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Solar System Exploration Today
~ ver the last four decades, robotic spacecraft have visited nearly every planet, from torrid
I 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 Mars's 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:
Answering these questions requires several simultaneous approaches. On Earth, theoretical
models and research with telescopes can improve our understanding. In space, small space-
craft like those in NASA's ongoing and highly successful Discovery series can 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 discipline's
first long-range, community-generated strategy and set of mission priorities: New Frontiers ir' the
Solar System: Ar' 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.
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After studying input from the scientific community, the SSE Survey developed four themes
to guide the prioritization process:
ff~f~ffff/~fff.f~ fi.~y 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 system's history is only
dimly glimpsed at present.
~i ~~ addresses the reality that life requires
organic materials and vocatives, 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.
"habitable zone" 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.
~~K seeks deeper understanding of the
fundamental mechanisms operating in the solar system today. Comprehending such
processes and how they apply to planetary bodies is the keystone of planetary science.
~t will provide deep insight into the evolution of aid 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 activities including
the development of medium- and large-class, flight-mission concepts as well as supporting
ground-based facilities were 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.
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Europa Geophysical
Explorer
Mars Missions
Kuiper Belt-Pluto
Explorer
South Pole-Aitken Basin
Sample Return
'of/
. ¢
The First
Billion
; Years of
83~3 Swum 2;
~ ~~$~
Volatiles
and
Organics:
The Stuff
of Life
The
Origin
and
Evolution
of
Habitable
Worlds
Processes:
How
Planetary
Systems
Work
Large Synoptic Survey
Telescope
Comet Surface
Sample Return
Venus In Situ Explorer
_
. ~ _
~ CW~K _
~ 3~as3ns33 __
_ ~ 11__
_ ~ _
~ . ~ ~
i ;
_1 ~ _
_ 1 _ _
_ 1 _ _
Jupiter Polar Orbiter with
Probes
3
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Six Continuing Mysteries About the Solar System
O 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 ancl close to the Sun. The gas giants
(Jupiter ancl Saturn) are tens of times larger than Earth ancl are made mostly of hydrogen ancl heli-
um. Beyoncl the gas giants are the ice giants, Uranus ancl Neptune, which are composed of frozen
methane ancl 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?
O The sharp contrast between Earth and Venus. Although similar in size, mass, composition, ancl
distance from the Sun, Venus is hellish while Earth has life. Why are Venus ancl Earth so clifferent?
Dicl Venus once have an ocean? Is the uniqueness of Earth's Moon a factor in making Earth hos-
pitable to life? What basic factors control a planet's climate?
O 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, ancl
the development ancl subsequent decay of a strong magnetic fielcl. When ancl how clicl these changes
occur? How clicl they affect Mars's environment, ancl what was the impact of such changes on the
possible origin, evolution, ancl survival of life?
O 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 clicl asteroids ancl comets play in the origin of life
on Earth by delivering amino acids ancl water during Earth's formation? What role have small bodies
played in shaping the course of evolution through globally devastating impacts? Will these objects
determine our ultimate fate?
O Distant worlds of fire and ice, and possible life. Activity abounds on the moons of the outer
solar system, from lo's fiery volcanoes to Triton's frigid geysers. Much of this activity is clue to the
fact that these satellites orbit within the strong gravitational pull of the ice ancl gas giants. What is the
role of tidal heating (heating of the satellites' interiors by the pull of gravity)? How many of the large
icy moons hide oceans beneath the surface? Are these oceans habitable?
O Nature of the Kuiper Bell 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 clear 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 ancl asteroicls? How far out from the Sun does the
Kuiper Belt extencl?
The rocky bodies Mercury (upper left), Venus, Earth and the Moon, and Mars and the gaseous bodies-
Jupiter, Saturn, Uranus, and Neptune (lower left) of the inner and outer solar system. Not to scale.
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Representative terms from entire chapter:
sse survey
