Basic Solar Sail Mechanics

Introduction

The purpose of this paper is to show how a slow to moderate performance solar sail can travel closer or farther away from the sun. The mechanics of the situation are greatly simplified for the purpose of showing people new to solar sails and/or orbital mechanics how a force directed away from the sun can bring an object closer to it.

Solar sails receive a thrust normal to their sail surfaces from reflected sunlight. There are two components to this pressure. One component acts tangential to the sun, and the other acts radially outward from the sun. For this document, low to moderate performance sails are assumed, because they have accelerations from sunlight far less than the sun's gravity. Faster sails will have much more capability than will be shown here.

Objects in orbit around the sun are perpetually falling due to residual angular momentum imparting at the formation of the solar system. In order to make an object travel away from the sun by expanding its orbit, angular momentum must be added to the spacecraft by accelerating the spacecraft in the direction of the orbit. Moving closer to the sun by shrinking the orbit is accomplished by accelerating the spacecraft in the opposite direction of the orbital motion.

Traveling to Locations out from Earth

To reach orbits farther away from the sun, such as that of Mars, a sail needs to orient itself so that the normal of the sail points at an angle forward along the orbital path. This produces a force component in in the direction of the sail's orbit which pushes it forward and raises the orbit away from the sun.

Traveling to Locations in from Earth

To reach orbits closer to the sun, like Mercury or Venus, a sail points its normal back along the orbit. This creates a force component back along the sail's path which pushes the spacecraft back and lowers the orbit.