OK, you have definately got a lubrication problem in orbit as andyf alludes. But if the weapon is only expected to be used infrequently then it could be sealed and charged with a inert gas so to prevent lubricant degredation prior to firing.

How cold actually is it in Space?

another thought, on the ground is this piston damped by a spring? if so then the pistion doesn't negate the recoil but instead smears it out over a longer time so you don't feel it so much. In orbit the recoil still happens unless you eject something of similar momentum in the oposite direction to the bullet/pellet aimed at the target. When using a rail gun you can accellerate things to enormous velocities and so you need less mass in the recoil absorbing pellet. Infact less mass in both projectiles to achieve the same momentum. I think UK has a rather good rail gun research facility but I think the latest model isn't manpackable!

It's probably best to forget about any weapon that uses kinetic energy from the launch platform (gun) to kill the target. The second problem is that a projectile fired at any kind of distance is still going to have drop due to gravity. The target is also following the arc of the earth as well, so targetting at distance is complicated. Better to use something that tracks a target and corrects for minor errors till impact. Think more like a torpedo then a bullet. Liguid fuel would probably best for flexibility, and a warhead with a shaped charge, sort of like a claymore fired from 10 feet. Bound to hit something important. Another option is to use a warhead that creates an aerosol mist of acid that would eat through most of the target particularly the solar panels. Of course all of the above would be moot is a laser/HPM were used. It would be the ultimate space bullet. Straight to target and no warhead needed. Also allows instant BDA. OK, enough caffiene for me ;)

"Better to use something that tracks a target and corrects for minor errors till impact. Think more like a torpedo then a bullet." We could still launch the torpedo in the general direction using a rail gun sled. If we have the weapon mounted in a magazine satelite then conventional chemical rocket ejecta would splatter the launcher as the weapon flies away. Problem goes away if each weapon is self contained, kind of like a torpeado mine moored on the seafloor. "Of course all of the above would be moot is a laser/HPM were used. It would be the ultimate space bullet. Straight to target and no warhead needed. Also allows instant BDA. " How so, the instant BDA? If I was to design a system that might come under HPM attack I might shield it well enough to survive (maybe an upset occurs but reboot would follow), but sense the HPM and using that trigger shut down my transmitting antennas until I feel like turning them on again. How does a laser damage a satelite which is rotating fast? How much power would such a laser need? Are the US DOD seriously going to put power supplies for such beasts into orbit!

How does a laser damage a satelite which is rotating fast? How much power would such a laser need? Are the US DOD seriously going to put power supplies for such beasts into orbit! Satellites don't rotate fast, they are actually very stable. They need to keep their solar arrays facing the sun, as well as their antennas pointed to the earth. Best way to destroy one with a laser would probably be to target it's solar panels. You'd know very quickly if it worked. Space is the ideal place for using lasers, no atmosphere to attenuate the strength of the light. Also lasers do not follow the inverse square rule for power since the light is coherent. So a modest size laser would provide real destructive power at long distances. I suspect we already have some up there for test purposes. A laser communications system is just a low powered weapon, and IIRC we already have satellite to satellite laser communications. A weaponized space satellite would probably be in GEO orbit possibly using a radioactive power source, with solar panels for added kick (and for deception). While you could shut off your signal when under HPM attack, soon as it comes one again it gets zapped again. So you don't really have a reliable usefull satellite.

I dunno, if the satellite was for secure comms and it could receive all day whilst you thought it was dead then chirp the outgoing messages to recipiant using a directional antenna I think it might still be useful. Alternativly if it was for image gathering it could chirp data down to a isolated ground station some time after you thought it died, thoughput would be reduced but function maintained. I don't think the input amplifiers of a sigint bird could be protected though so HPM wins over that. Lasers pah! my point is that by spinning a satellite you reduce the damage a laser can do to it from most angles, kind of like spinning a ICBM in the launch phase to reduced laser effectiveness. Also a satellite coming under laser attack could just puff some particulate in the general direction of attack it might buy some time to change orbit slightly and get out of the beam. My feelings on Nuclear material in LEO are quite negative. In GEO, well I think its fine as long as it get up there OK. I think the whole cost benifit analysis on launching nuclear material into orbit is skewed by the present practice of comparing benfit now of lauch and death now if launch goes pear shaped. The debit side of the balance should be years of productive life lost over the lifetime of the radioactivity released. Not as is presently the case the deaths absolutly atributable to accident in the year (or generation) it happens. Same with Nuclear Power. I am thinking of the unranium mine tailings releasing radioactive gases which normally would not be in the biosphere. I am pro -nuclear, just not pro stupid fuel cycle.

FWIW there are quite a few nuclear powered satellites in space, just not the type of nuclear power you are thinking of: http://www.nuc.umr.edu/nuclear_facts/spacepower/spacepower.html And these are public domain satellites, we can only guess what the USAF puts on those enormous boosters. How would a satellite know it is under attack. The first sypmtom is that something is not working. That could be too late. But even then if you blast off some particulate matter you've just decreased your solar panel capability. BTW - you also can't just spin a satellite, the solar panels would not work nor would the antennas. A satellite is not an ICBM warhead, they are not designed to reenter the earths atmosphere, most are actually rather delicate.

EW3, thanks for the link on RTG's. I conclude when SNAP-9A burnt up the increase in background radiation prompted the designers to modify the design philosophy and contain the fuel on reentry. The following quote outlines the fisson reactors which I believe are a Sword of Damocles if deployed as part of a orbital weapons system which would be targeted in the event of conflict. "Over 100 kWe, fission systems have a distinct cost advantage over RTGs. The US SNAP-10A launched in 1965 was a 45 kWt thermal nuclear fission reactor which produced 650 watts using a thermoelectric converter and operated for 43 days but was shut down due to a satellite (not reactor) malfunction. It remains in orbit. The last US space reactor initiative was a joint NASA-DOE-Defence Dept program developing the SP-100 reactor - a 2 MWt fast reactor unit and thermoelectric system delivering up to 100 kWe as a multi-use power supply for orbiting missions or as a lunar/Martian surface power station. This was terminated in the early 1990s after absorbing nearly $1 billion. It used uranium nitride fuel and was lithium-cooled. There was also a Timberwind pebble bed reactor concept under the Defence Dept Multi-Megawatt (MMW) space power program during the late 1980s, in collaboration with DOE. This had power requirements well beyond any civil space program. Between 1967 and 1988 the former Soviet Union launched 31 low-powered fission reactors in Radar Ocean Reconnaissance Satellites (RORSATs) on Cosmos missions. They utilised thermoelectric converters to produce electricity, as with the RTGs. Romashka reactors were their initial nuclear power source, a fast spectrum graphite reactor with 90%-enriched uranium carbide fuel operating at high temperature. Then the Bouk fast reactor produced 3 kW for up to 4 months. Later reactors, such as on Cosmos-954 which re-entered over Canada in 1978, had U-Mo fuel rods and a layout similar to the US heatpipe reactors described below. These were followed by the Topaz reactors with thermionic conversion systems, generating about 5 kWe of electricity for on-board uses. This was a US idea developed during the 1960s in Russia. In Topaz-2 each fuel pin (96% enriched UO2) sheathed in an emitter is surrounded by a collector and these form the 37 fuel elements which penetrate the cylindrical ZrH moderator. This in turn is surrounded by a beryllium neutron reflector with 12 rotating control drums in it. NaK coolant surrounds each fuel element. Topaz-1 was flown in 1987 on Cosmos 1818 & 1867. It was capable of delivering power for 3-5 years for ocean surveillance. Later Topaz were aiming for 40 kWe via an international project undertaken largely in the USA from 1990. Two Topaz-2 reactors (without fuel) were sold to the USA in 1992. Budget restrictions in 1993 forced cancellation of a Nuclear Electric Propulsion Spaceflight Test Program associated with this." http://www.uic.com.au/nip82.htm It may take many years for radioisotopes to re-enter after any orbital conflict, perhaps hundreds/thousands if the reactors are in high orbits. Nevertheless the stuff would still reenter into the atmosphere of whatever civilisation humans salvaged from the conflict. A more pressing problem though is the habit of distributing poorly maintained RTG's around the earths surface in isolated locations! T's could use the material as poison and contaminant if they aquired them. "How would a satellite know it is under attack. The first sypmtom is that something is not working."-EW3 Unless a set of microbolometers could be arranged to view the spacecraft skin and their output fed to a protection suite control system. "But even then if you blast off some particulate matter you've just decreased your solar panel capability."-EW3 I am thinking of the satellite effectivly poping smoke and displacing to a new firing position like a AFV would when faced with incoming ATGM. The solar panels on such a satellite would be moved clear of the particulate cloud by the change of orbit. "BTW - you also can't just spin a satellite, the solar panels would not work nor would the antennas. " - EW3 Granted some applications can't be spin stabilized, however for example two old quotes; "Most working low earth orbit satellites are three-axis stabilized, magnetically stabilized or gravity graident stabilized. A couple of smaller satellites are 'tumblers' (e.g. purposely not stabilized). The satellites which are spin stabilized in LEO are rather rare. At higher altitudes many satellites, in particular the ones made by Hughes Aircraft, are spin stabilized spacecraft, using gyroscopic principles for stabilization. (the only non-spinners built by Hughes are the lunar Surveyor spacecraft and the HS-601 model). Secondary benefits of a spinner are easier thermal control since all of the outer surfaces are evenly heated and better stability during rocket burns." http://satobs.org/seesat/Feb-1996/00

the whole recoil problem stems from newton's 3rd law. if you fire anything off, theres a recoil. on earth recoil is damped quite a lot.the airgun mentioned below still recoils, just not much. and in a freefall enviroment 'not much' wouldnt be dampened by the friction of the firer's feet on the flor for example. its not the motion of the piston etc ( I think a pre-charged pnematic would be a lot better)- its the projectile coming out of the pointy end. it has moentum, and you firing it , wil also have that momentum. a muzzle brake is a good idea, but seeing as it can't actually be 100% efficient, some of the gas is going in the same direction as the bullet. you'd need a weird bit hanging over your shoulder with an RCS in it.. would look odd all right. but precharged airguns can develop some lethal energies. on the subject of guided projectiles, how about a railgun launched slow spinning round? contains a small solid state IR imager slighly off axis and a exhaust on the opposite side of the shel. fitt the inside of the shel with a tank of a suitable propellant, water would work. and a small heating device, every time the slow spin of the shell brought the target into view , the motor would fire a blip. eventually it would guide it in. no moving parts.. additional benefit- the more accurately the shell was fired in the first place the more mass is left when it hits.