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 survey1 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 (RS) instead of 4 RS (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 RS.
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.