3HIGH PERFORMANCE FLAT-TOP ULTRA NARROWBAND
4FASTEST CUSTOM COATING TURNAROUND
1HIGHEST PERFORMANCE THIN FILM COATING
2ULTRA SERIES BANDPASS, DICHROICS, AND
SPACE AND ASTRONOMYcontinued
mission). Tilting, however, lessens the power and, hence, the
force on the sail by 20%. Roughly 75% of the available electromagnetic momentum may be converted into the tangential component.
Finally, for a reflective sail, the photons are generally used
only once—that is, they are either reflected back into space or
absorbed in the metal.
Advantages of using a diffractive surface
In principle, these disadvantages may be addressed by replacing the reflective coating with an active diffractive surface. The rainbow-like spectral dispersive properties of diffraction gratings are familiar to anyone who takes notice of
the color separation of light scattered from the surface of a
Recent advances in grating design and manufacturing provide
various means to efficiently diffract light into a diffraction order, and active control has been demonstrated. A transmissive
diffraction grating is particularly attractive for a lightsail because the diffracted photons may be used again and again. For
example, some transmitted light may be converted to solar-elec-tric power, with the rest diffracted again by a second grating to
impart additional momentum to the sailcraft. Heating of the
sail material is less of a concern because a transmissive diffractive sail may absorb very little light.
Tilting of the sailcraft may also become unnecessary, since
a diffraction grating naturally redirects photon momentum
even when the grating is directly facing the sun (see Fig. 2).
This is understood by use of the well-known grating equation
that relates the diffraction angle to the ratio of the wavelength
and grating period. For a first order grating, the diffraction
angle can approach 90° as the ratio approaches unity. Unlike
a reflective sail, a grating may therefore convert up to 100%
of the electromagnetic momentum into the desired transverse
component to raise or lower an orbit. Maintaining a high efficiency over a band of solar wavelengths where the momentum is greatest (for example, 250–750 nm) is a topic of further research.
Achieving an optimal trajectory is part of the art called
“rocket science.” In principle, the momentum-transfer efficiency components must be varied throughout portions of a
spaceflight to achieve desired objectives. As suggested above,
such changes require attitude adjustments of a large reflective sail. However, an active diffractive sail may maintain the
sun-facing orientation to provide a large force, while changing the efficiency components with nonmechanical means.
One way of doing this is by the construction of an array of
individually controlled gratings. A simple binary control
scheme may be envisioned whereby each grating is switched
from the + 1 to - 1 order, in effect providing a varying tangential efficiency.
As materials scientists develop space-hardened arrays
of lightweight active diffractive sheets, the attraction of