Inspired by Charles Townes, the inventor of the laser, Young K. Bae, CEO of Bae Institute, has devised a scheme for ultra-large space platforms consisting of tethers with typical dimensions from 100 meters to 10 kilometers. Bae believes that his concept could enable a "revolutionary/disruptive technology in the astronomical dimension" to increase telescope, radar and detector resolution by two to four orders of magnitude. He calls his concept Photon Tether Formation Fight (PTFF).

Others have proposed formations of free flying components to increase resolution. Almost a dozen formation flying mission concepts have emerged. They vary in size and in the accuracy with which the shape of the formation can be controlled. One can view dimensions and attainable accuracies associated with these concepts using the Archive of Speaker Presentations at Chapter Meetings "button" on our AIAA Enterprise Network home page. None of the others can match the control accuracy of the PTFF, a "disruptive" breakthrough technology which Bae claims to be capable of a baseline dimensional accuracy within one nano-meter, and a pointing accuracy within 0.1 micro-arcsec (1 arsec = 1/3600 of a degree).

How does Bae do it? A wide variety of constellations can be assembled, depending on the number and position of free flying detector satellites. The satellites are connected to each other by tethers. The formation is kept in the desired configuration by balancing tension forces tending to contract the tethers between the satellites with laser beam photon thrust forces tending to push the satellites further apart and stretch the tethers. In addition to the tether and photo thruster systems, there is a system for ranging.

The tether system consists of a reel for coarse control (mm) of tether length, an inchworm or stepper motor for fine control (µm), and a piezo translator for very fine control (nm). The proper tension in the tether is maintained by the photon-thruster system, which consists of lasers and high-resolution mirrors on each of the satellites providing two-way laser beams and ultrahigh precision photon thrust. The photons are bounced between the spacecraft at extremely rapid rates, a process which amplifies the photon thrust. Nonetheless we are still speaking of very small forces, typically tens or hundreds of micro newtons (1 newton ~ .227 pounds), and very small deflections measured in nano-meters. The ranging system is the result of dual usage of the lasers. Using a system of high-resolution mirrors and photo-detectors on the satellites in conjunction with the lasers provides interferometric ranging and enhanced resolution at lower weight and cost than competitive systems.

Design challenges for the tether system include thermal effects depending on orientation with respect to the sun, vibrations from constellation reorientation, and meteoroid impacts. The longitudinal damping and compensation for thermal effects will be provided by the tether motors and the photon thruster modulation. Lateral vibration will be handled by electromagnetic dampers. Thermal management, atmospheric absorption and beam scattering are parameters which the laser system must deal with.

The potential uses of an effective system are very impressive. Imagine a PTFF telescope 1 km wide "hovering" at Lagrangian Point L2, offering a 300 meter resolution at a distance of 10 parsecs (~309*10^12 km). This telescope could map asteroids and civilizations supported by other stars. Or imagine a 200 m PTFF at GEO with an optical resolution of 10 cm offering a Goggle map in real time, or an ultra large syntethic aperture radar (SAR). Other uses include early warning of military or natural disaster events.

Where does an inventor with such revolutionary ideas get his sponsorship? Bae’s contract is with NIAC, the NASA Institute for Advanced Concepts. Some of you entrepreneurs out there might consider contacting them with your far-out ideas.

Guido Frassinelli 04/26/07