Solar Probe Plus will be an extraordinary and historic
mission, exploring what is arguably the last
region of the solar system to be visited by a spacecraft,
the Sun’s outer atmosphere or corona as it
extends out into space. Solar
Probe Plus will repeatedly sample the near-Sun
environment, revolutionizing our knowledge and
understanding of coronal heating and of the origin
and evolution of the solar wind and answering
critical questions in heliophysics that have been
ranked as top priorities for decades. Moreover,
by making direct, in-situ measurements of the
region where some of the most hazardous solar
energetic particles are energized, Solar Probe Plus
will make a fundamental contribution to our
ability to characterize and forecast the radiation
environment in which future space explorers will
work and live.
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 development—and
observational confirmation—of the theory of the
corona’s 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
and competing 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 corona’s million-degree temperature and
in the early 1960s by the proof of the supersonic
solar wind’s 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-andfields
environment, was first recommended in 1958,
at the dawn of the space age, by the National Academy
of Science’s “Simpson Committee.” Since
then, NASA has conducted several studies of possible
implementations of a Solar Probe mission, and
a Solar Probe has remained at the top of various National Academy and NASA science priority lists. |