  The exponential rise in small-satellites and CubeSats in Low Earth Orbit
(LEO) poses important challenges for future space traffic management. At
altitudes of 600 km and lower, aerodynamic drag accelerates de-orbiting of
satellites. However, placement of satellites at higher altitudes required for
constellations pose important challenges. The satellites will require on-board
propulsion to lower their orbits to 600 km and let aerodynamic drag take-over.
In this work we analyze solutions for de-orbiting satellites at altitudes of up
to 3000 km. We consider a modular robotic de-orbit device that has stowed
volume of a regular CubeSat. The de-orbit device would be externally directed
towards a dead satellite or placed on one by an external satellite servicing
system. Our solutions are intended to be simple, high-reliability devices that
operate in a passive manner, requiring no active electronics or utilize
external beamed power in the form of radio frequency, microwave or laser to
operate. Utilizing this approach, it is possible for an external, even ground
based system to direct the de-orbit of a spacecraft. The role of an external
system to direct the de-orbit is important to avoid accidental collisions. Some
form of propulsion is needed to lower the orbit of the dead satellite or
orbital debris. We considered green (non-toxic) propulsion methods including
solar radiation pressure, solar-thermal propulsion using water steam,
solar-electrolysis propulsion using water and use of electrodynamic tethers.
Based on this trade-study we identify multiple solutions that can be used to
de-orbit a spacecraft or orbital debris.
