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SUMMARY OF PRINCIPAL FINDINGS l. Study of the outer solar system offers a rich field for the discovery of new phenomena, for the development of new concepts, and for definitive solution of long-recognized problems in physics, astronomy, and chemistry. 2. In situ observations by deep-space missions offer great advantages in directness, sensitivity, and resolution. 3. In many areas such as planetary magnetism, physics of the interplanetary medium, physics of nonthermal radio sources, and composition of deep planetary atmospheres, there is no known alternative to the techniques of direct observa- tion near, or within, the object of investigation. 4. Considerable advances in our knowledge of the outer solar system can also be made by use of earth-based radio, radar, and optical telescopes, and of equipment flown on rockets, balloons, aircraft, and earth-orbiting satellites. 5. The successful conduct of flight missions during the next two decades to all the outer planets and, indeed, to heliocentric radial distances of the order of l00 astronomical units is well within the technological capabilities of the United States. 6. Chemical propulsion systems, such as the Titan HID/ Centaur/Burner II launch vehicles, and radioisotope thermal generators are adequate for a large variety of flyby, orbit- ing, and probe missions, as well as missions involving solar- captive orbits in planes at inclinations up to 90° to the ecliptic and trajectories that escape the solar system. Ad- vanced propulsion schemes, ion propulsion, for example, may be important for certain specialized missions such as prolonged flight within the asteroid belt. 7. Exceptionally favorable astronomical opportunities occur in the late l970's for multiplanet missions. Equally favorable opportunities will not recur for approximately l80 years, although there will be some multiplanet opportunities in the period l989 to l996. 8. Professional resources for full utilization of the outer-solar-system mission opportunities in the l970's and l980's are amply available within the scientific community,
and there is a widespread eagerness to participate in such missions. 9. A vigorous national program of exploration of the outer solar system can be mounted for a small fraction of the total cost of the program of the National Aeronautics and Space Administration.
