3.3.4 Other missions

Many other important space missions have been proposed, each with unique capabilities for solar and related sciences. We do not prioritize these here and recognize that some of them will have to wait for some time, but we do think that their scientific content strongly warrants their eventual development. Some require deep space, and these will have increased priority in the eventuality of human travel to the Moon and the planets. Some of these missions have been well-studied, and others are just ideas at present.

  • The Pinhole/Occulter Facility: The P/OF scientific objective is to provide sensitive, high-resolution observation of the solar corona, incorporating large-aperture UV and visible imagers as well as the crucial high-energy observations (hard X-rays and gamma-rays) at 0.1 arc sec resolution. The HESP mission defined above serves as a predecessor of the full-fledged Pinhole/Occulter high-energy experiments. The P/OF coronal observations would provide an ideal Earth-orbital complement to the Solar Probe, for its observations would record the ''context" of the Probe's in situ measurements. As mentioned, P/OF has been approved as a concept study for the Space Station Freedom , and its development schedule will depend upon that of the Station.

  • Solar Polar Orbiting Imager: Solar observations from above the solar poles, including imaging and stereoscopy in conjunction with observations from near Earth. The polar regions of the Sun play a unique role in the evolution of solar activity, particularly on solar-cycle time scales. A solar polar orbit would let us look directly at these regions for the first time, without the major uncertainties caused by projection from ecliptic-plane observations. A polar orbiter will also give a global view of the mid-latitude regions of activity and permit us to follow their evolution on the crucial few-week time scales not accessible from a single perspective in the ecliptic.

  • Solar Probe/Context: While not a solar remote-sensing mission in the classical sense of solar astronomy, Solar Probe can make in situ observations from its unique perspective, within a few solar radii of the solar surface. True scientific productivity from Solar Probe demands the existence of facilities for "context" observations of the corona from one A.U., with state-of-the-art coronal imaging and spectroscopy; the scientific return from Solar Probe can be amplified many times by conducting high-resolution coronal observations before, during, and after the perihelion passage -- it may be possible to carry out true tomographic remote-sensing observations, capable of defining the three-dimensional structure through which Solar Probe would fly. This is the aim of the High-resolution Coronal Imager (Context). The Context mission addresses a major problem in astrophysics: How are hot plasmas in stellar atmospheric structures created? Through high-resolution imaging and spectroscopy, Context can identify the structures through which Probe has flown, in order to understand the environment of Probe's particles-and-fields measurements. Such a combination of in situ and remote sensing will be unprecedented in astrophysics.

  • Advanced Solar Observatory (ASO): The deployment of advanced instrumentation in all areas of observation, into a comprehensive facility for space observation of the Sun, represents a natural goal of the solar space program. Concepts for an ASO have been studied for the Space Station. The Pinhole/Occulter Facility and the Ultra High Resolution XUV Spectroheliograph represent the initial steps toward a comprehensive Advanced Solar Observatory.

  • Mercury Orbiter: Unique solar physics observations can be made from a spacecraft orbiting Mercury. High-energy neutral emissions, such as hard X-rays, gamma-rays, and neutrons, will be considerably more intense than at 1 A.U. (about 10x for photons, but more than 1000x for neutrons). Solar energetic particles, when observed at 0.3 A.U., are less affected by the intervening interplanetary medium, and hence will more directly reflect the propagation of the particles accelerated in the flare region than they do at 1 A.U. Long-term observations of these flare emissions from Mercury could be carried out for a large part of Cycle 23 after the year 2000. Additional observations at other solar aspect angles can give valuable stereoscopic information bearing on the directivity of solar emissions or their height of origin. These observations can be made with small instruments (<25 kg) with modest power and telemetry requirements.

  • Solar-Stellar Activity Mission: The stars teach us about solar activity by allowing the study of analogs to the solar mechanisms; they also give us rich fields for new discovery. Stellar magnetic activity can be sensitively studied with wide-field (one degree), high-resolution (few arc sec) soft X-ray observations, preferably arranged with observing sequences that allow broad ranges of time scales. A dedicated solar-stellar activity mission might also carry EUV instrumentation for monitoring stellar chromospheric activity, and visible-light instrumentation for stellar seismology.

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