To Jupiter At
Last! Galileo's Longest Day: December 7, 1995On the long ride to Jupiter, six instruments are resting inside the Probe, a mere 1.25 m in diameter and 0.86 m high. A July 12 JPL press release read the probe is "packed like an interplanetary paratrooper." And so it is. During this first and brief invasion of the Jovian atmosphere, these science instruments--the atmosphere structure instrument (ASI), helium abundance detector (HAD), lightning and radio emission detector and energetic particles instrument (LRD/EPI), nephelometer (NEP), net flux radiometer (NFR), and neutral mass spectrometer (NMS)--will capture diverse sets of data. The instrument science teams gathered at the July Probe Pre-Encounter Atmospheric Science Meeting to discuss their plans for analysis and interpretation of the anticipated new information on the composition, meteorology, and structure of the planet Jupiter; they also hope to gain evidence of how the solar system evolved.
In previous issues of The Galileo Messenger, four of the instruments--ASI, NEP, NFR, and NMS--have been discussed. Now we will examine the LRD/EPI and HAD.
LRD is well equipped for the task; it was designed to take into account large uncertainties in the nature of possible Jovian lightning. The LRD hardware, funded and built by the Federal Republic of Germany, consists of three basic sensors: a radio frequency antenna and two photodiodes located behind fish-eye lenses. On the approach to Jupiter, it will take measurements at about 4, 3, 2, and 1 planetary radii above the cloud tops and then acquire data continuously during the parachute descent into the atmosphere. The radio frequency antenna will measure RF magnetic signals in the range from about 10 Hz to 100 kHz in three principal channels: a waveform analyzer for snapshots and statistics of lightning RF waveforms, a spectrum analyzer at three frequencies (3, 15, and 90 kHz), and a determination of the magnetic field/probe spin rate in one plane. There is also a single channel "superbolt" detection mode.
Meanwhile, back on Earth, the science team--PI Louis J. Lanzerotti, Co-PI Klaus Rinnert, Gunter Dehmel, Martin A. Uman, E. Philip Krider, and Fritz O. Gliem--has conducted numerous campaigns to study lightning on our planet and to "calibrate" a duplicate LRD. These results will enable a better interpretation of the Jovian data, and they have already contributed to our understanding of the ongoing electrical processes in Earth's atmosphere.
Lanzerotti graduated from Harvard with a PhD in applied engineering. He went directly to AT&T Bell Labs seeking a career with a future-the study of space. His work there on communications satellites and pure research on the ionosphere (and his association with the University of Florida) continues to be "where the action is."
Rinnert (see photo) of the Max Planck-Institut für Aeronomie commented on the strong community that has developed among the German and American members of the LRD science team, representing the universities of Florida and Arizona and two German institutes at the University of Braunschweig. Close personal ties were also established with engineering colleagues at NASA Ames, JPL, Hughes, and Dornier Systems. "I have made good friends and have had much fun in doing this investigation. Besides the rewarding science and management part of it, there has been a nice person-to-person relationship."
Four tiny moons circle inside the orbit of Io; two of these intercept the dust ring surrounding Jupiter at about 2 Jovian radii. Scientists believe that these moons and the dust ring influence the particle population between Io and the inner edge of Jupiter's radiation belt by sweeping up particles as they cross the moon's orbital path. The Probe's EPI will be the first to measure particles in this region where Jupiter's radiation belts are populated with relativistic (very fast, comparable with the speed of light) electrons that emit synchrotron radiation. In fact, observing Jupiter's synchrotron radiation has provided the only means of "seeing" into Jupiter's inner magnetosphere and is routinely monitored by the NASA/JPL Deep Space Network (DSN) radio telescopes. Data from the EPI will be compared with models based on the DSN and Pioneer observations. One recent theory concerning the synchrotron radiation suggests that an interaction between waves produced by atmospheric lightning and the relativistic electrons dictates the distribution of energetic particles in the radiation belts. The combined investigation of the LRD and EPI is well suited to test this concept, since the LRD will make some wave measurements in the magnetosphere.
Co-PI Harald M. Fischer (see photo), Institut für Reine und Ange-wandte Kernphysik, Universität Kiel, has been studying high energy particles in space since 1963; his first experiment flew on the German satellite DIAL (1969). Co-I John D. Mihalov, EPI team member at Ames, worked on energetic particle experiments for studies of the Van Allen radiation belts (1962-1965) and participated in the Pioneer flybys of Jupiter in 1973 and 1974. The EPI is a cooperative effort between the Ames Research Center and the Universität Kiel. Other science team members are Co-Is Jörg Bach, Fritz O. Gliem, Klaus Rinnert, Eckhard Pehlke, and Gerd Wibberenz.
In 1977, when the instruments were chosen, the weight of scientific opinion was that the helium abundance in the Jovian atmosphere is the same as that created in the Big Bang and present in the solar nebula from which the Sun and the planets were formed. An accurate measurement, therefore, would tell us something about conditions at the instant of the Universe's creation; but later, the Voyager results for Saturn and Uranus, as well as more detailed knowledge of Jupiter and the Sun, have all suggested that processes in Jupiter could have modified the original ratio. The accurate measurement by the HAD is now seen as telling us about the origin and evolution of the planet itself.
After its arrival in the Jupiter atmosphere the HAD will begin to admit samples of Jupiter's atmosphere near the 2.5-bar level and will make repeated measurements as the pressure increases to about 10 bars. These measurements will become progressively more accurate; about 30 of them will be transmitted to the Orbiter. Analysis of these data will be based on the simulations of the descent made in the laboratory with a high-pressure chamber also capable of realistic simulations of the temperature profile.
The HAD was adapted from a tiny commercial instrument used to measure methane in coal mines and converted into a highly precise, space-qualified instrument for the helium measurement. The principle is to measure the refractive index of a sample of Jovian atmosphere by comparing it with a standard gas mixture carried from Earth. Technically it is a folded Jamin interferometer, working at a wavelength of 900 nm. The mass is 1.4 kg and it consumes 900 mW.
Along with this small size goes a small team of investigators: the PI is Professor Ulf von Zahn (see photo), formerly of Bonn Universität and now at the Institut für Atmospärenphysik an der Universität Rostock, and the Co-I is Professor Donald Hunten (see photo) of the University of Arizona. Hunten replaces Dr. Hans Hoffmann who left Bonn Universität to join the instrument contractor. Other important members of the team and their contributions have been W. Mett, who was responsible for radiation hardening of all the instrument subsystems; W. Schulte, who developed the apparatus for laboratory simulations of the instrument descent into the Jovian atmosphere; and H. Schuetze, who performed the calibration and environmental testing. Integration of the instrument into the spacecraft and systems tests were supported by H. Schuetze, K. Pelka, and G. Lehmacher. The interferometer part of the HAD instrument was developed by Carl Zeiss (Oberkochen, Germany), the other portion of the HAD by Messerschmitt-Boelkow-Blohm (Ottobrunn, Germany).
To Jupiter At
Last! Galileo's Longest Day: December 7, 1995
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