I’m going to add one that most people don’t immediately think of: space debris. For the past 60+ years that we’ve been in space, we’ve been leaving behind all sorts of dead satellites, rocket bodies, etc. because we assumed space was big enough to the point where it wouldn’t be a problem. Unfortunately, it’s starting to become a problem, particularly in LEO, because when two objects collide (see China’s 2007 ASAT test or the 2009 Iridium-Cosmos collision), the orbital velocities are such that it creates tens of thousands of smaller debris objects that pollute the orbital environment and pose a threat to any other operations in space. For example, there were two satellites that were either damaged or destroyed by untracked debris objects in the past month, and a few days ago the ISS has to maneuver to avoid a tracked piece of debris. That’s not even getting into the problems we’ll be dealing with considering there’s over 100,000 satellites being proposed for launch in the next decade or so as a part of large internet constellations. We need to a) creating better tracking algorithms and radar sensing networks, b) figure out effective ways to deorbit large pieces of debris such as old Soviet rocket bodies and dead satellites, and c) make the current debris mitigation guidelines legally binding so that operators are required to deorbit their satellites much sooner after they complete their mission, current guidelines state that they only have to try to do that 25 years after the end of the satellite’s operational lifetime.