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How feasible was Project Orion and how feasible would it be if started today? Specifically, what would be the real world impact of nuclear fallout? And, would there be methods to mitigate or eliminate the fallout, so Project Orion could became a realistic method of space travel?

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    $\begingroup$ IIRC the original studies were done assuming a pure fusion bomb would become available by the time it was built; so I'd be highly skeptical of any environmental impact citations from the original work. $\endgroup$ – Dan Neely Jul 31 '13 at 18:00
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    $\begingroup$ Larry Niven and Jerry Pournelle have already outlined the conditions in "Footfall": breakdown of international law, do-or-die environment when you don't care about the fallout any longer. $\endgroup$ – Deer Hunter Jul 31 '13 at 18:57
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    $\begingroup$ @geoffc with current or near future technology not taking off from a planetary surface is even more unrealistic. Orion was huge, in a dwarf the Saturn V way, constructing one in orbit from conventional rocket launches would be staggering difficult. For any mission in the solarsystem you'd also lose most of the benefit doing it that way; the energy needed to achieve Earth orbit being much larger than that needed to leave Earth orbit and go elsewhere. $\endgroup$ – Dan Neely Jul 31 '13 at 19:12
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    $\begingroup$ It seems just as unrealistic that we could build a large space ship like the ones suggested in Project Orion in space. $\endgroup$ – matthew spear Jul 31 '13 at 19:12
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    $\begingroup$ @matthewspear why would be build it in space? The main benefit of Project Orion is that its scales well for very large, heavy spacecraft. Unlike most, Project Orion works better if it's very large and heavy. It doesn't make sense to build in space. $\endgroup$ – Jeff-Inventor ChromeOS Aug 11 '14 at 3:05
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How feasible was Project Orion and how feasible would it be if started today?

In general, the only limitation on space flight has been political capital. Project Orion would thrive in an environment in which there was substantial interest (~500B USD range) in interstellar but not enough resources (~3-5T USD) or time (requiring space based solar) for development of antimatter propulsion. If you gave NASA half a trillion dollars and told them to build an interstellar Project Orion, I'm sure that all the technical problems would be worked out relatively quickly. Specifically addressing some common concerns, I'd expect it to be assembled in orbit from predominately asteroids pulled in by gravity tractors. There isn't any particular reason to assembly such a large craft in a gravity well. While the proposed pure-fusion detonators have not been developed, there hasn't been reason to. Regardless, launching large amounts of fissile material to seed detonators, while costly, is still exclusively a question of political capital. Space elevator is also always an option, especially for multiple vehicles.

I think the main difference would be that, given this implementation rather than a from surface launch, there's little to no reason not to employ existing crew to orbit methods and then use nuclear reactors powering ion thrusters, as it scales considerably better. However, if nuclear pulse detonation was, for any reason (including surface launch) desired, it certainly could be done.

Specifically, what would be the real world impact of nuclear fallout?

I don't think a surface launch would be pursued today under almost any circumstances, including systemic failure of the planet to sustain life, due to the increased efficiency of orbital assembly. However, polar launches would largely reduce fallout, as would using conventional detonations from the launch pad for the initial pulse. Overall, however, this in an unresolved problem. However, Freeman Dyson estimated on the order of a single death (from cancer through radiation) from each launch, and this is likely fairly reasonable, especially given the astounding number of nuclear detonations that have already occured worldwide.

And, would there be methods to mitigate or eliminate the fallout, so Project Orion could became a realistic method of space travel?

While I've outlined a few things, I'll summarize here:

  1. Polar launch
  2. Conventional initial pulse
  3. Orbital assembly
  4. Antimatter propulsion

The most important thing to stress though is that Project Orion is a realistic method of space travel. The consequences of launch would likely be negligible, the investment would likely generate enough new knowledge to pay for itself, and the value of having life off-planet as an insurance policy is incalculably valuable. Personally I favor holding off for antimatter for humans and nuclear powered ion for cargo, but if we had to launch in 2020 (possibly sooner?), Project Orion could do it.

Happy travels!

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"How feasible" is a very large question. Technically feasible? They had found no show-stoppers by the time the project was cancelled, but acknowledged that it could only be proven by taking the next step and testing hardware. Testing might have (or probably would have, depending on who you ask) reveal that the pusher plate would be destroyed, even with the oil spray between blasts. What makes Orion so heartbreaking is that it was cancelled at the peak of its promise, before we had a chance to see a pusher plate in an underground cavern get torn to shreds by Test Pulse Number Five. Or survive. We may never know.

Environmentally feasible? Fission bombs can be made cleaner, but not actually clean. There would be fallout. Even if the launch scenario minimizes ground burst effects, and even if the bombs were engineered with boron to "eat" a lot of their own radiation, fallout would still be orders of magnitude above what society is prepared to accept. Any hint of "fourth generation nukes" that initiate fusion without a fission trigger is empty hype. Design a "low-fallout" Orion and launch it, and within 48 hours radiation monitors worldwide would know it.

Geo-politically, the only entities who could build and test Orion are the ones most invested in nuclear non-proliferation. Even if it's launching a peaceful scientific payload, Orion carries nukes into space, thereby "militarizing" it. It requires treaty exceptions or violations, and those treaties work as well as they do because there are no exceptions.

So as I see it (sorry, speculating here) the only way Orion could move forward would be in response to an extinction-level threat. Asteroid collision is the most commonly cited one, but global warming might be more realistic. Sea levels rise and we start losing cities, and the global perception of risk changes. Suddenly we want to move hundreds of tons of particulates into the upper atmosphere, or deploy a hundred massive solar sails at the earth-sun L1 point to reduce solar heating by 2% worldwide. Then, and only then, do treaty exceptions get made and the project gets authorization to build a pusher plate test article and begin underground testing with real nukes. Sadly, that's probably when some nice people at Las Alamos come forward and say "Just for kicks we computer modeled this in 2008. The pusher plate definitely does not survive. Don't bother building it."

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