In , Dr. W.H. Pickering called attention to the possibility that undiscovered small natural earth satellites might exist. In 1952, after a long period of searching for trans-neptunic planets and "lost" asteroids, during which the planet Pluto was found, Dr. Clyde W. Tombaugh began a search for small satellites which might be in circular geocentric orbits having radii between 5,000 and 26,000 mi.
In searching for small, high-velocity bodies having a luminance close to the photographic threshold, it is essential to avoid "trailing"; that is, the image must be kept stationary with respect to the film. For example, if a star image 0.04 mm. in diameter trails over the emulsion for a distance of 10 mm., its brightness at any point will be diminished in the ratio 0.04/10.0 = 1/250 times. The resulting trail image may be below the film's threshold. Therefore, Dr. Tombaugh's experimental method was based on searching the surfaces of a large number of spherical shells, each concentric with the earth. The angular velocity of the search in each shell was made equal to the angular velocity a body moving in the gravitational field of the earth would have at a geocentric distance equal to the radius of that shell. (Tombaugh, 1959).
| State | O | NI | NO | total | NC | M | CI | II | conclusively astronomical | inconclusively astronomical |
|---|---|---|---|---|---|---|---|---|---|---|
| S Dak | 0 | 0 | 0 | 5 | 2 | 0 | 3 | 2 | ||
| Neb. | 4 | 6 | 0 | 10 | 8 | 1 | 1 | |||
| Kans. | 3 | 9 | 1 | 14 | 9 | 1 | 0 | 3 | 1 | |
| Mo. | 10 | 7 | 2 | 27 | 17 | 1 | 3 | 3 | 3 | 2 |
| Iowa | 3 | 2 | 1 | 11 | 8 | 0 | 2 | |||
| Ill. | 11 | 16 | 5 | 33 | 27 | 0 | 1 | 1 | ||
| Okla | 3 | 3 | 3 | 14 | 9 | 0 | 2 | 2 | ||
| Total | 34 | 43 | 12 | 114 | 80 | 2 | 4 | 14 | 4 | 8 |
| % Total | 30 | 38 | 11 | 70 | 2 | 4 | 12 | 100 | 67 |
The minimizing of trailing permitted the recording of images down to the Mpg = +15 in a 2 min. exposure. A dark rock, four feet in diameter, having a reflectivity equal to that of the moon, at a geocentric distance of 26,200 miles, would produce an image of this photographic magnitude.
The project was terminated at the end of June 1956. The number of concentric shells searched was over 100, resulting in a collection of 13,450 photographs. A few dozen possible natural satellite images having photographic magnitudes lying between +16 and +14 were found and attempts were made to recapture them by repeatedly photographing the shells in which they occurred, but with no success. The conclusion is that these images were either film defects, very small asteroids in elliptical orbits around the sun, or natural satellites in elliptical, rather than circular, orbits around the earth.
As a by-product of this project, a search for moon satellites was made during the lunar eclipse of November 1956. Three telescopes, monitored by a sky photometer, produced a total of 25 plates, recording point images down to about Mpg = +17. Some 500 candidates were found in the region between the moon's surface and a lunicentric distance of 37,000 miles, but none survived a detailed analysis.
A program of visual observation for nearby objects at very low latitudes began at the end of 1955 and continued through 1958. The equatorial plane, at distances between 600 and 2,500 miles from the surface of the earth, was searched with a twelve inch Newtonian reflecting telescope and 10 X 80 binoculars. The telescope had a limiting visual magnitude of +11 at 100 miles and +13 at 2,400 miles, while the binoculars could detect objects of MV = +8 at 100 miles and of MV = +9 at 2,800 miles. No satellites were seen. In the words of the report:
It is most unlikely that any objects larger than two feet in diameter at an altitude of 100 miles or twenty feet at 2,500 miles as seen by binoculars, and several inches at 100 miles or three feet at 2,500 miles as seen by the telescope existed ... during 1956, or that any natural objects have since entered these regions.
The method used by Dr. Tombaugh, while admirably suited to orbiting bodies, is not appropriate for the observation of aerial phenomena that are not constrained in circular orbits. If their distances from the cameras were large they would not be detected due to the effect of trailing. For this reason a search on satellite survey films for reported UFOs was not attempted.