for the most famous extinction event of all time. DON YEOMANS: For the Cretaceous
tertiary extinction event, the one that took out the
dinosaurs 65 million years ago, the jury is still out. That was thought to be a
10-kilometer sized object. And there are no asteroids in
near-Earth space that large that could impact the Earth. And there are a number of
comets that are that large. And since it occurred
65 million years ago, and you would expect a cometary
impact every 40 million years or so, it may
well have been a comet. NARRATOR: While
comets most likely slammed into Earth
countless times in the past, it's been difficult to determine
if an impact crater was made by a comet or asteroid,
because the two bodies appear to be similar. When we study the fossil
records, a lot of the material is gone, and just we
simply can't find it. And it turns out that
asteroids and comets have a lot of materials in common. So even if you do find
extraterrestrial material, it's really hard to tell
whether it came from an asteroid or from a comet. NARRATOR: And to
complicate matters, a comet doesn't even have
to impact Earth's surface to ignite a catastrophe. In 1908, a fireball
exploded in the atmosphere above the Tunguska
wilderness in Siberia. The heat and energy
from the air burst propelled downward
like a hot tornado. It propagated across the
forest, flattening over 800 square miles of trees. For over a century,
some scientists have wondered if
the cosmic intruder was a comet or an asteroid. If it were caused by a comet,
you would imagine you'd find in the sediment some record
of unusual things, like-- like a ratio of
helium 3 to helium 4. That could be indication that
it may have been a comet. Another possibility is if
you find some strange isotope buried in lakes somewhere. That's tough. This is not easy. NARRATOR: While it's been
difficult to substantiate cometary impacts on
Earth, physical proof of their colossal power exists
on the gas giant planets. And these dramatic events
warn that far worse collisions loom in the future, with Earth
as the potential bullseye. As we've followed the trail
of comets through space, we've passed through the
chilliest and warmest places in our solar system. We've also investigated whether
these icy bodies have been deliverers and
destroyers of life. And new observations prove
many of these ancient bodies haven't quietly retired
to the frigid outer limits of our solar system. AMY MAINZER: We think there
are several possible end states for comets. In one case, they can actually
get pulled right into the sun. In other cases,
they can actually get scattered by
one of the planets, and kind of get kicked out
maybe back into the Oort cloud, and never seen again. And finally, they can also
actually impact a planet. NARRATOR: We now shuttle
back to July 1994. We follow a string of
21 comet fragments, called Shoemaker-Levy 9,
as they're gravitationally pulled toward the gas
giant, planet Jupiter. If we were riding on one
of the fragments of comet Shoemaker-Levy 9, it would
have been truly spectacular, because we're heading
toward Jupiter. And this giant planet
is looming ever bigger. And then splat,
we crash into it, and we throw up a whole bunch
of material from the insides. It would be really
an amazing journey. The plumes of material
coming out of Jupiter were super-heated gas heated
by this collision, and also excavated from the
interior of Jupiter. NARRATOR: Shoemaker-Levy 9 left
impact scars the size of Earth, driving home the
scale of violence that comets can produce if
they slam into our planet. DON YEOMANS: If any
of those fragments had hit the Earth,
rather than Jupiter, we'd have been in serious
trouble, because they are large and they were coming
in extremely rapidly. But fortunately,
big brother Jupiter took all the hits for us, and
didn't seem to suffer much in the way of damage. NARRATOR: But even with Jupiter
acting as a planetary shield, comets still sneak
by the gas giants and have close
encounters with Earth. DON YEOMANS: The good news is,
asteroid impacts are far more likely to occur, because
asteroids outnumber comets 100 to one in near-Earth space. The bad news is, if
we do find a comet on an Earth-impacting
trajectory, we wouldn't realize
it till I got inside the orbit of Jupiter,
when it started throwing off gas and dust. AMY MAINZER: In the very
unlikely event that a comet could get close to the Earth,
you would worry about it because they tend to have
high average velocities relative to an asteroid-- as
high as tens of miles a second. So in other words, they
would pack a bigger punch. NARRATOR: While the frequency
of near-Earth asteroids heightens the risk of
impacts, the speed of comets is equally troubling. This sobering fact has prompted
viewer Robin W from Boulder, Colorado to Ask the Universe. What poses a greater hazard to
humans, asteroids or comets? Robin, that's an
important question. The asteroids are more numerous. So there's more of
them that could hit us. But we can track
their trajectories, and maybe do something about
one that's going to hit us. Deflect it, for example. The comets or more rare, but
we have very little warning when they come in. And also, they move much
faster than asteroids. So there is more
energy impacting Earth. So I would say comets are
the most hazardous objects. NARRATOR: NASA is taking the
threat of comets seriously. So far, tracking satellites have
tagged 84 near-Earth comets, objects with orbits that
come within 28 million miles of Earth's path around the Sun. There are no periodic
comets that currently have Earth's name written on them. But Jupiter and the
other giant planets occasionally perturb
the orbits of comets. So there could be a periodic
comet in the future that will collide with Earth. [explosion] NARRATOR: Since the beginning
of recorded history, over 4,200 comets have been
observed, a mere fraction of the total number
of these icy bodies. This means billions
of unknown comets still lie in wait in the
outer reaches of space. One day, a doomsday comet
could be nudged out and sent on a collision
course with Earth. If we're on that
cosmic highway, and it's getting
into our onramp, we need to worry about it. And we need to worry about it
with very little lead time. ALEX FILIPPENKO: There
is very little we can do, other than sending
a giant bomb toward them and trying to deflect them. But that's very difficult
when there's only a few months warning. NARRATOR: Just as comets
have inspired awe and fear among our ancestors, they
remain a force to reckon with. By continuing to track
their tails of dust, we may uncover more clues
about these frozen artifacts of our ancient past. AMY MAINZER: It's
pretty remarkable if you think about it. Just a few hundred year ago,
we had no idea what they were. We had no idea what they meant. And now we've actually been
to the surfaces of comets. We've seen up close and personal
what they're really like and what they're made out of. So these mysterious objects
have meant a lot to humans throughout our history. They've been agents of change. They've been agents of
destruction sometimes. They've maybe been the
agents of creation.
