system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] How is it even possible to fight Tyranid Hive fleets when they contain so much mass they could literally de-orbit target planets without even landing on them?

The Tau I could see managing something (albeit at great cost due to their relatively low numbers) due to being able to adjust their weapons to be exactly as efficient as necessary, but for say... a Space Marine, a lot of their weapons put out way more energy than is necessary at times, and they're bound to run out of ammo.

Do many races beyond the Necron have weapons that can reduce the targets to submatomic particles?  Because even if the targets were atomized surely the Tyranid could still hoover that up while swarming the entire planet after a victory.

Are the fleets not quite as dense as depictions on galactic maps tend to indicate?
gpt
This would be why they often don’t. Afaik, only a few successful defenses of major tyranid attacks are covered in the lore. Baal was definitely an unconventional victory, but the hive fleet was legit defeated in Ultramar so that’s at least one.

The short guide to repelling tyranid invasion is trying to last-out the ground war until another friendly fleet can reestablish orbital supremacy. If you can’t eventually win the war in space, you just can’t win the war on the ground against tyranids.