Well I did say tens-of-thousands of multi-gigaton, that’s not so much of a stretch there. Looking at the rough outlines of the tasr bomba design my guess is that a half-a-teraton (5000x the yield of tsar bomba) stationery bomb is not outside our technological limits. It’d be very big and extremely hard to build, yes, but it’s not like it’s going to intrinsically impossible to build because of technological or material limitations. For 48 million gigatons you’d need 96 thousand of those. As long as we have enough (or can make/mine enough) fissionable material to start the fusion reactions we’re set.
Eh, it’s a stretch but then again if all of that impulse is magically applied in one direction without causing massive disintegration or losses to heat you’re going to give the earth it’s own escape velocity. If you point that at the sun, that’s a very very quick and fun spiral into oblivion (off the top of my head probably about a couple of standard earth months). This should tell you that to crash the earth into the moon you probably don’t need nearly that much nor any impulse redirection magic.
Theoretically, there’s no limit to how big you can make a staged thermonuclear device, particularly if you don’t care how large it gets; each state just acts as a neutron producer for the next stage while adding its energetic yield to the total. You just keep adding more stages to get the desired yield. In practice, you’d have to design it in such a way that you obtain the desired reactions before it blows itself apart. I suspect there’s a feasibility limit to how much yield you could practically get. I doubt a half-teraton yield is practical, nor could you hope to focus the energy in one direction.
Plus you’d have to bury them sufficiently that none of the energy gets wasted making pretty mushroom clouds. Each charge would also have to be designed to direct all it’s energy away from the center of the earth, or you’d get a leftover core were downward directed material from one blast meets downward directed material from other blasts. It’d be very disappointing to go to all that trouble, and end up left with a white hot, Ceres sized chunk circling the sun in Earth’s original orbit.
Hmm…this reminds me of the original H-Bomb testing. One concern was that the atmosphere would become a part of the fusion reaction and essentially the whole atmosphere would start undergoing fusion reactions. Is this still believed to be possible given enough energy, or is this no longer a valid concern? In other words could we ignite all of Earth’s atmosphere in a gigantic fusion reaction?
Not a chance. The triple product required for the CNO cycle are much greater than those involved in fusion of lightweight elements (hydrogen, helium), and the majority of the net energy budget you get from a thermonuclear bomb is from the D-T (deuterium-tritum) reaction. Fusion of heavier elements only occurs in larger or off-sequence stars.