(Apologies for a slight threadjacking, but I can’t resist this, one of my favorite spaceflight topics)
Back in school, we had a little device in science class that blew my mind, and I can't remember what it's called (what with having a blown mind and all). But bottom line, it was a little four-bladed propeller thingy mounted inside a vacuum bulb, similar in size and shape to a light bulb. The one side of each propeller blade was black, whilst the other side was white. It hung on a magnetic pin, with very little friction. In any case, when you put that sucker in light, it started spinning. And the brighter the light, the faster it spun. It was awesome.
You’re describing a Crookes radiometer. (My parents bought me one in a planetarium gift store, and it was one of my most prized childhood possessions
Its inventor, Crookes, believed its vanes turned due to the impact of light particles (ca. 1870, before quantum physics and the photon had been imagined). He was wrong, as was soon argued by people who noticed that, if that were the case, they would rotate away from the white side of the vanes (reflecting the particles gains twice the momentum of absorbing them), not away from the black sides, as actually happens.
What really makes the Crookes radiometer spin is heating of the little air inside the globe, which is greater near the warmer black sides of the vanes than near the cooler white sides. If made with a better vacuum, these little toys don’t have enough force to overcome the friction of their pin-on-glass bearings, and don’t work, though, in principle, if you could make the bearing low-friction enough, they would, turning backward from their usual direction.
Nonetheless, the idea of light-pressure propulsion is a valid one that can actually be measured by more precisely built devices
But here's my point:
Instead of trying to accellerate ions, why not use some form of nuclear reactor to build up juice to fire the brightest and harshest possible floodlights available? You fire them away from your direction of travel - surely, there must be some kick there?
This is known as a nuclear photonic rocket
. It’s basic shortcoming is too low a power/mass ratio – the mass of a nuclear reactor is too great, and the force of light, too small.
This problem can be eliminated by having the harsh light stay put, and only the reflector move as part of the spacecraft. This is the essence of the space exploration scheme described by (and to a some extent patented by) the late Robert Forward. His designs ranged from tiny “Starwisp
” probe spacecraft that could be propelled by microwave masers on Earth-orbiting solar power satellites that could serve double-duty to beam power to electric power plants on the surface, to http://en.wikipedia....pulsion_System'
>massive, 3-sail designs to carry human explorers on missions to explore nearby star systems, powered by an array of thousands of Sun-orbiting lasers focused thought a moon-sized Fresnel lens
I’m very enthusiastic about such systems, and have posted several times on the subject here at hypography. I highly recommend Forward’s writing on this and other spaceflight topics, particularly his alternating fiction/non-fiction book, “Indistinguishable from Magic”.
An obvious limitation of such systems is that the spaceship must trust the station beaming it power not to lose interest, funding, political stability, etc, and stop beaming power when and if its needed. This limitation has, I think, limited such systems popularity among spaceflight enthusiasts, many of whom are lone individualist types who imagine the role of interstellar spacecraft to be getting away from Earth before some calamity ensues.
The major technical challenge, after the preliminaries of building giant space lasers and lenses, is aiming them with sufficient precision to hit the target ship with a sufficiently narrow, intense beam at interstellar distances. Note that hitting a target with a roughly moon-diameter (3000000 m) beam at a 1-light year (about 10^15 m) requires an angular precision of 3000000/10^15 radians, or about .00006 arc-seconds. For reference, the Hubble Space Telescope
, one of the most accurately pointable spacecraft yet flown, has a pointing accuracy of about .007 arc-seconds, 100 times too course.