If memory serves me right you only get about a theoretical 6 kilotonnes of yield per kg weight of a 2 stage conventional hydrogen bomb mostly due to weight of the plutonium tamper. With the newer 4 th gen tritium boosted anti-hydrogen weapons you get about 60-100 kilotonnes per kg. So even small warheads that weigh 6000 kg i.e. payload for a old Soviet SS18 etc will get you a approx 1/2 Gigatonne explosion. Enough to take out a 5 km wide asteroid or comet. Off course if the aliens arrive on planet Niburu next year then all bets are off.