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The Role of Explorers in Astronomy and Astrophysics 8 1. a modestly scaled option for scientific research, 2. a quick response to new scientific opportunities, 3. an extended observing period, 4. access to regions of space optimum for astronomical observations, 5. hands-on experience for university researchers and students, 6. international collaborations, 7. a first step to define and guide the requirements for major missions, and 8. complementarity and extension of the science of maJor m1ss1ons. These are laudable and achievable goals, but to the frus tration of all space scientists, the frequency of flight opportuni ties in the Explorer line has diminished substantially. In the 1 960s, rates of launch were three to four per year. In the late 1970s this figure was reduced to less than one per year. This rate of less than one Explorer per year for all space science dis ciplines is the rate we face in the 1 980s. Moreover, the time elapsed from selection of an Explorer mission to its launch now exceeds 15 years. In part, these problems result from the larger sizes of recent Explorer spacecraft. In astronomy and astrophysics, more complex instruments are frequently required to continue with detailed study after the first exploratory measurements. Also, the funds allocated for the Explorer Program have not kept pace with inflation. Allowing for inflationary growth since 1 965 would lead to a sum of $87M for the Explorer line, when in fact it now stands at $48.2M in FY 1 986. We are deeply worried that the frequency of flight oppor tunities has fallen below the critical level necessary to attract and ensure participation by the best and most imaginative scientific groups. The lead time for Explorer experiments is now so long and the likelihood of timely flight so low that large segments of the community are discouraged. The astronomical community has urged, through CSAA, the Space Science Board
The Role of Explorers in Astronomy and Astrophysics 9 SAS-1 (Uiauru): Discovery of one of the first x-ray pulsars (Cen X-3 ) by Ulauru, the first Small Astronomical Satellite . These systems consist of neutron stars in close of the system via mass transfer to the neutron star . The pulsations seen here are at binary companionship with other stars that can supply the enormous x-ray luminosity the rotation period of the neutron star, 4.84 s. (SSB), and, most recently, the Astronomy Survey Committee, a major influx of funds for the Explorer Program to ensure a minimum healthy level of flight opportunities in astronomy (e.g., one astronomy and astrophysics Explorer opportunity every year). Budgetary restrictions have thus far precluded a major augmentation in the Explorer line and in fact, it was reduced in FY 1 986. As a result, the queue of experiments awaiting flight is long and there appear to be no new dedicated opportunities for flights prior to 1991. This backlog of waiting programs has profound implications: creative new ideas may become stifled for want of opportunity; resources committed to maintaining programs waiting for initiation may be wasted; international endeavors cannot be encouraged; the energetic scientists required to design and operate the instruments may be difficult to find if the lack of creative new endeavors causes them to abandon the field; and the intellectual framework for training new scientists will be in serious jeopardy. The Explorer Program has provided an opportunity for young
The Role of Explorers in Astronomy and Astrophysics 10 research scientists to develop their talents. Their activities include actual hands-on experience with flight hardware, or careful reduction and theoretical analysis of measurements obtained with Explorer missions. Long gaps between Explorer opportunities, and stretched-out schedules will compromise the education and training of this new generation of scientific and technical researchers. It is urgent that this situation be addressed immediately-the programs that have been waiting for the past several years, and new concepts, must be initiated soon if we are to maintain a healthy space science program, and if the United States wishes to continue to be a leader in space science and technology. This report addresses some of the problems stated above, presents an assessment of critical areas, and recommends a phi losophy and guidelines to increase the productivity of the Explorer missions. DEVELOPMENTAL COMPONENTS OF SPACE ASTRO PHYSICS The intellectual and technological developments of space astrophysics contain four components in a synergistic partner ship. ⢠In the discovery component we find qualitatively new phenomena as a direct consequence of a technological advance that allows a wholly new type of observation. Such technologi cal advances may permit observations in a new wavelength range or with greatly improved spectral resolution, sensitivity, angular resolution, or timing ability. Suborbital programs (sounding rockets, balloons, and aircraft) have usually played leading roles in this activity. In many spectral regions astro physical research has accomplished the discovery measurement, but the EUV and submillimeter spectral regimes still have a large component of discovery. ⢠In the survey component, long duration flight experi- ments are used to survey the whole sky at moderate sensitivity
The Role of Explorers in Astronomy and Astrophysics 11 in order to catalog the number of each type of source in order to learn the range of phenomena in the universe, in what environments the sources are located, and what their evolution ary status and physical properties might be from comparison with observations at different wavelengths. In their own indi vidual ways, specific Explorers have played this role for cosmic rays, gamma rays, x rays, ultraviolet, and the infrared . A future Explorer, EUVE, is expected to play this role for the extreme ultraviolet. The only remaining gap that cannot be accomplished through ground-based observations is the submil limeter wavelength interval that another future Explorer, COBE, can partially fill. ⢠Once the different types of sources are located and identified and some of their interesting properties recognized, one can design space missions for specific, detailed studies, the third component in space astrophysics. Such missions will typi cally emphasize one or several of the following: high sensi tivity, high spectral resolution, high timing precision, or high angular resolution. With the greatly enhanced capability in one or more of these areas, such missions can determine the physical properties of the interesting astrophysical objects in great detail and thus begin to answer the major questions of astrophysics and also discover new phenomena for analysis. None of the space missions in this third component is now being built, although XTE is in an instrument study phase awaiting authority to begin development. ⢠The observatory component, represents permanent facilities in space built to accommodate many focal plane instruments, each with extraordinary sensitivity, spectral reso lution, timing precision, and angular resolving power. These observatory-class facilities are designed for periodic refurbish ment with new focal plane instruments at the forefronts of their fields. Each facility services a mature subdiscipline within astrophysics, consisting of a large number of users who will propose, as Guest Investigators, to obtain data for their pro grams. One of these observatory-class facilities, the Hubble Space Telescope ( HST) , is nearly ready to be launched, and another, the Gamma Ray Observatory ( GRO ), is at an advanced stage of construction. Others, the Advanced X-ray
The Role of Explorers in Astronomy and Astrophysics 12 Astrophysics Facility ( AXAF), the Space Infrared Telescope Facility ( SIRTF ), and the Large Deployable Reflector (LDR) , are high priority major missions endorsed by the Astronomy Survey Committee. While each of these components is necessary, this report stresses the contributions to the overall program that can be accomplished with Explorer missions. Explorers are the vehicle of choice for two of these components: surveys and specific studies. RELATION TO OTHER FLIGHT OPPORTUNITIES The NASA program now provides opportunities for space research through a broad range of flight programs, ranging from suborbital vehicles through long duration, observatory class spacecraft. The Explorer Program plays a vital role in the mid-region of this spectrum, providing extended duration expo sure for small to moderate size instruments aboard dedicated free flying spacecraft. It has also provided access to special orbits which are dictated by scientific objectives. Examples include a halo orbit about the Ll libration point in the Earth Sun system, ISEE; geosynchronous orbit, IUE; and polar Sun synchronous orbit, IRAS. Another key aspect of the Explorer Program is its level of-effort character, which has had positive results: very sub stantial scientific returns at a modest cost and a relatively rapid response to new scientific opportunities including interna tional efforts for small instruments. This feature of the pro gram has an even greater importance for astronomy and astro physics as we enter the era of large space observatories. The infrequency and uncertainty of new start opportunities would otherwise preclude the initiation of moderate but scientifically important missions. Level-of-effort programs do have a nega tive aspect as well-the delay and stretching out of all but the smallest missions. In this new era of the Space Station, the Explorer Pro gram, with its high degree of scientific productivity, might well
The Role of Explorers in Astronomy and Astrophysics 13 serve as a model for future level-of-effort programs to utilize the Station and associated platforms for carrying out small, timely experiments.
The Role of Explorers in Astronomy and Astrophysics 14 HOâ¢<b<IOâ¢) - COSMIC RAY ,MATTER INTERACTIONS VELA AND COMPTON RADIATION 0 2 O 10-4 IPSR0833-45l --- COSMIC RAY ,MATTER INTERACT IONS 1 ��EM��C;��N:TOMIC HYDROGE N � ········· ···· ---DIFFUSE + <I> CYGNUS X-3 t t ' � 1.0110 -4 t � ,! � � o m � m � w � � 1 SAS-t: Comparison of '"t-ray brightness of the galactic plane with the distribution of diffuse matter, as represented by several model lines. Discrete sources as well as the diffuse galactic structure appear at different galactic longitudes, with a broad, major excess around the galactic center (.l= 0).
ill. ACCOMPLISHMENTS OF ASTRONOMY AND ASTROPHYSICS EXPLORERS Eight Explorer missions pursued mainly astronomical goals. Many cosmic-ray and gamma-ray experiments were car ried on Explorers devoted to other research disciplines. Explorer activities have also provided for international coopera tion in spacecraft launches and the inclusion of U.S. x-ray instruments on ANS and Ariel-5. These past and planned future Explorer astrophysics missions are listed in Table 1. The major accomplishments of these missions have sub stantially enlarged our understanding of practically every observable quantity in astronomy and astrophysics from radio astronomy to infrared, ultraviolet, x-ray, '""f-ray, and cosmic-ray physics. A brief summary of scientific highlights from the astronomy Explorers follows, and many results illustrate this report. EXPLORER 11: GAMMA-RAY ASTRONOMY Explorer 11, the first astronomical satellite, carried a directional detector designed to search for high energy gamma rays from the decay of neutral pions produced in the interac tions of cosmic-ray nuclei with interstellar matter. Thirty 15
