Using extensive light measurements made by the Pioneer 10 and 11 spacecraft, a NASA scientist has produced "celestial constants" that will be highly useful to astronomers and physicists. The new constants are the first "pure" measurements of the various kinds of background light in our solar system, galaxy and universe.
Work conducted by Dr. Gary Toller, Goddard Space Flight Center, Greenbelt, Md. and General Sciences Corp, Laurel Md., indicates that background light from beyond the solar system is made up of approximately 82 percent light from faint stars. Most of the remainder is galactic light diffused by dust, the final proportion, less then 0.6 percent of background light, originates beyond the galaxy.
Since much of the knowledge of the universe comes from visible light, the data will provide a benchmark in many fields of astronomy and physics. The Pioneer 10 and 11 photo polarimeter measurements have provided the first observations of incoming light without interference of solar system light. The Pioneers are managed by NASA's Ames Research Center, Mountain View Calif., for the agency's office of Space Science and Applications.
The new work combined with other measurements also provides a clue to chemical composition of solar, galactic, and cosmic dust. It gives an accurate measure of the Sun's position above the plane of the galaxy (about 12 parsecs). It describes how cosmic dust scatters light. For the entire celestial sphere, 60 percent of light is scattered, not absorbed, predominantly in the same direction it had been traveling in.
Toller has used his data as another way to calculate total amounts of visible matter in the universe. These calculations confirm other estimates that 90 percent of matter in the universe is missing or unseen dark matter.
Toller and others used a variety of observations from Earth for the analysis, combining data on the quantities of stars and types of stars with computer models of light scattering in the galaxy, amounts of dust and gas and size of particles. Then he compared these models to measurements made by the Pioneers as the two spacecraft moved out of the solar system.
The new data will help investigators study diffuse celestial light sources such as zodiacal light which reaches earth after being reflected by nearby dust. For an astronomer on earth, looking in a random direction in space, 40 percent of incoming light is zodiacal light.
Once the Pioneers were beyond 300 million miles, the zodiacal light diminished to a negligible level and scientists were able to make the first pure measurements of background light from beyond the solar system in the mid 1970's. Since then the long flight paths of the Pioneers have made it possible to make very exact measurements of this outside light.
Background light from beyond the solar system breaks down into integrated starlight from stars too faint to be seen by the eye, diffused galactic light reflected by dust particles in the galaxy and light coming from outside the galaxy.
Toller who reported his work at an international conference on galactic and extragalactic background radiation in Germany earlier this year is continuing to refine and apply the data.