Subject: Space-tech Digest #68 Contents: Lou Adornato Mars Conveyor Henry Spencer Re: Mars Conveyor Phil Fraering Re: Mars Conveyor Phil Fraering Re: Mars Conveyor Edmund Hack Re: Mars Conveyor Kevin Driscoll Re: Aim For The Moon Lou Adornato Re: Aim For The Moon Edward Wright Re: Aim For The Moon Nick Szabo Re: Aim For The Moon Henry Spencer Re: Aim For The Moon Vince Cate Rocket Contest moving to another email list Keith Henson Slung into Space! Nick Szabo Jupiter-powered science Paul Dietz Re: Jupiter-powered science ------------------------------------------------------------ Date: Tue, 26 Jun 90 12:09:42 CDT From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Re: Mars Conveyor Donald.Lindsay@mathom.gandalf.cs.cmu.edu write: >There's an old idea, which I have always called the Mars Conveyor. >It's quite simple: we put a large structure into an orbit that >(approximately) visits both Earth and Mars. Once established, we >leave it that way, and use our delta-v in getting to it/from it. From the discussion on this I gather that it could work, but I don't see how. In order for an object to match speeds with the conveyor, it would have to be in the _same orbit_, so what's the sense. If the conveyor where to "catch" the conyeyee, there would be some serious problems in dealing with the velocity differences (we're talkin' BIG shock absorbers, here). And finally, even if you did snag an object on flyby, you would have to return the delta-v (ok, the momentum) that the (presumably) larger conveyor gave to the transit vehicle. TANSTAAFL. If you're going to have fuel and engines enough to return the momentum, why bother with the conveyor at all? NOTE: I am _not_ saying this won't work, only that I don't understand how it is supposed to work. As always, I'm willing (eager) to be convinced. Lou Adornato | Statements herein do not represent the opinions or Cray Research | attitudes of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 26 Jun 90 15:07:18 EDT To: lfa@vielle.cray.com Cc: space-tech@CS.CMU.EDU Subject: Re: Mars Conveyor > From the discussion on this I gather that it could work, but I don't see > how. In order for an object to match speeds with the conveyor, it would > have to be in the _same orbit_, so what's the sense... The conveyor doesn't help the orbit changes, but it means you have to take less mass through those changes. All the living quarters, power systems, solar-flare shielding, etc. needed for the long trip out and back stay on the conveyor, instead of being hauled along through all the orbit changes on every trip. Henry Spencer at U of Toronto Zoology [uunet!] henry@zoo.toronto.edu ------------------------------ Date: Mon, 25 Jun 90 17:10:41 -0500 From: Fraering Philip To: davidsen@crdos1.crd.ge.com Cc: space-tech@CS.CMU.EDU Subject: Cyclers vs. Nuclear Propulsion Personally I doubt that relativistic exhaust velocities will be neccesary, at least until we're ready to go to Alpha Centauri. Even ion engines, which have about a 40% efficiency in converting grid energy to exhaust energy (which I think can probrably be made higher), would be much better than what we now have if a small high power density power source were developed. Keep in mind this relation: if you want twice as much momentum out of a given amount of reaction mass, you must spend twice the energy. It will probrably be much cheaper to put up with an Isp of 6000 or so and buy a lot of reaction mass (a small comet will have enough water to last an industry a _long_ time, and it ought to be energy-cheap, relatively speaking, to retrieve) than to spend the effort and money to develop and the energy to power something to push reaction mass up to .9 c (which gives an Isp of 2.7*10^7 lbs.). An "easy" :-) tenfold improvement from 450Isp (RL-10) to 4500 Isp (microwave remote, laser remote, gas-core reactor, or nuclear fusion powered) would give us the solar system. Philip Fraering dlbres10@pc.usl.edu Standard disclaimer applies, although everyone around here doesn't seem to like much of anybody or anything anyway... ------------------------------ Date: Tue, 26 Jun 90 12:07:28 -0500 From: Fraering Philip To: space-tech@CS.CMU.EDU OOPS! What I meant in the previous posting, is that you must spend _four_ times the energy to double exhaust velocity. I was corrected in E-mail. If it only took twice the energy to get twice the Isp, the tradeoff would actually be worthwhile... which was not what I was saying... Also, I do know about the relativistic mass increase. However, look at the current mechanisms used here on earth to raise atoms to those velocities: particle accelerators. They are not very efficient in transferring power to kinetic energy. In Van de Graff machines, most of the power is lost in friction. In cyclotrons, a lot of it goes into the magnet or into the oscillating field generator (never mind about how much is wasted bringing slightly out of synchronization particles back into synch.) Synchotrons lose a lot of energy depending on the driving mechanism; waveguide-channeled E-M fields lose a great deal of energy in eddy currents in the irises, etc... And all of those machines are _very_ heavy in terms of the sort of things we currently put in spacecraft. As I said before, a 4000 Isp rocket would be such an improvement over what we currently have, especially if it has a low Isp (1000-2000) high-thrust mode, that it could keep us very busy for a long while. Also, it would make reaction mass on orbit much cheaper than it is now. Philip Fraering dlbres10@pc.usl.edu ------------------------------ Date: Mon, 25 Jun 90 13:18:20 PDT From: Edmund Hack X-Vmsmail-To: AMES::"space-tech@cs.cmu.edu" Subject: Cycling Spaceships To: space-tech@CS.CMU.EDU Work on cycling spaceships for Earth-Mars transport is being done in support of NASA by Buzz Aldrin for the New Initiatives Office (SEI is in this domain, but the studies have been going on for a while). Aldrin has a Ph.D. from M.I.T. in Aerospace Engineering, with his dissertation being on orbital rendezvous. This is one reason he was a Gemini and Apollo astronaut. You may remember that he specifically called for development of cyclers in the remarks that he made at the Apollo landing celebration at the Air & Space museum last year. He has been doing specific studies on delta-V requirements, which launch windows can be used in support of missions, etc. Rendezvous and other orbital ops are still somewhat of a black art to design and experience helps develop a feel for what to explore. Edmund Hack, speaking for myself, not for Lockheed ESC, Houston, TX hack@lock.span.nasa.gov ------------------------------ Posted-Date: Thu, 28 Jun 90 04:12:32 CDT Date: Thu, 28 Jun 90 04:12:32 CDT From: Kevin Driscoll To: space-tech@CS.CMU.EDU Subject: Re:Aim For The Moon Sponsorship Try one of these three sources of prize money. 1) General Advertising If the Russians can sell fast food advertising and the like, why can't an amateur rocket contest? 2) Technology Advertising There are probably companies that would like to say "our product went to the moon". 3) Vested Interests There are probably groups that would benefit from having a successful amateur rocket go to the moon. For example, those that want to show space can be commercialized could say "See, it is easy, even amateurs can not only get into space but get to the moon." The latter two groups might be persuaded by the competing teams to make donations in kind (material to construct rockets, services, loan of equipment and facilities). I wonder if it would be worth it to make the whole effort eligible for tax deductions. Might be a way for donors to get full value for items for which there really isn't a market where they can be unloaded. >------------------------------ >From: Lou Adornato >The big advantage of the system I proposed is that it greatly reduces the mass >and expense of onboard systems. The whole race has to pay for ONE guidance >system, and the onboard (and expendable) equipment for each rocket is limited >to enough logic to decode incomming command sequences and the actuator systems It is really the actuators and their power supply that is the big component of guidance system mass. The computer needed for this has negligible mass compared to the actuators and power supply. In fact, the mass of a computer would be essentially the same as the radio receiver and logic decode needed for this scheme. >I mentioned, the shuttle takes regular state vector updates from the ground. >If NASA can't find an accurate enough gyro system for LEO, given the mass, >volume, and cost allowances of the shuttle (positively liberal in comparison >to what we're talking about), then it's time to look at alternatives. Depends on time on orbit. The INS drifts with time. A short enough mission would need not updates. >BTW, the _best_ we could do would be a ring laser gyro. These buggers use >some kind of Doppler shift algorithm measured over a ludicusly long internal >path (somehow the laser travels several miles inside a package the size of >a pacemaker) to provide pretty awesome accuracies. A few years back the >Honeywell corporate jet flew from Minneapolis to Buenos Aires and back with >a cumulative RLG error measured in inches. And that was one of the early >models. It is *NOT* Doppler. The "source" and detector are part of a single solid block. They cannot move with respect to each other. There can be no Doppler shift. It is a relativistic effect. When the RLG is rotating, the path length around the loop is actually different depending if you measure it clockwise or counter-clockwise. This path length difference causes the counter-rotating laser beams to have a different frequency. The beat of these frequencies is detected to measure the angular rate. An RLG's internal path is measured inches not miles. You were probably thinking of a fiber optic gyro (FOG) which is a less mature technology than RLGs. We are just beginning to sell FOGs. >Unfortunately RLGs are expensive, and I think they use a lot of power. Compared to other gyros, RLGs do not use a lot of power. Alas, if you you don't care about accuracy or reliability, and if low cost is your main concern, RLGs are not the answer. >------------------------------ >From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU >I think we want to have a continuous burn into an almost straight trajectory >heading towards where the moon will be, say, two days after launch. Probably the best plan. >Vince has almost convinced me that spinning the rocket is good enough to >maintain direction. If maintaining an up direction is all you want to do; getting close to the moon is another story altogether. I think you will have to carry a guidance package (position and attitude sensors, computer, and actuators) all the way up through the atmosphere and to the point where the vehicle is on its final coast phase to the moon. This is going to be your biggest weight problem. And, as long as you have to carry it that far, you might as well send it all the way to the moon. >Military missiles have to work in bad weather, and LEO satellites have >to maintain orientation while the sun and moon were periodically >blocked by the earth. A probe in a direct ascent trajectory to the >moon could avoid both these restrictions. Why couldn't it determine >its orientation by optically tracking the moon and sun? With wide field >of view optics, the system might not even need moving parts. How about >peephole optics for the sun? (Maybe for the moon too - it might >actually be easier to find the centroid of a fuzzy image.) I was thinking about using peepholes or staring arrays too. My design would be along the following lines. Spin stabilize the vehicle through the first stage burn, use ram air and fluidic amplifiers to vector the thrust of the remaining in-atmosphere stages, cover the outside of the exo-atmospheric stage(s) with miniature squibs for final guidance actuation. Use GPS for position and rough attitude sensing. Special purpose ICs are making these receivers quite small now. Mount these ICs and the computer ICs as raw die on a substrate. You probably won't want to use Class S parts. :-) Mount the substrate directly to the highest energy density battery you can get. Encase all this in paraffin to absorb the electronics' heat on the launch pad and give it back to the battery in space. Use the peephole optic sensors for fine attitude determination. The vehicle could send an RF pulse for tracking exactly every X minutes. As long as it can hear the GPS time signal, this is accurate to fractions of a microsecond. The tracking receivers on earth should be able to look for a very narrow pulse in a precise time window. By making the duty cycle low, the vehicle should be able to put substantial power in the narrow pulse. Doublet PPM pulses using this accurate timing could be used to send low bandwidth information back. Kevin R. Driscoll, Principal Research Scientist (612) 782-7263 FAX: -7438 POST: Honeywell M/S MN65-2500; 3660 Technology Drive; Mpls, MN 55418-1006 INTERNET: driscoll@SRC.Honeywell.com or driscoll@altura.Honeywell.com UUCP: driscoll@srcsip.uucp or {umn-cs, or any smart host}!srcsip!driscoll ------------------------------ Date: Thu, 28 Jun 90 11:43:33 CDT From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Re:Aim For The Moon Regarding the idea of a race: I suggest that it be a race from LEO to the moon. All contestants get boosted at the same time, in what and at whose expense we leave as an exercise for the reader (for now). The advantage of this is that the entrants don't have to worry about the structural weight needed to survive the initial boost, they don't have to worry about the aerosurfaces and actuators for the ascent, and they don't have to carry enough fuel for the entire trip. Eliminating this greatly improves the chance for designs within a reasonable budget. The other advantage of this is that it allows some more exotic design work. Maybe someone could come up with a lightweight ion engine, or some kind of lightsail/laser system. Guidance and control operations would yeild more interesting solutions, as well. Perhaps a prize for the _slowest_ vehicle, or for the best Isp could be awarded. As for getting the orbital boost, perhaps if this was billed as a way to glamorize science and math education (perhaps by requiring that the entrants be from college/high school teams), then maybe our education president would be able to get the gov't to allocate one expendable (or shuttle space) for the contest. Lou Adornato | Statements herein do not represent the opinions or Cray Research | attitudes of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ Date: Thu, 28 Jun 90 15:46:41 CDT From: "Edward V. Wright" To: lfa@vielle.cray.com, space-tech@CS.CMU.EDU Subject: Re:Aim For The Moon >As for getting the orbital boost, perhaps if this was billed as a way to >glamorize science and math education (perhaps by requiring that the entrants >be from college/high school teams), then maybe our education president would >be able to get the gov't to allocate one expendable (or shuttle space) for the >contest. The safety requirements for putting a payload aboard the Space Shuttle are very strict (i.e., paperwork intensive) and might be difficult for a school to handle. An expendable booster would be a better bet, but they've all been privatized, so NASA isn't in a position to provide one except for piggyback payloads on a NASA launch. You might talk to one of the manufacturers about donating a launch, but the Air Force has placed new restrictions on the number of launches from Cape Canaveral, so they might not be able to supply a launch even if they wanted to. ------------------------------ Date: Thu, 28 Jun 90 15:41:25 -0700 From: "Nicholas J. Szabo" To: uunet!cs.cmu.edu!space-tech@uunet.UU.NET Subject: Re:Aim For The Moon Edward V. Wright writes: >You might talk to one >of the manufacturers about donating a launch, but the Air Force has >placed new restrictions on the number of launches from Cape Canaveral, >so they might not be able to supply a launch even if they wanted to. Orbital Science Corp.'s Pegasus does not require Cape Canaveral (its "launch pad" is a B-52 or 747). Pegasus can launch 450 lbs. to LEO for $6 million. If there is large publicity it would be in OSC's interest to underwrite some or all of the launch cost. There are also non-U.S. boosters that are more expensive (>= $30 million) but lift bigger payloads. Nick Szabo uunet!ibmsupt!szabonj ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 29 Jun 90 12:44:36 EDT To: cs.cmu.edu!space-tech@zoo.toronto.edu Subject: Re:Aim For The Moon > ... If there is large publicity it would be in OSC's interest to > underwrite some or all of the launch cost.... Maybe. Emphasis on the "large". OSC/Hercules (there are two companies involved, not just one) is not big enough and profitable enough to casually consider giving out a free launch just for publicity. They might have done so if they'd needed a payload for an early test launch... but DoD bought the first launch and options (which will probably be used) on the next several. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Fri, 29 Jun 1990 00:30-EDT From: Vincent.Cate@SAM.CS.CMU.EDU To: space-tech@CS.CMU.EDU Subject: Rocket Contest moving to another email list The idea of using small rockets to launch a very small payload into space has generated enough interest that we have started a mailing list just for this topic. The list is "space-project@cs.cmu.edu". The purpose of this list is to get together people who are interested in turning this idea into reality. All of the issues will be discussed (designs, engines, safety, cost, transmitters, materials, balloons, funding, legal issues, insurance, ...). There are enough people on rec.models.rockets that are only interested in the kind of rockets that fit the legal definition of "model" that we really should move the discussion from there to this new list. If you are interested in this discussion please send your name and email address to: space-project-request@cs.cmu.edu This name is used because Marc Ringuette already had the list sitting around unused. I will just be maintaining the list while Marc is out of town. Thanks Marc!!!! -- Vince ------------------------------ Date: Fri, 29 Jun 1990 12:57-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Slung into Space! [ This is from sci.space. A pretty wild way to get some launch velocity! Keith has joined our list. --M. ] From: hkhenson@cup.portal.com (H Keith Henson) Subject: Slung into Space! Date: 27 Jun 90 05:11:02 GMT I hate to post a half-baked idea, but raising money for an email privacy lawsuit has wiped out my time to bake it. (Though I did get a patent application filed.) If you drop a 200-ton aircraft from 16 km (say 50,000 feet) to 6 km (about 20,000 feet) the potential energy available is enough to raise a one-ton payload to 2,000 km or to accelerate it to about 6.5 km/sec, neglecting losses, of course. The reason I choose 6.5 km/sec is that it permits landing at the low end of a tether in a two-hour orbit without a rocket stage. It may turn out that it is better to use rockets for about half the boost; a boost of 3 km/sec can be done with a very modest mass ratio. Using a cable and a sling maneuver to transfer energy from plane to payload leads to an endpoint where the payload is going 6.5 km/sec in a nearly circular path. (At that point, you cut the cable at the payload end with an explosive bolt). Assuming firewalled engines develop 1/2 g, the available force (weight plus engines) to keep the payload in a circular path is about 300 tons. A little high-school physics tells us that a massless cable needs to be almost 15 km long to balance the forces, and of course, the payload is undergoing about 300 gees of acceleration. While this is a little rough for people :-), a lot of payloads, and all bulk materials would take it fine. (The ham sandwiches might be a little mashed . . . ) Real tapered cables for this application seem to be marginally within the state of the art; available materials give a section area at the payload end of perhaps 6 square cm and an effective (accelerated) mass perhaps twice that of the payload (implying three times as much aircraft or one third of a ton payload). The cable frontal area of about 300 square meters for a simple cylinder is too high, but presumably it can be streamlined in section. A 20-to-one reduction in effective area would bring the frontal area down to 15 square meters. The highest velocity occurs when the cable is almost vertical. This considerably reduces drag effects (the payload and fastest section of the cable should be about 20 km high at release.) The sling process starts by reeling a payload out from the aircraft (not necessarily to the end of the cable) and entering a series of banking turns to get the payload pendulumed out from the aircraft path. Drop tanks on the payload might be needed to increase the inertial/drag ratio in the early part of this maneuver. This would be followed by an involute spiral turn with the bank turning into a roll before most of the altitude loss. The majority of the energy would be fed into the payload as the plane followed an increasingly steep upside-down dive. Forward acceleration on the payload would average 20-30 gees over 30 seconds, while the plane lost 10 km of altitude. This is a bit toward the edge of what you want to do with a big subsonic aircraft, but well within what could be done with a specialized supersonic one. The payload path would be bent through approximately 3/4 of a circle in the horizontal plane and a quarter circle in the vertical. This is as far as I have time to take it at the moment. Obviously it need to be modeled. Does anyone know of a program which can model a sling? Anyone have pointers to airfoil section data over the range of velocities involved? How about roll, etc. performance data on C5's and 747's? Anybody have an opinion about hard points on these aircraft as to where you anchor the rope? Know anybody interested in funding some studies or bootlegging them and co-authoring a paper? Line forms over there for pilots who want to volunteer. H. Keith Henson (hkhenson@cup.portal.com) ------------------------------ Date: Fri, 29 Jun 90 11:00:14 -0700 From: "Nicholas J. Szabo" To: uunet!cs.cmu.edu!space-tech@uunet.uu.net Subject: Jupiter-powered science Galilean System Mapping Project Nick Szabo, June 29 1990 INTRODUCTION Last year, Paul Dietz on sci.space proposed taking advantage of the fact that Jupiter's four inner moons, Metis, Adrastea, Almathea, and Thebe, move at high velocity through Jupiter's strong magnetic field. He proposed generating electricity with a tether attached to one of the moons. I propose to use this technique to perform experiments requiring power levels not currently practical. CAVEAT This is in the "idea stage" only. There are lots of WAGS, a lot of numbers still need to be worked out, and there are undoubtedly many more ideas that could be incorporated. Comments are strongly solicited! MICROWAVE AND RADIO EXPERIMENTS Our main transmitter would consist of a 100-kilowatt maser similar to the Goldstone Solar System Radar. GSSR is based at the DSN's 70-meter radar at Goldstone, and is used for studying everything from Titan to Mercury, by transmitting radar and studying the echo. Our radar would be aimed at Jupiter's moons in turn. The Galileo orbiter, or another probe launched specifically for this purpose, would receive the echo signature and transmit it back to Earth. By WAG, this method would give us (10^5)^4 = 10^20 or better radar resolution for each of Jupiter's moons, for the same amount of power, over Earth-based radar. Recieving on Earth would still give us a factor of 10^10 improvement. For Saturn and its moons, and some main belt asteroids, we might get a typical factor of 4^4 = 256 improvement. The goals of the radio/microwave experiments would be: --Penetrate hundreds of kilometers into Jupiter's clouds to provide a 3D view of its weather patterns --Characterize Titan's surface (moon of Saturn: feasibility depends on relative positions of Jupiter and Saturn at the time of the mission) --Radio and microwave reflection characteristics of Jupiter's moons, rings, and radiation belts (including the 2-terawatt Jupiter/Io flux tube) --Characterize the surface of some asteroids --Radar pictures of Io's volcanic plumes OPTICAL EXPERIMENTS Our main laser might consist of a 10 gigawatt pulsed laser. 100 kilowatts of electrical power would charage a capacitor to create the pulses. The experiments would also include ultraviolet, x-ray, and a smaller tunable optical laser. Lenses would provide the desired beam spread (we might wish to illuminate large or small areas over a wide range of distances). The goals of the optical experiments would be: --Provide illumination where it does not exist or needs enhancement. For example, we could study weather patterns on the night side of Jupiter with a series of photographs timed with the flashes. Jupiter's moons, Saturn and its moons, and asteroids could also be photographed in this manner. --Spectroscopic studies of clouds, surfaces, rings, radiation belts, etc. taking advantage of the specific wavelengths of our lasers. As with the radar, imaging could be performed by Galileo, a follow-on probe, or telescopes at Earth. ELECTRIC POWER GENERATION A lander on Metis, the innermost moon, contains a pointable maser, a length of wire, and a hopper that unreels the wire parallel to the equator (perpendicular to Jupiter's magnetic field). After each hop of a few meters, the hopper drives a "staple" into the surface to keep the wire from moving around. (How well this works depends on the forces on the wire from the magnetic field interaction, and the consistency of the surface). After the wire is extended, a metal mesh net is spread on each end, to catch electrons for current. A potential (volts) = B*v*s is set up across the wire, where B = magnetic field (5*10^-4 Teslas) (this is just the textbook value, not adjusted for distance from Jupiter!) v = velocity of moon through magnetic field Jupiter's rotational velocity at surface: 9,800 m/s revolution velocity of moon (from memory): 12,000 m/s velocity through field: 2,000 m/s s = length of the wire (meters) Thus potential = 5*10^-4 * 2,000 * s, so we get 1 volt of potential for every meter of wire (nice!). If the hopper strings 2,000m per cable, we get a 2,000 volt potential. If we assume (WAG warning!) our net can catch 100 amps of electrons, we get 200 kilowatts of power which should be enough for our experiments. If our wire weighs .4 kg/m, we need 800 kg of cable. Add 400 kg for the hopper/reel, 400 kg for the lander and experiments, and 400 kg (WAG!) for the collecting net; we need to put 2,000 kg on the surface of Metis. Add 500 kg for the landing fuel; we must put 2,500 kg in low orbit around Jupiter. For comparison, the Galileo mission puts 3,003 kg in high Jupiter orbit. The mission uses Jupiter and its moons as gravity brakes to get into low orbit. Our mission can be launched with one Titan 4 rocket using TOS or Centaur as a transfer stage from LEO to Jupiter orbit. Whadya think? Nick Szabo uunet!ibmsupt!szabonj ------------------------------ To: "Nicholas J. Szabo" Cc: cs.cmu.edu!space-tech@uunet.uu.net, dietz@cs.rochester.edu Subject: Re: Jupiter-powered science Date: Fri, 29 Jun 90 16:33:39 -0400 From: dietz@cs.rochester.edu Comments: You don't need to attach the spacecraft to a moon. The conducting tether can be held more or less aligned with Jupiter by tides. Orbits closer to Jupiter are *much* better, especially for viewing the Jovian cloud tops, because (a) the vehicle is moving faster w.r.t. the magnetic field, (b) the field is stronger, and (c) the Jovian atmosphere is much closer. Electrodynamic drag can be used to spiral in towards Jupiter, once you are within synchronous altitude. If you start in an eccentric orbit, drag can be induced at periapsis even if the apoapsis is beyond synchronous altitude. I don't think chemical rockets could get you down far enough -- a *lot* of delta-V is required to rendevous with Metis. Eccentric orbits might have problems with debris, though. Even if the spacecraft is not attached to a moon, kinetic energy can be extracted from the rotation of the planet at distances beyond synchronous. However, even if this is countered by applying drag closer to the planet, some angular momentum transfer would still occur. The spacecraft orbit could be tweaked by slingshots off moons. A likely scheme would be to use Io as an angular momentum source/sink, then fire up the tether at periapsis of an eccentric orbit. Your figures for Metis are faulty. Metis is moving at about 31.5 km/s around Jupiter. The Jovian magnetic field in the vicinity of Metis is moving at 22.4 km/s (I believe this is the right way to calculate this). So, the net velocity is 9.1 km/s, not 2 km/s. The magnetic moment of Jupiter is 1.55 gauss cm^3, so (if I've calculated it correctly) the magnetic field at Metis is 7.5e-5 T. Combining these, you get a voltage of .68 volts/meter, which is close enough to your number. A similar calculation at the Jovian equatorial cloud tops gives a field of 12 V/m (circular orbit) or 19 V/m (highly eccentric orbit) -- about 20 times greater. How much force would be exerted on the spacecraft drawing 200 kilowatts? Acceleration equals power / (velocity x mass), or 2e5 / 9.1e3 x 2.5e3 = 8.8e-3 m/s^2. This is probably less than the surface gravity of Metis, although I imagine Jovian tides could foul things up. Another possibility is to station the vehicle near Io. The electric field there is about .1 V/m in circular Jovian orbit. In Io-centric orbit itself we might be able to avoid problems with orbital decay, and the Io plasma torus might have higher conductivity, assisting in the use of a tether (as well as getting lots of nice shots of the eruptions -- not *too* low an orbit!). Paul ------------------------------ End of Space-tech Digest #68 *******************