Solar Sailing in Space

Handsake, You're wrong about no gravity in space. Gravitational force exists between planets and galaxies. Utilizing phonons from Sun (or stars) in space to propelling long distance (n light years) space travel is efficient enough as there are many stars and suns in space. 

There is a tradeoff between mass efficiency and energy efficiency. Ion drives are quite power hungry devices and use enormous amounts of power to produce rather little thrust...but they can produce that thrust for years on end with a tiny tank of fuel. Compressed air driving a stream of water out the back could achieve enormous amounts of thrust, but the exhaust velocity will be so low that utterly impractical amounts of water and compressed air would be required to keep it up for any amount of time.

Chemical rockets are somewhere in the middle, trying for a high "specific impulse" so they have to lift as little fuel off Earth as possible, but also requiring high thrust so they can exceed one full gravity of acceleration. Once in orbit, you don't need to exceed any particular amount of acceleration to get anywhere, because even the smallest gains add up, so highly fuel-efficient engines become desirable (except when trip time is a particular problem...a manned craft has requirements that supersede fuel efficiency).

Higher fuel efficiency is achieved by using a higher exhaust velocity, but since (non-relativistic) momentum is m*v and kinetic energy is 0.5*m*v^2, energy requirements increase faster than the thrust produced for a given mass of fuel. You can not have an exhaust velocity equal to the speed of light with any fuel possessing mass, so a photon drive is the limit of fuel efficiency, requiring only energy and no fuel.

You could collect light with solar panels and use the electricity to power an ion drive, but you lose most of the light energy and require fuel, even if it is small amounts. (This *is* actually done, however, due to the relative simplicity and the lack of development of solar sails, and the occasional need for the higher thrust of an ion drive.) You could do the same, using the energy to power a photon drive to eliminate the fuel requirement, but photon drives and solar panels are far heavier than a thin film mirror of the same area, on top of the inefficiencies of conversion along the way. Solar sails are really the best way to harness sunlight for propulsion without requiring fuel.

phlyerphil, you are indeed missing something very large -- the sun, and its gravitational attraction. Solar sails can be used to increase orbital velocity, and spiral out from the sun. They can also be used to decrease orbital velocity, simply by tilting them in the opposite direction, causing the spacecraft to spiral in toward the sun. No need to row.

Maximum speeds for solar sailing spacecrafts are not exactly known; 100 miles per hour is just the speed the spacecraft could reach in a day. The Planetary Society estimates that in 100 days a sail could reach 10,000 miles per hour, in one year 36,000 miles per hour, and in 3 years could reach a speed over 100,000 miles per hour. This means it could reach Pluto in less than five years.

I have added this information to the article.

Fluctuating solar winds won't do much, the solar wind doesn't carry much momentum in comparison to the light that solar sails use. The luminance of the sun does vary over time, but by a fraction of a percent. It would certainly be taken into account, though.

Yes, the numbers do account for the loss of sunlight with distance. Trajectories have even been developed which involve starting out in another direction and then backtracking to do a flyby of the sun, allowing more time close to the sun during which to accelerate. An interesting and related fact: the intensity of light from the sun drops with the inverse square of distance, but so does the gravity of the sun. A sail that exactly counters the force of gravity of the sun on a payload at Mercury will also do so at Pluto. A sailcraft can thus carry just enough sail to counter the sun's gravity, drop toward the sun with the sail on edge, and turn it face on for the trip out, keeping all the velocity it gained during the flyby. It needn't even have enough sail to fully support it against the sun's gravity, just enough to provide that asymmetry between the incoming and outgoing trips.

The sail with one reflective side and one absorbing side would accelerate until it reached equilibrium temperature, at which point the emission from the black side would match reflection from the mirrored side. Unfortunately for spacecraft, good reflectors also tend to be poor radiators. If the sail were a heat pump and actively heated the mirrored side and cooled the black side to compensate for the difference in emissivity, or if the entire sail were actively cooled, it would continue to produce net thrust but would require energy input.

edit: And something more immediately relevant...the launch failed. It appears the second stage did not separate when it was supposed to. Experimental solar sail craft seem to have hard luck with launch vehicles...