A neutronic analysis has been performed for a thorium fusion breeder with a special task of burning minor actinides Np-237, Am-241, Am-243 and Cm-244 and production of U-233, Pu-238, Am-242m and Cm-245 for spacecraft application. Pu-238 is an important radioisotopic energy source for spacecraft generators. As potential nuclear fuels in the foreseeable future, U-233, Am-242m and Cm-245 would allow one to build extremely compact space reactors. Natural lithium has been selected as the coolant medium for the nuclear heat transfer out of the fuel zone. Minor actinides out of 5 and 10 units of LWRs per metre of blanket height have been mixed with ThO2. Higher fission rates in minor actinides enables one to realise a power flattening in the fissile zone over three years of plant operation by a gradual increase in the radial direction at start-up. This has significant advantages with respect to plant operation over the long term and also with respect to a uniform utilisation of the nuclear fuel in the fissile zone. After three years of plant operation, the net U-233 production is similar to300 kg per metre of blanket height. The Pu-238 yield is 21 and 41 kg for a waste actinide charge out of 5 and 10 units of LWRs per metre of blanket height, respectively, and the Cm-245 yield is 1.1 and 2 kg, respectively. The net Am-242m production is practically nil. With waste actinides out of 10 reactor units per metre of blanket height, the flattening of the nuclear heat production density in the fissile zone is almost perfect. Waste actinides out of five reactor units per metre of blanket height allow still an excellent power flattening. The quasi-constant power shape is saved over 36 months.