The old Orion project had a few potential problems. There were doubts regarding the stability of the system, but with modern simulation technology this can be verified rather easily and without the need for an actual empirical investigation. The main problem however was the possible nuclear fallout. This was the most important reason due to which the project was shelved. The treaty of 1963 came as the final blow.
Even with the newer “versions” of Orion like projects there are all sorts of problems.
While the Orion was important for its time (in terms of stimulating possible engineering concepts), I would currently view it as “post period”.
A better possible concept is when will we have the technology to effectively observe “everything”? If you can observe it you do not have to “go there”. There are of course limits, and of course these should be discussed. But current planned satellites allow us to “go there” much more effectively without the need for an Orion like project.
I believe the Mars rovers can provide a good example of how to do things now. They have lasted something like 2-3 years longer than their design life. So long as one builds in fault tolerance bots could have even longer lifetimes.
We are not going to really get there until we have true nanorobots that can be organized to operate collectively because these can be launched with very small rockets.
The solution to managing things is what is known as a “broadcast architecture” (which is very similar to what NASA uses now with satellites but on a somewhat larger scale).
Even with the mini-mag there are problems. First, basing a propulsion system on 245 Cm presumes that you could synthesize sufficient amounts of it. That is a massive undertaking.
Second, it assumes one needs to navigate 100ton spaceships around the solar system. We do not. We need to be able to launch nanorobots into orbits that can easily be transported to various places in the solar system to manage development. That requires lots of micro-rockets — not the huge spaceships designed to transport humans to places they were they are not adapted to live. This i have already touched upon in the previous paragraph. The paper (cited in the previous entry) is a classic example of good physicists doing good work who have little understanding of nanotechnology or microbiology. They also are stuck in 1960’s era concepts that “we” should go there when we have to completely alter the human genome before we should even consider that. And by then we will likely be dead or uploaded and so it is pointless to attempt 1960’s era transport of us.
Related Articles:
Death of a project: Project Orion
Possible Rebirth of Project Orion?
Onionesque Reality 
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Thanks for the kind words!
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Nanorobots can’t do mining and colonization. If we ever want to lift off this planet for real, Orion is the only way.
Yes Mick you are right,
Orion is the only way if we would ever have to set up up a permanent base with minimal effort. However it might not be suited for exploration of unknown worlds, it is better in that case to first explore them “without going there”, the idea of the Orion is dated in that sense. At that (the time it was being tried to be made) time it was thought that to explore a planet, humans MUST go as the technology was not up to the mark (remember even Sputnik was not launched). That’s what i meant in this post.
But yes you are right in case we have to colonize! But not needed for exploration.
Mick is right, absolutely. project like orion are important for space colonization, not for observation. Earth population is costantly growing, and the lack of resources is a big problem also today. there are 2 solution: global war or space colonization. that is the reason why NASA want to return on the moon, that is why is trying to go on mars with manned missions. that is why they are trying to make engines able to burn helium3 and other substances (co2) that can be easily found on solar system planets. it’s obvious that orion or orion like projects have a meaning if we consider space colonization. and they are actual, more than actual. every other propulsion technologies are uneffective or unavailable. nuclear pulsion is only a step far.
Nanobots perform poorly in any place where there is low radiation shielding, due to their small size. They also have other limitations: http://www.stardestroyer.net/Empire/Tech/Myths/Nanotech.html
And unless you want to send them into Venus or a gas giant, the other rocks in the solar system provide very little protection.