Summary

In May 2003 the Space Studies Board’s Committee on Solar and Space Physics held the Workshop on Exploration of the Outer Heliosphere to synthesize understanding of the physics of the outer heliosphere and the critical role played by the local interstellar medium (LISM)1 and to identify directions for the further exploration of this challenging environment. What emerged was a palpable sense of excitement about the field’s progress in the past 8 to 10 years.

It was only in the mid-1990s that the fundamental role of neutral interstellar hydrogen in determining the global structure of the heliosphere was elucidated and the hydrogen wall predicted. With the later discovery of the hydrogen wall, and then, the discovery of hydrogen walls about other stars in our galactic neighborhood and the associated discovery of stellar winds from solar-like stars, the field of solar and space physics underwent dramatic change.

Coupled to the theoretical advances were the increasingly exciting observations being returned by the Voyager Interstellar Mission, Ulysses, ACE (Advanced Composition Explorer), and Wind—ranging from observations of cosmic rays signaling the approach to the termination shock, to the large- and small-scale magnetic fields responsible for guiding and scattering energetic particles, to name only two types. At the workshop, the greatest excitement was generated by the suggestion that the low-energy cosmic rays showed evidence that Voyager may have crossed the termination shock—completely unexpected observations illustrating the Voyagers’ promise for returning results with a capacity to surprise and baffle for years to come.

To further the exploration of the outer heliosphere four strategic directions became clear in workshop discussions:

1  

The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Summary In May 2003 the Space Studies Board’s Committee on Solar and Space Physics held the Workshop on Exploration of the Outer Heliosphere to synthesize understanding of the physics of the outer heliosphere and the critical role played by the local interstellar medium (LISM)1 and to identify directions for the further exploration of this challenging environment. What emerged was a palpable sense of excitement about the field’s progress in the past 8 to 10 years. It was only in the mid-1990s that the fundamental role of neutral interstellar hydrogen in determining the global structure of the heliosphere was elucidated and the hydrogen wall predicted. With the later discovery of the hydrogen wall, and then, the discovery of hydrogen walls about other stars in our galactic neighborhood and the associated discovery of stellar winds from solar-like stars, the field of solar and space physics underwent dramatic change. Coupled to the theoretical advances were the increasingly exciting observations being returned by the Voyager Interstellar Mission, Ulysses, ACE (Advanced Composition Explorer), and Wind—ranging from observations of cosmic rays signaling the approach to the termination shock, to the large- and small-scale magnetic fields responsible for guiding and scattering energetic particles, to name only two types. At the workshop, the greatest excitement was generated by the suggestion that the low-energy cosmic rays showed evidence that Voyager may have crossed the termination shock—completely unexpected observations illustrating the Voyagers’ promise for returning results with a capacity to surprise and baffle for years to come. To further the exploration of the outer heliosphere four strategic directions became clear in workshop discussions: 1   The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Making use of existing assets. ACE, SOHO (Solar and Heliospheric Observatory), Wind, Ulysses, and the Voyagers are all currently furthering understanding of the outer heliosphere. In particular, the importance of the Voyagers cannot be overstated as Voyager 1 is capable of lasting another 16 years, allowing it to reach 150 AU. The Voyager Interstellar Mission is impossible to replace at its current location in the next 20 years, making it a uniquely valuable platform. The spacecraft are reasonably well instrumented for the mission, making the Voyagers the best and only near-term hope for exploring the heliospheric boundaries and interstellar medium in situ. The other vital mission is Ulysses, since it is currently the best-situated, best-instrumented mission that directly addresses the fundamental question of how the solar wind couples to the LISM. Scientific understanding of the physics of pickup ions, their relation to interstellar atoms and anomalous cosmic rays, and their influence on the solar wind has been advanced almost entirely by Ulysses (and to a lesser extent ACE), and it is the only spacecraft to directly measure neutral interstellar material. Workshop participants agreed that continued support and a long-term vision from NASA Headquarters, and the provision of continuing data coverage of the Voyagers and Ulysses missions from the Deep Space Network, are essential. Developing new outer heliosphere missions. New missions should be developed that can use current and moderately improved in situ and remote techniques to conduct heliospheric studies from 1 to 3 AU and beyond. The possibilities include in situ studies and remote observations. For example, the ability to image the region of the termination shock and heliosheath remotely is steadily increasing, with several laboratories now working on experiments in this area. Energetic neutral atom imaging is a promising avenue, with technology outstripping theory at present. Other possibilities for remote observations involve the use of Lyman-alpha absorption and backscatter techniques; the former is possible at 1 AU with space-based spectroscopic telescopes, and the latter offers the possibility of monitoring the temporal response of interstellar gas to the solar cycle as it flows into the heliosphere. In situ studies within 1 to 4 AU will remain critical to furthering understanding of the fundamental coupling of LISM material and solar wind plasma. Such studies will require spacecraft with instrumentation that can study the inflowing neutral gas and dust, pickup ions, cosmic rays (anomalous and galactic), energetic particles, and magnetic field directly since the measurements of these variables are essential if we are to eventually probe both the LISM and the heliospheric boundaries with new spacecraft (and even remotely). Understanding of the critical microphysics will be advanced best by in situ measurements. It is now possible to build instruments that allow orders-of-magnitude more accurate measurements than those made by the instruments that currently fly on missions. Both in situ and remote measurement could be accomplished within the MIDEX (Medium-Class Explorer) program or perhaps the SMEX (Small Explorer) program. For the interim period, this approach would complement current Voyager and Ulysses activities well. Continuing support of theory and modeling. Continuing theoretical and modeling studies are essential to ensure progress in understanding the interaction of the solar wind and the LISM. Numerous questions raised by Voyager, Ulysses, and other spacecraft missions remain unanswered, and theoretical studies continue to lag observations. Optimal planning for a mission of the magnitude of Interstellar Probe requires a sufficient understanding of the physics of the remote outer heliosphere and local interstellar medium, which in turn requires far more elaborate modeling of the outer heliosphere, and incorporation of current and future in situ and remote sensing results. In particular, remote sensing techniques, because they are by nature integrated line-of-sight observations, produce results whose interpretation depends on theoretical models of the global heliosphere. Preparing for Interstellar Probe. Interstellar Probe, a mission characterized by both enormous scientific potential and technical challenge, will be one of the most exciting undertakings of NASA in the new millennium. For a mission as ambitious as Interstellar Probe, the technical requirements, the scientific payload, including instrument and communications requirements, and feasibility have to be addressed far

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report in advance. Developing the required propulsion technology is the primary technical challenge of this mission. At least three approaches—nuclear-electric propulsion, solar sail propulsion, and powered Sungravity assist—are well suited for and, in principle, capable of accelerating Interstellar Probe to the speeds needed to reach the heliopause within 15 years or less from launch.2 However, as detailed in the text, none of these options are currently available and all present significant hurdles in their development. Because Interstellar Probe will require only a rather straightforward trajectory with little need for precise navigation, it could be regarded as an ideal demonstration of nuclear-electric propulsion or solar sailing. The development of a mission as scientifically and technologically far-reaching as Interstellar Probe will require considerable planning. The eventual scientific payload must be guided by current missions, Pathfinder missions, and theory. A crucial role will be played by Pathfinder missions, which may explore interstellar material such as pickup ions, neutral atoms, or anomalous cosmic rays directly, or explore the boundary regions using remote measuring techniques such as Lyman-alpha or energetic neutral atoms, or explore physical processes induced by the complex partially ionized plasma populations that make up the outer heliosphere beyond some 10 AU. Sending a well-equipped spacecraft to the boundaries of the heliosphere to begin the exploration of our galactic neighborhood will be one of the great scientific enterprises of the new century—one that will capture the imagination of people everywhere. Significant questions about the outer heliosphere and the LISM still to be addressed include the following: What are the nature, structure, and temporal character of the termination shock, and is the termination shock the same in all directions? How do pickup ions and solar wind plasma evolve at the shock and in the heliosheath? What are the size and shape of the heliopause? Does reconnection between the solar and interstellar magnetic fields at the heliopause affect the structure and dynamics of the heliosphere? What are the physical state and the degree of ionization of the LISM? What is the elemental and isotopic composition of the LISM? What are the direction and magnitude of the interstellar magnetic field? Is the interstellar wind subsonic or supersonic? These are among the most fundamental of the questions that can be addressed by space and planetary physics in the next 20+ years, and the answers will have far-reaching implications, not only revealing the nature of the heliosphere but also informing theories on the evolution of our galaxy, and, indeed, the entire universe. 2   Radioisotope electric propulsion is another propulsion option that should not be ruled out yet.