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I've read about NASA Glenn's Next's and JPL's DAWN ion engines using xenon (or cheaper argon and krypton) to go on trips to asteroids, bring a small one back to lunar orbit, bring back mined minerals, and Ad Astra's one to deflect asteroids.

These ion engines all use neutral gases, bringing several tons of fuel with them. If the craft could use gases extracted from the asteroid itself, there would be plenty for deflecting the asteroid or bringing it nearer to earth for mining, and it wouldn't have to launch with as much fuel.

While an ion thruster that uses in-situ resources sounds at first glance like a good idea, there's probably a reason I haven't seen articles about them.

Would such an ISRU ion thruster be infeasible or otherwise inferior to other ways for moving an asteroid?

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    $\begingroup$ The issue is that ion engine have very low thrust to weight ratio. With equipment to land and extract/process the resources it would make it even worst. Especially if you have high thrust requirement like ... landing somewhere $\endgroup$ – Antzi Oct 5 '16 at 3:45
  • $\begingroup$ @Antzi - Landing on an asteroid doesn't require high thrust. It's a microgravity environment. $\endgroup$ – Ben Crowell Oct 5 '16 at 14:40
  • $\begingroup$ Deflecting an asteroid is a very different application from bringing an asteroid near earth. Deflecting an asteroid can be done with an arbitrarily small impulse, provided that you start early enough. In many scenarios the spacecraft doesn't even need to touch the asteroid in order to get the necessary deflection. It can simply move near the asteroid, and the gravitational attraction provides enough of an impulse. Moving a large asteroid into a radically different orbit is a whole different thing. $\endgroup$ – Ben Crowell Oct 5 '16 at 14:44
  • $\begingroup$ @BenCrowell landing yes. But before you can land you need a capture. And this one needs higher thrust on micro gravity environments. $\endgroup$ – Antzi Oct 5 '16 at 17:06
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Using an asteroid's mass for propulsion is definitely possible. You can chop up pieces of one and use it in a mass accelerator, or ablate it with a nuclear weapon, or use material as fuel in some sort of rocket - provided the body in question has water or gas which can be used.

The problem with ion engines is time. Ion engines are efficient but achingly low thrust. It would take decades or centuries to move an asteroid in this way.

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  • $\begingroup$ Sounds like if you have the whole asteroid and need a lot of thrust (bringing back to earth on a schedule), an ion engine is not the best choice to move the asteroid around anyway. Meanwhile for just visiting and returning, the mass and complexity of ISRU gear would make it inferior to just bringing the fuel. Oh well. $\endgroup$ – Graham Oct 26 '16 at 17:59
  • $\begingroup$ I think if you are talking about moving asteroids around you have to think in terms of decades or centuries. We humans need to start thinking in much longer terms if we are going to explore space. $\endgroup$ – GdD Oct 27 '16 at 7:17
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Thats a good way to look at it, and a good way to create solutions in unexpected ways. Indeed the ION engine uses inert or otherwise incombustible material to gain what I tend to view as "traction" rather than calling the material a "fuel"... As the "fuel" I would say was the source of the electrical field employed to eject the inert material for "thrust" The material being carried along in similar fashion as the fuel burned in a combustion reaction process on conventional vehicles, does need to be replaced at some point, if you want to keep going... Why not dust or dirt from intercepted objects (micro meteorites) harvested parasitically for use as the inert material being thrown out the rear of craft for "traction" ??? It doesn't take a whole lot of the inert stuff to do what it does in the ION engine application... Sounds like something I would want to investigate.

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    $\begingroup$ Re "traction," I think the term you're looking for is reaction mass. What you seem to mean by "fuel" is energy, and the energy source for ion drive engines is not the electric field used to accelerate the ions. In current ion drive systems, the source of energy is sunlight. $\endgroup$ – Ben Crowell Oct 5 '16 at 14:39
  • $\begingroup$ a power source like what voyager used ( parasitic nuclear decay "battery" ) could provide the electromotive force to maintain an Ion engine, given the source and the drain were proportionally compatible. $\endgroup$ – Russ Reed Oct 7 '16 at 6:08

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