The Drexler lightsail

SOLAR SAILING By Eric Drexler

Solar Sailing

Solar sails are a way of moving things around in space, from one orbit
to another. After a year's work, they are beginning to look like the
best means of space transportation for a wide range of uses: they may
be both cheap and fast. Before discussing clipper ships vs. canoes,
however, we should first discuss boats and the ocean.

Space is big It would take as many Earths to fill the solar system
(500,000,000,000,000,000) as elephants to fill the sea (an unpleasant
prospect). The Earth's orbit around the Sun is 23,000 times the
Earth's circumference. Driving to the Moon (1/400 of the distance to
the Sun) would take six months, at 55 mph of course. Driving to the
nearest star would take 50,000,000 years, and so on. Space is Big. To
get anywhere you have to go fast.

But, you say, since there is no air resistance in space, perhaps a
patient traveler (or load of freight) could start out slowly and
simply take whatever time was needed, drifting along. But, alas,
gravity is in control. Objects in space don't really go anywhere, if
left to themselves; they simply go around in orbits. Unless you kick
something so hard that it stops completely (in which case it falls
into whatever it was orbiting) or kick it so hard in the other
direction that it can fly away despite gravity, never to return, the
object will simply grunt at the kick, and shift its orbit somewhat. To
get from one orbit to another generally takes at least two pushes: the
first to put the object onto an orbit that crosses the orbit you're
trying to reach, and another at the crossing point, to make the object
start following the orbit you want, instead of the transfer orbit that
the first push put it on. Another way to do the same thing is to push
gently for a long time, and slowly twist and stretch the first orbit
until it matches the second. Either way, you can add up the change in
velocity that all the pushing would produce, in the absence of
gravity, play around with different directions and times of push, and
find that the total velocity change needed has a minimum that can't be
beaten for a given trip. This requirement is usually measured in
kilometers per second (1 km/sec is about 2,200 mph). One of the lowest
requirements of any interest is 2.4 km/sec: the velocity needed to get
off the moon.

Rockets have limits, because they must carry mass to throw away. A
rocket can reach the same velocity as its exhaust fairly easily; not
much fuel is needed to reach a few kilometers per second. The problem
is that fuel has mass, just like the payload. Let's say you have a
rocket with enough fuel to reach 1 km/sec, and to take a ton of
payload with it. How much fuel would you need to reach 2 km/sec?
Enough fuel to take the ton of payload to 1 km/sec, and enough fuel to
take the fuel needed for the second km/sec to 1 km/sec. The total fuel
mass needed turns out to increase exponentially with the velocity
reached, just as population has been increasing exponentially with
time. Both increases can gobble up more resources than you can afford
to provide. Using the Saturn V moon rocket as a first stage, and
piling up rockets from there, we could have reached 30 km/sec with
enough payload to drop one haunch off an elephant into the Sun (an
unpleasant prospect).

Rockets burning chemical fuels run out of ability fast when measured
against the solar system, although they were decent for getting us as
far as the Moon. The exponential curve that gets rockets into trouble
can be made less steep, however, if more energy can be put in the
exhaust. This is the principle of the electric rocket; by soaking up
solar energy in space and using it to throw small amounts of mass away
fast (a mass driver is particularly efficient and versatile at this
job), payloads may be pushed around the solar system in a reasonable
way. The main problem is the cost and mass of the solar power plant.
To use it efficiently accelerations must be low and trips long. Costs
are also low: freight rates from Earth orbit to Mars orbit might be as
little as $.20 per pound.

Asteroid mining facility with moored sail. Top, right: solar sail(10
km diameter). Top, left; Bernal sphere colony (1/2 km diameter).
Bottom, left; asteroid (1 km diameter). Bottom, center; industrial
complex. Behind asteroid; mooring tower with shroud lines extending to
sail in distance. The pit on the right side of the asteroid has
supplied enough material to build this colony, the industrial complex,
50 power satellites and many, many sails like the one shown. The solar
system contains thousands of similar asteroids. The sail shown is one
of a fleet used for asteroid mining; when loaded (2,000 tons). It will
depart for a two year trip to Earth.

Solar sails don't work on the rocket principle, but on light pressure.
Like stage magic, this trick is all done with mirrors. Because E =
mc2, energy, including light, has mass. For light in particular, that
little bit of mass moves very fast through space; when it is bounced
off a mirror it exerts a force, just like fast ping-pong balls
bouncing off a wall. If you wanted that wall to move quickly, even
without friction, you'd want it to have little mass and be hit by many
ping-pong balls. Similarly, the mirror that makes up a solar sail
should be very thin and lightweight, and have a large area - a square
mile of reflected sunlight exerts enough force to support the weight,
not of a building, not of a car, a person, or a large dog, but of a
medium-sized cat. The name of the game, then, is to maximize
acceleration by minimizing the mass per unit area of the mirror.

People have looked at this problem, off and on, for about 20 years.
They set themselves the problem of stuffing about a square mile of
folded reflecting surface into the nose of a rocket, of launching it,
and of making it unfold and stretch into a reasonably flat surface in
space. A design for a kite-like sail, with thin, aluminized plastic
film for the reflecting surface, has finally reached an advanced
planning stage at the Jet Propulsion Laboratory in Pasadena,
California. (See illustration on inside back cover.) Their design can
accelerate at about 1/7,000 of a gravity, which is actually fairly
good: the sail can reach 1 km/sec in about eight days. This lets you
get around, and because it needs no fuel, and no fuel to help carry
fuel, and so on, it doesn't peter out at high velocities like a rocket
does. They want to use it to reach Halley's comet (an object which is
going around the sun the wrong way compared to the Earth; a huge
velocity is needed): the flight would take four years. They may not
get to do it, because solar electric rockets, mentioned above, still
look good by comparison (1/7000 of Earth's gravity isn't spectacular)
and because these rockets have been sitting in everybody's "come on,
let's do it" file for many years. They have seniority.

Can solar sails be made better? The answer seems to be yes, if you
forget about folding them up and launching them from the ground. I
came to suspect this in the summer of 1976, and now, a year later, it
looks as if it may be true: solar sails can be made in space, not as
aluminized plastic sheet, but as aluminized nothing, which weighs far
less. Designs now worked out on paper use aluminum foil as the
reflecting surface, but foil 1/1000 the thickness of the kitchen kind.
These sails are over 40 times as light, and therefore over 40 times as
fast, as previous designs. This is spectacular.

If I had to draw a sail today, it would be a hexagon about six miles
across, and weighing 20 tons. This is somewhere between the size of
Manhattan and San Francisco, but the metal of the sail could be wadded
up to the size of a Volkswagen bug. They could be made both much
larger and much smaller. The sail itself would be a spinning (to keep
it taut) metal mesh with long, parallel strips of very thin metal foil
glued to it. At regular intervals across the front, wires would come
up, and be bundled to form groups, with each group having a wire
coming from it, with these wires, in turn, bundled to form groups
still farther in front of the sail. After this bundling and
re-bundling has concentrated the load of light pressure on the sail
enough (that's what the wires are for), shroud lines take the
concentrated force to the payload (see drawing).

The sail would be made on a large, lightweight framework, like a loom.
Wires would be laid down, and fastened to each other where they
crossed. As the wires go down, a device would travel back and forth,
producing thin metal foil by vapor deposition on wax, vaporizing the
wax for recycling, and laying the foil on the wire mesh. The whole
process would take about six months; building the "loom" would require
several flights of the Space Shuttle. Additional sails require about
one flight apiece to provide needed raw materials. Eventually, sails
would be built from extraterrestrial materials.

What can you do with a solar sail? First, how can you "tack"? Boats
can go in any direction by using both wind and water; solar sailing
vessels can go in any direction by using both light pressure and the
Sun's gravity. Light pressure on a mirror is always at right angles to
the mirror's surface, even when the mirror tilts and bounces the light
at an angle. As the mirror tilts towards being edge-on to the light,
the force becomes smaller and approaches zero. This means that the
mirror can collect some force in any direction that would take it
farther from the light source, in this case the Sun. So how can a
solar sail reach, say, Venus, which is closer to the Sun than Earth?
By using light pressure to slow down in its orbit, then letting the
Sun's gravity pull it in. Solar sails can go anywhere in the solar
system, and, in the inner solar system (where we are), they can get
there faster than almost anything proposed.

The 20 ton solar sail mentioned above could take 180 tons of payload
to any place in the solar system, stop (not orbit, but stop) and hang
there on light pressure. With 800 tons of payload to slow it down, it
would finally have the same acceleration as a plastic film sail with
no load at all. With 6 tons of payload, it could fly to Pluto in one
and a half years. Pioneer 11, launched over four years ago, won't
reach Saturn until two years from now, and Saturn is only 1/3 the
distance of Pluto.

Rough cost estimates suggest that solar sails will cost between $.03
and 1/3 cent a square foot. Kitchen wrap costs about $ .01 a square
foot. If nobody throws them out of the solar system, toasts them too
close to the sun, or crashes them into something, they should last for
thirty to three hundred years. Maintenance costs should be about nil
(you don't fix the sail at all, and there are only about two dozen
reels for the shroud lines to keep track of). Each sail, without fuel
expenses, can cruise around the solar system almost indefinitely.
While a rocket must be built differently for almost every mission, the
same sail that flies from low Earth orbit to geosynchronous orbit and
back can do a perfectly good job of flying twenty times the freight to
the asteroid belt, out beyond Mars. Not only that, but the sail costs
above, with 10% real rate of interest on capital, can give costs like
$.10/lb for transportation around the solar system. And sailboats have
always had little environmental impact....

How do (apparently) good ideas like this arise? Well, they never seem
to be new. Solar sails are an old idea The literature even contains
references to metal film solar sails, although not of the high
performance discussed here. It even contains a reference to the idea
of making the material in space. When I first had the idea, my reasons
were not to seek high performance, but to try to make a sail out of
metals, which are readily available in space. My background was
oriented towards manufacture in space, towards materials properties,
and towards vapor deposition.

Previous workers concentrated on hauling sails up from the ground, but
metal film sails are too delicate for that, so they were never
studied. The few who considered making the films in space considered
inappropriate manufacturing techniques, which either didn't work or
produced films about 500 times as thick as optimum.

Many questions come up about the new design:

* Can films be made that thin (thinner than a soap bubble )? Yes.
* Do they have decent strength? Yes, I've made some, and taken
them from coast to coast in. my luggage.
* Are the films still reflective when so thin? Yes.
* Will fast moving atoms from the sun knock enough atoms from the
film to destroy them quickly? No.
* Will this stream of atoms exert destructive forces on the sail
during solar storms? No.
* Will electric charging or magnetic fields cause problems? No.
* Will the films be able to stand the temperatures they reach in
the inner solar system? Yes.
* Will meteoroids destroy the films quickly enough to matter? No.
* Will they destroy the wire mesh that is the structure of the
sail? No.
* Can the films be mass-produced in space at low cost? Yes
* How much does the device for making the films weigh ? It's
manageable.
* What trajectories can the sails fly on? Name it.
* Could the sails possibly find use for near Earth transportation,
as well as deep space? Maybe.
* Will the sails ripple under light pressure and gradually tear
themselves apart? No.
* How well can the sails be turned to point where you want them
to? Well enough.
* Will stresses from changing temperatures cause problems? No.

And so on. Work of this kind never really stops until something has
been built and running for a while and people take it for granted.
Solar sails of this design have two things going for them: they have
passed many tests, and no one has examined the idea and rejected it in
the past. At the time of this writing, formal publication and
widespread examination and criticism are about to take place. Time
will tell, but the chances seem good that we're on to something
interesting.

How interesting? As interesting as cheap space transportation, free of
fuels and complex maintenance. As interesting as moving Earth's
industry into deep space and scattering her life to the Sun's light.
Arthur Clarke said: "If man survives for as long as the least
successful of the dinosaurs - those creatures whom we often deride as
nature's failures - then we may be certain of this: For all but a
vanishing instant near the dawn of history, the word 'ship' will mean
- 'spaceship.' " And, those ships may yet have sails.

http://www.nas.nasa.gov/About/Education/SpaceSettlement/CoEvolutionBook/EDUC.HTML#Solar%20Sailing