We think of spacecraft as these solid, expertly engineered emissaries of humanity exploring the universe, but really, they're just machines with computers, and like your laptop they can be hacked, and so can NASA, which is a little insane to think about.
When it comes right down to it, NASA is just like any modern business, relying on a network of computers, both on the ground and in space. And because it's all computer-based, NASA has to protect its network from cyber attacks like any other big company does or else risk the loss or theft of very important and sensitive data.
And it's not just access to computers, hackers can access NASA satellites. Hackers gained access to the Landsat-7 satellite twice in 2007 and 2008, but only gained access; they couldn't control it. Two more serious attacks were waged on NASA's Terra EOS satellite in 2008, where the hackers actually gained control for 2 minutes in June and 9 minutes in October. They were able to issue commands to the satellite but didn't actually do it.
But hacking satellites isn't always nefarious. Sometimes hacking has good intentions behind it! Say NASA's funding for a mission runs out but the satellite is still active. It's possible for private citizen scientists or lesser-funded space organizations to talk to it with radio waves the same way NASA does!
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