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ROWE: An invisible danger
hurtles towards Earth

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at close to the speed of light.

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These are intergalactic alien
interlopers on our Milky Way.

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ROWE: Cosmic rays.

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Getting hit by
a cosmic ray is like

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getting hit by a cosmic bullet.

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Cosmic rays are billions of
times more energetic than any

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other types of particles,

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vastly more energetic
than anything

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we can even create in

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a laboratory, in a nuclear
fusion reactor, anywhere.

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ROWE: They pierce spaceships,
putting our astronauts

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in danger.

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But the source of their power
is a mystery.

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Are they coming from
other galaxies?

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Are they coming from things in
between the galaxies?

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BULLOCK: Where do cosmic rays
come from?

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Truth is,
the most powerful ones,

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we haven't got a clue.

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ROWE: The race is on
to solve the mystery

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of the fastest particles
in the universe.

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PLAIT: If I were to make a list
of the dangers of space,

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it would be a long list.

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You know, there's hard vacuum,
huge swings in temperatures,

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micro meteorites,
all kinds of things.

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But probably at the very top
of that list, cosmic rays.

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ROWE: These space invaders
are not what they seem.

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When you hear the name
cosmic rays,

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you might think
it's like a beam,

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like a laser beam of light.
No, no, no, no, no, no.

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It's a tiny little
death particle.

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ROWE: To fight them,

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we must first understand them.

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April 2019.

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NASA's Parker probe flies
closer to the sun than

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ever before.

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We know the sun produces some of

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the cosmic rays that fill
the solar system,

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but we don't know how.

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Our sun looks like
a beautiful glowing orb,

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bringing energy and light to
Earth and allowing life

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to thrive.

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But if you look at it up close,

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you'll see a tumultuous storm
of events.

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PLAIT: The amount of energy
the sun is emitting every

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second is the equivalent of
100 billion one megaton bombs.

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It's a dangerous neighborhood.

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ROWE: Suddenly,
the probe is caught head-on

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in a powerful blast.

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It's perfectly
positioned to monitor

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the outburst from the inside.

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OLUSEYI: The entire outer
third of the sun is a boiling

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cauldron, and tied up in that
plasma are magnetic fields.

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They get tied and twisted,
and energy is stored in them,

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so they rise towards
the surface, and there,

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they rearrange, they reconnect,
they twist, they spin.

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ROWE: When the magnetic
field lines snap,

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energy bursts out.

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And sometimes, that energy
release is explosive

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and that's what
results in flares,

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which are these huge
bursts of light.

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ROWE: The probe discovers
that after a solar flare,

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the sun's surface stores
electrically charged particles.

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But sometimes, there's
a second explosion called

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a coronal mass ejection,
releasing superheated,

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electrically charged gas
called plasma.

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These giant balls of plasma
go flying off the surface of

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the sun,
and in those balls of plasma

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are contained
these charged particles.

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ROWE: The charged particles move
fast, but they hit a roadblock,

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a cloud of
slower-moving particles

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that always surrounds
the sun, the solar wind.

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Well, the coronal mass ejection
is moving into the solar wind

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much faster
than the wind is moving.

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So it sort of runs into it
and creates this shockwave

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and ends up piling up
particles at the edge.

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ROWE:
The shockwave and particles

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slam together --
In the collision,

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the particles steal
energy and speed,

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like a baseball accelerating
off a bat.

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The particles transform into
something far more powerful --

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A solar cosmic ray.

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They're light,
but they're moving

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incredibly fast.

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The Earth is on average
93 million miles away from

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the sun, and these guys reach
us in about an hour.

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That's 93 million miles an hour.

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That's pretty fast.

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ROWE: The cosmic rays speed
towards Earth.

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We're under attack.

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Cosmic rays are, by far,
the most energetic

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particles that we know
to exist in the universe.

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And when things with very high
energy, no matter how small

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they are, impact something else,

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they deposit that energy, right,
and so cosmic rays

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can be very dangerous.

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ROWE: Solar cosmic rays aren't
the only threat we face.

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Other space bullets arrive
from beyond our solar system.

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There are different kinds of
cosmic rays,

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just like there are different
kinds of bullets.

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At the lowest end
of the spectrum

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are these solar cosmic rays.

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These are like the BBs,
and when a BB hits you,

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it might sting for a little
bit, but you're not gonna get

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too worried about it.

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ROWE: A bigger concern --
Galactic cosmic rays.

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They travel faster
and have more energy.

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If solar rays are like BBs,

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the galactic cosmic rays are
like rifle bullets.

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They're far more dangerous.

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They're moving a lot faster.

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But they're also more rare.

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ROWE: Faster still are
the universe's most wanted --

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Ultra-high-energy cosmic rays.

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If you thought galactic
cosmic rays were bad, it's

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because you haven't met
an ultra-high-energy cosmic ray.

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These are the biggest, baddest,

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meanest cosmic rays in
the universe.

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These ultra-high-energy
cosmic rays

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are like hypersonic missiles.

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They are screaming,
and they come from

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the most energetic events in
the universe.

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ROWE: The ultra-high-energy
cosmic missiles

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are the rarest
but also the swiftest.

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These cosmic ray particles
are moving fast.

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These mysterious particles
are moving incredibly

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close to the speed of light.

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I'm not talking about
99 percent the speed of light.

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PLAIT: They're moving through
space at

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like 99.999...

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- 9999...
- 9999...

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- 99999...
- 999999...

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99999 -- 21 nines.

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That's fast.

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That's wild. That's scary.

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ROWE:
All three types of cosmic rays

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are racing
through the solar system.

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If I were to hold up a golf
ball in the middle of space,

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almost 100 cosmic rays pass
through that golf ball every

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single second.

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ROWE: It's a deadly hail of
particle bullets,

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and out in space,

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our astronauts are caught in
the crossfire.

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Cosmic rays represent one of

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the greatest dangers for
human space flight.

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ROWE: NASA plans to send
astronauts back to the moon,

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where radiation levels
-OVER RADIO: Lift-off.

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From cosmic rays are
200 times greater than

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on Earth,
and that is just the start.

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One of NASA's big goals is to
send humans to Mars,

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and that is a long way away,

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at least a six-month journey,
and more often,

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about a nine-month journey.
That's a big problem.

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I am hoping that one day, I can
go to Mars as an astronaut,

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but I'm definitely afraid of
cosmic rays,

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and the more that I read
about it,

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the bigger of a threat it seems.

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So I think that NASA and other
space organizations

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are going to need to work on

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how to protect
their astronauts in these

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really dangerous situations.

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ROWE: Only one group
of people have been

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exposed to these high levels
of cosmic rays,

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the crew members of
the Apollo missions

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July 1969.

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NEIL ARMSTRONG: That's one
small step for man,

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one giant leap for mankind.

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ROWE: One of the astronauts,
Buzz Aldrin,

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sees something strange.

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PLAIT:
During Apollo 11,

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Buzz Aldrin reported
seeing tiny little

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flashes sometimes
when he was looking around.

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That's pretty weird.

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But what's weirder
is that he saw them

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when his eyes were closed.

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ROWE: Later missions also report
seeing odd flashes of light.

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ASTRONAUT: A streak in
the lower left side of the...

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left eye, moving down.

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ROWE: The astronauts describe
the flashes as spots,

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streaks, and clouds.

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Apollo 15 Commander David
Scott reported seeing one that

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was blue with a white cast,
like a blue diamond.

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What's happening is that
a cosmic ray is entering

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the eyeball

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and then striking molecules
and giving off

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a flash of light.

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An alternative theory is that
it triggers the layer of

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sensitive cells in your retina,

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so you perceive
a streak of light even though

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no light ever actually existed.

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ROWE: The cosmic rays
cause long-term damage.

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Inside of the eye's lens,

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there are these fiber cells
that are transparent.

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Well, when a cosmic ray
travels through them,

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it can damage those cells
and make them cloudy,

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causing cataracts.

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When NASA examines
ROWE: the astronauts' helmets,

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they find tiny tracks etched
through them,

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evidence of cosmic ray impacts.

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00:10:07,545 --> 00:10:10,647
When we say that cosmic rays
are like tiny little bullets,

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we're not joking around.

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And some of these burrowed all
the way through the helmet,

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which means it ended up
in the astronaut's brain,

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which just makes me feel weird
to think about.

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DARTNELL: What might that
long-term radiation

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do to your brain,
to your ability to reason

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00:10:30,635 --> 00:10:33,670
and problem solve in one of
the most dangerous environments

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that humanity has ever
placed itself?

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ROWE: The farther we venture
from our home planet,

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the more danger we face.

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ROWE: Cosmic rays, highly
energetic space particles,

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may be the most serious threat
to human space exploration.

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DARTNELL: The Hollywood
conception of outer space is

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00:11:02,934 --> 00:11:05,301
it's full of dangers like aliens

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00:11:05,303 --> 00:11:08,104
wielding ray guns or black holes

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00:11:08,206 --> 00:11:09,539
or asteroid showers.

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00:11:09,608 --> 00:11:11,307
But in reality, the biggest

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00:11:11,309 --> 00:11:13,943
danger facing astronauts
is invisible.

219
00:11:14,012 --> 00:11:15,245
It's the cosmic radiation.

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Cosmic rays damaged
ROWE: Apollo astronauts' eyes

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00:11:19,918 --> 00:11:22,752
after just a few days' exposure.

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00:11:22,821 --> 00:11:26,656
A one-way trip to Mars takes
nine months.

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Future missions are going to be

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00:11:28,326 --> 00:11:30,460
spending much longer times
in space,

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which means we really need to

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consider how cosmic rays will
impact us.

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CHRISTIAN: We don't understand
all the long-term effects

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from a steady rain of
cosmic rays,

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but the astronauts are gonna
have to deal with it.

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ROWE: To find out more,
scientists bombarded

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00:11:48,714 --> 00:11:52,716
human cells with manmade
cosmic ray particles.

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They discovered cosmic rays
physically cut through DNA,

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chopping it apart.

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Damage to DNA in your cell is

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by far the worst kind,
because your DNA

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is the cell's operating
manuals, the blueprints

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so the cell knows how it
should be functioning normally.

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You can trigger that cell to

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00:12:17,943 --> 00:12:21,311
turn tumorous,
to start producing cancer.

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In 2019, scientists
ROWE: took the experiment

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00:12:26,318 --> 00:12:29,119
further and simulated
a trip to Mars...

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00:12:30,122 --> 00:12:32,555
for mice --
For six months,

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00:12:32,624 --> 00:12:34,657
they blasted the rodents with
a steady

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00:12:34,726 --> 00:12:38,762
stream of lab-made
cosmic ray particles.

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00:12:38,863 --> 00:12:42,465
The experiment found
profound alterations

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00:12:42,534 --> 00:12:45,435
to the mice's normal behavior.

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They learned new tasks
much more slowly.

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Their memory was affected,
and they

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00:12:50,609 --> 00:12:53,643
forgot things they had
already learned.

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ROWE: They were more anxious
and prone to

251
00:12:55,547 --> 00:12:58,915
giving up on tasks
they'd normally complete.

252
00:12:58,917 --> 00:13:01,317
If you put some of these
irradiated mice into

253
00:13:01,452 --> 00:13:05,622
a swimming test, rather than
trying to swim to safety,

254
00:13:05,624 --> 00:13:07,223
many of them just simply
gave up.

255
00:13:09,060 --> 00:13:10,727
CHRISTIAN: This is important,
because we need

256
00:13:10,829 --> 00:13:13,129
our astronauts to be
fully functioning.

257
00:13:13,131 --> 00:13:16,633
The reason why you do
crewed missions is because

258
00:13:16,735 --> 00:13:20,837
the human brain is much better
than any computer.

259
00:13:20,939 --> 00:13:23,139
PLAIT: If even one of them
has a problem,

260
00:13:23,208 --> 00:13:26,209
it can even put the mission
and their lives in jeopardy.

261
00:13:27,712 --> 00:13:31,114
ROWE: Other studies discover
cosmic rays can accelerate

262
00:13:31,216 --> 00:13:35,318
aging, alter genes, and cause
cardiovascular disease.

263
00:13:37,055 --> 00:13:38,855
That sounds bad enough,

264
00:13:38,957 --> 00:13:41,958
but there's
a more immediate danger.

265
00:13:42,027 --> 00:13:44,794
When cosmic rays
penetrate spaceships,

266
00:13:44,796 --> 00:13:47,931
they can fry electronic
systems, and that's enough to

267
00:13:47,933 --> 00:13:49,566
jeopardize a mission.

268
00:13:49,634 --> 00:13:54,904
Our operations in space depend
on electronics, on computers.

269
00:13:54,906 --> 00:13:59,609
And the worst case scenario is
that the wrong cosmic ray comes

270
00:13:59,711 --> 00:14:01,644
at the wrong time and hits

271
00:14:01,746 --> 00:14:06,349
the wrong circuit, and it leads
to a cascading series of

272
00:14:06,451 --> 00:14:10,019
failures that can totally
jeopardize a mission.

273
00:14:11,056 --> 00:14:13,923
ROWE: We see evidence of this
onslaught in mission cameras.

274
00:14:15,260 --> 00:14:17,126
Even when we have
a detector in space

275
00:14:17,262 --> 00:14:18,828
like on
the Hubble Space Telescope,

276
00:14:18,930 --> 00:14:20,363
if you saw a raw image,

277
00:14:20,465 --> 00:14:23,066
it doesn't look like
the beautiful images

278
00:14:23,134 --> 00:14:24,334
that are shown to the public.

279
00:14:24,469 --> 00:14:26,870
They're just crossed with
cosmic rays, and those

280
00:14:26,971 --> 00:14:31,207
cosmic rays are destroying
that detector slowly over time.

281
00:14:33,111 --> 00:14:36,045
ROWE: So how can we protect
astronauts and their equipment?

282
00:14:37,449 --> 00:14:39,716
The obvious answer is to
add shielding.

283
00:14:41,720 --> 00:14:45,054
SUTTER: It's one thing to say
like, oh, just add more stuff.

284
00:14:45,156 --> 00:14:48,825
But have you seen rocket
launches and how hard they are,

285
00:14:48,927 --> 00:14:51,828
how expensive it is to get
stuff up into space?

286
00:14:53,231 --> 00:14:55,965
ROWE: NASA does have a plan.

287
00:14:56,067 --> 00:14:58,768
The spacecraft for the Artemis
moon landing mission

288
00:14:58,836 --> 00:15:02,038
will be packed for optimum
cosmic ray protection.

289
00:15:02,107 --> 00:15:05,041
CHRISTIAN: So one of the ways
you can get around

290
00:15:05,143 --> 00:15:08,311
the mass limit is
to basically get dual use

291
00:15:08,413 --> 00:15:10,046
out of everything --
Your supplies,

292
00:15:10,115 --> 00:15:13,816
your fuel, your water, and you
can use those as shielding.

293
00:15:13,818 --> 00:15:15,818
ROWE:
But it's not that simple.

294
00:15:15,820 --> 00:15:19,022
Just as more powerful bullets
penetrate armor,

295
00:15:19,024 --> 00:15:22,358
more energetic cosmic rays
pierce the shielding

296
00:15:22,427 --> 00:15:24,027
on spaceships.

297
00:15:24,029 --> 00:15:26,129
The solar ones --
Yeah, you can just put up some

298
00:15:26,264 --> 00:15:28,965
material, some shielding, and
it'll generally block them.

299
00:15:29,067 --> 00:15:31,868
But the higher energy ones,
they can just burrow

300
00:15:32,003 --> 00:15:33,202
on through.

301
00:15:33,238 --> 00:15:37,307
If they hit one of the atoms
in the shielding that is

302
00:15:37,409 --> 00:15:38,841
protecting our astronauts,

303
00:15:38,943 --> 00:15:42,045
it can create
a shower of particles.

304
00:15:42,146 --> 00:15:44,714
DARTNELL: That radiation
particle might have missed

305
00:15:44,816 --> 00:15:46,215
any of the cells in your body.

306
00:15:46,318 --> 00:15:47,650
But you've now turned it into

307
00:15:47,752 --> 00:15:51,554
a blast, shredding through
everything in the spacecraft.

308
00:15:51,623 --> 00:15:55,692
And so it turns out
your shielding becomes

309
00:15:55,694 --> 00:15:59,228
the weapon that the cosmic
rays use against you.

310
00:15:59,330 --> 00:16:04,534
ROWE: But NASA is recruiting
an unexpected ally -- the sun.

311
00:16:04,636 --> 00:16:09,305
Can we protect our astronauts
by fighting fire with fire?

312
00:16:17,916 --> 00:16:22,418
ROWE: Powerful cosmic rays
smash through spaceships.

313
00:16:22,520 --> 00:16:27,523
But how can objects smaller
than an atom carry enough

314
00:16:27,658 --> 00:16:30,460
energy to be
dangerous to astronauts?

315
00:16:30,561 --> 00:16:33,429
Moving objects carry energy.

316
00:16:33,531 --> 00:16:35,965
We call this kinetic energy,
and when

317
00:16:36,034 --> 00:16:39,535
they strike something,
they transform that energy.

318
00:16:40,939 --> 00:16:44,207
When I hit my hand,
the kinetic energy of

319
00:16:44,342 --> 00:16:49,645
my fist transforms into sound
and heat and vibration.

320
00:16:49,714 --> 00:16:51,748
My hand hurts a little from

321
00:16:51,816 --> 00:16:55,618
that impact, from
the transformation of energy.

322
00:16:55,720 --> 00:16:58,154
ROWE: It's the same thing
with cosmic rays.

323
00:16:58,223 --> 00:17:02,258
When they slam into a human
brain cell or a computer chip,

324
00:17:02,360 --> 00:17:04,961
they dump some energy,
causing damage.

325
00:17:06,331 --> 00:17:09,732
How much damage depends on
their kinetic energy,

326
00:17:09,734 --> 00:17:12,068
and that comes down
to two things --

327
00:17:12,137 --> 00:17:13,970
Mass and speed.

328
00:17:15,006 --> 00:17:18,941
Intuitively, things that are
moving at the same speed,

329
00:17:19,043 --> 00:17:21,411
if they're more massive,
they carry more energy.

330
00:17:21,413 --> 00:17:23,946
A bigger asteroid slamming
into Earth will

331
00:17:24,015 --> 00:17:26,716
do more damage than
a smaller asteroid.

332
00:17:26,818 --> 00:17:29,419
ROWE: If you double
the mass of an object,

333
00:17:29,521 --> 00:17:32,321
its kinetic energy also doubles.

334
00:17:32,323 --> 00:17:35,058
Although mass is important,

335
00:17:35,159 --> 00:17:36,726
it's not as important as speed.

336
00:17:39,631 --> 00:17:42,732
OLUSEYI: Speed matters
even more than mass.

337
00:17:42,734 --> 00:17:46,069
The kinetic energy depends
directly on the mass,

338
00:17:46,137 --> 00:17:48,704
but it depends on
the square of the speed.

339
00:17:48,706 --> 00:17:51,407
Here's what that means --
You double the mass,

340
00:17:51,409 --> 00:17:54,444
you have double the kinetic
energy, you double the speed,

341
00:17:54,545 --> 00:17:56,813
you have four times
the kinetic energy.

342
00:17:59,717 --> 00:18:05,655
ROWE: When it comes to speed,
cosmic rays are the elite.

343
00:18:05,756 --> 00:18:07,924
An ultra-high-energy cosmic ray

344
00:18:07,926 --> 00:18:12,528
detected in 1991 hit
the atmosphere so fast,

345
00:18:12,530 --> 00:18:15,965
scientists called it the
"Oh My God Particle."

346
00:18:19,237 --> 00:18:22,505
This particle was higher
energy than they thought

347
00:18:22,607 --> 00:18:24,340
they would ever, ever see.

348
00:18:24,442 --> 00:18:27,810
ROWE: Until this fluorescent
streak in the Utah sky,

349
00:18:27,912 --> 00:18:31,214
no one believed a particle
could reach the Earth traveling

350
00:18:31,216 --> 00:18:33,316
so close to the speed of light,

351
00:18:33,318 --> 00:18:36,619
making cosmic ray particles
far more dangerous

352
00:18:36,721 --> 00:18:37,954
than expected.

353
00:18:38,055 --> 00:18:40,223
As you approach
the speed of light,

354
00:18:40,325 --> 00:18:42,758
energy, momentum, mass,
they start

355
00:18:42,827 --> 00:18:44,727
to act a little bit differently.

356
00:18:44,862 --> 00:18:47,530
PLAIT: Einstein's equations of
relativity become important,

357
00:18:47,632 --> 00:18:50,333
because the physics changes,
and the energy it has

358
00:18:50,468 --> 00:18:53,469
becomes much, much,
much stronger.

359
00:18:53,538 --> 00:18:56,405
OLUSEYI: If a particle is moving
at close to the speed of light,

360
00:18:56,407 --> 00:19:00,510
that means that its energy is
almost at the maximum allowed

361
00:19:00,612 --> 00:19:02,612
by the laws of physics.

362
00:19:04,716 --> 00:19:06,816
THALLER: It's amazing to think
that something as tiny as

363
00:19:06,818 --> 00:19:09,819
a proton could actually be
dangerous to a human being.

364
00:19:09,954 --> 00:19:13,222
But amazingly, that proton is
moving so fast,

365
00:19:13,324 --> 00:19:15,591
it carries as much energy
as a baseball

366
00:19:15,593 --> 00:19:17,260
thrown at 100 miles an hour.

367
00:19:17,328 --> 00:19:21,164
ROWE: A baseball contains over
a trillion, trillion protons.

368
00:19:21,265 --> 00:19:25,234
Imagine all that energy
carried by just one particle.

369
00:19:25,336 --> 00:19:29,205
So now you get a sense of just
how risky these can be.

370
00:19:29,207 --> 00:19:31,607
ROWE: Ultra-high-energy
cosmic rays like

371
00:19:31,709 --> 00:19:35,711
the Oh My God Particle are
like supersonic missiles.

372
00:19:35,713 --> 00:19:38,614
They are the fastest,
but they're so rare,

373
00:19:38,716 --> 00:19:41,817
astronauts are unlikely
to be hit by one.

374
00:19:41,920 --> 00:19:46,022
Solar cosmic rays are like
BB pellets -- abundant,

375
00:19:46,024 --> 00:19:48,724
but our spacecraft can
block them.

376
00:19:48,793 --> 00:19:50,459
The biggest threat to
our astronauts,

377
00:19:50,528 --> 00:19:53,129
however, are galactic
cosmic rays.

378
00:19:53,231 --> 00:19:56,132
They come from elsewhere
in the Milky Way.

379
00:19:56,134 --> 00:19:57,867
The combination of their speed

380
00:19:58,002 --> 00:20:01,604
and frequency makes them
the most dangerous.

381
00:20:01,606 --> 00:20:05,708
These galactic cosmic rays
are much more powerful than

382
00:20:05,710 --> 00:20:07,410
the solar cosmic rays,

383
00:20:07,512 --> 00:20:10,813
and they've traveled
enormous distances

384
00:20:10,815 --> 00:20:13,549
to mess you up.

385
00:20:13,618 --> 00:20:18,020
ROWE: Luckily, our astronauts
have a surprising protector,

386
00:20:18,022 --> 00:20:21,624
a guardian of the solar system,
the sun.

387
00:20:23,161 --> 00:20:24,627
DARTNELL:
As well as spitting out these

388
00:20:24,729 --> 00:20:29,131
high-energy solar cosmic
ray particles,

389
00:20:29,133 --> 00:20:33,436
the sun is also streaming out
lots of much lower energy

390
00:20:33,504 --> 00:20:36,038
particles of the solar wind.

391
00:20:36,140 --> 00:20:38,207
That outward moving
solar wind acts

392
00:20:38,209 --> 00:20:41,777
as a force field, and
the cosmic rays have to work

393
00:20:41,779 --> 00:20:47,016
their way upstream to get to
Earth far inside this bubble.

394
00:20:47,018 --> 00:20:51,721
ROWE: The solar wind extends
11 billion miles

395
00:20:51,823 --> 00:20:53,122
around the solar system,

396
00:20:53,257 --> 00:20:57,126
generating a magnetic field
that repels incoming

397
00:20:57,128 --> 00:20:59,428
galactic cosmic rays.

398
00:20:59,530 --> 00:21:01,897
DARTNELL: It's almost like
the deflector shield

399
00:21:01,899 --> 00:21:04,133
of the Starship Enterprise.

400
00:21:04,135 --> 00:21:08,938
So the sun's magnetic field
partially helps protect

401
00:21:09,007 --> 00:21:11,340
the Earth and any astronauts

402
00:21:11,409 --> 00:21:12,808
from the incoming radiation.

403
00:21:12,910 --> 00:21:16,712
OLUSEYI: Not long ago,
our Voyager spacecraft

404
00:21:16,814 --> 00:21:19,949
made it to that boundary
between the sun's bubble

405
00:21:20,051 --> 00:21:21,450
and the galaxy and was able

406
00:21:21,552 --> 00:21:24,654
to study that region, and we
see the difference between

407
00:21:24,755 --> 00:21:27,523
inside the sun's bubble
and what's going on

408
00:21:27,525 --> 00:21:29,125
outside the sun's bubble.

409
00:21:29,127 --> 00:21:31,527
The sun has our back, billions

410
00:21:31,629 --> 00:21:34,664
of miles away,
and that's pretty cool.

411
00:21:34,765 --> 00:21:37,833
ROWE: The Voyager
space probes discovered

412
00:21:37,835 --> 00:21:40,670
a moving battlefield.

413
00:21:40,738 --> 00:21:44,607
The solar wind behaves a bit
like a storm front on Earth.

414
00:21:44,609 --> 00:21:46,509
Sometimes, it advances.

415
00:21:46,511 --> 00:21:48,744
Sometimes, it retreats.

416
00:21:49,814 --> 00:21:51,814
OLUSEYI: When the sun's
activity is the highest,

417
00:21:51,916 --> 00:21:54,817
it's spitting out more
solar energetic protons,

418
00:21:54,919 --> 00:21:58,754
but those solar cosmic rays
are much less damaging

419
00:21:58,823 --> 00:22:00,022
than the galactic ones,

420
00:22:00,024 --> 00:22:02,525
so the net is a benefit.

421
00:22:03,695 --> 00:22:06,529
DARTNELL: So actually,
ironically, you might find that,

422
00:22:06,531 --> 00:22:10,132
for astronauts, it is safer
to launch missions to Mars

423
00:22:10,134 --> 00:22:14,070
during a period of higher
solar activity, because although

424
00:22:14,138 --> 00:22:18,607
you have more of
the solar particle radiation,

425
00:22:18,609 --> 00:22:22,111
you also get a better
shielding effect

426
00:22:22,113 --> 00:22:23,813
from the solar wind.

427
00:22:23,815 --> 00:22:25,848
ROWE: The sun's activity
goes through

428
00:22:25,950 --> 00:22:28,417
an 11-year cycle
of highs and lows.

429
00:22:28,419 --> 00:22:32,121
The protective bubble follows
the same cycle,

430
00:22:32,123 --> 00:22:36,625
allowing NASA to predict
the safest times to launch.

431
00:22:36,627 --> 00:22:38,527
This is a thorny problem,
and, you know,

432
00:22:38,629 --> 00:22:41,764
we have very smart people
working on it, but we want to

433
00:22:41,866 --> 00:22:44,233
explore space as much as we can,

434
00:22:44,235 --> 00:22:45,835
but we have to lower the risk

435
00:22:45,937 --> 00:22:47,837
to the astronauts
as much as possible.

436
00:22:49,941 --> 00:22:53,442
ROWE: NASA's fight against
the cosmic invaders continues,

437
00:22:53,511 --> 00:22:56,245
but the biggest mystery remains.

438
00:22:56,314 --> 00:22:59,749
What exactly is launching
the deadliest

439
00:22:59,817 --> 00:23:02,051
galactic cosmic rays?

440
00:23:10,328 --> 00:23:12,928
ROWE: Every second,
quadrillions of bits

441
00:23:12,930 --> 00:23:15,364
of space shrapnel
race towards Earth

442
00:23:15,433 --> 00:23:17,366
at close to
the speed of light --

443
00:23:17,435 --> 00:23:20,035
Galactic cosmic rays.

444
00:23:21,105 --> 00:23:24,407
The galactic cosmic rays
are like a rifle bullet.

445
00:23:24,409 --> 00:23:27,410
You do not want to get hit
by one of these.

446
00:23:27,545 --> 00:23:31,313
ROWE: They are invaders from
outside the solar system.

447
00:23:31,416 --> 00:23:34,049
We know they're made by
something powerful

448
00:23:34,118 --> 00:23:35,418
within our galaxy,

449
00:23:35,420 --> 00:23:39,054
so the source should be
easy to detect.

450
00:23:39,156 --> 00:23:41,557
HOPKINS: You'd think
if one of them

451
00:23:41,659 --> 00:23:44,326
hits a detector on Earth,
that we'd just be able

452
00:23:44,429 --> 00:23:46,061
to point back
in a straight line and say,

453
00:23:46,130 --> 00:23:47,296
"It came from over there."

454
00:23:47,331 --> 00:23:49,131
And then look, is there
something else over there,

455
00:23:49,133 --> 00:23:51,033
like a supernova explosion that

456
00:23:51,035 --> 00:23:52,968
could explain the source
of this?

457
00:23:53,037 --> 00:23:55,604
The problem is
that cosmic rays get

458
00:23:55,606 --> 00:24:00,009
bent as they move
by magnetic fields.

459
00:24:00,011 --> 00:24:01,944
ROWE: The electric charge
on a cosmic ray

460
00:24:02,046 --> 00:24:04,113
makes it act like
a little magnet,

461
00:24:05,183 --> 00:24:08,050
and the Milky Way
is full of other magnets.

462
00:24:08,119 --> 00:24:12,254
SUTTER: If I'm a cosmic ray just
barreling through the galaxy,

463
00:24:12,356 --> 00:24:14,757
and I encounter
a magnetic field,

464
00:24:14,826 --> 00:24:17,126
I'm gonna slightly
change directions.

465
00:24:17,228 --> 00:24:20,229
Maybe here, maybe there,
maybe up there.

466
00:24:20,231 --> 00:24:23,265
My trajectory is going to
become scrambled.

467
00:24:23,334 --> 00:24:25,968
HOPKINS: And after
a few million years or so,

468
00:24:26,070 --> 00:24:28,737
basically all the information
about where it started has

469
00:24:28,806 --> 00:24:30,339
been lost --
It's going in a completely

470
00:24:30,408 --> 00:24:34,310
random direction for all
practical purposes.

471
00:24:34,412 --> 00:24:38,314
ROWE: But galactic cosmic rays
also have a sidekick,

472
00:24:38,416 --> 00:24:44,053
one that is far less elusive --
Gamma rays.

473
00:24:44,154 --> 00:24:47,356
When a galactic cosmic ray
hits a regular atom out

474
00:24:47,458 --> 00:24:50,326
in space,
it causes this big reaction.

475
00:24:51,629 --> 00:24:53,896
HOPKINS: It emits all sorts
of other particles,

476
00:24:53,898 --> 00:24:55,231
including gamma rays,

477
00:24:55,366 --> 00:24:58,734
which are basically extremely
energetic photons of light.

478
00:24:58,836 --> 00:25:02,705
Critically, gamma rays don't
get bent by magnetic fields,

479
00:25:02,840 --> 00:25:04,507
because they don't have
an electric charge,

480
00:25:04,609 --> 00:25:07,543
so they just beeline off in
a straight line along whatever

481
00:25:07,645 --> 00:25:10,846
direction the cosmic ray was
moving in the first place.

482
00:25:10,948 --> 00:25:14,950
So we can look back at where
gamma rays are coming from in

483
00:25:15,019 --> 00:25:18,420
the sky, and that tells us
where there are a lot of

484
00:25:18,422 --> 00:25:19,755
cosmic rays having collisions.

485
00:25:21,058 --> 00:25:24,860
ROWE: And they've led us
to a prime suspect...

486
00:25:26,130 --> 00:25:27,129
supernovas.

487
00:25:30,701 --> 00:25:31,967
Supernova are some of

488
00:25:32,069 --> 00:25:34,403
the most powerful explosions
in the universe,

489
00:25:34,405 --> 00:25:37,439
and so they're ripe grounds
for these highly energetic,

490
00:25:37,541 --> 00:25:41,710
extremely fast particles
to be created.

491
00:25:41,812 --> 00:25:44,046
ROWE: When a giant star
runs out of fuel,

492
00:25:44,115 --> 00:25:46,982
it can no longer support its
own weight.

493
00:25:46,984 --> 00:25:48,617
It collapses inward,

494
00:25:50,555 --> 00:25:54,056
triggering a huge explosion,
powerful enough

495
00:25:54,125 --> 00:25:57,726
to smash atoms into tiny pieces.

496
00:25:57,728 --> 00:26:01,030
The explosion pushes out
an expanding cloud

497
00:26:01,032 --> 00:26:04,833
of gas and dust,
the supernova remnant.

498
00:26:04,835 --> 00:26:06,969
HOPKINS: And that material,
as it's moving out at

499
00:26:07,071 --> 00:26:08,170
1,000 miles a second,

500
00:26:08,238 --> 00:26:11,540
generates an incredibly
powerful shockwave.

501
00:26:11,609 --> 00:26:14,043
And that shockwave
could be where

502
00:26:14,111 --> 00:26:19,448
a particle swept up in
the shock gets accelerated.

503
00:26:19,550 --> 00:26:22,017
ROWE: The magnetic fields
inside the cloud

504
00:26:22,019 --> 00:26:24,620
trap the subatomic particles.

505
00:26:24,689 --> 00:26:27,556
Cosmic rays inside of
the supernova remnant

506
00:26:27,625 --> 00:26:29,458
are a lot like being
in a pinball machine.

507
00:26:31,529 --> 00:26:35,531
So you have the shockwave as
the flipper, and then your

508
00:26:35,533 --> 00:26:37,833
magnetic fields
are these bumpers,

509
00:26:37,835 --> 00:26:40,069
prohibiting it from
actually leaving.

510
00:26:41,439 --> 00:26:42,905
They're bouncing back
and forth across

511
00:26:42,907 --> 00:26:45,908
this incredibly energetic shock,
and each time

512
00:26:45,910 --> 00:26:47,042
they bounce back and forth,

513
00:26:47,111 --> 00:26:49,612
the key is they pick up
a little more energy.

514
00:26:50,648 --> 00:26:53,949
ROWE: When a galactic cosmic ray
gains enough energy,

515
00:26:54,018 --> 00:26:57,286
the magnetic fields can no
longer hold on to it.

516
00:26:57,288 --> 00:26:58,520
It escapes.

517
00:27:01,158 --> 00:27:03,826
The supernova theory
explains the birth

518
00:27:03,828 --> 00:27:06,462
of many of these cosmic bullets.

519
00:27:06,563 --> 00:27:09,398
But then we discovered
a super gamma ray

520
00:27:09,400 --> 00:27:13,135
so powerful, it must have
a completely different

521
00:27:13,237 --> 00:27:14,570
origin story.

522
00:27:14,638 --> 00:27:17,506
So this gamma ray was
incredibly high energy,

523
00:27:17,641 --> 00:27:20,809
which means that the cosmic
ray responsible for it was

524
00:27:20,811 --> 00:27:23,545
probably also extremely
high in energy.

525
00:27:23,647 --> 00:27:25,748
If you fire a bullet into
a pinball machine,

526
00:27:25,783 --> 00:27:27,449
it's not gonna bounce back
and forth.

527
00:27:27,518 --> 00:27:29,118
[gunshot sound effect]

528
00:27:29,253 --> 00:27:30,953
I'm just gonna break
through the machinery.

529
00:27:31,055 --> 00:27:32,421
The problem is that these

530
00:27:32,423 --> 00:27:35,224
are vastly more energetic
than that.

531
00:27:35,326 --> 00:27:38,127
So there's no way they could
have been bouncing around all

532
00:27:38,129 --> 00:27:41,563
the way up to their current
energies inside of that

533
00:27:41,632 --> 00:27:45,034
particular pinball machine.

534
00:27:45,036 --> 00:27:48,070
ROWE: There must be
something else in the Milky Way

535
00:27:48,138 --> 00:27:50,439
creating galactic cosmic rays,

536
00:27:50,508 --> 00:27:54,710
something more powerful
than a supernova.

537
00:27:56,313 --> 00:27:59,515
The question is, what?

538
00:27:59,617 --> 00:28:03,052
January 2021.

539
00:28:03,153 --> 00:28:04,820
At an observatory

540
00:28:04,822 --> 00:28:07,823
high up on the side of
a Mexican volcano,

541
00:28:07,825 --> 00:28:11,326
blue light zaps through
water tanks,

542
00:28:12,430 --> 00:28:15,030
signs of incoming gamma rays.

543
00:28:16,834 --> 00:28:19,835
Their trail stretches back
across the Milky Way,

544
00:28:19,837 --> 00:28:24,106
crossing billions of miles,
but suddenly goes cold.

545
00:28:25,342 --> 00:28:28,444
Instead of originating
in a huge explosion,

546
00:28:28,512 --> 00:28:33,449
the trail ends in a cold,
sparse cloud of dust.

547
00:28:33,550 --> 00:28:36,719
SUTTER: Molecular clouds,
at first glance, seem like

548
00:28:36,821 --> 00:28:38,320
one of the most boring,

549
00:28:38,322 --> 00:28:41,123
innocuous places in
the universe.

550
00:28:41,225 --> 00:28:45,527
You can barely even see them
without an infrared telescope.

551
00:28:45,529 --> 00:28:48,263
They're not events
like supernova

552
00:28:48,365 --> 00:28:50,332
that have enormously
high energies.

553
00:28:50,434 --> 00:28:52,468
So you wouldn't expect it
to create super

554
00:28:52,536 --> 00:28:54,303
energetic particles.

555
00:28:54,305 --> 00:28:57,039
ROWE: Something must be
hidden in the cloud,

556
00:28:57,108 --> 00:29:01,410
something powerful enough to
accelerate the cosmic rays.

557
00:29:01,412 --> 00:29:03,512
We just don't know what.

558
00:29:03,514 --> 00:29:06,415
We can't see inside
the molecular clouds,

559
00:29:06,550 --> 00:29:09,752
so it could be that,
deep inside them,

560
00:29:09,853 --> 00:29:13,322
there are clusters of newborn
stars that are cranking out

561
00:29:13,424 --> 00:29:14,823
these cosmic rays,

562
00:29:14,825 --> 00:29:18,761
but we don't know if even
the crankiest of stars are

563
00:29:18,829 --> 00:29:23,532
capable of producing cosmic
rays at these energies.

564
00:29:23,534 --> 00:29:25,667
ROWE:
Just two months later,

565
00:29:25,769 --> 00:29:29,238
in March of 2021,
we get another clue.

566
00:29:29,339 --> 00:29:31,940
Scientists detect
gamma rays coming

567
00:29:32,042 --> 00:29:35,244
from the Cygnus Cocoon Nebula.

568
00:29:35,345 --> 00:29:37,946
It's a dense molecular
cloud with

569
00:29:38,048 --> 00:29:41,617
a difference --
At the center is a cavity.

570
00:29:41,719 --> 00:29:46,455
Hundreds of closely packed
stars push against the dust

571
00:29:46,557 --> 00:29:48,824
and gas, including

572
00:29:48,893 --> 00:29:52,861
huge bright stars
called spectral type O and B

573
00:29:53,931 --> 00:29:56,165
Spectral type O and B stars are

574
00:29:56,233 --> 00:29:58,367
some of the hottest stars in
our universe.

575
00:30:00,037 --> 00:30:02,604
ROWE: The massive stars
blast out solar winds far

576
00:30:02,739 --> 00:30:05,040
stronger than the wind
produced by our sun.

577
00:30:06,710 --> 00:30:09,044
When you think about all these
stars forming together,

578
00:30:09,146 --> 00:30:12,347
they are all putting off
a wind of high-energy particles

579
00:30:12,449 --> 00:30:13,649
from their surfaces.

580
00:30:13,684 --> 00:30:16,718
These winds collide
and form big shock structures

581
00:30:16,821 --> 00:30:18,654
between all of
these young stars.

582
00:30:18,755 --> 00:30:20,956
CHRISTIAN: You're getting
so much energy from

583
00:30:21,058 --> 00:30:22,758
so many different winds,
coming from so many

584
00:30:22,860 --> 00:30:27,362
different directions, that it
forms a boiling mass of

585
00:30:27,431 --> 00:30:29,498
shockwaves and magnetic fields.

586
00:30:29,500 --> 00:30:30,966
ROWE:
It's a pinball machine

587
00:30:31,035 --> 00:30:34,970
on a far bigger scale --
The magnetic fields are stronger

588
00:30:35,039 --> 00:30:36,305
than a supernova's,

589
00:30:36,307 --> 00:30:40,609
trapping and accelerating
the more energetic cosmic rays

590
00:30:40,611 --> 00:30:41,810
for longer.

591
00:30:41,812 --> 00:30:44,246
One important thing about
star clusters is that

592
00:30:44,348 --> 00:30:47,015
they are around for millions
and millions of years.

593
00:30:47,017 --> 00:30:49,952
It's not just a one-off event
like a supernova.

594
00:30:49,987 --> 00:30:52,921
CHRISTIAN: And so you've got
this magnetic field

595
00:30:52,923 --> 00:30:56,124
and these shocks happening
over a long period of time,

596
00:30:56,260 --> 00:30:58,527
and that may be what you need
to accelerate cosmic rays.

597
00:30:59,930 --> 00:31:04,433
ROWE: Molecular clouds may
shoot out galactic cosmic rays,

598
00:31:04,535 --> 00:31:07,669
but what fires the hypersonic
space missiles,

599
00:31:07,738 --> 00:31:10,806
the ultra-high-energy
cosmic rays?

600
00:31:10,941 --> 00:31:14,810
The culprit may be hiding out
in distant galaxies --

601
00:31:14,945 --> 00:31:18,113
Supermassive black holes.

602
00:31:25,222 --> 00:31:27,522
ROWE: Ultra-high-energy
cosmic rays are

603
00:31:27,625 --> 00:31:31,226
the hypersonic missiles of
the particle world.

604
00:31:31,295 --> 00:31:33,328
If a photon of light,
the fastest thing in

605
00:31:33,330 --> 00:31:37,232
the universe, had a race with
an ultra-high-energy cosmic ray,

606
00:31:37,334 --> 00:31:41,670
it would be so close,
that after 200,000 years,

607
00:31:41,705 --> 00:31:44,506
that photon would be
half an inch ahead of

608
00:31:44,641 --> 00:31:46,408
the ultra-high-energy
cosmic ray.

609
00:31:46,410 --> 00:31:50,946
ROWE: They appear to come from
beyond our Milky Way galaxy.

610
00:31:51,048 --> 00:31:55,817
OLUSEYI: Our galaxy is
100,000 light years across.

611
00:31:55,920 --> 00:31:59,621
The next nearest galaxy to us
is two million light years away.

612
00:31:59,723 --> 00:32:01,757
So these are traveling to us

613
00:32:01,858 --> 00:32:04,826
across millions and billions
of light years.

614
00:32:04,929 --> 00:32:09,331
How do you accelerate this tiny

615
00:32:09,466 --> 00:32:13,969
little particle to such
insane velocities?

616
00:32:14,071 --> 00:32:15,904
What is the power source?

617
00:32:15,906 --> 00:32:19,541
What in the universe has
that kind of capability?

618
00:32:19,610 --> 00:32:22,411
Where's the Death Star here?

619
00:32:22,546 --> 00:32:25,147
ROWE: Their speed makes
them dangerous,

620
00:32:25,215 --> 00:32:29,051
but it also makes it easier to
find their source.

621
00:32:29,119 --> 00:32:32,421
Ultra-high-energy cosmic rays
are moving so rapidly,

622
00:32:32,556 --> 00:32:34,523
that they're really not
affected that much by

623
00:32:34,625 --> 00:32:35,757
magnetic fields.

624
00:32:35,826 --> 00:32:38,026
It's like a bullet going
through a fisherman's net.

625
00:32:38,028 --> 00:32:41,830
CHRISTIAN: And so they're coming
mostly in a straight line.

626
00:32:41,832 --> 00:32:44,166
When they're coming in a
straight line, and we can point

627
00:32:44,234 --> 00:32:45,734
back to their origin,

628
00:32:45,736 --> 00:32:49,504
and that's something we can
use to figure out where

629
00:32:49,506 --> 00:32:50,939
and how they're getting
accelerated.

630
00:32:52,910 --> 00:32:54,609
ROWE:
In the Argentinian desert,

631
00:32:54,712 --> 00:32:58,246
the Pierre Auger Cosmic Ray
Detector completes

632
00:32:58,315 --> 00:33:00,415
a 12-year study of the sky.

633
00:33:02,653 --> 00:33:05,554
It confirms that most
galaxies have a supermassive

634
00:33:05,655 --> 00:33:07,356
black hole at their center,

635
00:33:07,424 --> 00:33:11,159
but only a few are active,
shooting out energy.

636
00:33:11,261 --> 00:33:15,063
These active
supermassive black holes also

637
00:33:15,132 --> 00:33:18,433
blast out ultra-high-energy
cosmic rays.

638
00:33:19,436 --> 00:33:23,305
Supermassive black holes are
already extremely powerful.

639
00:33:23,307 --> 00:33:25,040
So it makes a lot
of sense to me that

640
00:33:25,142 --> 00:33:28,243
the ultra-high-energy cosmic
rays could originate at

641
00:33:28,345 --> 00:33:30,445
supermassive black holes.

642
00:33:30,514 --> 00:33:35,717
ROWE: The M87 galaxy is
54 million light years away.

643
00:33:35,719 --> 00:33:38,320
It's famous
because we took a photo

644
00:33:38,322 --> 00:33:41,723
of the supermassive black hole
at its core.

645
00:33:41,725 --> 00:33:45,994
So the event horizon telescope
image of the swirling vortex of

646
00:33:45,996 --> 00:33:48,463
gas around
that central black hole,

647
00:33:48,532 --> 00:33:50,565
that shadow that
you can't actually see,

648
00:33:50,634 --> 00:33:55,437
that could be a site for
the unbelievably energetic

649
00:33:55,539 --> 00:33:56,938
acceleration of cosmic rays.

650
00:33:57,040 --> 00:34:00,308
ROWE: In March 2021,

651
00:34:00,310 --> 00:34:03,311
scientists analyze
the data further.

652
00:34:03,447 --> 00:34:07,315
MINGARELLI: This new image
of M87 shows very clear

653
00:34:07,451 --> 00:34:08,950
magnetic field lines,

654
00:34:09,053 --> 00:34:11,820
which is really stunning and
reminds us of how much energy

655
00:34:11,922 --> 00:34:15,057
could be contained close to
the supermassive black hole.

656
00:34:15,158 --> 00:34:17,826
ROWE: Black holes have
enormous power,

657
00:34:17,928 --> 00:34:20,929
but how do they transfer some
of that energy

658
00:34:20,931 --> 00:34:22,164
to a tiny particle?

659
00:34:22,265 --> 00:34:25,333
HOPKINS: One possibility for how
supermassive black holes

660
00:34:25,469 --> 00:34:27,636
could accelerate such
enormously energetic

661
00:34:27,738 --> 00:34:31,606
cosmic rays is that they
actually drag or capture via

662
00:34:31,741 --> 00:34:32,941
their gravity

663
00:34:33,076 --> 00:34:36,411
preexisting normal cosmic rays,
which are already

664
00:34:36,546 --> 00:34:37,846
extremely energetic,

665
00:34:37,948 --> 00:34:41,817
and then give them an extra
boost to even higher energies.

666
00:34:41,919 --> 00:34:45,053
MINGARELLI: So supermassive
black holes bend the fabric

667
00:34:45,122 --> 00:34:46,455
of spacetime around them,

668
00:34:46,556 --> 00:34:50,625
and even light particles can
get stuck, and cosmic rays are

669
00:34:50,627 --> 00:34:53,662
no different -- they can also be
attracted by the supermassive

670
00:34:53,763 --> 00:34:57,365
black holes and get drawn
into their orbit.

671
00:34:57,467 --> 00:35:00,836
ROWE: It makes sense that
the black hole captures

672
00:35:00,838 --> 00:35:02,504
passing cosmic rays,

673
00:35:02,606 --> 00:35:05,640
but how do the particles
escape its clutches

674
00:35:05,742 --> 00:35:07,109
and hurtle towards us?

675
00:35:08,479 --> 00:35:12,280
M87 has a fearsome weapon
in its arsenal.

676
00:35:12,282 --> 00:35:17,319
Enormous jets of energy
blast out of its poles.

677
00:35:17,321 --> 00:35:20,856
TREMBLAY: So M87's jets are
spectacularly large,

678
00:35:20,991 --> 00:35:23,959
larger than the entire galaxy
that houses this black hole

679
00:35:24,061 --> 00:35:25,427
that's launching those jets.

680
00:35:25,529 --> 00:35:28,163
ROWE: The powerful jets
may give the cosmic rays

681
00:35:28,198 --> 00:35:29,564
a speed injection,

682
00:35:29,666 --> 00:35:33,168
transforming them from
galactic rifle bullets into

683
00:35:33,270 --> 00:35:36,505
ultra-high-energy
hypersonic missiles.

684
00:35:36,607 --> 00:35:41,042
So imagine if you had
a regular bullet that

685
00:35:41,144 --> 00:35:43,812
you fired out of a gun
at high speed,

686
00:35:43,814 --> 00:35:47,949
and as its flying,
a little rocket motor in

687
00:35:48,084 --> 00:35:52,320
the bullet kicks in and takes
it up to even higher speeds.

688
00:35:52,322 --> 00:35:54,656
That's sort of what's
happening to

689
00:35:54,758 --> 00:35:57,125
the cosmic rays in these jets.

690
00:35:58,462 --> 00:36:00,362
ROWE: Black holes may be
the supervillains

691
00:36:00,463 --> 00:36:02,030
we've been looking for,

692
00:36:02,032 --> 00:36:04,933
firing out the fastest
cosmic bullets,

693
00:36:04,935 --> 00:36:08,904
but cosmic rays have
a superpower of their own.

694
00:36:08,906 --> 00:36:10,405
They're time travelers.

695
00:36:18,115 --> 00:36:20,715
ROWE: Cosmic rays race through
the universe at close

696
00:36:20,850 --> 00:36:24,819
to the speed of light --
Like subatomic bullets,

697
00:36:24,922 --> 00:36:28,924
they can pierce spaceships
and harm astronauts.

698
00:36:31,929 --> 00:36:34,629
But down on Earth,
we're protected.

699
00:36:36,567 --> 00:36:41,703
Out of all of the rocky inner
planets in the solar system,

700
00:36:41,705 --> 00:36:44,206
the Earth is the only one
to generate

701
00:36:44,341 --> 00:36:48,310
its own deflector shield
against this cosmic radiation.

702
00:36:48,378 --> 00:36:50,612
That's amazing.
And that's where life is.

703
00:36:50,747 --> 00:36:54,216
I don't think that's actually
all that much of a coincidence.

704
00:36:54,351 --> 00:36:58,053
ROWE: The Earth creates
its own magnetic field.

705
00:36:58,121 --> 00:37:01,223
The Earth has this wonderful
active molten core of metal.

706
00:37:01,325 --> 00:37:04,793
All of that metal is moving
around inside the Earth,

707
00:37:04,795 --> 00:37:07,562
and that moving metal generates
a strong magnetic field.

708
00:37:07,664 --> 00:37:09,931
These cosmic rays are
electrically charged.

709
00:37:09,933 --> 00:37:11,566
They follow a magnetic field.

710
00:37:11,668 --> 00:37:13,969
So our magnetic field
deflects most

711
00:37:14,070 --> 00:37:15,937
of the cosmic rays around it.

712
00:37:16,039 --> 00:37:18,206
ROWE:
The shield is not perfect.

713
00:37:18,208 --> 00:37:21,276
Some cosmic rays do get through,

714
00:37:21,278 --> 00:37:24,312
but then they hit
our second line of defense --

715
00:37:24,314 --> 00:37:25,814
The atmosphere.

716
00:37:25,816 --> 00:37:27,849
One of the things we have to
be thankful for is

717
00:37:27,984 --> 00:37:30,518
our atmosphere -- not only
does it give us air to breathe,

718
00:37:30,654 --> 00:37:32,520
but it protects us
from these space bullets.

719
00:37:32,522 --> 00:37:37,759
ROWE: The atmosphere is like
a missile defense system.

720
00:37:37,827 --> 00:37:40,629
Cosmic rays collide
with air molecules,

721
00:37:40,631 --> 00:37:44,132
shattering into
safer, smaller particles.

722
00:37:44,267 --> 00:37:48,370
The most common ones
are called muons.

723
00:37:49,706 --> 00:37:54,209
The muons are the children
of the cosmic rays.

724
00:37:54,311 --> 00:37:57,545
They're produced by
these high-energy collisions

725
00:37:57,614 --> 00:37:58,913
in our upper atmosphere that

726
00:37:58,915 --> 00:38:02,550
create these showers of
muons that then come down to

727
00:38:02,652 --> 00:38:03,784
the surface.

728
00:38:03,786 --> 00:38:06,021
There's as many as four of
these cosmic rays passing

729
00:38:06,023 --> 00:38:07,622
through my hand every second.

730
00:38:07,624 --> 00:38:09,724
They're passing through
your body right now.

731
00:38:09,793 --> 00:38:11,760
ROWE:
Muons are so abundant,

732
00:38:11,861 --> 00:38:14,829
we don't need a high-tech
observatory to detect them,

733
00:38:14,964 --> 00:38:18,767
just a few things you'd find
in a high school science lab.

734
00:38:18,835 --> 00:38:23,138
A small aquarium that I've
attached a small piece of felt

735
00:38:23,239 --> 00:38:24,806
to the bottom.

736
00:38:24,808 --> 00:38:27,809
Some frozen carbon dioxide,
some dry ice,

737
00:38:27,911 --> 00:38:33,815
hence the safety gloves,
a flat piece of metal like this,

738
00:38:33,817 --> 00:38:35,650
some isopropyl alcohol.

739
00:38:37,421 --> 00:38:40,055
Then I flip the whole thing over

740
00:38:40,156 --> 00:38:43,325
onto the bottom, and I wait.

741
00:38:45,529 --> 00:38:49,531
So what's happening is that
the alcohol in the felt is

742
00:38:49,533 --> 00:38:52,934
evaporating in sinking down,
and because

743
00:38:52,936 --> 00:38:55,737
that bottom layer is
so cold from the dry ice,

744
00:38:55,739 --> 00:39:00,141
it forms a super saturated
cloud of alcohol vapor.

745
00:39:01,511 --> 00:39:05,313
ROWE: When the charged particles
pass through the cold vapor,

746
00:39:05,315 --> 00:39:09,718
they create tiny ghostly trails.

747
00:39:09,820 --> 00:39:12,620
What we're looking for
are the muons,

748
00:39:12,622 --> 00:39:16,224
the subatomic particles
generated when a cosmic ray

749
00:39:16,359 --> 00:39:18,727
strikes the upper atmosphere.

750
00:39:18,729 --> 00:39:21,830
ROWE: Each silvery thread
in the cloud chamber

751
00:39:21,965 --> 00:39:25,033
is the sign of a cosmic ray.

752
00:39:25,035 --> 00:39:28,903
These muons should never
make it down to Earth at all.

753
00:39:28,905 --> 00:39:34,008
They only live for 2.2
microseconds before breaking up,

754
00:39:34,010 --> 00:39:35,710
not enough time to travel

755
00:39:35,812 --> 00:39:38,713
through six miles of
Earth's atmosphere.

756
00:39:38,715 --> 00:39:41,850
Naively, we would think there's
no way that a muon could make it

757
00:39:41,985 --> 00:39:44,152
from the upper atmosphere
to where we are now

758
00:39:44,254 --> 00:39:45,220
without decaying.

759
00:39:45,222 --> 00:39:47,021
It turns out they do,
and the only

760
00:39:47,023 --> 00:39:50,925
way they do this is
they effectively time travel.

761
00:39:50,927 --> 00:39:54,429
The muons move at 98 percent
the speed of light.

762
00:39:55,832 --> 00:39:57,532
They move so fast,

763
00:39:57,534 --> 00:40:01,069
they experience what Einstein
called time dilation.

764
00:40:01,137 --> 00:40:04,739
Albert Einstein taught us that
we live

765
00:40:04,808 --> 00:40:06,741
in a space time,
and so that means

766
00:40:06,810 --> 00:40:10,345
that all measurements of
lengths and durations of time

767
00:40:10,414 --> 00:40:11,946
are relative.

768
00:40:12,048 --> 00:40:13,815
ROWE: From
a muon's perspective,

769
00:40:13,917 --> 00:40:17,452
we humans move
incredibly slowly.

770
00:40:17,521 --> 00:40:20,822
They're moving so fast that,
for them,

771
00:40:20,824 --> 00:40:23,725
time is stretched out.

772
00:40:23,827 --> 00:40:27,862
What we found by measuring
the energy and the lifetime of

773
00:40:27,964 --> 00:40:33,234
muons is that as muons got
closer to the speed of light,

774
00:40:33,336 --> 00:40:37,739
their lifetime increased,
because to them, time is

775
00:40:37,808 --> 00:40:41,242
slowing down, exactly the way
Einstein predicted.

776
00:40:41,344 --> 00:40:45,413
OLUSEYI: Their lifespan is
extended by more than 20 times

777
00:40:45,415 --> 00:40:46,614
from our perspective,

778
00:40:46,716 --> 00:40:49,017
so they make it to the ground.

779
00:40:51,955 --> 00:40:55,957
ROWE: Cosmic rays are
the ultimate space travelers.

780
00:40:56,059 --> 00:40:59,727
They're awe-inspiring speed
allows us to unlock

781
00:40:59,830 --> 00:41:03,431
hidden processes and test
our theories of physics.

782
00:41:04,734 --> 00:41:06,468
They're way more energetic
than anything

783
00:41:06,536 --> 00:41:08,470
we can do in a laboratory
on Earth.

784
00:41:08,538 --> 00:41:10,405
So that means we can unlock
all kinds of

785
00:41:10,507 --> 00:41:13,041
new domains about physics
at the highest,

786
00:41:13,142 --> 00:41:15,243
most extreme energies.

787
00:41:15,312 --> 00:41:17,879
ROWE: They're our best link
to the farthest reaches of

788
00:41:17,881 --> 00:41:19,647
the cosmos.

789
00:41:19,716 --> 00:41:23,451
To me, it's really exciting
that we're actually sampling

790
00:41:23,553 --> 00:41:26,321
pieces of matter from distant

791
00:41:26,423 --> 00:41:30,225
stars, from distant galaxies,
and we're getting them here at

792
00:41:30,227 --> 00:41:31,960
Earth and studying them.

793
00:41:33,930 --> 00:41:37,432
There are so many amazingly
violent events in the universe,

794
00:41:37,434 --> 00:41:40,335
the birth of black holes,
exploding stars.

795
00:41:40,337 --> 00:41:42,370
These cosmic rays that are
going through your body

796
00:41:42,438 --> 00:41:45,607
right now are messengers
from those events.

797
00:41:45,609 --> 00:41:47,509
In some way,
you're still connected to

798
00:41:47,611 --> 00:41:49,944
those events, millions
of light years away.

799
00:41:51,848 --> 00:41:54,048
These are messengers
from the universe,

800
00:41:54,117 --> 00:41:56,017
telling us about how it works.