http://www.spaceref.com/news/viewnews.html?id=1237

reprints an interesting Aviation Week article suggesting a push for proposals for new Discovery-sized missions to use closed-loop Stirling cycle nuclear power systems, aiming for launch around 2013. It's unclear how the shortage of 238Pu is being resolved, though the Stirling-cycle devices use two 250W_thermal units of 238Pu for 143W output, whilst the Cassini/New Horizons RTG uses eighteen units for 285W electrical output.

http://nspires.nasaprs.com/external/viewre...ationDocument=1 is a summary of the specs of the Advanced Stirling Radioisotope Generator; http://esto.nasa.gov/conferences/nstc2007/...ations/D2P1.pdf is quite a nice presentation of the technology development.

The engineering history seems to be by way of Stirling-type cryocoolers, of which ten have been running for more than 10k hours each.

Good improvement in RPS System efficiency between the RTG powered by Cassini Spacecraft and the present model ASRG: 6.35% versus over 30%. However, the electric output power (Watts) versus weight has improved by 67% (285 Watts for 56 kg -5.09 W/kg versus 160 Watts (estimate) for 19 kgs (estimate) -8.5 W/kg).

The probably future spacecraft: Titan Explorer, REP(?) & Europe Geophysical Explorer will be powerd by ASRG.

Seems like the Russians ought to have plenty of plutonium lying about which they'd be happy to sell to a legitimate buyer - or is there some rule prohibiting Nasa from buying Pu from foreign nations?

NASA has bought 238Pu from Russia for years (or at least, the Department of Energy does so on its behalf).

I am worried about how many new non-Mars missions this technology will allow. I've just been reviewing the Juno website, and it is about as simple of an outer planets mission as possible, and it required the New Frontiers level of funding. (One possible caveat: Perhaps the costs of the instruments drives the mission, rather than the need to reach and operate beyond Mars). Will cheap, low wattage (~250 w) nuclear power really enable new types of missions?

I have no doubt that this will be very useful at Mars for long-lived geophysical (seismology and weather) stations and for small rovers. But the cost of Juno suggests that the lack of RTGs is not what is eliminating outer planet missions from Discovery.