4 ms·

The key to a successful interstellar probe is probably miniaturization. You only need about 5kg of Pu-239 (energy density ~8.8e13 J/kg) to accelerate a 1kg payl

by zeteo 15y ago

The key to a successful interstellar probe is probably miniaturization. You only need about 5kg of Pu-239 (energy density ~8.8e13 J/kg) to accelerate a 1kg payload to 10% of the speed of light (energy requirement ~4.5e14 J). A single large H bomb (2.1e17 J) could accelerate 100kg to 0.2c.

Groxx 15y ago
As shrapnel, yeah. Not much can survive those kinds of accelerations. So you'd need to carry a bunch of smaller ones with you, or shoot them at the probe as it accelerates (harder and harder to do), at which point we have the same current problems - large masses, or shooting things at other things to propel them. -- After thinking about it for a bit, I don't think miniaturization for very-long-range probes would work very well anyway. You can't focus signals beyond a certain degree, dictated by your reflector size, so you either need a lot of power to send signals long distances or very large reflectors to compensate for lower power. The reflectors could conceivably be something like a solar sail - huge and light - but you still have to carry it and deploy it and ensure it stays focused as you approach other stars (solar winds). You're stuck with rigid and heavier or flexible and useless, and if it's big enough it'll push you away from the star regardless of its ability to hold a shape. At which point we're pretty much stuck with using, thus carrying, more power beyond the inflection point. No clue where that would be, but I doubt it would bring us past all but the closest stars, if any.
- zeteo 15y ago
  Yes, of course you wouldn't directly use an H bomb, the idea was that the quantities of fissile materials needed to propel a 100kg interstellar probe to 0.2c are easily within reach of several nations. Controlled fusion can be achieved in very confined spaces: http://en.wikipedia.org/wiki/Hohlraum http://en.wikipedia.org/wiki/Hohlraum I agree that communication equipment is a major problem. The best way to solve it is probably to oversize the local equipment: a dedicated space telescope and a powerful orbital laser. This may let you get away with a very lightweight emitter/receiver pair on the probe itself.
  - Joakal 15y ago
    Assuming there's no microscopic debris in space and it's pretty empty, wouldn't it be possible to accelerate faster than c relative to Earth and get there sooner in terms of body age? I mean gradual increase of acceleration, of course. I understand that Earth will just see the ship time flow more slowly.
    - johngalt 15y ago
      Lorentz factor only makes a large enough difference when you're very close to the speed of light. Edit: changed from 'c' to 'the speed of light'
      - Joakal 15y ago
        c, as in Earth's view of ship's speed?
    - prewett 15y ago
      You can't go faster than c from anyone's reference frame (that's why space and time contract, to prevent that). "I understand that Earth will just see the ship time flow more slowly." You have that reversed: Earth will see time same as usual, but time for you will slow down.
  - Groxx 15y ago
    I haven't seen the Hohlraum article before - interesting stuff. And for the earth-side receiver, I was generally assuming hugely-sensitive equipment. But I may be underestimating the sensitivity of what we can build - clearly we can detect individual photons, so the probe emitter would just need to ensure it sends enough that the receiver can capture enough to reconstruct the message... In any case, interesting stuff :) I'm sure there's a calculation that we could do to figure out the actual power requirements given background radiation and receiver sizes, but I don't know the associated math. -- A thought just occurred to me - given a big reflector, you could emulate a transmitter by varying its reflectiveness / target, beaming a massively-powerful, regular pulse from Earth, and watching what comes back. That could eliminate transmitters (and their power requirements) entirely from the probe, though you still have the solar wind problem if it's really big.

by zeteo 15y ago

Groxx 15y ago

As shrapnel, yeah. Not much can survive those kinds of accelerations. So you'd need to carry a bunch of smaller ones with you, or shoot them at the probe as it accelerates (harder and harder to do), at which point we have the same current problems - large masses, or shooting things at other things to propel them. -- After thinking about it for a bit, I don't think miniaturization for very-long-range probes would work very well anyway. You can't focus signals beyond a certain degree, dictated by your reflector size, so you either need a lot of power to send signals long distances or very large reflectors to compensate for lower power. The reflectors could conceivably be something like a solar sail - huge and light - but you still have to carry it and deploy it and ensure it stays focused as you approach other stars (solar winds). You're stuck with rigid and heavier or flexible and useless, and if it's big enough it'll push you away from the star regardless of its ability to hold a shape. At which point we're pretty much stuck with using, thus carrying, more power beyond the inflection point. No clue where that would be, but I doubt it would bring us past all but the closest stars, if any.

zeteo 15y ago
Yes, of course you wouldn't directly use an H bomb, the idea was that the quantities of fissile materials needed to propel a 100kg interstellar probe to 0.2c are easily within reach of several nations. Controlled fusion can be achieved in very confined spaces: http://en.wikipedia.org/wiki/Hohlraum http://en.wikipedia.org/wiki/Hohlraum I agree that communication equipment is a major problem. The best way to solve it is probably to oversize the local equipment: a dedicated space telescope and a powerful orbital laser. This may let you get away with a very lightweight emitter/receiver pair on the probe itself.
- Joakal 15y ago
  Assuming there's no microscopic debris in space and it's pretty empty, wouldn't it be possible to accelerate faster than c relative to Earth and get there sooner in terms of body age? I mean gradual increase of acceleration, of course. I understand that Earth will just see the ship time flow more slowly.
  - johngalt 15y ago
    Lorentz factor only makes a large enough difference when you're very close to the speed of light. Edit: changed from 'c' to 'the speed of light'
    - Joakal 15y ago
      c, as in Earth's view of ship's speed?
  - prewett 15y ago
    You can't go faster than c from anyone's reference frame (that's why space and time contract, to prevent that). "I understand that Earth will just see the ship time flow more slowly." You have that reversed: Earth will see time same as usual, but time for you will slow down.
- Groxx 15y ago
  I haven't seen the Hohlraum article before - interesting stuff. And for the earth-side receiver, I was generally assuming hugely-sensitive equipment. But I may be underestimating the sensitivity of what we can build - clearly we can detect individual photons, so the probe emitter would just need to ensure it sends enough that the receiver can capture enough to reconstruct the message... In any case, interesting stuff :) I'm sure there's a calculation that we could do to figure out the actual power requirements given background radiation and receiver sizes, but I don't know the associated math. -- A thought just occurred to me - given a big reflector, you could emulate a transmitter by varying its reflectiveness / target, beaming a massively-powerful, regular pulse from Earth, and watching what comes back. That could eliminate transmitters (and their power requirements) entirely from the probe, though you still have the solar wind problem if it's really big.