space@mit-mc (03/12/85)
From: FIRTH@TL-20B.ARPA Most of the information about light sails has been said, but you might like a short summary of how to get around with them. First, the propulsive force is created almost entirely by the reflection of light from the sail. Assuming perfect reflection, this force is normal to the sail (by Heron's law), and can to a good first approximation be calculated classically. Secondly, a sailship in the vicinity of a star can use two forces: gravity, directed always radially towards the star, and light, directed normally to the sail, but of course never towards the light source (the star). Whenever you have two forces acting in different directions, you can tack. If you can tack, you can go anywhere. Consider a typical initial state, of a ship in circular orbit, counterclockwise when viewed from local North, oriented in the direction of motion, ie with the star on the port beam. Do nothing; keep those sails furled; by Newton's First Law you will remain in stable orbit. Now unfurl the sails perpendicular to the ship, ie deployed port - starboard. Nothing happens, for the simple reason that the sail is edge-on to the light. So deploy the sail fore and aft, and the light will exert a force directly outward from the star. This pushes you away; unfortunately you gain potential energy but not angular momentum, so your orbit becomes elliptical, with a greater apastron distance but shorter periastron distance. Now cant the sail so the force is away and forward - away from the star and forward round the orbit. The sail now stretches from port bow to starboard quarter. The light force will give you both potential energy and angular momentum; with the correct angle you will move away from the star in a smooth Archimedean spiral. Please note that it is NOT necessary for your craft to be light enough, or big enough, for radiation pressure to overcome gravitation. Any extra force will move you away from the star, in time. Note also that a sail configuration that works close in will also work far out (in the classical approximation) since both forces weaken equally with distance. Finally, the manoeuvre that separates the women from the girls - moving to an INNER orbit. You cannot do that just by canting the sail the other way. Remember that every configuration has a component away from the star, that tries to increase potential energy. Three steps are necessary. First, deploy the sail so as to reduce angular momentum, ie starboard bow - port quarter. There will be an inevitable outward thrust also. Secondly, wait. Your orbit will have become elliptical, and you will gradually fall towards the star, trading potential for kinetic energy. Finally, near periastron, deploy the sails again, similarly. The light force will push you away, but also reduce your angular momentum; if all has gone well, you will be in a circular orbit closer than before. If you made a mistake, your sails melt. Unfortunately, there are some technical difficulties. Those sails are BIG, and moving them around is not easy. Moreover, when you are close to a star, there are gravitational tidal effects that cannot be ignored. If you doubt that, go to the seacoast of your choice and watch the water slosh about! Many proposed sail designs (eg "aluminised nothing") would tear apart even in Earth-Trojan orbit. Robert Firth -------