WEBVTT

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- DO YOU LOOK THE SAME

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AS YOU DID
4 1/2 YEARS AGO?

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I SURE DON'T.

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NOW IMAGINE
HOW MUCH OUR SOLAR SYSTEM

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HAS CHANGED
OVER 4 1/2 BILLION YEARS.

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SO HOW CAN WE LEARN ABOUT
WHAT OUR SOLAR SYSTEM WAS LIKE

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ALL THOSE YEARS AGO?

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FIND OUT NEXT ON "REAL WORLD."

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ONE OF THE WAYS WE CAN LEARN

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ABOUT THE HISTORY
OF OUR SOLAR SYSTEM

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IS BY STUDYING COMETS.

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YOU'VE DEFINITELY HEARD
OF SOME COMETS TOO.

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DO THE NAMES HALLEY,

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SHOEMAKER-LEVY,

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OR HARTLEY-2 RING A BELL?

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WELL, THERE ARE THOUSANDS MORE
WHERE THOSE CAME FROM.

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SO WHAT IS A COMET?

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LET ME LET DR. PAUL CHODAS,
A RESEARCH SCIENTIST

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AT NASA'S JET PROPULSION
LABORATORY, EXPLAIN.

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- A COMET IS AN ICY SMALL BODY.

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IT CONTAINS A LOT OF DIRT
AND OTHER STUFF IN IT.

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BUT IT CONTAINS FROZEN ICE--
MOSTLY WATER ICE,

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BUT THERE COULD BE OTHER ICES
IN THERE.

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SO THE ICE IS VERY COLD,
OF COURSE,

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AND WHEN THE COMET
GETS CLOSE TO THE SUN,

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THE ICE GETS WARMED UP,

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AND IT STARTS TO BOIL OFF,

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AND AS IT BOILS OFF,

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IT FORMS STREAMS
OF GASES COMING OFF,

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AND THESE ARE WHAT FORM
THE TAIL.

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THEY HAVE BEEN
IN KIND OF A DEEP FREEZE.

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MANY OF THEM HAVE BEEN
FAR BEYOND PLUTO.

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THEY'VE BEEN WAY OUT

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AND GRADUALLY COME
INTO THE INNER SOLAR SYSTEM,

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AND THESE OBJECTS CONTAIN
NOT JUST THE MINERALS

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OF THE EARLY SOLAR SYSTEM.

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THEY CONTAIN THE ORIGINAL ICES
OF THE EARLY SOLAR SYSTEM.

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SO IF WE STUDY COMETS,
WE CAN LEARN

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ABOUT ALL OF THE MATERIALS
THAT WERE THERE

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WHEN THE SOLAR SYSTEM
WAS FORMED.

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- COOL, SO COMETS ARE MADE

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OF FROZEN GASES, ICE,
DUST, AND ROCK.

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WHEN THE COMET
IS FAR AWAY FROM THE SUN,

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EVERYTHING STAYS FROZEN.

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WHEN IT GETS CLOSER
TO THE SUN,

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IT WARMS UP,
CREATING A COMA.

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AS IT GETS EVEN CLOSER
TO THE SUN,

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THE SUN'S RADIATION PUSHES
SOME OF THE COMA

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AWAY FROM THE COMET,
CREATING THE TAIL.

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SO IF COMETS CAN BE
AS FAR OUT IN OUR SOLAR SYSTEM

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AS BEYOND PLUTO

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OR CLOSE ENOUGH TO BE SEEN
BY THE NAKED EYE HERE ON EARTH,

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HOW DO WE KNOW
HOW FAR AWAY A COMET IS?

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WHENEVER YOU LOOK
AT A DISTANT OBJECT,

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YOU SEE THE OBJECT'S
ANGULAR SIZE,

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NOT ITS ACTUAL PHYSICAL SIZE.

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FOR INSTANCE, THE MOON ISN'T
REALLY THE SIZE OF A QUARTER.

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IT JUST LOOKS LIKE THAT

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BECAUSE OF ITS DISTANCE
FROM THE EARTH.

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ANGULAR SIZE IS THE MEASURE
OF AN ANGLE

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FORMED BY EXTENDING
TWO IMAGINARY LINES

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OUTWARD FROM OUR EYES
TO SPAN THE OBJECT.

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THERE IS A SIMPLE FORMULA
THAT RELATES

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THE ACTUAL SIZE
AND THE DISTANCE TO AN OBJECT

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TO ITS APPARENT ANGULAR SIZE:

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THE TANGENT OF THE QUANTITY
THETA DIVIDED BY 2

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IS EQUAL TO D DIVIDED BY 2R,

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WHERE D IS THE OBJECT'S DIAMETER
IN KILOMETERS

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AND R IS THE DISTANCE
OF THE OBJECT

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FROM THE OBSERVER IN KILOMETERS.

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[record scratches]

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NOT READY
FOR THAT KIND OF MATH YET?

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THAT'S OKAY.

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BUT UNDERSTANDING SCALE RATIO,

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YOU CAN USE A MILLIMETER RULER
AND AN IMAGE OF A COMET

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TO SET UP A PROPORTION

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AND DETERMINE JUST HOW BIG
THAT COMET IS.

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SO WHY SHOULD WE CARE
ABOUT WHAT A COMET IS,

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HOW BIG IT IS, HOW IT MOVES,
OR EVEN HOW FAR AWAY IT IS?

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WELL, COMETS CAN TELL US A LOT
ABOUT OUR EARLY SOLAR SYSTEM.

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- THE MAIN REASON
TO STUDY COMETS

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IS TO LEARN
ABOUT THE CONDITIONS

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OF THE FORMATION
OF THE SOLAR SYSTEM.

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WHAT ARE THE MATERIALS
THAT FORMED THE SOLAR SYSTEM?

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WHAT DID IT LOOK LIKE
ORIGINALLY?

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WE CAN'T LOOK AT THE PLANETS,
LIKE THE EARTH,

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BECAUSE IT'S UNDERGONE
SO MUCH PROCESSING,

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HEATING UP AND WEATHERING,
ALL KINDS OF THINGS.

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ALL THE PLANETS
HAVE BEEN PROCESSED HEAVILY.

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IF WE TAKE A LOOK AT COMETS,

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WE CAN SEE
THE ORIGINAL MATERIALS

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FROM WHICH THE SOLAR SYSTEM
FORMED.

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- AND WE HAVE TAKEN
A LOOK AT COMETS

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WITH SOME REALLY COOL
SPACECRAFT.

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BUT HOW DOES SOMEONE
LIKE YOU OR ME

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GO ABOUT GETTING INVOLVED
IN SOMETHING THIS COOL?

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ASSOCIATE ADMINISTRATOR

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FOR THE SCIENCE MISSION
DIRECTORATE,

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DR. ED WEILER,
HAS GOT THE ANSWER.

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- I HAVE A MESSAGE
FOR ANY SCHOOLKIDS OUT THERE

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WHO MIGHT BE WONDERING
HOW NASA CAN SEND A SPACECRAFT

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BILLIONS OF MILES
THROUGH THE SOLAR SYSTEM

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AND SOMEHOW WIND UP FLYING
SO CLOSE TO A TINY COMET

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ONLY A FEW KILOMETERS
IN DIAMETER.

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IT'S DONE WITH MATH.

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- SO A COMET IS BASICALLY LIKE
A REALLY OLD TIME CAPSULE,

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AND AS WE LEARN
ABOUT WHAT'S INSIDE

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THESE TIME CAPSULES,
OR COMETS,

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WE CAN BEGIN
TO ANSWER QUESTIONS LIKE,

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"WHERE DID WE COME FROM?"

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AND, "HOW DID WE GET HERE?"

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I DON'T KNOW ABOUT YOU,

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BUT I WOULDN'T MIND
FINDING OUT THOSE ANSWERS

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BEFORE THE NEXT
4 1/2 BILLION YEARS PASS.

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SEE YOU NEXT TIME
ON "REAL WORLD."