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Solar and Space Physics: A Science for a Technological Society
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SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
BOX 6.1 SOLAR PROBE PLUS—A MISSION TO THE CORE OF THE HELIOSPHERE
Solar Probe Plus (SPP) will travel closer to the Sun than any other spacecraft and explore the innermost
region of our solar system—the corona. The 2003 decadal survey 1 recommended a solar probe intended to
“determine the mechanisms by which the solar corona is heated and the solar wind is accelerated and to understand how the solar wind evolves in the innermost heliosphere.” The present survey found that the scientific
rationale for a solar probe remains compelling and concluded that SPP meets that challenge. SPP will study
the streams of charged particles the Sun hurls into space from a vantage point where the processes that heat
the corona and generate the solar wind actually occur. SPP will repeatedly sample the near-Sun environment
and, using in situ measurements, reveal the mechanisms that produce the fast and slow solar winds, coronal
heating, and the transport of energetic particles.
The region inward of 0.3 AU is one of the last unexplored frontiers in our solar system. Discovering how the
solar wind originates and evolves in the inner heliosphere requires in situ sampling of the plasma, energetic
particles, magnetic field, and waves as close to the solar surface as possible. SPP measurements will determine
how energy flows upward in the solar atmosphere, heating the corona and accelerating the solar wind. SPP will
also reveal how the solar wind evolves with distance in the inner heliosphere. During the past decade remote
observations have revealed much about particle acceleration, heating, plasma turbulence, waves, and the flows
of mass and energy in the corona. In the survey committee’s view, these observations only increase the need
for measurements from this critical region.
The current solar probe differs in several respects from the mission envisioned in the 2003 survey. The
closest approach for SPP is 9.5 solar radii (R S ) instead of 4 R S (Figure 6.1.1). The loss in proximity is significant
but is more than compensated for by the opportunity to spend far more time close to the Sun and gather
observations spanning half a solar cycle. This latter feature in particular makes the timing of the mission with
respect to the solar activity cycle a less significant issue. SPP orbits in the ecliptic plane instead of a polar plane
and thus leaves the polar regions unexplored. Nevertheless, fast solar wind streams typical of polar coronal
holes will still be adequately sampled, albeit at larger radial distances from the Sun. Only one remote sensing
instrument remains on SPP, but the most significant impairment comes from the lack of in situ solar wind
composition measurements; the survey committee urges NASA to consider restoring this capability. However,
in most other aspects SPP offers the prospect of a substantially enhanced science return compared to the
earlier solar probe concept.
The four science investigations provide a combination of in situ and remote sensing capability. Three in
situ instruments will make comprehensive measurements of the solar wind ion and electron thermal plasma,
of suprathermal and energetic particles, and of magnetic and electric fields from DC to high frequencies. A
side-looking imager will provide both global context and quasi-in situ measurements of density and dust in
the corona from 2.2 to 20 R S .
1 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National
Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel Reports, The National
Academies Press, Washington, D.C., 2003.
The committee emphasizes that what is called for is a review by NASA, in which the expected outcome
will be actions by NASA with regard to SPP that preserve a program of balanced progress in heliophysics
throughout the decade.
Solar and Space Physics: A Science for a Technological Society
132
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
BOX 6.1 SOLAR PROBE PLUS—A MISSION TO THE CORE OF THE HELIOSPHERE
Solar Probe Plus (SPP) will travel closer to the Sun than any other spacecraft and explore the innermost
region of our solar system—the corona. The 2003 decadal survey 1 recommended a solar probe intended to
“determine the mechanisms by which the solar corona is heated and the solar wind is accelerated and to understand how the solar wind evolves in the innermost heliosphere.” The present survey found that the scientific
rationale for a solar probe remains compelling and concluded that SPP meets that challenge. SPP will study
the streams of charged particles the Sun hurls into space from a vantage point where the processes that heat
the corona and generate the solar wind actually occur. SPP will repeatedly sample the near-Sun environment
and, using in situ measurements, reveal the mechanisms that produce the fast and slow solar winds, coronal
heating, and the transport of energetic particles.
The region inward of 0.3 AU is one of the last unexplored frontiers in our solar system. Discovering how the
solar wind originates and evolves in the inner heliosphere requires in situ sampling of the plasma, energetic
particles, magnetic field, and waves as close to the solar surface as possible. SPP measurements will determine
how energy flows upward in the solar atmosphere, heating the corona and accelerating the solar wind. SPP will
also reveal how the solar wind evolves with distance in the inner heliosphere. During the past decade remote
observations have revealed much about particle acceleration, heating, plasma turbulence, waves, and the flows
of mass and energy in the corona. In the survey committee’s view, these observations only increase the need
for measurements from this critical region.
The current solar probe differs in several respects from the mission envisioned in the 2003 survey. The
closest approach for SPP is 9.5 solar radii (R S ) instead of 4 R S (Figure 6.1.1). The loss in proximity is significant
but is more than compensated for by the opportunity to spend far more time close to the Sun and gather
observations spanning half a solar cycle. This latter feature in particular makes the timing of the mission with
respect to the solar activity cycle a less significant issue. SPP orbits in the ecliptic plane instead of a polar plane
and thus leaves the polar regions unexplored. Nevertheless, fast solar wind streams typical of polar coronal
holes will still be adequately sampled, albeit at larger radial distances from the Sun. Only one remote sensing
instrument remains on SPP, but the most significant impairment comes from the lack of in situ solar wind
composition measurements; the survey committee urges NASA to consider restoring this capability. However,
in most other aspects SPP offers the prospect of a substantially enhanced science return compared to the
earlier solar probe concept.
The four science investigations provide a combination of in situ and remote sensing capability. Three in
situ instruments will make comprehensive measurements of the solar wind ion and electron thermal plasma,
of suprathermal and energetic particles, and of magnetic and electric fields from DC to high frequencies. A
side-looking imager will provide both global context and quasi-in situ measurements of density and dust in
the corona from 2.2 to 20 R S .
1 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National
Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel Reports, The National
Academies Press, Washington, D.C., 2003.
The committee emphasizes that what is called for is a review by NASA, in which the expected outcome
will be actions by NASA with regard to SPP that preserve a program of balanced progress in heliophysics
throughout the decade.