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Solar System Exploration Today ver the last four decades, robotic spacecraft have visited nearly every planet, from torrid O Mercury to frigid Neptune. The data returned by these Pioneers, Mariners, Vikings, and Voyagers have revolutionized our understanding of the solar system. These achievements rank among the greatest accomplishments of the 20th century. Now, at the opening of the 21st, it is appropriate to ask, where do we go from here? The scientific potential is limitless; the text on page 4 highlights just a few of the mysteries our explorations have turned up to date. We could send spacecraft to roam Marsâs surface, return pieces of a comet to Earth, explore Pluto, or probe the hellish atmosphere of Venus. By making careful choices about which of those tantalizing targets are most important, we set our- selves on the path to realizing all our scientific dreams concerning the solar system. From a scientific perspective, we must consider how future missions will help answer a set of fundamental questions that reach beyond just planetary exploration: â¢ Are we alone? â¢ Where did we come from? â¢ What is our destiny? Answering these questions requires several simultaneous approaches. On Earth, theoretical models and research with telescopes can improve our understanding. In space, small space- craftâlike those in NASAâs ongoing and highly successful Discovery seriesâcan pursue limited objectives. Larger probes, like the U.S.-European Cassini/Huygens mission currently en route to Saturn, have comprehensive goals. With the wide variety of subjects and approaches, the ques- tion remains: Which missions are the most scientifically significant, technically ready, and fis- cally viable? In 2001, NASA asked the National Academies to study the current state of solar system exploration in the United States and devise a set of scientific priorities for missions in the upcoming decade (2003-2013). After soliciting input from hundreds of scientists around the nation and abroad, the Solar System Exploration Survey (SSE Survey) produced the disciplineâs first long-range, community-generated strategy and set of mission priorities: New Frontiers in the Solar System: An Integrated Exploration Strategy (National Academies Press, Washington, D.C., 2003). The key mission recommendations made in the report, and the scientific goals from which the recommendations flow, are summarized in this booklet. 1
After studying input from the scientific community, the SSE Survey developed four themes to guide the prioritization process: The First Billion Years of Solar System History covers the formative period that features the initial accretion and development of Earth and its sibling planets, including the emergence of life on our globe. This pivotal epoch in the solar systemâs history is only dimly glimpsed at present. Volatiles and Organics: The Stuff of Life addresses the reality that life requires organic materials and volatiles, notably, liquid water. These materials originally condensed in the outer reaches of the solar nebula and were later delivered to the planets aboard organic-rich comets and asteroids. The Origin and Evolution of Habitable Worlds recognizes that our concept of the âhabitable zoneâ has been overturned, and greatly broadened, by recent findings on Earth and elsewhere throughout our galaxy. Taking inventory of our planetary neighborhood will help to trace the evolutionary paths of the other planets and the eventual fate of our own. Processes: How Planetary Systems Work seeks deeper understanding of the fundamental mechanisms operating in the solar system today. Comprehending such processesâand how they apply to planetary bodiesâis the keystone of planetary science. It will provide deep insight into the evolution of all the worlds within the solar system and of the multitude of planets being discovered around other stars. With the assistance of six expert panels, the SSE Survey developed a list of eight recom- mended activities for the decade 2003-2013 (see adjacent diagram). These activitiesâincluding the development of medium- and large-class, flight-mission concepts as well as supporting ground-based facilitiesâwere ranked in priority order according to their relevance to the themes outlined above and their role in answering important scientific questions. A program based on these recommendations will provide a strong backbone for the continuation of solar sys- tem exploration through 2013. Subsequent pages outline a set of scientific topics in which exciting research is being con- ducted today, matching those topics with mission concepts proposed by the SSE Survey to fur- ther our knowledge in these areas. The description of each proposed mission includes a list of important science questions the mission should address, the measurements needed to do so, and the guiding themes in solar system exploration that the mission would help elucidate. 2 New Frontiers in Solar System Exploration
Europa Geophysical Large Synoptic Survey Explorer Telescope The First Billion Years of Solar System History Comet Surface Mars Missions Sample Return Volatiles and Organics: The Stuff of Life Kuiper Belt-Pluto Explorer Venus In Situ Explorer The Origin and Evolution of Habitable Worlds South Pole-Aitken Basin Jupiter Polar Orbiter with Sample Return Probes Processes: How Planetary Systems Work 3
Six Continuing Mysteries About the Solar System â¢ The diversity of bodies in the solar system. There are several distinct classes of objects in the solar system. The terrestrial planets such as Earth are rocky and close to the Sun. The gas giants (Jupiter and Saturn) are tens of times larger than Earth and are made mostly of hydrogen and heli- um. Beyond the gas giants are the ice giants, Uranus and Neptune, which are composed of frozen methane and ammonia. Farther out are relatively small icy fragments called Kuiper Belt objects. These bodies (Pluto is one) are thought to have undergone relatively little change since their forma- tion some 4.6 billion years ago. Is this diversity of objects a common feature of planetary systems? If so, what is its cause? â¢ The sharp contrast between Earth and Venus. Although similar in size, mass, composition, and distance from the Sun, Venus is hellish while Earth has life. Why are Venus and Earth so different? Did Venus once have an ocean? Is the uniqueness of Earthâs Moon a factor in making Earth hos- pitable to life? What basic factors control a planetâs climate? â¢ The potential habitability of Mars. Mars, the planet with the most Earth-like environment, is a potential abode of life. Throughout its history, Mars has undergone significant changes, including massive climatic shifts, enormous volcanic eruptions, loss of volatiles like water vapor into space, and the development and subsequent decay of a strong magnetic field. When and how did these changes occur? How did they affect Marsâs environment, and what was the impact of such changes on the possible origin, evolution, and survival of life? â¢ The effects that asteroids and comets have on Earth. The small, wandering bodies of the solar system may determine the fate of Earth. What role did asteroids and comets play in the origin of life on Earth by delivering amino acids and water during Earthâs formation? What role have small bodies played in shaping the course of evolution through globally devastating impacts? Will these objects determine our ultimate fate? â¢ Distant worlds of fire and ice, and possible life. Activity abounds on the moons of the outer solar system, from Ioâs fiery volcanoes to Tritonâs frigid geysers. Much of this activity is due to the fact that these satellites orbit within the strong gravitational pull of the ice and gas giants. What is the role of tidal heating (heating of the satellitesâ interiors by the pull of gravity)? How many of the large icy moons hide oceans beneath the surface? Are these oceans habitable? â¢ Nature of the Kuiper Belt and its myriad objects. What is the composition of the Kuiper Belt objects found in the outer reaches of the solar system? Is there a great deal of diversity in their makeup? How many Kuiper Belt objects are Pluto-size or larger? What is the relationship of Kuiper Belt objects to other small bodies like comets and asteroids? How far out from the Sun does the Kuiper Belt extend? The rocky bodiesâMercury (upper left), Venus, Earth and the Moon, and Marsâand the gaseous bodiesâ Jupiter, Saturn, Uranus, and Neptune (lower left)âof the inner and outer solar system. Not to scale. 4