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B Can radiation in space be captured and used in an Ion drive?

  1. Apr 5, 2017 #1
    Since NASA is currently under way with the deep space Ion drive, they plan on using xenon to power this propulsion system. My question is could they harness the already existing radioactive particles in space to power it? If they could then there would be almost an unlimited supply of fuel for the craft.
     
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  3. Apr 5, 2017 #2

    russ_watters

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    Welcome to PF!
    What radioactive particles in space? And how would "radioactive particles" be a fuel?
     
  4. Apr 6, 2017 #3

    mfb

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    Radiation and radioactive particles (one source of radiation) are different things. You have a lot of radiation in space, but nearly no radioactive particles.

    There is no reasonable way to extract the ions that hit the spacecraft, and their amount is way too small to be interesting as fuel.
     
  5. Apr 6, 2017 #4
    In principle a craft could continue for ever using something like a light sail.
    However most parts of the Universe doesn't really have that much light available,
    it's only effective as a power source close to highly energetic sources of radiation (primarily stars),
    but then you get into problems with gravity.
    It is possible in principle though to make a craft of that sort which can go onward forever,
    if taking millions of years between destinations is not a problem.
     
  6. Apr 9, 2017 #5
    Wouldn't a spaceship just accelerate infinitely, due to the rate and force of acceleration being far greater than the rate and force it encounters space dust over time?
    And at some point, wouldn't all spaceships be basically stuck at a minimum of 99% c (Or whatever velocity is so fast that it's acceleration force cannot exceed the density and force of spacedust)? So you'd basically end up with almost the same time to travel long distances with a spaceship of low/medium acceleration as high acceleration, give or take a few days or months.
    The massive energy of suns could be stored in capacitors, but if the distance between stars is too far, you could be stranded when the energy runs out and you slowly slow down from space dust.
     
  7. Apr 9, 2017 #6
    Yes, you could get the craft to accelerate continually (up to some point limited by gas dust and so on), although it wouldn't accelerate very quickly.
    My earlier reply assumed that it would want to slow down to take a look at stuff like solar systems here and there rather than zipping past collecting little useful data.
     
  8. Apr 9, 2017 #7
    Well, how fast a car can go from solar power is about 105 mph. But if you had a spaceship with disproportionately large solar panels, thats a lot more horsepower. Also, the star power should be more because there is no atmosphere. You'd have to slingshot past each sun to get the most power in the capacitors. If you are too far you won't get too much power at all.
     
    Last edited: Apr 9, 2017
  9. Apr 9, 2017 #8

    OmCheeto

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    Sounds a bit like the Bussard Ramjet.
     
  10. Apr 9, 2017 #9

    mfb

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    @quickquestion: Spacecraft are not cars.

    Capacitors are heavy, don't store much energy, and what do you want to do with that energy without a reaction mass?

    Without lasers to help, solar sails accelerate very slowly and don't reach high final velocities.
     
  11. Apr 10, 2017 #10

    ChrisVer

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    https://en.wikipedia.org/wiki/Solar_sail
    reading that the total force an 0.8km * 0.8km object would experience is about 5N (and that's at 1AU distance from the sun) can give you an idea of how reasonable reaching any velocity near c is...
    The spacecraft would have to be EXTREMELY large, covering as much area as possible, so that it can keep getting as much radiation from the sun as possible.... and the bigger you make it, the more massive it becomes, and so the more difficult it's to move it (as well as the damping from outside becomes more important and dangerous)
     
  12. Apr 10, 2017 #11

    mfb

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    Just as an example: Paper typically has a mass of 80 gram per square meter, or 51 tons for 0.8 km * 0.8 km. Add a support structure and it gets even heavier. We get 5 N of radiation pressure. At the same time, we get 300 N of gravitational acceleration from the sun - we are still in an orbit, we can just change it slowly.
    Even if we ignore the support structure and the payload, the 5 N just produce an acceleration of 0.0001 m/s2. Over 1 day, the paper changes its speed by 9 m/s. Let's wait longer! After 1 year, we have a speed change of 3.2 km/s. This has to be compared to the orbital velocity at 1 AU, about 30 km/s. Our literally paper-thin sail just managed to change its orbit a little bit - probably not even enough to reach Mars. We can probably reach Mars within 2 years. Unfortunately radiation pressure goes down if we go further outwards - at Mars it is a bit less than half the original value. At Jupiter it would be 4% its original value.

    A solar sail has to be much thinner than paper. And even then it won't reach high speeds with realistic thickness unless we increase the radiation pressure massively via laser.
     
  13. Apr 10, 2017 #12
    Well, maybe we should start research on how to transport consciousness itself through space rather than heavy bodies. If consciousness could simply warp to other bodies space travel wouldn't have the mass or gravity problem to deal with.
     
  14. Apr 10, 2017 #13

    ChrisVer

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    I think if you opened the closet and enter Narnia, it would still count as problem solved.
     
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