Now let’s turn to the Soviet space program, and consider the Proton roc ket configured with a Blok-D upper stage with a fission thermal engine. We’ll call it Blok-N. The direct Soviet counterpart to the NERVA flight engine was the RD-0410. Thrust=35.3 kN. Dry mass=2,000 kg Specific impulse= 910 s. This engine is lighter than the American 75K NERVA and even more efficient, though it has much lower thrust. The Proton/Blok-N configuration was used in a series of launches to construct a space station called Zarya (“Dawn”) at the Earth-Moon L-4 Lagrange point. After reaching L-4, the Blok-N upper stage would circle behind the Earth through L3 and reach L-5, where it would be safely parked to be reutilized at a future date as an orbital tug. No manned missions would utilize the Blok-N stage due to radiation hazards.