Satellites and the services enabled by them, like GPS, real-time world-wide imaging, weather tracking, and worldwide communication, play an increasingly important role in modern life. To support these services satellite software is becoming increasingly complex and connected. As a result, concerns about its security are becoming prevalent. While the focus of security for satellites has historically been on encrypting the communications link, we argue that a fuller consideration of the security of satellites is necessary and presents unique challenges. Satellites are becoming increasingly accessible to attackers–thanks to supply chain attacks and Internet connected ground stations–and present a unique set of challenges for security practitioners. These challenges include the lack of any real ability for a human to be physically present to repair or recover these systems, a focus on safety and availability over confidentiality and integrity, and the need to deal with radiation-induced faults. This work characterizes the cyber threats to satellite systems, surveys the unique challenges for satellite software, and presents a future vision for research in this area.