TABLE B.1 Instruments for the Interstellar Observatory


Measurement or Objective

In Situ Package



Magnetic fields of heliosphere and interstellar medium

Plasma and radio wave detector

Interaction of solar wind and interstellar medium

Solar-wind plasma ion and electron detector

Thermal ion composition and charge state; ion and electron distribution functions

Interstellar medium plasma ion and electron detector

Thermal ion composition and charge state; ion and electron distribution functions

Pickup and interstellar ion mass spectrometer

Interstellar neutral atom mass spectrometer

Density, composition of neutral species in the interstellar medium

Suprathermal ion mass spectrometer

Anomalous and galactic cosmic ray element/isotope spectrometer

Molecular analyzer for organic material

Organic material in outer heliosphere and interstellar medium

Dust composition analyzer

Suprathermal ion charge states detector

Gamma-ray burst detector

Complement long-baseline grid to locate gamma-ray bursters accurately

Imaging Package

Infrared spectrometer—scans via spin

Structure of solar system dust disk; cosmic infrared background radiation

Energetic neutral atom imager

Structure and dynamics of heliosphere

Ultraviolet spectrometer (Lyman alpha)

Backscatter from neutrals in the interstellar medium; heliospheric structure

Space Physics and the Heliosphere

The heliosphere is a large and complicated structure whose dimensions are not definitively known. Recent measurements suggest that the Voyager 1 spacecraft may have crossed the “termination shock” of the solar wind and passed intermittently into the interstellar medium (Figure B.1). Although the interpretations of the observations are controversial, it is quite certain that this region is unlike anything ever sampled previously. Continued tracking of the two Voyager spacecraft should provide the size of our heliospheric cavity within the next several years.1

The solar wind continually rams into the local interstellar medium, and through complex interactions forms the large-scale outer boundaries of our solar system. The latter has three distinct components:

  • The termination shock where the solar wind is abruptly slowed and heated prior to being deflected by the interstellar medium;

  • The heliopause that separates the solar wind from the interstellar medium; and

  • The bow shock or bow wave where the interstellar flow is slowed, heated, and deflected by the solar wind.

Between these boundaries that separate layers of solar wind from the interstellar medium, this “interstellar interaction” generates an approximate factor-of-two enhancement of neutral hydrogen (the “hydrogen wall”) in the upstream direction—the direction from which interstellar material is moving toward the Sun at ~25 km/s. Understanding the structure and dynamics of the interstellar interaction and our solar system’s outer boundaries is a primary goal of heliospheric science, and it is also relevant to understanding how other stars interact with the interstellar medium.

A variety of indirect measurements have provided only limited information on the nature of the interstellar interaction. These indirect techniques include measurements in the inner heliosphere of interstellar neutral atoms

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