Can satellites be hacked?

Read an article http://www.information-age.com/email/channels/security-and-continuity/perspectives-and-trends/2105738/can-satellites-be-hacked.thtml which discuss the report that hackers believed to be operating from China, had managed in 2008 to interfere with two US government satellites. The hackers reportedly took control of the Landsat-7 and Terra AM-1 satellites for a grand total of 12 minutes and two minutes respectively, the report claimed.

The article goes onto say it is doubted whether hackers would have had the wherewithal to send tracking and telemetry data up to the Skynet satellite. This is implying that the hackers would of used there own infrastructure to do the hack, which I think is misleading, why build your own infrastructure when NASA and other organisations already have the infrastructure in place. What is there to prevent hackers from controlling a satellite via a compromised system in one of the many military or space agencies systems around the world with the capability to communicate with satellites.

Although Gary McKinnon who is one of many that have been accused of hacking NASA systems, NASA have always denied anyone has had access to critical resources. Would NASA admit a breach, has one gone undetected, could one occur in the future.