There are many fundamental science questions that can only be addressed by instrumentation that actually penetrates outside of the heliosphere. Specific goals include:

• Explore the nature of the interstellar medium and its implications for the origin and evolution of matter in the Galaxy.
  
  We know amazingly little about the nature of the Very Local Interstellar Medium (VLISM). For example, measurements of rotation measures and dispersion measures of pulsars suggest a large scale magnetic field of ~1.4 µG, but we have no idea of the field structure or its variations within 0.1 or even 0.01 light years (LY). Similarly, the properties of the nonthermal portion of the medium (including the low-energy galactic cosmic rays) remain unknown.

• Explore the structure of the heliosphere and its interaction with the interstellar medium.
  
  The Voyager Interstellar Mission has recently established the distance to the termination shock, but a farther-ranging probe is required to understand the dynamics of the interaction and how it is influenced by the conditions in the VLISM.

• Explore fundamental astrophysical processes occurring in the heliosphere and the interstellar medium.
  
  Shock acceleration of particles has profound impacts upon many branches of astrophysics. In addition, the structure of the solar wind interface with the VLISM has analogs in many other astrophysical settings.

• Determine fundamental properties of the Universe.
  
  Measurements of ³He, D, and ⁷Li would give constraints on big-bang nucleosynthesis and on how these key indicators have been processed in the interstellar medium. Extremely accurate tracking of a probe can be used to look for gravitational waves and a non-zero cosmological con-stant, and/or other anomalous forces such as that inferred to be acting on several deep-space missions. Polarization measurements of the downlink carrier can be used to look for inherent anisotropies in the structure of space.