Outer planet missions worry particularly about the ability to command the spacecraft (uplink) at long ranges in the event something goes awry. One factor that works in these missions' favor is that in most failure modes it remains usually straightforward to get some part of the spacecraft pointed at the Sun. Usually (but not always) this part is the main antenna boresight axis. Stacked on this axis are often a high-gain antenna (HGA), a medium gain antenna (MGA), and a low-gain antenna (LGA). The low-gain antenna is typically useful only well within 1 AU from Earth (1 AU = 150 million km, or 1.5E8 km, the average distance of the Earth from the Sun). The high-gain antenna requires tight pointing to place its beam on the Earth, which means we have to know where Earth is to get a command in, which would be a problem if we lost knowledge of time on-board (that happens)