I hadn't heard of the 9M730. That page links to one for the US Air Force's Supersonic Low Altitude Missile (SLAM) or "flying crowbar" of the 50s and 60s, which makes for fascinating reading: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...

This case is different though, using the fission reaction to generate electricity which in turn powers the electric motors among other things.

The nuclear powered missiles use the fission reaction to heat incoming air similar to how a jet engine works.

No fission reaction, just radioactive decay in Dragonfly's case. It will use an RTG (radioisotope thermoelectric generator) not unlike Curiosity and most missions to the outer planets to date.

Just a note - radioactive decay is is fission, since it involves a nucleus splitting apart.

No, not quite. One of the types of radioactive decay is spontaneous fission, but it's not relevant in the context of RTGs. Alpha decay (the emission of a 4He nucleus) might in principle be called "fission" but in practice is not. Beta decay involves no splitting whatsoever.

RTGs are heavy; I suppose this means that it'll park itself for a long time to charge its batteries and then do a quick flight and repeat. Which sounds like it could work pretty well for this use case.

On the other hand, a hot-air balloon kept aloft by excess heat from radioactive metal might be a more attractive option if we were selecting proposals based on awesomeness.

Does that mean a nuclear powered cruise missile could stay airborne until mechanical failure? That would make future warfare... interesting.

It isn't all that much of a game changer. Mid-air refueling means that bombers can stay airborne a long time. Four or five bombers rotating back and forth can maintain one "on station" continuously. So the capability is already there. This tech would just make it more scifi.

Why bother? Rocket fuel is cheaper than nuclear fuel and rocket missiles can already hit anywhere.

Because a nuclear-powered missile can in principle fly in circles at low altitude off your enemy's shores for months at a time without landing or refueling, ready to be called down at any time with no detectable warning.

Well, if I am your enemy, I sure as hell won't let your missile cruise around my shore for a month

The difference between rocket fuel as a power source and nuclear radioactive decay as a power source is like comparing C4 explosive and a log fire in your fireplace. Once it great for providing a huge, fast release of energy, the other is great for providing a slow, steady release of energy.

Cheaper, easier to mass produce, doesn't need an unusually special launcher, hard to distinguish from air to air and other defensive missiles, easier to disguise the number of warheads waiting in readiness, can take an indirect/stealthy course, hard to shoot down, can include smart tracking for moving targets like carrier groups.

Maybe so it can double as a shitty nuclear bomb. I'm gonna crash my small nuclear reactor into you.