[106] Part II - Student Experiments   Chapter 9: Celestial Objects in Space.

[107] In the 18th century, Joseph Lagrange, the French 1 astronomer, theorized that in a three-body system of celestial objects there exist points at which all gravitational forces on the third body cancel out. These Lagrangian or libration points, then, offer locations in which interplanetary matter might be found.

Two Skylab student investigators were interested in exploring the possibilities of observing such locations. One student wanted to search for the existence of matter which might accumulate in the stable Lagrangian points of the Earth-Moon system. Another sought to verify or disprove the theory that there is a planetary body orbiting the Sun closer than the planet Mercury.

Lunar Libration Clouds

Lagrange had shown that small particles could be placed at five points where gravitational and centrifugal forces acting on the particles would balance almost exactly in the Earth-Moon system. While forces at three of these points are not stable, balanced forces acting at the other two points could trap any small particles. An accumulation of these small particles in these regions is called a "libration cloud."

Alison Hopfield of Princeton Day School, Princeton, N.J., proposed an observation of the region of Lagrange points L₄ and L₅ of the Earth-Moon system by photographing them with the aid of Skylab solar observatory's white-light coronagraph. This instrument was designed to produce artificial eclipses of the Sun by means of circular disks centered on the instrument line of sight and blocking the bright Sun. The region thus seen by the coronagraph was the outer edges of the Sun's total atmosphere or corona. She felt that any particles in the libration points would be identi-.....

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[108]

Alison Hopfield, seen below right with science adviser Harry Coons, felt that interplanetary dust might be detected as clouds reflecting sunlight at Lagrangian points 4 and 5. She suggested that photographs from Skylab might reveal them.

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....-fiable only by the scattering of sunlight striking them.

The Skylab investigators with whom Ms. Hopfield was affiliated utilized the forward scattered light from material in the libration regions, since they observed the regions when they were near the Sun. At smaller scattering angles, the brightness from any small particulate clouds in these regions would be enhanced over that seen at the large angle.

The observations of the libration clouds reported by the OS0-6 satellite indicated that the clouds are of an angular size of approximately 6 degrees (compared to the 3.2-degree field of view of the Skylab instrument). The coronagraph's field of view was such that the predicted location of the libration points could not be included. The areas investigated were generally 1 to 3 degrees away from the libration points, although in at least one observation the coverage was within 1 degree of....

....the position predicted from the OS0-6 observations.

Ms. Hopfield spent the summer of 1974 at the High Altitude Observatory, in Boulder, Colo., helping analyze the coronagraph photographs. No dust clouds could be distinguished against the solar coronal background. An upper limit to the libration cloud radiance of 2.5 x 10^-11 of the mean radiance of the solar disk was determined. When this upper limit was combined with past measurements of the libration region backscattered radiance, certain candidates for the nature of the interplanetary dust could be eliminated. It was calculated that the radiance contrast of a possible libration cloud composed of remaining candidate materials to the background zodiacal light would be maximized at about 30 degrees from the Sun (which was not within the field of view of the coronagraph). Thus, the observations serve as a guide for future ground or spaceborne instruments which will examine the nature of the libration regions.

Objects Within Mercury's Orbit

By the mid-19th century astronomers understood the motion of the planets of the solar system well enough that any pertubations between observed orbits and calculated ones had to be caused by some gravitational force. Such perturbations in the orbit of Mercury caused many astronomers, notably Jean Joseph Laverrier, in Paris, to assume....

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[110]

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Daniel C. Bochsler felt that pictures from Skylab might answer the centuries-old question of whether there is an object in orbit between Mercury and the Sun. Below, he discusses the Skylab program with science adviser John Humphreys.

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.....that there must be another planet (or asteroid belt) between it and the Sun. He even proposed the name "Vulcan" for the planet. For many years the search for Vulcan went on, but there was never any positive observational proof of its existence.

A Skylab experiment to look for possible bodies in orbit between Mercury and the Sun was proposed by Daniel C. Bochsler of Silverton Union High School, Silverton, Oreg. The Skylab whitelight coronagraph provided the capability, and it took thousands of frames of film of the near-Sun region. Analysis of these photographs revealed no new objects other than Comet Kohoutek (1973f). It is theoretically possible that one or more extremely small objects could be orbiting very near the Sun but were too dim for the coronagraph to detect.

While Bochsler's experiment was not conclusive,....

....his results strongly suggest that Vulcan does not exist, since the proposed planet should have been easily visible several times during the Skylab mission. Indeed, the results indicate that no object larger than 104 miles in diameter is orbiting the Sun inside Mercury's orbit.

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