Citation
Abstract
The umbra of a planet may serve asa sun shield for a space-based optical communications terminal or for a space-based astronomical observatory. An orbit that keeps the terminal or observatory within the umbra is desirable. There is a corevolution point behind every planet. A small body stabilized at the planet corevolution point will revolve about the Sun at the same angular velocity as the planet, always keeping the planet between itself and the Sun. This corevolution point is within the umbra of Mars but beyond the end of the umbra for Mercury, Venus, and Earth. The Mars corevolution point is an ideal location for an astronomical observatory. There Mars obstructs less than 0.00024 percent of the sky at any time, and it shades the observatory completely from the Sun. At the Earth corevolution point, between 51 percent and 84 percent of the solar disk area is blocked, as is up to 92 percent of the sunlight. This provides a reduction from 3 dB to 11 dB in sunlight that could interfere with optical communications if scattered directly into the detectors. The variation is caused by revolution of the Earth about the Earth-Moon barycenter. Corevolution is also possible closer to the planet than the corevolution point, but only if continuous outward thrust is provided or if a tether is attached to another body located beyond the corevolution point. Corevolution close enough to the Earth to make the Earth disk appear 75 percent larger in diameter than the solar disk (in order to block sunlight at any position of the Earth and Moon) is practically unobtainable, since it would require a tether 900,000 km long, or 5.6 newtons (1.3 pounds) of continuous outward thrust for a 10,000-kg terminal or observatory.
Details
- Volume
- 42-91
- Published
- November 15, 1987
- Pages
- 133–140
- File Size
- 384.9 KB