I am surprised that I have heard no serious suggestion to build at least one more of the spacecraft used in the next mission to study an asteroid or comet(eg the DAWN project), and put the extra spacecraft in storage.

That way, if there is ever a serious asteroid threat, you can launch it on the first heavy-lift launcher available. An Ariane 5 could give a heck of allot of Delta V to a spacecraft intended for a much smaller launcher. You have a proven spacecraft available to give you a very quick look at anything that comes up.

It would also be available for any extraordinary scientific oportunity, or to act as a low-cost communications relay in the event of a Galileo-type failure in a more expensive spacecraft.

You may have to design the craft from the start with this potential use in mind, so it might add a bit to development costs. But the cost for an extra copy of the spacecraft is really minimal compared to the first one, or so I have heard.

You could go even further and build 3-4 spacecraft for the next mission. One goes into storage, the others are launched and return far more science per dollar because of the low additional cost of building the extra spacecraft. This is why NASA is using 2 MARS rovers, I believe...

As good an idea as I think this is, to build multiple crafts on the same design (afterall, surely the fact that unit costs will inevitably drop when producing more and more of something, and one would think quite considerably in relation to spacecraft), I think there are a few things that stop this from happening.

One is simply cost, building a second one is still going to be an expensive proposition, so it needs to be justified, and with NASA's neverending budgetary problems, cost-cutting is the mantra rather than being able to spread the largesse in perhaps more sensible ways, and the other problem is the fact that there's probably not much point having one waiting on the rack as within a few years a much better design is going to come along and render the former obsolete.

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

You ALREADY resurrected a 2002 thread with this EXACT, SAME TOPIC !! What is the point of bumping ancient threads with a 1- or 2- sentence re-introduction, and then abandoning the "new" thread, so that you can bump other threads ???