MISSION:
Solar Probe will
be a historic mission, flying into one of the last unexplored regions
of the solar system, the Suns atmosphere or corona, for the
first time. Approaching as close as 3 RS above the Suns surface,
Solar Probe will employ a combination of in-situ measurements and
imaging to achieve the missions primary scientific goal: to
understand how the Suns corona is heated and how the solar
wind is accelerated. Solar Probe will revolutionize our
knowledge of the physics of the origin and evolution of the solar
wind. Moreover, by making the only direct, in-situ measurements
of the region where some of the deadliest solar energetic particles
are energized, Solar Probe will make unique and fundamental contributions
to our ability to characterize and forecast the radiation environment
in which future space explorers will work and live. Solar Probe
is currently under study as part of the Sun-Solar
System Connection within NASA's
Science Mission Directorate.
Our First Visit
to a Star:
Two of the transformative advances in our understanding of the Sun
and its influence on the solar system were the discovery that the
corona is several hundreds of times hotter than the visible solar
surface (the photosphere) and the developmentand observational
confirmationof the theory of the coronas supersonic
expansion into interplanetary space as a solar wind.
In the decades that have
followed these important milestones in solar and space physics,
the composition, properties, and structure of the solar wind have
been extensively measured, at high heliolatitudes as well as in
the ecliptic and at distances far beyond the orbit of Pluto. The
corona and the transition region above the photosphere have been
imaged with unprecedentedly high resolution, revealing a complex
architecture of loops and arcades, while photospheric magnetography
has uncovered the magnetic carpet of fine-scale flux
bundles that underlies the corona. Observational advances have been
accompanied by advances in theory and modeling, with a broad range
of models offering plausible scenarios to explain coronal heating
and solar wind acceleration.
We now know more about
the corona and the solar wind than ever before. And yet the two
fundamental questions, raised in the 1940s by the discovery of the
coronas million-degree temperature and in the early 1960s
by the proof of the supersonic ES-1 solar winds existence,
remain unanswered: why is the solar corona so much hotter than the
photosphere? And how is the solar wind accelerated?
The answers to these
questions can be obtained only through in-situ measurements of the
solar wind down in the corona. A mission to provide these measurements,
to probe the near-Sun particles-and fields environment, was first
recommended in 1958, at the dawn of the space age, by the National
Academy of Sciences Simpson Committee. Since then,
NASA has conducted several studies of possible implementations of
a Solar Probe mission, and Solar Probe has remained at the top of
various National Academy and NASA science priority lists. Most recently,
the National
Research Councils decadal survey in solar and
space physics recommended implementation of a Solar Probe mission
as soon as possible (NRC, 2003), while NASAs Sun-Solar
System Connection Roadmap identifies Solar Probe as a Flagship
mission that is ready to fly and is our highest priority for
new resources (NASA, 2005).
To date, however, nearly
50 years after the Simpson Committee report and despite strong and
repeated endorsements of a Solar Probe by the National Academy,
NASA, and the solar and space physics community, the closest any
spacecraft has come to the Sun is 65 RS, far outside the region
where the acceleration of the solar wind occurs. Thus the need for
a Solar Probe remains. Solar Probe will be the first spacecraft
to venture into the unexplored inner reaches of the heliosphere
where the solar wind is born. Through high-cadence in-situ measurements
of the solar wind plasma, energetic particles, and fields as close
to the Sun as 3 RS, supplemented by coronal and photospheric imaging,
Solar Probe will provide the data needed to solve, finally, the
twin mysteries of coronal heating and solar wind acceleration. This
historic mission will transform our understanding both of our Sun
and of other stars with hot, x-ray-emitting coronas and supersonic
winds as well.
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