- RADIOACTIVITY.
THERE ARE LOTS OF THINGS THAT CAN GIVE OFF
SMALL OR LARGE AMOUNTS OF ENERGY AND PARTICLES
THAT WE REFER TO AS BEING RADIOACTIVE,
INCLUDING MINERALS THAT COME OUT OF THE EARTH
AND HELPFUL MEDICAL TECHNOLOGIES.
EVEN BANANAS AND YOUR OWN BONES
CONTAIN SUBSTANCES THAT ARE SLIGHTLY RADIOACTIVE.
IN 1898, MADAME MARIE CURIE
FIRST USED THE WORD "RADIOACTIVITY"
TO DESCRIBE THE ABILITY OF CERTAIN ELEMENTS
TO GIVE OFF INVISIBLE ENERGY.
BUT DID YOU KNOW THIS TYPE OF ENERGY
CAN BE USED TO EXPLORE THE SOLAR SYSTEM?
FIND OUT HOW, NEXT ON "REAL WORLD."
WHEN MOST PEOPLE HEAR THE WORD "RADIOACTIVITY"
THEY THINK OF MYSTERIOUS GLOWING SUBSTANCES,
NUCLEAR POWER PLANTS, OR WEAPONS.
BUT YOU MIGHT NOT BE AWARE OF THE VAST DIFFERENCES
BETWEEN THE DIFFERENT RADIOACTIVE SUBSTANCES.
THESE DIFFERENT SUBSTANCES
GIVE OFF DIFFERENT AMOUNTS OF ENERGY
AND HAVE ALL KINDS OF PRACTICAL USES
FROM BONE IMAGING
TO SMOKE DETECTORS;
ARCHAEOLOGICAL DATING
TO FOOD PROCESSING.
BUT WHAT MAKES A MATERIAL RADIOACTIVE TO BEGIN WITH?
LET'S START WITH SOME BASICS.
ALL MATTER IS MADE UP OF PARTICLES CALLED ATOMS.
AND ATOMS ARE MADE UP OF SMALLER PARTICLES--
PROTONS, NEUTRONS, AND ELECTRONS.
ATOMS OF A GIVEN ELEMENT
ALL HAVE THE SAME NUMBER OF PROTONS,
BUT THE NUMBER OF NEUTRONS CAN VARY.
ATOMS OF THE SAME ELEMENT
WITH A DIFFERENT NUMBER OF NEUTRONS
ARE KNOWN AS ISOTOPES OF THAT ELEMENT.
NOW, SOME ISOTOPES ARE STABLE,
MEANING THEY DO NOT CHANGE OVER TIME;
OTHERS ARE UNSTABLE.
WE CALL THESE UNSTABLE ISOTOPES "RADIOISOTOPES"
BECAUSE, TO BECOME MORE STABLE,
THEY RADIATE, OR GIVE OFF ENERGY,
USUALLY MOVING PARTICLES PLUS HEAT.
AND THIS PROCESS IS CALLED RADIOACTIVE DECAY.
- RADIOACTIVE DECAY IS WHAT HAPPENS
WHEN AN ATOM IS TRYING TO GET TO A STABLE STATE,
THAT IS, A BALANCED STATE.
- YOU HAVE A VERY UNSTABLE ATOM
THAT LIKES TO EMIT THINGS
FROM ALPHA PARTICLES TO BETA PARTICLES
TO SOMETIMES GAMMA RAYS.
BUT WHAT IT'S DOING IS,
IT'S TAKING AN UNSTABLE ATOM
AND GOING TO A MORE STABLE STATE
AND RELEASING ENERGY DURING THE PROCESS.
- ONE OF THE MOST IMPORTANT THINGS TO KNOW
ABOUT THIS WHOLE PROCESS
IS THAT THE RATE AT WHICH ANY RADIOISOTOPE DECAYS
IS PREDICTABLE.
UNDER THE SAME CONDITIONS,
YOU CAN COUNT ON IT TO HAPPEN THE SAME WAY
OVER AND OVER.
NOW, FOR UNSTABLE ATOMS,
THERE'S NO WAY OF TELLING WHEN AN INDIVIDUAL ATOM WILL DECAY,
BUT THROUGH CAREFUL MEASUREMENT,
IT IS POSSIBLE TO PREDICT HOW LONG IT WILL TAKE
FOR A LARGE NUMBER OF THEM,
SAY HALF OF THE ATOMS IN A GIVEN SAMPLE, TO DECAY.
AS THE ATOMS DECAY,
THEY CHANGE FROM ONE ELEMENT INTO ANOTHER.
THE TIME IT TAKES
FOR HALF AN AMOUNT OF ANY RADIOISOTOPE
TO DECAY INTO ANOTHER TYPE OF ELEMENT
IS CALLED ITS HALF-LIFE.
- HALF-LIFE IS THE AMOUNT OF TIME IT TAKES
FOR HALF OF THE RADIOISOTOPES TO DECAY
FROM A MORE UNSTABLE STATE TO A MORE STABLE STATE.
AND ATOMS OF DIFFERENT TYPES HAVE DIFFERENT WAYS OF DECAYING
FROM THIS MORE UNSTABLE STATE TO THE MORE STABLE STATE.
- IT IS JUST THE NATURE OF THE ELEMENT
AND OF THE ISOTOPE OF THAT ELEMENT
HOW LONG THAT HALF-LIFE IS.
AND IT'S A NATURE OF HOW--
WHAT KIND OF ENERGY IT'S RELEASING,
WHETHER IT'S GAMMA RAYS, WHETHER IT'S HEAT,
WHETHER IT'S OTHER PARTICLES.
- HALF-LIVES VARY GREATLY--
FROM MILLISECONDS TO BILLIONS OF YEARS--
BUT EACH RADIOISOTOPE HAS ITS OWN RATE OF DECAY.
FOR EXAMPLE, IODINE-131
HAS A HALF-LIFE OF ONLY EIGHT DAYS,
WHILE CHLORINE-36
HAS A HALF-LIFE OF 300,000 YEARS.
SUPPOSE YOU HAVE 32 GRAMS OF SODIUM-24,
WHICH HAS A HALF-LIFE OF 15 HOURS
AND DECAYS INTO MAGNESIUM-24,
A STABLE FORM OF MAGNESIUM.
AN UNDERSTANDING OF HALF-LIFE AND SOME SIMPLE MATH
CAN HELP YOU DETERMINE
HOW MUCH OF THE ORIGINAL MATERIAL EXISTS
AFTER A GIVEN PERIOD OF TIME.
AFTER 15 HOURS,
HALF OF THE SODIUM-24
WOULD HAVE DECAYED INTO MAGNESIUM-24,
LEAVING 16 GRAMS OF THE ORIGINAL ISOTOPE.
AFTER 30 HOURS,
THERE WOULD ONLY BE 8 GRAMS OF SODIUM-24 LEFT,
AND SO ON,
UNTIL THERE'S NOT ENOUGH SODIUM-24 LEFT IN THE SAMPLE
TO DETECT ITS RADIOACTIVITY.
SO NOW THAT YOU KNOW MORE ABOUT RADIOACTIVITY AND HALF-LIFE,
HOW DOES NASA USE MADAME CURIE'S "INVISIBLE ENERGY"
TO EXPLORE THE SOLAR SYSTEM?
WELL, SINCE HALF-LIFE IS PREDICTABLE,
SCIENTISTS AND ENGINEERS CAN USE THIS KNOWLEDGE
TO CREATE POWER SOURCES
THAT RELY ON THE PREDICTABLE HEAT
GIVEN OFF BY RADIOISOTOPES AS THEY DECAY.
NASA'S RADIOISOTOPE-POWERED SPACECRAFT
USE A UNIQUE ISOTOPE CALLED PLUTONIUM-238,
WHICH IS PROVIDED BY THE DEPARTMENT OF ENERGY.
IT HAS A HALF-LIFE OF 88 YEARS
AND GIVES OFF HEAT
PLUS LOW-ENERGY PARTICLES CALLED ALPHA PARTICLES.
THE FACT THAT IT'S A LONG-LIVED SOURCE OF HEAT
MAKES THIS ISOTOPE AN IDEAL FUEL FOR LONG-LIVED SPACE MISSIONS.
- A RADIOISOTOPE POWER SYSTEM IS A SPECIAL SYSTEM
THAT USES THE DECAY OF A RADIOISOTOPE,
IN OUR CASE, PLUTONIUM-238, TO GENERATE HEAT.
THAT HEAT THEN IS CONVERTED INTO POWER OR ELECTRICITY
THAT WE USE ON SPACECRAFT.
- THIS MAKES RADIOISOTOPE POWER SYSTEMS--
OR AN R.P.S. FOR SHORT--
IDEALLY SUITED FOR MISSIONS
IN THE EXTREME ENVIRONMENTS OF SPACE
OR ON PLANETARY SURFACES,
LIKE SOME DESTINATIONS ON MARS.
THE MARS SCIENCE LABORATORY ROVER,
ALSO KNOWN AS CURIOSITY,
HAS BEEN DESIGNED TO USE A RADIOISOTOPE POWER SYSTEM
AS IT EXPLORES THE RED PLANET.
THE R.P.S. CAN GENERATE ELECTRICITY NEEDED
TO OPERATE THE ROVER AND ITS INSTRUMENTS,
AND EXCESS HEAT FROM THE POWER SOURCE
CAN KEEP THE ROVER WARM ON THIS COLD, DUSTY PLANET.
THE STEADY AND RELIABLE OUTPUT OF ITS R.P.S.
SHOULD ENABLE THE LARGE ROVER
TO TRAVEL FARTHER, LAST LONGER,
AND DO MORE SCIENCE
THAN ANY PREVIOUS MOBILE MARTIAN EXPLORER.
CURIOSITY IS DESIGNED TO ROAM MARS
FOR A FULL MARTIAN YEAR--
THAT'S ABOUT TWO EARTH YEARS.
WITH ITS RELIABLE, LONG- LASTING POWER SOURCE,
THE MISSION COULD GO EVEN LONGER.
AND THAT LONGEVITY
IS THANKS TO OUR UNDERSTANDING OF RADIOACTIVE DECAY.
SO HOW ABOUT THAT?
FROM LABORATORIES ON EARTH TO ROVERS ON MARS,
SCIENCE AND MATH
CAN HARNESS THE POWER OF RADIOACTIVE DECAY
TO HELP MAKE EXPLORATION TRULY A COSMIC ENDEAVOR.