"Robert J. Bradbury" wrote:
> On Sun, 25 Mar 2001, zeb haradon wrote:
> > If you are going on a one way trip, the sails will become useless at some
> > distance away from the sun (or the lasers, as they disperse). If you're
> > making a trip to another solar system, you could then reel the sails in,
> > grind them up, and use them as propellant. This would dictate what type of
> > material you can make the sails out of.
> Zeb, I think the general strategy for interstellar trips should be to
> take as little mass with you as possible. You want to use "matter
> beams" driven by mass drivers to provide the fuel over long distances.
> Lasers will spread out, but matter bonded together will not. You
> could even ship alternating packages of matter and anti-matter.
> You can provide sufficient on-board navigational intelligence
> in the matter shipments to keep them targeted properly over
> many parsecs of travel.
> So much of the discussion of space travel has always focused on
> the problem of taking your fuel with you, while constraining
> yourself to accelerations that humans can tolerate. If you
> split the problem so the humans accelerate at whatever velocity
> they can tolerate and the fuel is accelerated to a velocity
> such that when it reaches the ship it can provide momentum transfer,
> but not at a velocity that exceed the heat dissipation capacity
> of the spaceship that is the best strategy. Divide and conquer.
> Of course this isn't 'solar sailing' so its off the topic that
> Spike was dealing with.
The best suggestions along these lines are, as is often the case, in
Drexler's Engines of Creation. See the notes for chapter six at
... Robert Forward ... suggests ... See his article "Roundtrip
Interstellar Travel Using Laser-Pushed Lightsails," (Journal of
Spacecraft and Rockets, Vol. 21, pp. 187-95, Jan.-Feb. 1984). Forward
notes the problem of making a beam-reversal sail light enough, yet of
sufficient optical quality (diffraction limited) to do its job. An
actively controlled structure based on thin metal films positioned by
nanometer-scale actuators and computers seems a workable approach to
solving this problem.
But nanotechnology will allow a different approach to accelerating
lightsails and stopping their cargo. Replicating assemblers will make it
easy to build large lasers, lenses, and sails. Sails can be made of a
crystalline dielectric, such as aluminum oxide, having extremely high
strength and low optical absorptivity. Such sails could endure intense
laser light, scattering it and accelerating at many gees, approaching
the speed of light in a fraction of a year. This will allow sails to
reach their destinations in near-minimal time. (For a discussion of the
multi-gee acceleration of dielectric objects, see "Applications of Laser
Radiation Pressure," by A. Ashkin [Science, Vol. 210, pp. 1081-88, Dec.
In flight, computer-driven assembler systems aboard the sail (powered by
yet more laser light from the point of departure) could rebuild the sail
into a long, thin traveling-wave accelerator. This can then be used to
electrically accelerate a hollow shell of strong material several
microns in radius and containing about a cubic micron of cargo; such a
shell can be given a high positive charge-to-mass ratio. Calculations
indicate that an accelerator 1,000 kilometers long (there's room enough,
in space) will be more than adequate to accelerate the shell and cargo
to over 90 percent of the speed of light. A mass of one gram per meter
for the accelerator (yielding a one-ton system) seems more than
adequate. As the accelerator plunges through the target star system, it
fires backward at a speed chosen to leave the cargo almost at rest. (For
a discussion of the electrostatic acceleration of small particles, see
"Impact Fusion and the Field Emission Projectile," by E. R. Harrison
[Nature, Vol. 291, pp. 472-73, June 11, 1981].)
The residual velocity of the projectile can be directed to make it
strike the atmosphere of a Mars- or Venus-like planet (selected
beforehand by means of a large space-based telescope). A thin shell of
the sort described will radiate atmospheric entry heat rapidly enough to
remain cool. The cargo, consisting of an assembler and nanocomputer
system, can then use the light of the local sun and local carbon,
hydrogen, nitrogen, and oxygen (likely to be found in any planetary
atmosphere) to replicate and to build larger structures.
An early project would be construction of a receiver for further
instructions from home, including plans for complex devices. These can
include rockets able to get off the planet (used as a target chiefly for
its atmospheric cushion) to reach a better location for construction
work. The resulting system of replicating assemblers could build
virtually anything, including intelligent systems for exploration. To
solve the lightsail stopping problem for the massive passenger vehicles
that might follow, the system could build an array of braking lasers as
large as the launching lasers back home. Their construction could be
completed in a matter of weeks following delivery of the cubic-micron
"seed." This system illustrates one way to spread human civilization to
the stars at only slightly less than the speed of light.
Hard to top that...
-- Doug Jones, Rocket Engineer XCOR Aerospace
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