Say WHAT!?
Nerva is more efficient than LH2/Lox rockets, but less than twice. (800sec vs 450sec). To move a 1km^3 asteroid (say ~10^9 tonnes, 10^12 kg) to miss the earth (move the asteroid 10, 000 km 10^7 m (around 2x earth radius, need to be well clear)) in two weeks ~10^6 seconds, we need delta-v of 10m/s. Nerva is 800 seconds (call it 1000), so ~10^4m/s velocity. SO the mass of LH2 you need for the nerva is ~10^5 tonnes - or about 1000 Saturn V loads, once you got to the asteroid and matched courses. Figure 3-10 times that load into earth orbit.
All very back of the enveloppe, and it'd work a lot better with an asteroid 100m on a side. (actually, it would be very, very expensive with that size asteroid, but might possibly be doable. I was surprised. Still, not 'easily')