WEBVTT

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- RADIOACTIVITY.

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THERE ARE LOTS OF THINGS
THAT CAN GIVE OFF

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SMALL OR LARGE AMOUNTS OF ENERGY
AND PARTICLES

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THAT WE REFER TO
AS BEING RADIOACTIVE,

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INCLUDING MINERALS
THAT COME OUT OF THE EARTH

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AND HELPFUL
MEDICAL TECHNOLOGIES.

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EVEN BANANAS AND YOUR OWN BONES

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CONTAIN SUBSTANCES
THAT ARE SLIGHTLY RADIOACTIVE.

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IN 1898, MADAME MARIE CURIE

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FIRST USED THE WORD
"RADIOACTIVITY"

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TO DESCRIBE THE ABILITY
OF CERTAIN ELEMENTS

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TO GIVE OFF INVISIBLE ENERGY.

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BUT DID YOU KNOW
THIS TYPE OF ENERGY

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CAN BE USED
TO EXPLORE THE SOLAR SYSTEM?

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FIND OUT HOW,
NEXT ON "REAL WORLD."

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WHEN MOST PEOPLE HEAR THE WORD
"RADIOACTIVITY"

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THEY THINK OF
MYSTERIOUS GLOWING SUBSTANCES,

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NUCLEAR POWER PLANTS,
OR WEAPONS.

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BUT YOU MIGHT NOT BE AWARE
OF THE VAST DIFFERENCES

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BETWEEN THE DIFFERENT
RADIOACTIVE SUBSTANCES.

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THESE DIFFERENT SUBSTANCES

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GIVE OFF
DIFFERENT AMOUNTS OF ENERGY

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AND HAVE
ALL KINDS OF PRACTICAL USES

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FROM BONE IMAGING

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TO SMOKE DETECTORS;

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ARCHAEOLOGICAL DATING

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TO FOOD PROCESSING.

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BUT WHAT MAKES A MATERIAL
RADIOACTIVE TO BEGIN WITH?

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LET'S START WITH SOME BASICS.

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ALL MATTER IS MADE UP
OF PARTICLES CALLED ATOMS.

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AND ATOMS ARE MADE UP
OF SMALLER PARTICLES--

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PROTONS, NEUTRONS,
AND ELECTRONS.

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ATOMS OF A GIVEN ELEMENT

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ALL HAVE
THE SAME NUMBER OF PROTONS,

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BUT THE NUMBER OF NEUTRONS
CAN VARY.

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ATOMS OF THE SAME ELEMENT

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WITH A DIFFERENT NUMBER
OF NEUTRONS

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ARE KNOWN AS ISOTOPES
OF THAT ELEMENT.

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NOW, SOME ISOTOPES ARE STABLE,

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MEANING THEY DO NOT CHANGE
OVER TIME;

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OTHERS ARE UNSTABLE.

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WE CALL THESE UNSTABLE ISOTOPES
"RADIOISOTOPES"

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BECAUSE, TO BECOME MORE STABLE,

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THEY RADIATE,
OR GIVE OFF ENERGY,

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USUALLY MOVING PARTICLES
PLUS HEAT.

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AND THIS PROCESS
IS CALLED RADIOACTIVE DECAY.

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- RADIOACTIVE DECAY
IS WHAT HAPPENS

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WHEN AN ATOM IS TRYING
TO GET TO A STABLE STATE,

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THAT IS, A BALANCED STATE.

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- YOU HAVE A VERY UNSTABLE ATOM

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THAT LIKES TO EMIT THINGS

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FROM ALPHA PARTICLES
TO BETA PARTICLES

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TO SOMETIMES GAMMA RAYS.

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BUT WHAT IT'S DOING IS,

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IT'S TAKING AN UNSTABLE ATOM

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AND GOING TO A MORE STABLE STATE

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AND RELEASING ENERGY
DURING THE PROCESS.

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- ONE OF THE MOST IMPORTANT
THINGS TO KNOW

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ABOUT THIS WHOLE PROCESS

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IS THAT THE RATE AT WHICH
ANY RADIOISOTOPE DECAYS

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IS PREDICTABLE.

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UNDER THE SAME CONDITIONS,

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YOU CAN COUNT ON IT TO HAPPEN
THE SAME WAY

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OVER AND OVER.

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NOW, FOR UNSTABLE ATOMS,

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THERE'S NO WAY OF TELLING WHEN
AN INDIVIDUAL ATOM WILL DECAY,

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BUT THROUGH CAREFUL MEASUREMENT,

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IT IS POSSIBLE TO PREDICT
HOW LONG IT WILL TAKE

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FOR A LARGE NUMBER OF THEM,

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SAY HALF OF THE ATOMS
IN A GIVEN SAMPLE, TO DECAY.

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AS THE ATOMS DECAY,

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THEY CHANGE FROM ONE ELEMENT
INTO ANOTHER.

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THE TIME IT TAKES

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FOR HALF AN AMOUNT
OF ANY RADIOISOTOPE

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TO DECAY
INTO ANOTHER TYPE OF ELEMENT

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IS CALLED ITS HALF-LIFE.

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- HALF-LIFE
IS THE AMOUNT OF TIME IT TAKES

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FOR HALF OF THE RADIOISOTOPES
TO DECAY

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FROM A MORE UNSTABLE STATE
TO A MORE STABLE STATE.

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AND ATOMS OF DIFFERENT TYPES
HAVE DIFFERENT WAYS OF DECAYING

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FROM THIS MORE UNSTABLE STATE
TO THE MORE STABLE STATE.

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- IT IS JUST
THE NATURE OF THE ELEMENT

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AND OF THE ISOTOPE
OF THAT ELEMENT

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HOW LONG THAT HALF-LIFE IS.

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AND IT'S A NATURE OF HOW--

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WHAT KIND OF ENERGY
IT'S RELEASING,

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WHETHER IT'S GAMMA RAYS,
WHETHER IT'S HEAT,

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WHETHER IT'S OTHER PARTICLES.

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- HALF-LIVES VARY GREATLY--

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FROM MILLISECONDS
TO BILLIONS OF YEARS--

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BUT EACH RADIOISOTOPE
HAS ITS OWN RATE OF DECAY.

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FOR EXAMPLE, IODINE-131

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HAS A HALF-LIFE
OF ONLY EIGHT DAYS,

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WHILE CHLORINE-36

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HAS A HALF-LIFE
OF 300,000 YEARS.

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SUPPOSE YOU HAVE
32 GRAMS OF SODIUM-24,

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WHICH HAS A HALF-LIFE
OF 15 HOURS

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AND DECAYS INTO MAGNESIUM-24,

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A STABLE FORM OF MAGNESIUM.

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AN UNDERSTANDING OF HALF-LIFE
AND SOME SIMPLE MATH

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CAN HELP YOU DETERMINE

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HOW MUCH OF THE ORIGINAL
MATERIAL EXISTS

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AFTER A GIVEN PERIOD OF TIME.

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AFTER 15 HOURS,

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HALF OF THE SODIUM-24

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WOULD HAVE DECAYED
INTO MAGNESIUM-24,

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LEAVING 16 GRAMS
OF THE ORIGINAL ISOTOPE.

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AFTER 30 HOURS,

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THERE WOULD ONLY BE
8 GRAMS OF SODIUM-24 LEFT,

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AND SO ON,

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UNTIL THERE'S NOT ENOUGH
SODIUM-24 LEFT IN THE SAMPLE

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TO DETECT ITS RADIOACTIVITY.

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SO NOW THAT YOU KNOW MORE ABOUT
RADIOACTIVITY AND HALF-LIFE,

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HOW DOES NASA USE MADAME CURIE'S
"INVISIBLE ENERGY"

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TO EXPLORE THE SOLAR SYSTEM?

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WELL,
SINCE HALF-LIFE IS PREDICTABLE,

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SCIENTISTS AND ENGINEERS
CAN USE THIS KNOWLEDGE

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TO CREATE POWER SOURCES

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THAT RELY
ON THE PREDICTABLE HEAT

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GIVEN OFF BY RADIOISOTOPES
AS THEY DECAY.

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NASA'S RADIOISOTOPE-POWERED
SPACECRAFT

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USE A UNIQUE ISOTOPE
CALLED PLUTONIUM-238,

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WHICH IS PROVIDED
BY THE DEPARTMENT OF ENERGY.

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IT HAS A HALF-LIFE OF 88 YEARS

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AND GIVES OFF HEAT

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PLUS LOW-ENERGY PARTICLES
CALLED ALPHA PARTICLES.

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THE FACT THAT IT'S
A LONG-LIVED SOURCE OF HEAT

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MAKES THIS ISOTOPE AN IDEAL FUEL
FOR LONG-LIVED SPACE MISSIONS.

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- A RADIOISOTOPE POWER SYSTEM
IS A SPECIAL SYSTEM

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THAT USES
THE DECAY OF A RADIOISOTOPE,

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IN OUR CASE, PLUTONIUM-238,
TO GENERATE HEAT.

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THAT HEAT THEN IS CONVERTED
INTO POWER OR ELECTRICITY

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THAT WE USE ON SPACECRAFT.

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- THIS MAKES
RADIOISOTOPE POWER SYSTEMS--

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OR AN R.P.S. FOR SHORT--

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IDEALLY SUITED FOR MISSIONS

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IN THE EXTREME ENVIRONMENTS
OF SPACE

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OR ON PLANETARY SURFACES,

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LIKE SOME DESTINATIONS ON MARS.

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THE MARS SCIENCE
LABORATORY ROVER,

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ALSO KNOWN AS CURIOSITY,

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HAS BEEN DESIGNED TO USE
A RADIOISOTOPE POWER SYSTEM

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AS IT EXPLORES
THE RED PLANET.

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THE R.P.S.
CAN GENERATE ELECTRICITY NEEDED

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TO OPERATE THE ROVER
AND ITS INSTRUMENTS,

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AND EXCESS HEAT
FROM THE POWER SOURCE

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CAN KEEP THE ROVER WARM
ON THIS COLD, DUSTY PLANET.

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THE STEADY AND RELIABLE OUTPUT
OF ITS R.P.S.

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SHOULD ENABLE THE LARGE ROVER

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TO TRAVEL FARTHER,
LAST LONGER,

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AND DO MORE SCIENCE

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THAN ANY PREVIOUS
MOBILE MARTIAN EXPLORER.

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CURIOSITY IS DESIGNED
TO ROAM MARS

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FOR A FULL MARTIAN YEAR--

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THAT'S ABOUT TWO EARTH YEARS.

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WITH ITS RELIABLE,
LONG- LASTING POWER SOURCE,

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THE MISSION
COULD GO EVEN LONGER.

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AND THAT LONGEVITY

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IS THANKS TO OUR UNDERSTANDING
OF RADIOACTIVE DECAY.

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SO HOW ABOUT THAT?

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FROM LABORATORIES ON EARTH
TO ROVERS ON MARS,

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SCIENCE AND MATH

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CAN HARNESS THE POWER
OF RADIOACTIVE DECAY

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TO HELP MAKE EXPLORATION
TRULY A COSMIC ENDEAVOR.