5
Strategic Directions

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 with it the prediction of the hydrogen wall—a completely new and somewhat unexpected result. With serendipitous discovery only a few years later (using a technique new to space physics as well) of the hydrogen wall and then, only a few more years after that, using the same Lyman-alpha absorption technique, 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 underwent dramatic change.

Coupled to the theoretical advances were the increasingly exciting observations being returned by the Voyager Interstellar Mission, Ulysses, ACE, and Wind—ranging from observations of cosmic rays signaling the approach to the termination shock, the remarkable radio emissions, the deceleration of the solar wind, and the important dynamical contribution of pickup ions to shock waves and pressure-balanced-structures, to the large- and small-scale magnetic fields responsible for guiding and scattering energetic particles, to name only a few. 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. Whether it indeed did is still under debate,1 but the completely unexpected observations illustrate Voyager’s capacity to surprise (Krimigis et al., 2003; McDonald et al., 2003; Burlaga et al., 2003). There is no doubt that the Voyagers will be returning results with a capacity to surprise and baffle for years to come as they explore the unknown.

To further the exploration of the outer heliosphere and to take advantage of the Voyagers, four strategic directions became clear as a result of the workshop discussions:

  1. Making use of existing assets. The highest importance must be given to preserving current missions. The Voyager spacecraft, while clearly not designed to optimize the exploration of the heliospheric bound-

1  

For a summary of the debate accessible to the general public, see http://www.sciencenews.org/articles/20040103/bob8.asp.



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report 5 Strategic Directions 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 with it the prediction of the hydrogen wall—a completely new and somewhat unexpected result. With serendipitous discovery only a few years later (using a technique new to space physics as well) of the hydrogen wall and then, only a few more years after that, using the same Lyman-alpha absorption technique, 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 underwent dramatic change. Coupled to the theoretical advances were the increasingly exciting observations being returned by the Voyager Interstellar Mission, Ulysses, ACE, and Wind—ranging from observations of cosmic rays signaling the approach to the termination shock, the remarkable radio emissions, the deceleration of the solar wind, and the important dynamical contribution of pickup ions to shock waves and pressure-balanced-structures, to the large- and small-scale magnetic fields responsible for guiding and scattering energetic particles, to name only a few. 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. Whether it indeed did is still under debate,1 but the completely unexpected observations illustrate Voyager’s capacity to surprise (Krimigis et al., 2003; McDonald et al., 2003; Burlaga et al., 2003). There is no doubt that the Voyagers will be returning results with a capacity to surprise and baffle for years to come as they explore the unknown. To further the exploration of the outer heliosphere and to take advantage of the Voyagers, four strategic directions became clear as a result of the workshop discussions: Making use of existing assets. The highest importance must be given to preserving current missions. The Voyager spacecraft, while clearly not designed to optimize the exploration of the heliospheric bound- 1   For a summary of the debate accessible to the general public, see http://www.sciencenews.org/articles/20040103/bob8.asp.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report aries, nonetheless offer a once-in-a-lifetime opportunity to investigate in situ the most remote and inaccessible region of the heliosphere. This should be viewed as a 25+ year investment of NASA (and national) resources that could not be replicated for at least 20 years (assuming optimistically that development and launch of an interstellar probe could occur within 10 years and that the spacecraft would take only 10 years to travel ~100 AU). Instruments on Ulysses measure neutral interstellar material (He and dust2), provide an essential basis for understanding the three-dimensional structure of the global heliosphere, and, like ACE, measure interstellar pickup ions, cosmic rays, and the magnetic fields that enable transport of energetic particles. Other spacecraft, such as ACE, SOHO, and Wind, are needed as 1-AU monitors to explore the solar influence on the outer heliosphere boundaries, and to provide insights into interstellar composition. Continuing support of theory and modeling. Although great progress has been made and key insights achieved, current theoretical understanding of the interaction of the solar wind with the LISM remains limited. The heliospheric boundary region is characterized by non-equilibrated distributions of neutral atoms over a large energy range. The mutual interaction and feedback of plasma and neutral interstellar and heliospheric atom populations, and the coupling of neutral atoms, pickup ions, solar and LISM plasma, solar and LISM magnetic fields, anomalous and galactic cosmic rays, and so on, make for an extraordinarily complex region to model theoretically. Further progress demands the development of more sophisticated models to capture the appropriate physics together with the development of three-dimensional, temporal numerical solutions to the models using realistic parameters. To refine theoretical models will require both observational input and observational testing of theoretical predictions. Since only the Voyagers are providing (limited) in situ data in the vicinity of the solar wind boundaries, modelers need to explore alternative approaches to either constrain or validate their models. For example, greater use of remote sensing measurements (see, e.g., Gayley et al., 1997; Mueller et al., 2000; Wood et al., 2002b; Gruntman, 2001a,b) such as Lyman-alpha absorption and backscatter techniques, imaging with energetic neutral atoms, x-ray measurements, and extreme ultraviolet mapping of the heliopause, should be explored. Workshop participants noted the vital role of theoretical studies in making the best use of past, current, and near-future measurements from existing spacecraft, in developing scientific directions that would allow smaller SMEX and MIDEX3 missions to be conceived and developed, and in providing the best scientific knowledge for optimizing an ambitious Interstellar Probe mission. Developing new outer heliosphere missions. New missions that use current and moderately improved in situ techniques to conduct heliospheric studies from 1 to 3 AU and beyond should be developed. The possibilities include in situ studies (neutrals, pickup ions, anomalous and galactic cosmic rays) and remote observations. For example, technology for imaging the region of the termination shock and heliosheath remotely is steadily improving, with several laboratories now working on detector systems in this area. Imaging with energetic neutral atoms is another promising approach. Technology is available for making these measurements, but other issues, such as heliospheric background contributions, need to be addressed soon. Remote detection missions by spacecraft located within 1 to 3 AU could be accomplished within the MIDEX program (e.g., Interstellar Pathfinder; McComas et al., 2003) or the SMEX program (e.g., the Interstellar Boundary Explorer). For the interim period, this would complement current Voyager activities well. Other possibilities for remote observation involve the use of Lyman-alpha absorption and back- 2   The Cosmic Dust Analyser that is on board the Cassini spacecraft is also currently making dust measurements. Cassini is approaching Saturn and will be inserted into orbit around that planet on July 1, 2004. 3   Information on NASA’s Small Explorer and Medium-Class Explorer programs is available at http://explorers.gsfc.nasa.gov.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report scatter techniques and extreme ultraviolet mapping of the heliopause; the first is possible at 1 AU with space-based spectroscopic telescopes. Cosmic rays (anomalous and galactic) and pickup ions continue to offer opportunities to probe both the LISM and the heliospheric boundaries. Preparing for Interstellar Probe. Interstellar Probe will be a mission that captures the imagination of the public as it takes humanity’s first steps into interstellar space. Scientifically, it will be breathtaking in scope; technologically, the mission will be extremely demanding, requiring very advanced propulsion systems and advanced data acquisition, processing, autonomy, and communication capabilities. Developing the required propulsion technology remains the primary technical challenge. At least three approaches—nuclear-electric propulsion, solar sail propulsion, and powered Sun-gravity 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.4 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 optimal approach requires further study and involves trade-offs among science requirements, launch vehicles, technology development, and system runout cost. Interstellar Probe will be one of the great NASA missions of the new millennium. Exploration of the vast new outer heliospheric frontier promises a rich dividend of scientific results and discoveries that will illuminate the way in which the Sun and the stars interact with our galaxy. With Interstellar Probe, we will “…slip the surly bonds of Earth,”5 leave our local neighborhood, and begin the exploration of interstellar space. 4   Radioisotope electric propulsion is another propulsion option that should not be ruled out yet. 5   From the poem “High Flight” by John Gillespie Magee, Jr. (killed in the Battle of Britain, age 19).