As Russia held its Victory Day parade this year, hackers backing the Kremlin hijacked an orbiting satellite that provides television service to Ukraine. Instead of normal programming, Ukrainian viewers saw parade footage beamed in from Moscow: waves of tanks, soldiers and weaponry. The message was meant to intimidate and was an illustration that 21st-century war is waged not just on land, sea and air but also in cyberspace and the reaches of outer space. Disabling a satellite could deal a devastating blow without one bullet, and it can be done by targeting the satellite’s security software or disrupting its ability...