Wow, this is pretty amazing: "... This month's Discover looks at "eight ways to kill a satellite" -- from space nukes to hack attacks -- and ranks "the threat level of each." The magazine declares "an interceptor launched from Earth, guided to its target by a ground-based laser or radar system" to be 'the gravest threat.'"
.. which is what most people would think. However, the journalist writing the blurb disagrees: "Most of the smart folks I've talked to about this think satellites' biggest vulnerability comes from the orbiters' "need for constant housekeeping from the ground," as MSNBC puts it. Those orders from back here on Earth are eminently spoofable."
So, is it the 21st century or what? :-) Hacking things is now easier than blowing them up...?