Stamatios Mike Krimigis, low energy charged particle Principal Investigator
The second LECP subsystem is a low energy charged particle telescope, designed to operate where the density of charged particles is low, such as in interplanetary space or the outer magnetosphere of Jupiter. For protons and positive ions, the energy range is from 50 keV to 40 MeV per nucleon. The energy and species resolution is again sufficient to determine the composition, both chemical and isotopic, of many ions encountered. In order to provide directional discrimination even on a spacecraft of fixed orientation, both LECP subsystems are mounted on a moving platform that steps through eight positions in a time that can be commanded to vary from 48 seconds to 48 minutes. The mass of the instrument and its platform is 6.7 kilograms.
Cosmic Rays
The solar system is constantly bombarded by extremely energetic charged particles. These are called cosmic rays, although they are particles, not photons—“rays” are only produced when the particles strike something, such as the molecules of the Earth’s atmosphere, and give up their energy in a flash of x-rays and gamma-rays. One of the Voyager instruments is designed to study these galactic cosmic rays, particularly to look from beyond the orbit of Saturn, where the cosmic ray particles will be less affected by the solar magnetic field and solar wind than they are near Earth.
The cosmic ray Principal Investigator is Rochus E. Vogt of the California Institute of Technology. Vogt has measured cosmic rays from the ground, from balloons, and from spacecraft for many years. During 1977 and 1978 he served as Chief Scientist at JPL, and then assumed the job of directing the physics, mathematics, and astronomy programs at Caltech. Among his six Co-Investigators is Ed Stone, the Voyager Project Scientist.
Because the cosmic ray instrument was not directed principally toward measurements of the Jovian system, it is described only briefly. Like the LECP, it is designed to determine the energy and composition of individual electrons and positive ions. For electrons, the energy range is from 3 to 110 MeV, and for ions from 1 to 500 MeV per nucleon; the corresponding velocities are from about 10 percent to 99 percent of the speed of light. For the positive ions, composition can be determined for elements from hydrogen to iron. At Jupiter, this system could be used to determine the nature of the rare particles accelerated to very high energies in the Jovian magnetosphere.
Radio Science
The final Voyager science investigation is in the field of radio science. No special instrument was required for this study; rather, NASA selected members of a Radio Science Team who proposed investigations that could be carried out using the already existing spacecraft telecommunication system.
The radio science Team Leader is Von R. Eshleman of the Center for Radio Astronomy at Stanford University. Eshleman is a radar physicist who has been interpreting spacecraft radio occultation data since the first such probe was carried out when Mariner 4 passed behind Mars in 1964. The Deputy Team Leader is G. Leonard Tyler, a colleague of Eshleman’s at Stanford. There are five other radio team members, four of them from JPL.
The radio science investigations are divided into two groups. The first deals with the atmosphere of Jupiter. During the Voyager flybys, the spacecraft passed behind the planet as seen from Earth, and the radio signal was dimmed by the atmosphere before it was finally extinguished. During an occultation, the propagation of the radio waves is slowed down by passage through the neutral atmosphere and is speeded up by passage through the electrically charged ionosphere. Because of the extreme stability of the ground-based and spacecraft radio transmitters, it is possible to measure these shifts in the signal with high precision. The shifts are proportional to electron density for the ionosphere, and to gas density for the atmosphere. From a careful study of the interactions of the transmitted beam with the Jovian atmosphere, Eshleman and his colleagues can reconstruct a temperature-pressure profile of the ionosphere and the upper atmosphere of Jupiter. The same approach can be used to search for tenuous atmospheres on the satellites.