[explosions]

NARRATOR: Our solar system has weathered

over 4 billion years of planet-altering catastrophes.

In some cases, the effects are obvious,

like the craters of the late heavy bombardment

that still litter the moon.

In others, the evidence has long since disappeared.

NASA has recently compared images of Jupiter

from the Cassini and New Horizons missions

and made a stunning discovery.

Like Saturn, Jupiter also has rings, though much fainter.

But something has disrupted them.

If you look at the rings of Jupiter,

they actually have little corrugations,

little ripples in them.

And those ripples are formed when a portion of the ring

is tilted.

And then as it continues to spin and evolve over time,

those ripples wander out, propagate

out through the ring system.

If you unwind that system and work back out the ripples,

you can find out the point in time

when that ring plane had gotten tilted over.

That point when that ring got tilted

was right around July of 1994.

NARRATOR: July 1994 marks the date of the Shoemaker-Levy 9

impact event.

The scientific sleuths had made a key discovery.

As the cometary fragments struck Jupiter itself,

much smaller debris passed through its rings,

tilting and twisting them into the ripples

that we still see today.

In the case of a disrupted comet like Shoemaker-Levy 9,

you've got an entire pall of dust,

a large mass of material distributed

across the disk of the rings raining through that system.

And so rather than displacing only one or two ring particles,

you can do the entire cloud at once, tipping it on the side.

NARRATOR: Shoemaker-Levy 9 wasn't

the only comet to leave its calling card mark

on Jupiter in recent times.

In July 2009, another asteroid smashed into the gas giant

near its south pole.

[explosion]

But when it impacted Jupiter, it brought up a lot of material

from deep within the atmosphere and created a huge scar

on the surface of the planet that

was visible for many weeks.

The size of this black ash cloud was

perhaps the size of the Pacific Ocean on the Earth.

It was quite large.

NARRATOR: It's now estimated that an asteroid or comet hits

Jupiter every 10 to 15 years, which

is 5,000 times higher than the rate of impacts on Earth.

And the fact that we've seen several of these

suggests that it's happening all the time because there

are all the ones we don't see.

So there's a lot more impact activity on the outer gas

giants than we ever thought.

If Jupiter was not in our solar system,

the Earth would be essentially a sitting duck

for all the debris, the comets and the asteroids

that were falling in towards the sun,

creating vastly larger numbers of catastrophes

on Earth than we've experienced through our history.

[roaring]

NARRATOR: But as our solar system ages,

new threats will likely arise, and Earth itself

will face a cosmic day of reckoning that nothing,

not even Jupiter, can prevent.

In several billion years, many scientists

believe Jupiter, the largest planet,

and Mercury, the smallest, will face off in an orbital duel.

And an innocent bystander, Earth, just might find

itself in the line of fire.

MICHAEL MISCHNA: Right now, our solar system

is kind of a paradigm of clockwork regularity.

But it turns out that the planets do affect

each other gravitationally.

The planetary orbits are, over very long periods of time,

vibrating in and out and turning.

Jupiter and Mercury will begin to turn

their orbits at the same rate.

And if that happens, Mercury's orbit becomes

progressively more eccentric.

It becomes progressively more elongated until the point

where, at its far point from the sun,

it's actually crossing Venus's orbit.

If Mercury's orbit ever gets to the situation

where it's crossing Venus's orbit, then basically,

all hell can break loose.

NARRATOR: Scientists have calculated one of four

disastrous consequences.

Mercury might collide with the sun,

might be ejected from the solar system, might smash into Venus,

or, in a worst case scenario, Mercury might collide

with the Earth, blasting away our mantle and atmosphere

and sterilizing our planet.

[explosion]

Mercury is hardly the only threat we face

from within the solar system.

According to some scenarios, Mars also

faces orbital chaos in the future,

and it too may slam into Earth, repeating the disaster

that gave rise to the moon.

And it's not just planets.

Nearby stars periodically disrupt comets

in the Oort cloud, which could send them on a kamikaze mission

through the solar system and set off

a new late heavy bombardment.

And if, as most expect, the Andromeda Galaxy ultimately

collides with the Milky Way, the galactic pileup

could shatter our solar system's deceptive calm.

The ultimate planetary catastrophe, I think,

is still in our future.