Project Cyclops, A Design... - Department of Earth and Planetary ...

4. POSSIBLE METHODS OF CONTACT
Several methods of achieving contact with intelligent
life beyond our solar system have been proposed.
These include actual interstellar space travel,
the dispatching of interstellar space probes, and the
sending and detection of signals of some form. Many
other suggestions invojving as yet unknown physical
principles (or a disregard of known principles) have also
been made but are not considered here.
INTERSTELLAR
TRAVEL
The classical method of interstellar contact in the
annals of science fiction is the spaceship. With our
development of spacecraft capable of interplanetary
missions it is perhaps not amiss to point out how far we
still are with our present technology from achieving
practical interstellar space flight, and indeed how costly
such travel is in terms of energy expenditure even with a
more advanced technology.
Chemically powered rockets fall orders of magnitude
short of being able to provide practical interstellar space
flight. A vehicle launched at midnight from a space
station orbiting the Earth in an easterly direction and
having enough impulse to add 8-1]2 miles/sec to its
initial velocity would then have a total orbital speed
around the Sun of 31-1/2 miles/sec. This would enable
the vehicle to escape the solar system with a residual
outward velocity of 18 miles/sec, or about 10-4 c. Since
the nearest star, a-Centauri, is 4 light-years away, the
rendezvous, if all went well, would take place in 40,000
years. Clearly we must have at least a thousandfold
increase in speed to consider such a trip and this means
some radically new form of propulsion.
Spencer and Jaffe (ref. I) have analyzed the performance
attainable from nuclear powered rockets using
(a) uranium fission in which a fraction e = 7X 10-4 of
the mass is converted to energy and (b) deuterium fusion
for which e = 4× 10-3. The mass ratios required for a
two-way trip with deceleration at the destination are
given in table 1 for various ratios of the ship velocity v
to the velocity of light c.
Table 1
v Uranium Deuterium
c fission fusion
0.1 3.8X104 8.1XI01
0.2 2.3X109 6.2X10 s
0.3 1.1XIO 6
0.4 1.5X 108
From these figures we would conclude that with
controlled fusion we might make the trip to a-Centauri
and back in 80 years, but that significantly shorter times
are out of the question with presently known nuclear
power sources.
Let us ignore all limitations of present day technology
and consider the performance of the best rocket
that can be built according to known physical law. This
is the photon rocket, which annihilates matter and
antimatter converting the energy into pure retrodirected
radiation. The mass ratio/a required in such a rocket is:
where
Id =
V/C
(1)
V
- = coordinate distance travelled per
Veff _ unit of ship's proper time.
Figure 4-1 shows/a,/a 2 , and/a 4 as a function of Vef_C.
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