ABSTRACT Spacecraft propulsion by reflection of beamed particles or photons represents a highly efficient means of achieving high velocities for interstellar spacecraft because the reaction mass and energy source do not have to be carried by the spacecraft. For reasonable levels of acceleration, the distance between the source of the beam and the spacecraft–on the order of hundreds to a few thousands of Astronomical Units by the completion of acceleration–creates the practical difficulty of ensuring that the beam reaches its target.
However, if the beam particles could steer themselves to a cooperating beacon on their target, this difficulty would vanish. The development of sub-micron technology gives hope that extremely small particles, bacteria-sized down to a few billions of atoms, might be made smart enough to perform their own course corrections. Anticipating that such homing systems would need to be as simple as possible, an algorithm for spinning particles was developed which requires only some means of photon absorption, energy storage, proportional circuitry, pulse triggering, and a flip flop.
A two-dimensional model of its performance indicates that a beam of particles a hundred kilometers wide might be focused down to a hundred meters or less over an astronomical unit, using light emitting diodes for attitude thrusters and single atom ejectors for cross-range velocity changes. In addition to forward momentum, reaction mass for deceleration and high density information might also be delivered by beams of such particles.
Given that relativistic velocities for interstellar probes may become accessible to humanity within a century, using scientific knowledge we already have, we would expect to find evidence of other civilizations which may have passed through a stage of industry and exploration similar to our own millions if not billions of years ago. Fermi’s question of “Where are they?” thus becomes all the more significant.
View for download the PDF with the entire article
