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My favorite speculation is that Oumuamua is a spent deceleration stage for a larger inbound multi-stage interstellar rocket. It would probably be a nuclear fusi

by api 8y ago

My favorite speculation is that Oumuamua is a spent deceleration stage for a larger inbound multi-stage interstellar rocket. It would probably be a nuclear fusion rocket or similar. Previous stages flew past us and we didn't notice. The next stages(s) and/or the payload should arrive shortly. Since it's decelerating the next stage will be traveling slower.

mfoy_ 8y ago
That's actually a fun thought... once you've got your probe up to a third of the speed of light or whatever... how are you going to slow down again in order to actually do anything useful? Or maybe this is a solar sail that was part of an ACCELERATION stage for a probe that already passed through our solar system millions of years ago.
- api 8y ago
  I'm not a rocket scientist (heh) but I'm imagining a many-stage nuclear rocket. You'd have a whole bunch of acceleration stages followed by a much smaller number of deceleration stages followed by the payload. You wouldn't need as many deceleration stages since you've already spent all the mass in your acceleration stages. Building such a thing in space would involve sending up the stages themselves with some kind of heavy lift vehicle, assembling them in orbit, and firing it. The number of stages you could assemble would be quite large since there's no air resistance or (relative) gravitational stress in space. You'd just have to not exceed the tensile strength of the material during acceleration. You could put together 100 stages or more I'm sure. It's one rather brute force but workable way of reaching some nontrivial fraction of the speed of light. Someone should do the math on this for fission (NERVA-like), thermonuclear pulse (Orion), and pure fusion.
- AtlasBarfed 8y ago
  If we were to do an interstellar ship with current physics it would probably rely on: - Preacceleration strategies that speed up the craft without using its onboard/deceleration reaction mass/fuel (gravity whips, launching lasers for a solar sail, cannon-fired fuel pods) - Pulse nuclear acceleration using "preseeded" fuel pods launched ahead of the craft and captured as the craft caught the pod - ion drive once the preseeded fuel pods weren't practical anymore - ion drive, solar sail, and pulse nuclear and more gravity whips to slow down Pulse nuclear/orion ships are the only thing that gets close to specific impulse and performance to get to Alpha Centauri in anything reasonable. It becomes a LOT more feasible if fuel pods are prelaunched, or seeded by a leading disposable fuel seeding ship (it doesn't have to slow down, it can just, uh ... .... fly through the target system once it's done. Uhoh.
  - mfoy_ 8y ago
    This kind of makes me want to play KSP again...
AnIdiotOnTheNet 8y ago
Something like that would probably be bright enough for us to see it by now if we looked, and we probably did because of our interest in Oumuamua.
- newsbinator 8y ago
  Assuming it's not intentionally obscured. If I were building an interstellar vehicle to send to/past possibly hostile alien worlds, it wouldn't have headlights.
  - AnIdiotOnTheNet 8y ago
    If it is decelerating toward us it will have to be dumping significant amounts of hot propellant at least roughly in our direction. You can't obscure that with known physics as far as I'm aware. If you're going to involve unknown physics, the theory becomes a lot more unlikely.
  - api 8y ago
    You can't do stealth in space. Thermodynamics won't let you. If you're using a whole lot of energy you will light up in the infrared. Parent's point is valid except for the fact that they might not be lighting off the deceleration stages one after the other. There could be a time gap, and maybe a significant one. Gaps between firings would be likely because that's one way you could steer. You could time firings to aim for e.g. an orbital insertion around Jupiter, which would be easier since Jupiter is so massive. You could also use Jupiter or another gas giant to aerobrake with some kind of ablative shield like in the film 2010. It's fairly likely that whatever physicists' nightmare propulsion system an expendable interstellar rocket stage would use would be something that is hard to turn off once you light the candle, so each stage might be an all or nothing thing. It would be interesting to use the JWST, an infrared telescope, to look at areas of the sky where an inbound craft coming along this sort of interstellar trajectory might be. A probe or spacecraft would have to have its own power source. This would be way dimmer than a lit deceleration stage, but JWST might be able to see it. If it's using energy at all it must have radiators due to the second law. Of course if this hypothesis were valid and they lit another deceleration stage, it would absolutely be visible if we looked in the right place. It would be very very bright in the infrared and probably in visible light too. Hell if it's something insane like an antimatter driven fusion rocket you might be able to see it with the naked eye.

by api 8y ago

mfoy_ 8y ago

That's actually a fun thought... once you've got your probe up to a third of the speed of light or whatever... how are you going to slow down again in order to actually do anything useful? Or maybe this is a solar sail that was part of an ACCELERATION stage for a probe that already passed through our solar system millions of years ago.

api 8y ago
I'm not a rocket scientist (heh) but I'm imagining a many-stage nuclear rocket. You'd have a whole bunch of acceleration stages followed by a much smaller number of deceleration stages followed by the payload. You wouldn't need as many deceleration stages since you've already spent all the mass in your acceleration stages. Building such a thing in space would involve sending up the stages themselves with some kind of heavy lift vehicle, assembling them in orbit, and firing it. The number of stages you could assemble would be quite large since there's no air resistance or (relative) gravitational stress in space. You'd just have to not exceed the tensile strength of the material during acceleration. You could put together 100 stages or more I'm sure. It's one rather brute force but workable way of reaching some nontrivial fraction of the speed of light. Someone should do the math on this for fission (NERVA-like), thermonuclear pulse (Orion), and pure fusion.
AtlasBarfed 8y ago
If we were to do an interstellar ship with current physics it would probably rely on: - Preacceleration strategies that speed up the craft without using its onboard/deceleration reaction mass/fuel (gravity whips, launching lasers for a solar sail, cannon-fired fuel pods) - Pulse nuclear acceleration using "preseeded" fuel pods launched ahead of the craft and captured as the craft caught the pod - ion drive once the preseeded fuel pods weren't practical anymore - ion drive, solar sail, and pulse nuclear and more gravity whips to slow down Pulse nuclear/orion ships are the only thing that gets close to specific impulse and performance to get to Alpha Centauri in anything reasonable. It becomes a LOT more feasible if fuel pods are prelaunched, or seeded by a leading disposable fuel seeding ship (it doesn't have to slow down, it can just, uh ... .... fly through the target system once it's done. Uhoh.
- mfoy_ 8y ago
  This kind of makes me want to play KSP again...

AnIdiotOnTheNet 8y ago

Something like that would probably be bright enough for us to see it by now if we looked, and we probably did because of our interest in Oumuamua.

newsbinator 8y ago
Assuming it's not intentionally obscured. If I were building an interstellar vehicle to send to/past possibly hostile alien worlds, it wouldn't have headlights.
- AnIdiotOnTheNet 8y ago
  If it is decelerating toward us it will have to be dumping significant amounts of hot propellant at least roughly in our direction. You can't obscure that with known physics as far as I'm aware. If you're going to involve unknown physics, the theory becomes a lot more unlikely.
- api 8y ago
  You can't do stealth in space. Thermodynamics won't let you. If you're using a whole lot of energy you will light up in the infrared. Parent's point is valid except for the fact that they might not be lighting off the deceleration stages one after the other. There could be a time gap, and maybe a significant one. Gaps between firings would be likely because that's one way you could steer. You could time firings to aim for e.g. an orbital insertion around Jupiter, which would be easier since Jupiter is so massive. You could also use Jupiter or another gas giant to aerobrake with some kind of ablative shield like in the film 2010. It's fairly likely that whatever physicists' nightmare propulsion system an expendable interstellar rocket stage would use would be something that is hard to turn off once you light the candle, so each stage might be an all or nothing thing. It would be interesting to use the JWST, an infrared telescope, to look at areas of the sky where an inbound craft coming along this sort of interstellar trajectory might be. A probe or spacecraft would have to have its own power source. This would be way dimmer than a lit deceleration stage, but JWST might be able to see it. If it's using energy at all it must have radiators due to the second law. Of course if this hypothesis were valid and they lit another deceleration stage, it would absolutely be visible if we looked in the right place. It would be very very bright in the infrared and probably in visible light too. Hell if it's something insane like an antimatter driven fusion rocket you might be able to see it with the naked eye.