They wouldn't so much be "at standby" as "in orbit". To enable them to intercept an object, they'd have to burn hard with a lot of fuel, which means we'd have p

They wouldn't so much be "at standby" as "in orbit". To enable them to intercept an object, they'd have to burn hard with a lot of fuel, which means we'd have place them in orbit with a lot of fuel. It would be more efficient to keep a rocket on standby on Earth ready to launch, since any orbital momentum we give them after launch that is not perfectly in line with their target would have to be canceled.