FROM J. POWELL AND R. TUNA
TO JILL TARTER, NASA/SETI
JULY 4, 1986

FROM ALL WE KNOW ABOUT CHEMICAL EVOLUTION, ABOUT PLANET FORMATION, AND THE PRESENCE OF HUNDREDS OF BILLIONS OF SUNLIKE STARS IN THE RIM OF THE GALAXY, THE EXISTENCE OF EXTRATERRESTRIALS IS PRACTICALLY A STATISTICAL CERTAINTY. ISAAC ASIMOV HAS PROPOSED THAT “PANCAKES” AND OTHER FORMS OF DARK MATTER MAY RENDER OUR FAST-MOVING VALKYRIE ROCKETS IMPOSSIBLE TO FLY EVEN IF THE MEANS CAN BE FOUND TO BUILD THEM, AND THAT THE ONLY WAY TO THE STARS WILL BE IN SLOW-MOVING, HOLLOWED-OUT ASTEROIDS OR “ARKS.” IT FOLLOWS THAT MORE TECHNOLOGICALLY ADVANCED CIVILIZATIONS SHOULD HAVE DISCOVERED LONG AGO THAT ISAAC IS RIGHT, THAT THERE IS TOO MUCH MATTER FLOATING BETWEEN THE STARS, AND THAT TRAVELING THROUGH IT AT AN APPRECIABLE FRACTION OF LIGHT SPEED IS SUICIDE. KNOWING THAT NO ONE CAN GET THERE FROM HERE, THEY SHOULD THEREFORE FEEL MUCH FREER TO COMMUNICATE THEIR PRESENCE, TO SEEK OUT LONG-DISTANCE CORRESPONDENCE WITH OTHER WORLDS. YET THE GALAXY, IN EVERY DIRECTION, REMAINS SILENT, EVEN OUT TO A SAFE DISTANCE (SAFE FROM EARTH, AT LEAST) OF 60 to 100,000 LIGHT-YEARS. IT SEEMS UNLIKELY THAT WE ARE TRULY ALONE IN THE GALAXY. THE GREAT SILENCE IS PERHAPS THE STRONGEST INDICATOR OF ALL THAT HIGH RELATIVISTIC VELOCITIES ARE ATTAINABLE AND THAT EVERYBODY OUT THERE KNOWS IT.

THE SOBERING TRUTH IS THAT RELATIVISTIC CIVILIZATIONS ARE A POTENTIAL NIGHTMARE TO ANYONE LIVING WITHIN RANGE OF THEM. THE PROBLEM IS THAT OBJECTS TRAVELING AT AN APPRECIABLE FRACTION OF LIGHT SPEED ARE NEVER WHERE YOU SEE THEM WHEN YOU SEE THEM. RELATIVISTIC ROCKETS, IF THEIR OWNERS TURN OUT TO BE LESS THAN BENEVOLENT, ARE BOTH TOTALLY UNSTOPPABLE AND TOTALLY DESTRUCTIVE. A STARSHIP WEIGHING IN AT 1,500 TONS (APPROXIMATELY THE WEIGHT OF A FULLY FUELED SPACE SHUTTLE SITTING ON A LAUNCHPAD) IMPACTING ON AN EARTHLIKE PLANET AT “ONLY” 30 PERCENT LIGHT SPEED WILL RELEASE 1.5 MILLION MEGATONS OF ENERGY—AN EXPLOSIVE FORCE EQUIVALENT TO 150 TIMES TODAY’S GLOBAL NUCLEAR ARSENAL.

THE FACT THAT WE MAY BE WITHIN ONLY A FEW DECADES OF BECOMING A RELATIVISTIC CIVILIZATION OURSELVES RAISES NEW PROBLEMS AND POSSIBILITIES THAT, FOR THE MOMENT AT LEAST, ARE SIMPLY INTERESTING TO THINK ABOUT, PERHAPS EVEN FUN TO JOKE ABOUT. BUT THE ABILITY TO LAUGH AT A PROBLEM IS DIRECTLY PROPORTIONAL TO ONE’S DISTANCE FROM THE PROBLEM. DURING THE NEXT SEVENTY TO ONE HUNDRED YEARS, WHEN A GENERATION ONLY NOW ABOUT TO BE BORN FINDS ITSELF STANDING ON THE BRINK OF INTERSTELLAR FLIGHT, THE POSSIBILITIES WILL HAVE TO BE CONSIDERED VERY SERIOUSLY. SUDDENLY OUR JOKES WON’T SEEM QUITE SO FUNNY ANYMORE.

FROM JILL TARTER, NASA/SETI
TO JIM POWELL, RICHARD TUNA, ISAAC ASIMOV
JULY 20, 1986

PERHAPS THE MOST SOBERING TRUTH IS THAT ANY EXPEDITION TO ANOTHER SOLAR SYSTEM, USING THE ROCKET YOU HAVE DESIGNED OR SOME VARIATION THEREOF, CARRIES WITH IT OVERWHELMING DESTRUCTIVE POTENTIAL; AND THERE IS NO WAY TO DISTINGUISH A PEACEFUL EXPLORATION FROM AN INTENDED ANNIHILATION—EXCEPT (AS YOU HAVE MADE TOO CLEAR) IN THE MINDS OF THE VISITORS.

FROM ISAAC ASIMOV
TO JILL TARTER, NASA/SETI
OCTOBER 4, 1986

WELL, I HOPE YOU, CARL SAGAN, AND THE OTHER SETI SCIENTISTS WHO HAVE LONG BELIEVED IN A BENEVOLENT UNIVERSE WILL FORGIVE ME THIS TRESPASS, BUT I’M AFRAID THE PICTURE POWELL AND TUNA HAVE BEGUN TO PAINT (UNLESS RELATIVISTIC FLIGHT SHOULD TURN OUT, AS I HOPE IT WILL, TO BE A PHYSICAL IMPOSSIBILITY) IS CLOSER TO THE TRUTH.