@S1-A.ARPA,@MIT-MC:redford%avoid.DEC@decwrl.ARPA (05/24/85)
From: redford%avoid.DEC@decwrl.ARPA (John Redford) Hmmm, what could we do to make a cheap interstellar probe? The parameters proposed were that the probe has to report back within a hundred years from a star no more than 20 light years away. That seems a long way away. How about if we just stick with Alpha Centauri at 4.3 ly, and say that it has to report back within fifty years? That implies a trip time of 46 years, and a velocity of about 0.1C, or 30,000 km/s . We're not going to get that with chemical rockets. One restriction was reasonably near-term technology, so matter-antimatter drives are out. Fission drives seem a bit bulky, and we don't know how to do controlled fusion drives, so we'll put those aside. How about a laser-driven light sail, like in "The Mote in God's Eye"? Light sails work by reflecting photons, thus gaining twice the momentum of the photon stream. The momentum, p, of a stream of photons is E/c where E is the stream's energy, so the change in momentum of the sail is 2E/c. The change in momentum per unit time is 2P/c, where P is the power of the stream. The acceleration of the sail, a, is the change in momentum per unit time divided by the mass of the sail, m, so: a = (2/c) * (P/m) We want to get the velocity up t0.1C2C in 46 years, so v = a*t = (2/c) * (P/m) * t P/m = v*c / 2*t = 6e6 W/kg For every kilogram of sail, we must apply 6 million watts for 46 years. That works out to 2.6 billion kW-hrs of energy per kilogram of sail, or (at seven cents per kilowatt-hour) 180 million dollars worth of energy per kg. This looks pretty grim. Never mind the problems of keeping a laser focussed on a sail 4 light-years away, or keeping the sail from melting in the beam. A 1000 kg probe will cost 180 billion bucks just in energy to get to the nearest star. Time to break out the hyperdrive. John Redford DEC-Hudson
dcn@ihuxl.UUCP (Dave Newkirk) (05/28/85)
> From: redford%avoid.DEC@decwrl.ARPA (John Redford) > > Hmmm, what could we do to make a cheap interstellar probe? The parameters > proposed were that the probe has to report back within a hundred years > from a star no more than 20 light years away. That seems a long way away. ... > John Redford > DEC-Hudson A cheaper way might be a simple ion drive. A mix of solar and nuclear power to provide electricity, and a cheap fuel source that can be ionized. It's not a big thrust, but it can be maintained for long periods, building up impressive speeds. I also think you gave up too soon on the laser-driven light sails. For more ideas on this, see Robert Forward's new book "Dragonfly". Remember, solar energy is cheap and plentiful in the inner solar system. -- Dave Newkirk, ihnp4!ihuxl!dcn
sewilco@mecc.UUCP (Scot E. Wilcoxon) (05/28/85)
In <1946@mordor.UUCP> redford%avoid.DEC@decwrl.ARPA (John Redford) writes: > ...[after showing well how he calculated it]... >For every kilogram of sail, we must apply 6 million watts for 46 years. >That works out to 2.6 billion kW-hrs of energy per kilogram of sail, or >(at seven cents per kilowatt-hour) 180 million dollars worth of energy >per kg. Maybe we need a cheaper laser. Or lasers..several lower power ones would have great maintenance advantages. Could always put a bank of lasers on a "Solar Power Satellite" (is SPS a recognized acronym?), but it would be nice to avoid the (photon-electron-photon) middleman. BTW, I assume these lasers would be in space. I don't care to have that much power going through the atmosphere we have to breathe. I'd also feel a bit safer if Earth's atmosphere were opaque to the laser's frequency. If a gas laser can be triggered by photons, maybe a group of mirrors can focus enough sunlight on the lasers. There's also the idea (wish I remembered the source) of using mirrors to focus sunlight on the sail. But since non-coherent light cannot be concentrated over long distances, this is only useful relatively near the sun. So to achieve significant acceleration, several (maybe even a ring) of mirrors are put around the initial circular orbit of the sail. As the sail starts getting too far from one mirror it is near enough to the next one for it to focus on the sail. When mirrors are not focusing on the sail, they are used as sails to accelerate nearer to the (ever expanding) orbit of the target sail. After the target sail uses a planet to direct itself onto the final course, lasers can be used to continue boost. If the mirrors can't be used to drive the lasers, they'll find some other use..SPS reflectors, asteroid smelter, Pluto probe...