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55 4. Supporting Research and Development An essential objective of the 1980's is the development of more powerful instrumental techniques that will achieve major improvements in sensitivities and in spectral and angular resolutions. Technical concepts already exist to achieve these goals, but they need extensive development before they are ready for space- flight. Thus, a vigorous research and development pro- gram with development flights on balloons and the Shuttle is an essential part of the strategy for gamma-ray astronomy in the 1980's. VI I . COSMIC-RAY ASTRONOMY A. Introduction Cosmic rays constitute a suprathermal gas of energetic charged particles that pervade the entire Galaxy. Con- sisting mostly of protons, alpha particles, and other bare nuclei of the elements with individual particle energies from 106 eV up to at least 102 eV, they are the only sample of matter from regions well outside the solar system that we can examine in detail. High-energy electrons, the source of Galactic synchrotron radio noise, constitute about 1 percent of the total cosmic-ray flux. Positrons and antiprotons produced in collisions of cosmic rays with interstellar matter have also been observed. m e astrophysical significance of cosmic rays stems from two considerations. On the one hand, they carry in the details of their composition and energy spectra inter- esting and unique information about their sources and the regions of space in which they have traveled; on the other hand, they are an important astrophysical entity in them- selves, having an energy density comparable with that of the Galactic magnetic field and of the turbulent motion of the interstellar gas. The pressure of the cosmic-ray gas and its heating of the interstellar medium affect the processes of star formation and influence the structure and evolution of the Galaxy. Observations of cosmic rays are made with a wide variety of instruments each tailored to the character- istics of a particular component and energy regime. Among the most important technical developments in recent years are instruments capable of resolving individual elements and isotopes, measuring the abundances of the

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56 exceedingly rare heavy nuclei up to uranium, observing high-energy electrons and positrons, and searching for particles of antimatter. Small detectors have been flown on satellites and space probes to investigate the prop- erties of low-energy cosmic rays in the solar system from the orbit of Mercury to beyond the orbit of Saturn. Large and heavy instruments have been used at high altitudes in balloon flights and in Earth orbit on the HEAD-3 satellite to measure the composition and spectra of cosmic rays up to energies of 101~ eV. Very large arrays of detectors on the ground have been used to study cosmic rays with energies as high as 1012 eV through the observation of showers of secondary particles produced in the atmosphere It is believed that most cosmic rays originate in pro- cesses associated with supernovae and are confined within the Galaxy for millions of years by the Galactic magnetic field. Some low-energy cosmic rays are produced within the solar system in solar flares, in planetary magneto- spheres, and in shock waves in the interplanetary medium. The rare cosmic rays with energies above 1019 eV probably originate outside our Galaxy. Magnetic force twists the trajectories of cosmic rays into helices around the lines of the magnetic field and thereby destroys any simple relation between the distri- bution of their arrival directions and the locations of their sources. Information about the sources of cosmic rays is therefore sought primarily from measurements of their composition and energy spectra. For example, the presence in the ultraheavy (Z greater than 28) cosmic rays of nuclei that can only be produced by rapid sequential absorption of free neutrons would indicate that they are synthesized during the nuclear phase of a supernova explosion. The isotopic compositions of the nuclei of individual elements such as neon, magnesium, silicon, sulfur, and iron can be related to the temperature, density, and mixing during nucleosYnthesis. - The existenc of a substantial difference between the compositions of cosmic rays with energies below and above the critical value near 1017 eV for magnetic confinement within the Galaxy would support the hypothesis that cosmic rays of very high energy have a different, and probably extra- galactic, origin. m e unambiguous detection of any nuclei of antimatter with Z greater than unity would be con- vincing evidence for the existence of stars of antimatter and would have profound implications for the origin of the Universe and the nature of the fundamental forces.

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57 Solar flares and planetary magnetospheres are sources of highly variable fluxes of low-energy cosmic rays within the solar system. Measurements of their elemental and isotopic abundances provide direct information about the composition of material in other bodies in our solar system and about the nature of the acceleration mechanisms. Charged particles are accelerated by electric fields induced by changing magnetic fields. The latter are caused, in turn, by collective motions of plasma or other conductors, such as rotating stars and planets, in which the current sources of the magnetic field are located O A necessary condition for this process to produce cosmic rays is that the density of plasma in the region of acceleration be low enough so that the particles gain energy faster from the induced electric field than they lose it in collisions. This condition evidently occurs throughout the Universe under a variety of circumstances. Much discussed cases are the outer layers of an exploding supernova remnant and interstellar shock waves. Informa- tion about the acceleration mechanisms can be derived from measurements of the energy spectra of individual components of cosmic rays. Particularly important is the range above 1011 eV/nucleon in which the spectra observed at Earth are not appreciably distorted by effects of the solar wind and relative abundances are apparently changed very little by interactions with interstellar matter. More specific information can be gained from measurements of the isotopic composition. For instance, the mean time between the nucleosynthesis of cosmic-ray nuclei and their acceleration to relativistic energies can be derived from measurements of the abundances of radioactive nuclides that decay by K-electron capture. Only those nuclides are present that were accelerated and stripped of their K-electrons by Coulomb collisions before they decayed. Cosmic rays interact with interstellar matter, with the magnetic field that controls their motions, and with photons of starlight and the microwave background. The average thickness, or pathlength, of the matter traversed before escape from the Galaxy is determined from measure- ments of the relative abundances of the secondary cosmic rays produced in these interactions, such as the nuclei produced by fragmentation of heavier nuclei in collisions with interstellar matter. If one also determines the average containment time, for instance through measure- ments of the abundances of various radioactive isotopes,

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58 one can find the average density of the matter in the regions traversed by cosmic rays. Cosmic-ray electrons with energies above 1013 eV, spiraling along magnetic-field lines in the Galactic disk, lose most of their energy within a few hundred parsecs owing to synchrotron emission and inverse Compton scattering with the microwave background radiation. Thus the shape of the energy spectrum of electrons in this energy range is a sensitive indicator of the distribution in distance of the sources of cosmic-ray electrons. Simi- lar considerations apply to the interpretation of the energy spectrum of cosmic-ray nuclei above the threshold for photodisintegration by collisions with photons of the microwave background. m is threshhold is near 1019 eV. While cosmic-ray nuclei with energies above this thresh- old almost certainly originate outside the Galaxy, their mean free path before photodisintegration is short com- pared with the Hubble distance so that the shape of their spectrum must be influenced by the spatial distribution of their sources. Interstellar matter is ionized and heated by cosmic rays that lose energy by Coulomb collisions. This may have significant effects on the evolution of molecular clouds and star formation. The importance of this effect depends critically on the flux of Galactic cosmic rays at very low energy. This flux is still unknown, because low- energy cosmic rays are swept out of the solar system by the solar magnetic field moving outward with the solar wind. A direct measurement could be made only by a deep space probe that leaves the solar cavity. B. Progress during the 1970's 1. Instrumentation and Vehicles . During the 1970ts balloon experiments were the principal and almost exclusive source of new information about the composition and energy spectra of Galactic cosmic rays at energies above 1 GeV per nucleon. At lower energies absorption and production of secondaries in the Earth's atmosphere limit the usefulness of balloon observations. Balloon experiments also played an essential role in the development of instrumentation for subsequent observations from space vehicles. Substantial progress was made in increasing the capabilities of balloon vehicles in regard

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59 to reliability' loads, and duration and in improving the quality of support facilities and data recovery. Typical flights carried payloads of 1000 or 2000 kg to altitudes above 40 km for about 1 day. Space experiments extended the range of cosmic-ray measurements to lower energies and yielded data of a precision limited only by counting statistics. The Interplanetary Monitoring Platform (IMP)-5, -6, -7, and -8, the Orbiting Geophysical Observatory (OGO)-5 and -6 and International Sun Earth Explorer (ISEE)-1 and -3 carried instruments outside the Earth's magnetosphere to measure the elemental and isotopic composition and the energy spectra of low-energy (E less than 109 eV/nucleon) cosmic rays from Galactic and solar system sources. In- struments on solar satellites and deep space probes, which include Pioneer-10 and -11, Mariner-10, Helios-1 and -2 and Voyager-1 and -2, measured the composition and spectra of cosmic rays over a wide range of heliocentric distances and surveyed the particle populations and acceleration phenomena in the magnetospheres of Jupiter and Saturn. Skylab, at the beginning of the decade, carried plastic track detectors in Earth orbit to measure the composition of the rare ultraheavy nuclei. At the end of the decade the third of the High Energy Astronomical Observatories, HEAD-3, was launched with heavy instruments to measure the mean isotopic composition of the cosmic-ray nuclei up to iron and the elemental composition beyond iron. Simi- lar measurements were undertaken with the British Explorer-class satellite, Ariel VI. Large air-shower detectors were operated on the ground in several countries to measure the energy spectrum, arri- val directions, and composition of the very rare cosmic rays in the region above 1017 eV where a transition may occur from Galactic to extragalactic sources. Novel approaches to the measurement of ultra-high-energy cosmic rays were taken in developing the "Fly's Eye" detector, designed to record the fluorescent light emitted along the trajectory of an air shower in the atmosphere, and the Homestake mine installation, which detects high-energy muons produced by interactions of primary cosmic-ray nuclei with air atoms near the top of the atmosphere.

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60 2. Scientific Accomplishments a. Elemental Composition and Energy Spectra (Z up through 28): Precise measurements of the elemental composition have been made up to energies of approximately 10 GeV/nucleon, and exploratory composition measurements have been performed to about 100 GeV/nucleon. mese measurements led to the discovery that the elemental composition of cosmic rays is strikingly similar to the of solar system material. On the other hand several characteristic deviations from the solar-system abundances have been found, and these deviations have led to these important conclusions: (i) The abundances of the secondary cosmic rays around 1 GeV/nucleon imply an average pathlength in interstellar matter of 7 g/cm before escape from the Galaxy. (ii) The average pathlength, and perhaps the containment time, decrease with increasing energy. Around 100 GeV/nucleon, the average pathlength may be as small as l g/cm2, a most surprising result whose interpreta- tion is currently a subject of intense study. (iii) Heavy elements are relatively more abundant in cosmic rays than in the solar system, possibly owing to the greater ease with which high-, atoms can be ionized prior to acceleration. An "anomalous component" of cosmic rays with very low energies of about 10 MeV/nucleon or less has been dis- covered. Its most likely origin is neutral atoms of interstellar matter, photoionized in the neighborhood of the Sun and accelerated by magnetohydrodynamic turbulence in the solar cavity. The composition of cosmic rays above 1012 eV is essen- tially unknown. However, measurements of the energy spec- trum were extended to about 1012 eV. Above 3 X 10 eV the spectrum begins to fall off more rapidly, possibly owing to a higher rate of leakage from the Galaxy. Above 1019 eV this trend reverses, and a small but significant anisotropy in the distribution of arrival direction is apparently present. It is likely that the origins of the particles above 1019 are extra-galactic, but the nature and location of their sources are unknown. b. Ultraheavy Nuclei with Z Greater Than 28 Before 1979 only very limited data were available on the extremely rare ultraheavy nuclei. The situation changed after HEAD-3 and Ariel VI were launched. The in-

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61 struments aboard these spacecraft were capable of resolv- ing, for the first time, the more abundant elements with nuclear charges up to those of the actinide elements. The preliminary data were not sufficient to establish definitely whether there exists an overabundance of heavy reprocess elements such as platinum and the actinides, as expected for supernova material. In the region of lower nuclear charges (Z up to 40) the elemental composition is clearly not dominated by the products of reprocess nucleo- synthesis, and the difference between cosmic-ray and solar-system abundances are roughly correlated with the values of the first ionization potentials. c. Isotopic Composition Recent investigations of low-energy cosmic rays have resolved individual isotopes of the elements from hydrogen to iron and have yielded information that cannot be derived from measurements of elemental abundances alone. For example, the neutron-rich isotopes of neon, magnesium, and silicon are significantly overabundant compared with solar-system material. This is clear evidence that cosmic-ray matter has a nucleosynthetic history that is different from that of solar-system material. Another example is the low abundance of the radioactive isotope 1OBe (half-life = 1-5 X 106 years) around 200 MeV/nucleon, which implies that the cosmic-ray containment time is about 10 years, much longer than previously assumed. This and the known average pathlength imply that cosmic rays are confined in low-density regions of interstellar space (with about 0.2 atom/cm3) or perhaps the galactic halo. d. Cosmic-Ray Electrons and Positrons The observations of cosmic-ray electrons and posi- trons lead to the following conclusions: (i) The energy spectrum of electrons above about 30 GeV is steeper than that of all cosmic-ray nuclei. At- tributing this effect to the influence of Compton- and synchrotron-energy losses in interstellar space, one finds the containment time of cosmic-ray electrons in the Galaxy to be of the order of 107 years, in good agreement with the containment time of cosmic-ray nuclei derived from the measurements of 1OBe. (ii) In the energy range 1-30 GeV, positrons have an intensity much smaller than that of negative electrons. Since positrons and negative electrons are produced in nearly equal numbers as a result of high-energy inter-

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62 actions of cosmic-ray nuclei with interstellar matter, this observation shows that only a small fraction of the cosmic-ray electrons arise from such interactions. (iii) Electrons observed near Earth at low energies (less than 50 MeV) include not only particles of inter- stellar and solar origin but also, and perhaps mostly, particles that originate in the magnetosphere of Jupiter. Scientific Goals for the 1980's Cosmic-ray research has reached the stage where it is now possible to make definitive measurements that bear on the origins of cosmic rays and on the interplay between cosmic rays, interstellar matter, and fields. The following observations are major goals of the 1980's. 1. Isotopic Composition from Hydrogen through Nickel A detailed comparison of the abundances of cosmic-ray isotopes with the solar-system abundances will provide critical information on the nucleosynthetic history of the material that becomes cosmic rays and, by comparison, a new perspective on the origin of the solar system. Particularly important are the abundances of the neutron- rich isotopes of neon, magnesium, silicon, sulfur, iron, and nickel, all of which are sensitive to circumstances of the initial phase of a supernova explosion. Information about the Galactic containment time of cos- mic rays can be derived from measurements of the relative abundances of radioactive isotopes. It is important that the abundances of a wide variety of isotopes be investi- gated in order to decide whether all cosmic-ray species experience the same propagation history. The measure- ments must cover a large energy range corresponding to a wide range of relativistic time dilations. 2. Elemental Composition of the Ultraheavy Nuclei The elemental composition in the range of atomic numbers above Z = 26 yields clues to the processes of nucleosyn- thesis and acceleration of cosmic rays. Precise abun- dance measurements should be made of individual elements including the rare odd-, elements. Even more valuable

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63 information could be derived from measurements of their isotopic abundances, but the technical problems of such measurements are severe. The relative abundances of interstellar secondaries of ultraheavy nuclei (Z values in the ranges 41-49 and 67-75) provide a particularly sensitive measure of the pathlength distribution of cosmic rays at short path- lengths since their interaction cross sections are much larger than those of the lighter cosmic rays. The radio- active actinide elements are potentially useful as chro- nometers for estimating the time elapsed since their nucleosynthesis. 3. Elemental Composition at High Energies The composition and the individual energy spectra of the major cosmic-ray components should be determined in direct measurements up to energies of at least 104 GeV/nucleon. As noted above, the average pathlength of cosmic-ray nuclei at these high energies is probably less than 1 g/cm2. Thus abundance changes due to interstellar spallation are almost negligible, and the measurements will yield direct information on the elemental compo- sition at the acceleration site. These measurements will also reveal how the average pathlength depends on energy and thereby cast new light on the confinement of cosmic rays in the Galaxy. The range of direct measurements should overlap the ultra-high-energy range of air-shower measurements in order to achieve an effective cross- calibration of the measurement techniques. 4. Energy Spectrum of Electrons at High Energies The energy spectrum of cosmic-ray electrons should be measured up to at least 104 GeV. Such measurements will provide information on the possible existence of cosmic-ray sources closer than about 1 kiloparsec. They will also yield more precise information on the contain- ment time of Galactic cosmic-ray electrons. Measurement of the positron spectrum is of special importance because the input spectrum of positrons can be calculated from knowledge of the interstellar nuclear collisions in which they arise. Currently available positron data do not yet cover the energy region above 30 GeV, where radiative energy losses are significant.

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64 5. The Composition and Origins of Ultra-High-Energy Cosmic Rays The origin of cosmic-ray particles with energies above 1018 eV remains a central problem of high-energy astro- physics that can only be approached through the detection and analysis of the very large showers that such particles produce in the atmosphere. Better knowledge of shower development and accurate calibrations of detectors are needed to improve the reliability with which the energy and composition of the ultra-high-energy primaries are deduced from observations of showers. Years of exposure time with detectors of the largest attainable effective collecting areas will be required in order to obtain adequate statistical accuracy in the measurement of the spectrum and the distribution in arrival directions. 6. Low-Energy Cosmic Rays (<300 MeV/Nucleon) in Interstellar Space The contribution of low-energy cosmic rays to heating the interstellar gas and the effects they have on the struc- ture of the Galaxy should be studied by measurements out- side the heliosphere. Direct measurements can be made only with a deep-space probe that leaves the solar system. 7. Solar-System Cosmic Rays Measurements of energetic particles originating in the solar system are of fundamental importance in the effort to gain a better understanding of the processes of par- ticle acceleration and propagation in less accessible regions of the cosmos. Such measurements may also reveal subtle differences in the isotopic composition between solar cosmic rays and terrestrial material and thereby cast new light on the origin and history of the solar system. In order to distinguish clearly between temporal and spatial variations the measurements must be made simultaneously at widely separated locations in the solar system and over several solar cycles.

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65 D. Inventory of Present or Approved Resources 1. Small Satellites and Space Probes Several spacecraft or deep-space probes with small cosmic-ray detectors aboard are expected to remain active into the 1980's. Detectors on ISEE-1 and -3 will con- tinue to measure the low-energy elemental composition and low-energy electrons in interplanetary space and provide the isotopic composition of the more abundant nuclides. Detectors on the deep-space probes Pioneer-10 and -11, and Voyager-1 and -2 will measure Galactic, solar, and planetary cosmic rays at very large distances from the Sun, and thereby probe the particle population and the solar modulation mechanisms in regions not pre- viously explored. During the next decade, these missions will reach distances out to 30 astronomical units (AU). The International Solar Polar Mission (ISPM), as originally approved, would be the first spacecraft to carry cosmic-ray and energetic-particle detectors far outside the ecliptic plane and over the poles of the Sun. It would thus explore fluxes and composition of particles from interstellar space and from the Sun in those parts of the solar system where no direct measure- ments could ever before be made. 2. Large Spacecraft HEAD-3 carried two large cosmic-ray detectors to measure the elemental composition of the more abundant ultraheavy cosmic rays and to measure the mean mass, i.e., the iso- topic mix of the elements around a few GeV/nucleon. This mission ceased operation in 1981. 3. Space Shuttle m e Space Shuttle can carry very large and heavy detec- tors. Unfortunately, the exposure times will be limited initially to only about 1 week. Nonetheless, experiments approved for Spacelab flights in the early 1980's will address key questions of cosmic-ray astrophysics. They will extend measurements of the elemental composition and energy spectra of the more abundant cosmic-ray species into the TeV/nucleon range, and they will provide informa- tion on the interactions of high-energy heavy nuclei at

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66 energies far above those currently attainable at accel Orators. 4. Balloons High-altitude balloons have been important vehicles for cosmic-ray measurements and will continue to be vitally important to development of the field. The balloon pro- gram is, however, severely underfunded. Its full poten- tial could be realized with an increase in funds in amounts that are small compared with the costs of space missions. 5. Air-Shower Detectors Ground-based observations of the extremely energetic cos- mic radiation will be pursued at several installations. In particular, the first phases of the Fly's Eye project is nearly completed and will provide pioneering data during the next few years. E. Recommendations for the 1980's Broad progress in cosmic-ray astronomy requires a wide variety of observations. Of central importance to the field in the 1980's are long exposures of large instru- ments in near-Earth orbit and high-sensitivity isotopic composition measurements on spacecraft beyond the inter- ference of the Earth's magnetosphere. Experiments on cur- rently active satellites and space probes should be fully utilized, and future planetary missions should be equipped with appropriate particle detectors. Instrumentation development and exploratory measurements on balloon vehi- cles must be continued. Progress at the highest energies requires the further development of air-shower installations. 1. The Cosmic-RaY Platform Definitive measurements in several important areas of cosmic-ray astronomy require exposures of massive (1000- 5000 kg) detectors with large collection areas (1-30 m2 sr) in Earth orbit for periods of at least 1 year. Such

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67 instruments can be developed and tested and will yield important preliminary results in Spacelab flights. How- ever, the long exposures required for definitive measure- ments could be provided by a relatively simple Cosmic-Ray Platform (CRP) that is launched, maintained, and refur- bished by the Shuttle Transportation System. The CRP does not need accurate celestial pointing. It should be able to carry one or two instruments and should be usable in either near-equatorial or high-inclination orbits. In typical missions, each individual instrument will be developed and supported by a group of invest) gators and institutions. Launch opportunities should exist at 1- to 2-year intervals, starting in the mid-1980's. The following investigations promise the most impor- tant scientific returns and should therefore be given highest priority: (a) Measurements of the composition of cosmic rays up to very high energies (104-105 GeV/nucleon). (b) Detailed composition measurements of ultraheavy cosmic rays, with resolution of individual elements and, perhaps, isotopes. (c) Precise measurements of the isotopic composition from hydrogen to iron at energies up to several GeV/nucleon. The experimental techniques to perform these measure- ments are currently available. In several cases, they have been verified on balloons or on HEA0-3 or are under development for Spacelab flights. 2. Missions outside the Magnetosphere We recommend that an Advanced Interplanetary Explorer be launched in the mid-1980's and that opportunities be made available to fly cosmic-ray instruments on this and other interplanetary spacecraft. Such spacecraft provide long- term (about 3 years) exposures outside the magnetosphere for detectors of modest size and cost. Highest priority should be given to detailed measurements of the isotopic composition of cosmic rays at low energies (1 GeV/nucleon) and of solar-flare-accelerated particles. Other scien- tific objectives include measurement of the elemental composition at low energies, detailed studies of the anomalous component, and investigations of particles of interplanetary origin. Simultaneous measurements at different locations in the heliosphere and over a long period of time are necessary. These measurements will

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68 require detectors with geometrical factors of about 100 cm2 sr (1 to 2 orders of magnitude larger than previous instruments) and good mass resolution (0.2 AMU) over the energy region from well below 1 MeV/nucleon to 1000 MeV/nucleon. The appropriate technology is at hand, and the expected scientific return is large. 3. Deep-Space Missions The study of the low-energy interstellar cosmic rays requires the operation of detectors outside the solar system to avoid the perturbing effects of the solar wind. During the 1980's Pioneer-10 will be beyond 20 AU from the Sun, Pioneer-ll will have passed Saturn, and the Voyagers will be traveling between 10 AU and 30 AU. The ISPM probes will be passing over both polar regions of the Sun at distances somewhat over 1 AU. In order to realize the astronomy objectives of these investigations, it is vital that collection of data from these missions by the Deep Space Network continue through 1990. Fur- thermore, in order to distinguish temporal from spatial variations, simultaneous measurements are needed near 1 AU. Opportunities for deep-space observations on future outer planetary missions should be utilized in order to enhance the probability that a properly functioning spacecraft will eventually leave the region of solar modulation, even though the time required for the journey significantly exceeds nominal mission and spacecraft design lifetimes. 4. Balloons High-altitude balloons have been exceedingly successful carriers of cosmic-ray instrumentation in the past, and they will continue to be important in the 1980's. They provide the means to develop and optimize innovative experimental approaches at relatively low cost and with rapid turnaround. Techniques to fly heavy payloads for weeks or months have been proposed. Their development should be supported along with the conventional balloon program. It is extremely important that adequate suppor be made available not only for the development of new instrumentation techniques but also for a broad range of basic experimental and theoretical studies.