2000 April 18, SPS 1020 (Introduction to Space Sciences) - Reading: today was PBD 19, 21 & 22. - Read TNSS 27 & 28 and PBD 5 & 20 by Thursday, April 20. Spaceflight Future (continued): ------------------------------ The Long-Term Future in Space ----------------------------- Anytime you have a brilliant, original vision about space, check Tsiolkovsky's books: he might have thought of it first. You'd be lucky to live long enough to see any of these, but then, surprise me! - O'Neill colonies: thought of by Tsiolkovsky, "rediscovered" by Gerard K. O'Neill in 1970s. Physics Today, September 1974, p. 32; also 1981 book, "The High Frontier" LARGE, rotating, self-sufficient habitats, made from lunar or asteroidal materials, in free space. O'Neill's unique contribution: the observation that a planetary surface may NOT be the preferred place for an expanding technological society. Once you go through all the trouble of climbing out of the strong gravity well of a planet, why climb back in? O'Neill stressed the need to make these large (hundreds of miles across), to help the inside ecosystem and society to be stable. "Not a penny for this nutty fantasy." -- Sen. William Proxmire (D, WI), champion of milk subsidies, to O'Neill But what would pay for O'Neill colonies? ----- - Solar Power satellites: interest heating up again! Large satellites with solar panels, to receive sunlight and beam power to Earth by microwave or laser link. (Prof. Raffaelle working on lightweight panels, useful for them.) O'Neill: Since the Moon has much weaker gravity than Earth, space colonies would be a lot less expensive to build if made of lunar materials, sent by mass driver (magnetic catapult, or rail gun) to Earth-Moon L4/L5. - Dyson spheres: thought of by Tsiolkovsky, "rediscovered" by Freeman Dyson in 1960. Swarm of O'Neill colonies or other structures around a star, capturing most of its light. => If such things existed in the local Universe, what would they look like? A fainter-than-usual G star (like the Sun), with an infrared excess? If we saw one, would we recognize what it was? - Terraforming Mars, other planets: discussed at length in PBD, Ch. 19. (Before you snicker: we may be unwittingly doing it to Earth, now...) - Space Elevators (also known as Skyhook cables, or Beanstalks): thought of by Tsiolkovsky, "rediscovered" by Arthur C. Clarke, others. Fix a cable from Earth's surface to a satellite in GEO. Take elevator up and down, would be vastly cheaper than rockets. Problem: no known material is strong enough. Needs to be 150x stronger than Kevlar. Possible solution?: Fibers made of carbon atoms in geodesic lattice, called "diamond fibers", or "Buckytubes" (like Fullerenes, the new C60 "soccer ball" molecules). Don't yet exist, though. Many nearer-term things to do with space tethers: - Produce sensation of weight (often wrongly called "artificial gravity") in two spacecraft, rotating at each end. Advocated by Zubrin for Mars Direct: has been tested, by Gemini XI and Gemini XII in 1966. - Generate power: current induced by Earth's magnetic field in conducting wire. Italian one on Space Shuttle (STS) generated > 3000 volts! - Satellite launcher and Shuttle de-orbiter: Uses Kepler's 3rd Law: shown off during STS-75 in 1996, when tether accidentally broke. - De-orbit satellites, reduce space debris problem (which is already getting fierce, in some orbits) - Starships: --------- "All this stuff about traveling around the universe in space suits---except for local exploration which I have not discussed---belongs back where it came from, on the cereal box." -- Edward Purcell, Harvard radio astronomer, 1960 "I still have dreams in which I fly up to the stars in my machine. It is difficult to work all on your own for many years, in adverse conditions, without a gleam of hope, without any help." -- Konstantin Tsiolkovky Flight between stars ISN'T IMPOSSIBLE. Four ships from Earth are doing it now: Pioneer 10 and 11, and Voyager 1 and 2. Will take many millennia... Great reference, with math and physics: The Starflight Handbook, by E. Mallove and G. Matloff -> How might we get people to the stars? 1) Mobile O'Neill colonies: flight times of centuries 2) Hibernation: unknown how to do it, or even whether possible 3) Faster-than-light travel: unknown how to do it, or even whether possible. Most physicists say it is prohibited by Einstein's Special Theory of Relativity, but who's ever done an experiment? 4) Advanced propulsion, to shorten flight times to decades or years: Space propulsion methods we have now: ------------------------------------ Chemical rockets: not up to the task of interstellar flight. - Solid fuel: simple, but once lit, can't be turned off; can't be throttled. Examples: common firecrackers; Space Shuttle Solid Rocket Boosters (SRBs) - Liquid fuel: usually more efficient than solids because of higher exhaust velocities (see chart) Cryogenic (must be kept cold): Liquid H_2 (or LH2) / Liquid oxygen, or LOX, oxidizer: powerful! Powers Space Shuttle Main Engines (SSME), Centaur upper stage. Problem: takes up lots of volume. => Kerosene used in 1st stage of Saturn V. Storable fuel: hydrazine and nitrogen tetroxide (don't need to be kept cold) This combination is also _hypergolic_: fuel and oxidizer ignite upon contact, don't need igniter. => Used often in upper stages What we might develop soon: -------------------------- - Nuclear thermal: THE U.S. TESTED ONE, Nerva, in 1968! H_2 gas heated with nuclear fission reactor, shoots out of nozzle Problem: the public is wary of anything nuclear that flies... - Ion drives: ionize gas and accelerate it with electricity, much more efficient than chemical rockets. Still needs power supply, though. Can be from solar panels (called "solar electric"; as in the Deep Space 1 probe, launched in 1998) or a nuclear reactor (called "nuclear electric"). - Solar sails: uses the pressure of sunlight (not the solar wind) to accelerate craft, resembling a large aluminum sail. Requires no propellant! Can move TOWARD Sun, too, by tacking like a sailboat. Problem with all 3 (nuclear thermal, ion drives, and solar sails): Can only be used in interplanetary space: None have thrust high enough to escape from Earth's surface. Advantage with all is fuel efficiency, from high specific impulse (Isp). Isp = v(exhaust)/acceleration This fuel efficiency lowes mass ratios (M(initial)/M(final), from rocket equation), and therefore cost, and also increases speed, but this says nothing about thrust (the sheer force the rocket generates). This also demonstrates why H_2 is such a powerful rocket fuel, and why H_2 gas is deemed useful as a moving fluid for a nuclear thermal engine: molecular weights are low, easier to accelerate. Advanced concepts: The details are left as an exercise to the student. ----------------- - Nuclear pulse propulsion: The Orion Project, by Freeman Dyson, early `60s. Nuclear bombs are thrown overboard & detonated, against heavy-duty pusher plate and high-quality shock absorbers. Would have worked: model with dynamite charges tested. Problem: Outer Space Treaty of 1964! - Nuclear fusion power: 1978 Daedalus study by British Interplanetary Society Problem: Don't know how to control fusion. ("Then a miracle occurs...") - Bussard ramjet: scoops in interstellar gas, "burns" it in nuclear fusion reactor. Obvious problem: don't know how to control fusion. - Anti-matter: uses matter/anti-matter annihilation, photon exhaust. No known process more efficient. Problem: Can now only make tens of anti-atoms. Also, don't have an accident! - Laser-pushed light sails: Could do this now! Technology exists to send probe to stars in 40 years. (cf. recent NASA study: see February 1999 Scientific American, p. 80) Can slow down near destination by detacting 2-piece sail, deflecting beam. Can even turn around and come home, with interstellar magnetic field.