An OpEd in this morning's NYTimes got me to thinking about the problem of non-intersecting orbits. https://www.nytimes.com/2018/03/19/opinion/space-race-regulation.html Avoiding Collisions in Outer Space By Yousaf Butt March 19, 2018 ----- There are multiple issues here, both practical & theoretical; I'm curious about the theoretical issues. In particular, I'm curious about satellites orbiting a perfectly spherical Earth (no gravitational anomalies), with no atmospheric drag, and no gravitational influences from the Moon. These conditions are intended to make satellite orbits as periodic as possible, so that hopefully zero fuel is required to avoid collisions. Most of you probably know that airplanes are restricted to various altitudes for various directions in order to cut down on the possibilities of "intersections". Furthermore, airplanes often use a "highway in the sky", wherein they follow one another at a fixed distance, just as if they were constrained to a physical roadway. But a simple altitude-sorting strategy won't work for satellites, even for those in perfectly circular orbits, because different circular altitudes produce different periods. The highway-in-the-sky approach works more-or-less for satellites in exactly the same orbit, but keeping them equidistant may require active maneuvers and hence fuel. But the biggest problem is that altitude-sorting rules out interesting orbits -- e.g., transfer orbits and Molniya orbits. (Molniya orbits are highly eccentric orbits used for satellite broadcasting at high latitudes.) So-called satellite "constellations" enable a number of satellites *under common control* to orbit without intersections, but it isn't clear whether every satellite needs or wants to live within the restrictions of these mostly circular orbits. https://en.wikipedia.org/wiki/Satellite_constellation There must be some incredibly beautiful orbit constellations left to discover -- e.g., periodic constellations of *elliptical orbits* s.t. communications satellites can periodically get close enough to one another for exceedingly high bandwidth communications. But if some international agreements prematurely rule out such constellations, then we may never be allowed to use them.