Moon Mirror
     (SFX: rocket taking off, 10-9-8-7-. . . )
     WHEN THE SPACECRAFT APOLLO LANDED ON THE MOON TWENTY FIVE YEARS
AGO, IT REPRESENTED THE CUTTING EDGE OF SCIENTIFIC AND ENGINEERING
CAPABILITIES. NOW IT'S A QUARTER OF A CENTURY LATER, AND SCIENTISTS ARE STILL
DOING INNOVATIVE RESEARCH BASED ON A PROJECT STARTED BY THOSE FIRST
ASTRONAUTS.  THE TWO MEN DID SOMETHING QUITE SIMPLE--THEY LEFT A MIRROR
SITTING ON THE MOON'S SURFACE.  THE MIRROR--WHICH IS A SPECIAL KIND CALLED A
CORNER CUBE--IS USED TO DETERMINE HOW FAR AWAY SOMETHING IS.  PHYSICIST
JAMES FALLER WHO HELPED CONCEIVED THE PROJECT, DESCRIBES HOW IT WORKS: 

     Faller: "What you do is basically you put this corner cube up there which has this unique property
that you send a pulse of light to it, it will come back to where the pulse of light originated."

     THEY SEND A LASER BEAM UP TO THE MOON, BOUNCE IT OFF THE MIRROR AND
THEN ABOUT TWO AND A HALF SECONDS LATER, A FEW PHOTONS WILL MAKE THEIR
WAY BACK TO EARTH. BY PRECISELY TIMING HOW LONG THE LIGHT BEAM TAKES TO
TRAVEL, SCIENTISTS CAN MEASURE THE DISTANCE TO THE MOON TO WITHIN A FEW
CENTIMETERS. FOR TWENTY-FIVE YEARS NOW, ASTROPHYSICISTS HAVE BEEN ABLE TO
PRECISELY MONITOR THE MOON'S ORBIT, SIMPLY BY DETECTING HOW FAR AWAY THE
MOON IS ON ANY GIVEN DAY. OVER SUCH A LONG TIME PERIOD, SCIENTISTS HAVE BEEN
ABLE TO FIND OUT SOME INTERESTING THINGS ABOUT THE MOON.  FOR ONE THING,
THE MOON'S ORBIT SUGGESTS IT HAS A LIQUID CORE--SOMETHING THAT FEW PEOPLE
SUSPECTED JUST A DECADE AGO.

     Faller: "If you have a ball filled with a lot of liquid and you try and bounce it, it doesn't bounce
very long.  So, the prescence of a liquid core changes the length of time the moon wants to rock and
rotate." 

     NOW FALLER AND HIS COLLEAGUES ARE ATTEMPTING TO IMPROVE THIS MIRROR
MEASURING SYSTEM TO A MILLIMETER MARGIN OF ERROR AS THEY EXAMINE OTHER
FACETS OF THE MOON'S SURFACE LIKE LUNAR TIDES.  EVEN AFTER TWENTY FIVE
YEARS, THE MIRROR IS STILL REFLECTING GOOD RESEARCH.