Planetary Exploration

The emphasis at NASA in the early 1960's was on manned lunar flights. But the Jet Propulsion Laboratory and other groups had already made extensive general studies of the ballistics of flight to other planets -- especially Venus and Mars. The interest in Mars was driven by the desire for geological studies of its surface and, perhaps more importantly, by the desire to search for any form of biological activity there. Also Mars and Venus were ballistically much easier to reach than was Mercury or the outer planets. The first planetary target to be adopted by JPL/NASA was Venus. I proposed a simple radiation detector for the first mission with the purpose of searching for the existence of a Venusian radiation belt and the consequent inferences on the magnetization of this planet, then completely unknown. My instrument was selected and was incorporated into the payload of Mariner I, an early in-flight failure, and of Mariner II, launched successfully on 27 August 1962. The cruise phase was quite successful, yielding, most importantly, the first continuous measurements of the solar wind by Conway Snyder and Marcia Neugebauer, the interplanetary magnetic field by Paul Coleman et al., and the detection of numerous solar energetic particle events by my apparatus and a companion instrument of Hugh Anderson and Victor Neher. Mariner II passed by Venus on 14 December 1962 at a radial miss distance of 41,000 km. In our measurements there was not the slightest indication of the presence of the planet, thereby implying an upper limit on its magnetic moment as 0.18 that of the earth, its "sister" planet. A casual, and perhaps even correct, interpretation of this result is that Venus is simply rotating too slowly (period 243 days) to drive an internal self-excited dynamo.

I had an improved model of the radiation instrument on Mariner III (launch failure) and Mariner IV (launched successfully on 28 November 1964) which made the first ever encounter with Mars on 15 July 1965. The interplanetary data yielded a nearly continuous record of solar x-ray flares and of the presence of energetic solar particles, including the discovery of energetic solar electrons and, in stereoscopic combination with data from Explorer 35 in lunar orbit, the first determination of the altitude in the sun's atmosphere at which 2 - 10 Å X rays are emitted. At Mars as at Venus, we got a null result and inferred an upper limit on Mars' magnetic moment as 0.001 that of the earth.

Meanwhile, as a member of the Space Science Board and from 1966-70 as a member of the Lunar and Planetary Missions Board, I had adopted the role of being a special and unremitting advocate of missions to the outer planets -- especially Jupiter. The first fruits of these efforts were the adoption by NASA of two missions to Jupiter -- later called Pioneer 10 and Pioneer 11 -- with emphasis on energetic particle and magnetic field measurements. A special motivation for this emphasis was the radio-astronomical evidence that Jupiter has a huge radiation belt whose population of relativistic electrons emits the observed synchrotron radiation in the decimetric wavelength range. The Pioneer 10/11 project was managed by the Ames Research Center of NASA with a keen concern for optimizing the scientific yield of the mission. The spacecraft were built by the TRW Company. My proposal for an energetic particle instrument was accepted, after some reduction in its scope, during the vigorous national competition for payload space. Pioneer 10 was launched successfully on 3 March 1972 and Pioneer 11 on 6 April 1973. For the subsequent l7 years, the in-flight data from these two spacecraft have been a central part of my research life and that of several of my students and associates. Pioneer 10 made the first ever encounter (December 1973) with Jupiter and yielded a large body of new knowledge, most especially on its magnetosphere. Pioneer 11 encountered Jupiter a year later along a different trajectory and confirmed and substantially expanded the earlier findings. Pioneer 10 has continued on an escape trajectory out of the solar system and, at the date of writing, is about 47 AU from the sun (nearly 7 billion km) -- the most remote man-made object in the universe -- still working well and providing daily data on cosmic rays and the interplanetary medium in the outer heliosphere. After its close encounter with Jupiter, Pioneer 11 made the first ever encounter with Saturn in September 1979, discovering its magnetosphere and yielding a rich body of new information on the planet itself and its system of rings and satellites. This spacecraft is now also on a solar-system-escape trajectory at a current distance of over 28 AU and it too is transmitting data on a daily basis.

In the late 60's and early 70's a Grand Tour of the Outer Planets was being advocated by the Jet Propulsion Laboratory, in particular, and by other planetary enthusiasts who were advising NASA on new programs. JPL had shown that the forthcoming configuration of the outer planets Jupiter, Saturn, Uranus, and Neptune (a once-in-179-year phenomenon) would make it ballistically feasible to have a single spacecraft fly by all four of these remote planets. The Grand Tour, as such, was a budgetary casuality of late 1970. Soon, thereafter, I was asked by JPL to chair a Science Working Group to develop a more modest-sounding mission, tentatively called MJS (Mariner/Jupiter Saturn). The two-spacecraft mission that we developed was eventually approved and came to life in 1974. It was later renamed Voyager. Although the term Grand Tour was now eschewed in polite conversation, it did not escape our attention that the configuration of the outer planets was independent of budgetary-political considerations in the White House and the Congress.

The successes of the two Pioneer missions produced a greatly enhanced interest in the Voyager missions, as well as in ground- based study of the outer planets. Competition for payload space brought forth a wealth of proposals of new and sophisticated instruments and eventual selection of an excellent complement.

Both Voyagers were launched successfully in the late summer of 1977. Each flew by Jupiter and Saturn and provided great advances, most notably in high-resolution imaging of the atmospheres of the planets and their satellites and rings and the plasma-physics of their magnetospheres. Since its Saturn encounter, Voyager 1 is on a solar-system-escape trajectory, but Voyager 2 made the first ever encounter with Uranus in early 1986 and is now approaching Neptune for a 25 August 1989 encounter -- thus prospectively achieving the objectives of the Grand Tour as visualized at the outset of this program.

I have had no part in the execution of the Voyager program but have been a guilty bystander, so to speak, and one of its enthusiastic fans.

In 1976-77, I chaired still another JPL/ARC science working group, called JOP/SWG (Jupiter Orbiter with (Atmospheric) Probe/Science Working Group). Our purpose was to develop a follow-on Jupiter mission of more advanced capability than the Pioneers and the Voyagers, with a deep atmospheric entry probe and an orbiter having a useful lifetime of at least two years in contrast to the limited period (days to weeks) of nearby observation available on a fly-by.

The mission, renamed Galileo, has suffered a plethora of delays -- financial, political, and technical -- principally as the result of the inadequacies and defaults of the shuttle launching system which had been adopted by NASA in the late 1970's and 1980's. The launch of Galileo is now scheduled for October 1989 but many uncertainties remain. Also because of the less-than- originally-planned capability of the shuttle, it has been necessary to adopt an ingenious but very long flight path to Jupiter, requiring over six years vs. the twenty-month flights of Pioneers 10/11. I recognize that the probability of my own survival to 1995 is substantially less than unity. Nonetheless, I still hope to function in my interpretative role as an Interdisciplinary Scientist beginning after Galileo's scheduled entry into orbit around Jupiter in that year.