Red Sun Radiation

When Superman – or indeed any Kryptonian – is on his home planet of Krypton, he is essentially the same as an Earth human. It is only when he’s on Earth – or anywhere else in the presence of our yellow sun – that he is empowered to his famous super-powered levels.

On Krypton, there is a red sun and whenever he is exposed to the radiation or energy from that, he loses all of his powers, which enemies can use to weaken and defeat him.

It doesn’t cause him any harm – and actually leaves him as healthy as any regular human – but without his powers he is effectively useless against his usual gallery of rogues.

Anyone who can access such energies can use it against him. Those characters have included Lex Luthor and Batman.