gravity is 28 times as great as ours and its escape speed is 55 + times as great-but
at the distance of Earth’s orbit that grasp has attenuated to about one thousandth
of a gee, and at Pluto at 31.6 A.U. it has dropped off to a gnat’s whisker, one
millionth of gee.
(No wonder it takes 21/2 centuries to swing around the Sun. By the way, some
astronomers seem positively gleeful that today Pluto is not the planet farthest from
the Sun. The facts: Pluto spends nine-tenths of its time outside Neptune’s orbit,
and it averages being 875,000,000 miles farther out than Neptune-and at maximum is
nearly 2 billion miles beyond Neptune’s orbit (1.79 x lO~ miles)-friends, that’s
more than the
ROUND TRIP BOOST
COMPARISON OF ELAPSED TIME
Earth-Mars-Earth – Earth- Pluto-Earth
@1 gee
4.59 days vs. 4.59 weeks
~w’Iio gee
14.5 days vs. 14.5 weeks
~~/too gee
45.9 days vs. 45.9 weeks
~1/tO0O gee
145 days vs. 145 weeks
distance from here to Uranus, nearly four times as far as from here to Jupiter. When
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Pluto is out there-l 865 or 2114 A.D.-it takes light 6 hours and 50 minutes to reach
it. Pluto-the Winnuh and still Champeen! Sour grapes is just as common among
astronomers as it is in school yards.)
-and the rabbit is out of the hat. You will have noticed that the elapsed-time
figures are exactly the same in both columns, but in days for Mars, weeks for
Pluto-i.e., with constant-boost ships of any sort Pluto is only 7 times as far away
for these conditions as is Mars even though in miles Pluto is about 50 times as far
away.
If you placed Pluto at its aphelion (stay alive another century and a
quarter-quite possible), at one gee the Pluto round trip would take 5.72 weeks, at
1/to gee 18.1 weeks, at 1/too gee 57.2 weeks-and at ‘Iiooo gee 181 weeks, or 3 yrs &
25 wks.
I have added on the two illustrations at ‘Iwoo of one gravity boost because
today (late 1979 as I write) we do not as yet know how to build constant-boost ships
for long trips at 1 gee, 1/10 gee, or even 1/too gee; Newton’s Third Law of Motion
(from which may be derived all the laws of rocketry) has us (temporarily) stumped.
But only temporarily. There is E = mc2, too, and there are several possible ways of
“living off the country” like a foraging army for necessary reaction mass. Be
patient; this is all very new. Most of you who read this
will live to see constant-boost ships of 1/10 gee or better-and will be able to
afford vacations in space- soon, soon! I probably won’t live to see it, but you
will. (No complaints, Sergeant-I was born in the horse & buggy age; I have lived to
see men walk on the Moon and to see live pictures from the soil of Mars. I’ve had my
share!)
But if you are willing to settle today for a constantboost on the close
order of magnitude of 1/1000 gee, we can start the project later this afternoon, as
there are several known ways of building constant-boost jobs with that tiny
acceleration-even light-sail ships.
I prefer to talk about light-sail ships (or, rather, ships that sail in the
“Solar wind”) because those last illustrations I added (l/t000 gee) show that we
have the entire Solar System available to us right now; it is not necessary to wait
for the year 2000 and new breakthroughs.
Ten weeks to Mars . . . a round trip to Pluto at 31.6 A.U. in 2 years and 9
months. . . or a round trip to Pluto’s aphelion, the most remote spot we know of in
the Solar System (other than the winter home of the comets).
Ten weeks-it took the Pilgrims in the Mayflower nine weeks and three days to
cross the Atlantic.
Two years and nine months-that was a normal commercial voyage for a China
clipper sailing out of Boston in the last century . . . and the canny Yankee
merchants got rich on it.
Three years and twenty-five weeks is excessive for the China trade in the