[FPSPACE] a colleague of mine responds to the question of the Nuclear Sysytems Initiative..

[thomsona]

> The near term solution under nuclear is radioisotope decay, as 
> this has already been demonstrated numerous times on almost all 
> of the deep space missions (outer planets) over the last 30 
> years. Fission is more problematic. The US of A has only 
> officially flown one space fission reactor, SNAP 10A, while the 
> Russians have flown a whole bunch of similar reactors (fission 
> heated thermoelectric generators) with higher power outputs. 

The power to weight ratio of radioisotope systems is much too low
for them to be other than low-power, er, power (or heat) sources.

> Fission is more problematic. The US of A has only officially 
> flown one space fission reactor, SNAP 10A, while the Russians 
> have flown a whole bunch of similar reactors (fission heated 
> thermoelectric generators) with higher power outputs. 

More problematic than radioisotope power supplies? The two are 
almost totally disjoint in their potential applications. And, as 
noted, we do know how to build fission reactors. (BTW, just to 
be picky, while the numerous RORSAT reactors did use 
thermoelectric generators, two test TOPAZ ones on Cosmos 1818 
and 1867 used thermionic converters.) 

> NASA has been supporting with internal funds, attempts to 
> store anti-matter in a magnetic bottle, a sort of giant Penning 
> trap like device. 

If we had some eggs we could have ham and eggs for breakfast if 
we had some ham. 

We're far, far away from being able to produce antimatter in the 
quantities that would be needed for propelling manned 
spacecraft. And if we could produce it, I sure wouldn't want to 
do it on Earth. What would Al Qaida would pay for a kilogram of 
anti-iron? 

Fusion rockets are probably closer than antimatter ones, and 
fusion is still way over the horizon. 

> Whether a direct fission rocket or a direct anti-matter rocket 
> (with no electrical intermediate) will be what is used for the 
> manned Mars mission depends on Isp, as Mr. Cole of MSFC told me 
> recently. Most likely, in order to meet the near term goals, 
> some form of fission-to-electrical conversion with some form of 
> electrical propulsion is what will get built. 

Antimatter propulsion is in fantasy land for a long time to 
come. Fission-based nuclear electric is attractive (I'm partial 
to MPD myself) but major development work has to be done on the 
thrusters to get reasonable thrusts and lifetimes for manned 
missions.  So, if you want to go beyond chemical rockets for 
manned spacecraft, fission-based nuclear thermal is really the 
only available choice now and for a good number of years to 
come.  But that isn't all bad, because there's a fair amount of 
development to be done there, starting with NERVA style systems, 
perhaps reviving TIMBERWIND pebble bed concepts and even (this 
is a stretch) seeing what can be done with gas/plasma-core 
reactors.  And certainly parallel work on electric propulsion
should continue.