The ultraviolet photometer investigated the ultraviolet reflective properties or emission from interplanetary hydrogen, helium and dust, and from Jupiter's atmosphere and satellites. It was hoped to provide data to resolve the origin of interplanetary neutral hydrogen and establish the boundaries of the heliosphere. From measurements of the interplanetary helium, experimenters hoped todetermine the percentage of this gas in the interstellar medium.
The ultraviolet photometer had a fixed viewing angle and uses the spin of the spacecraft to scan around the celestial sphere. When close to Jupiter, the photometer scaned the medium above the cloud tops. By measuring the changes in the intensity of ultraviolet light reflected into two photocathodes of the instrument -- one measuring radiation at 1216 Angstroms, the other at 584 Angstroms -- thephotometer detected light from hydrogen and helium respectively.
Within the Jovian system the photometer measured the scattering of solar ultraviolet light by the atmosphere of Jupiter. This scattering provided information on the amount of atomic hydrogen in Jupiter's upper atmosphere, the mixing rate of Jupiter's atmosphere, the amount of helium there, and therefore the ratio of helium to hydrogen at Jupiter. Prior to Pioneer 10's mission helium had notbeen identified at the planet.