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Current and Future Astronomy and Astrophysics Explorers 26 a LYR AOV >- z a LY R P H O TO S P H E R I C ( ' MO D E L ..... "' ""' a X � -' u.. 1 .0 V I S I BLE LIGHT ____. ..._ 14 13.5 L O G ( F R E Q U E NCY [Hz] l IRAS: A protoplanetary particle shell around the AO star Vega (Ct Lyr ) . The excess over the photospheric continuum at low frequencies indicates the existence or a cool (80 K) shell concentrated some 80 Astronomical Units from the star. ( ) provide a freedom from atmospheric opacity, em iSSion, and ( ) fluctuations in the emission, b full s ky coverage with a single ( ) complement of instruments, c a benign and controlled thermal environment to reduce systematic errors, d the ability to per ( ) form absolute primary calibration in flight without the neces sity of windows to avoid condensation of the atmosphere on () calibrators and instruments, and e sufficient time both to per form tests for systematic errors and to gain the increase in sen sitivity permitted by extended observation time.
Current and Future Astronomy and Astrophysics Explorers 27 COBE will map the spectrum and the angular distribution of diffuse radiation from the universe over the entire wavelength range from 1 p. to 1 em. It will carry three instru ments: a set of Differential Microwave Radiometers operating at 3 1 .4, 53, and 90 GHz ( 1 , 0.6, 0.3 em respectively ); a Far Infrared Absolute Spectrophotometer covering 1 to 100 c m - 1 (1 to 0.01 em) ; and a Diffuse Infrared Background Experiment covering 1 to 300 p. (10-4 to 0.3 em ) . They will use the ideal space environment, a one year lifetime, and standard instru ment techniques to achieve orders of magnitude improvements in sensitivity and accuracy, providing a fundamental data base for cosmology. The instruments are united by common purpose as well as by similar environmental and orbital requirements. The data from all three experiments will be analyzed together, to distinguish nearby sources of radiation from the cosmologi cally interesting diffuse background radiations. 2. Extreme Ultraviolet Explorer EUVE, an approved mission, will survey t)le whole sky in several filter pass bands between 80 and 900 A. The payload also includes three spectrometers which will be used to carry out spectroscopic observationJ> of brightest sources; the bandpass covered is 80-700 A with a resolving power of >.. j D.).. � 100. Until recently, the extreme ultraviolet spectral range was presumed to be of limited interest because the inter stellar medium was expected to block passage of radiation in these wavelengths. However, recent discoveries have shown the density of neutral species is quite low {lo-2 to 10-3 parti that the very local interstellar medium is partially ionized, and cles per cm3 ) in some directions. This low density extends for considerable distances and the interstellar medium in general is very patchy. A number of stellar objects were detected at extreme ultraviolet wavelengths by an experiment on the Apollo-Soyuz mission, by subsequent rocket experiments, by the Voyager spacecraft, and by the European X-Ray Astron omy Satellite ( EXOSAT ) mission. Hot white dwarfs, cool stars, binary systems, cataclysmic variables, some planets, the
Current and Future Astronomy and Astrophysics Explorers 28 interstellar medium and some extragalactic sources should be observable by EUVE. The EUVE spectrometer will be avail able to astronomers on a Guest Observer basis. 3. X-Ray Timing Explorer The XTE, to be launched about 1 992, will be devoted to temporal studies and broadband spectroscopy of compact x-ray sources over the energy range I to 200 keY. The experiments will feature a large photon collection area and an all-sky point ing capability with a complement of three instruments: a 1-m2 effective area xenon proportional counter array, an all-sky mon itor, and a high-energy array of phoswich Nai /Csl detectors with a total effective area of 2000 cm2. Previous studies of compact sources, based upon relatively brief observations with detectors of modest area ( typically a few hundred cm2 or in one case, HEA0-1 , with large areas on a spacecraft with very limited pointing capability ) , have proven to be very rich. The results point directly to fruitful studies possible with XTE, especially concerning neutron stars and black holes. For neutron stars, topics XTE will address include (a) new determinations of binary orbits ( including both very wide and highly compact systems ) and the masses of neutron stars, ( b) variations in spin rates of the accreting pulsars, ( c) "quasiperiodic" oscillations in x-ray intensity, ( d ) phenomena associated with the intense magnetic fields of x-ray pulsars and ( e) x-ray bursts from unstable thermonuclear burning of accreted material. For black holes, XTE will explore the inten sity and spectral variations ( such as the millisecond bursts reported in Cygnus X-1 ) due to accretion instabilities as matter approaches the black hole. Interest in rapid temporal varia tions has been greatly enhanced by the recent exciting discovery with EXOSAT of quasiperiodic oscillations on times cates of tens of milliseconds from a number of galactic bulge x ray sources. XTE will also conduct in-depth temporal and broad-band spectral studies of x-ray novae and other transient phenomena, precession and instabilities in accretion disks, and white dwarfs. The temporal variability of bright active galactic
Current and Future Astronomy and Astrophysics Explorers 29 nuclei is also amenable to study with XTE. In addition to current areas of x-ray timing research, XTE will also permit studies of qualitatively new phenomena on the shortest times cates. For the study of aperiodic phenomena, an increase in collecting area provides a fundamental advantage that cannot be obtained through longer observations. The entire XTE observing program will be dedicated to Guest Observers through competitive proposals. 4. U. S. Participation in the Roentgen Satellite The inclusion of a U.S.-provided high-resolution x-ray imaging detector in the Federal Republic of Germany's Roentgen Satellite ( ROSAT) program represents an important extension of the ROSAT capability to utilize the full angular resolution of the ROSAT x-ray telescope. Participation in ROSAT will provide an ongoing capability for the U.S. astro nomical community to obtain x-ray imaging data in the 12- to 15-year gap between the HEA0-2 (Einstein) and the AXAF missions. Both the High Resolution Imager and the German built position-sensitive proportional counter represent signifi cant improvements in sensitivity over the comparable instru ments on Einstein, but cover a narrower energy band. These imaging detectors can be used to carry out studies of the x-ray emission from stellar coronae, globular clusters, supernova rem nants, normal galaxies, active galactic nuclei and quasars, and clusters of galaxies. Access to both of these instruments will be through a Guest Observer program. 5. Cosmic-Ray Isotope Studies on the Combined Release and Radiation Effects Satellite A state-of-the-art solid state detector telescope for the ele mental and isotopic identification of low energy galactic cosmic rays is to be flown as part of the NASA /Department of Defense Combined Release and Radiation Effects Satellite ( CRRES ) mission. This experiment extends the techniques used on
Current and Future Astronomy and Astrophysics Explorers 30 ISEE-3 to resolve all of the isotopes of the elements from hydrogen through nickel and collect statistically significant samples of all but the rarest of these nuclides. It will be able to search for isotopic effects among elements heavier than silicon that may distinguish between various models proposed to explain the isotopic anomalies observed in lighter elements. 6. Heavy Nuclei Collector on the Long Duration Exposure Facility A large area of passive plastic track detectors ( the Heavy Nuclei Collector - HNC ) will be exposed on the second flight of the Long Duration Exposure Facility ( LDEF ) carrier, and later recovered for laboratory analysis of the tracks of ultraheavy cosmic-ray nuclei. This experiment will increase the total exposure for ultraheavy cosmic rays by more than an order of magnitude, and will individually resolve elements as heavy as uranium. It will be able to determine the relative contributions of r-process and s-process nucleosynthesis to galactic cosmic rays and look for connections between explosive nucleosynthesis and cosmic-ray acceleration. THE FUTURE PROGRAM The dedicated astronomy missions in the current Explorer Program were selected in the 1970s. There exists a wealth of innovative new concepts for Explorer-class missions, which could be initiated when funding becomes available. The Astronomy Survey Committee identified a number of scientific areas that appear to offer special promise for future Explorer-class missions. These include spectroscopic studies of high energy x-ray sources, determination of isotopic and ele mental composition of cosmic rays, soft x-ray measurements, high energy transient phenomena, submillimeter wavelength missions, and optical and infrared interferometers. The Astron omy Survey Committee also discussed several moderate new missions which could be carried out within the Explorer
Current and Future Astronomy and Astrophysics Explorers 31 Program if international collaboration could be enlisted. These comprised, in rough order of priority, a far-ultraviolet spectro graph in space, a space VLB interferometry antenna, and a series of cosmic-ray experiments in space. NASA has issued a Dear Colleague Letter that solicits mission concepts for new Explorers. We expect that the call for proposals to define future Explorer missions will be met with a great response, involving both those missions identified by the Astronomy Survey Committee and new concepts that have been developed since then. Many of the instruments used for remote sensing by astronomers can be profitably applied to problems in planetary science as well. The most difficult task in defining the future Explorer Program will be choosing among the many important missions that will be proposed.
Current and Future Astronomy and Astrophysics Explorers 32 I I I� w , , I I U IO I I z � I ffi , �\ <( / �o�t� � P w � I ' 1\ I a 1 � 1()- > ...J F I I \1 eo \1 � w a: 0 r Sc ·- IMP Serie1: (atomic numbers 2 through 28) in the cosmic radiation were measured on the IMP The relative abundances of heavy elements from helium through nickel series or Explorer satellites . These values (solid circles) are compared with solar system abundances (diamonds) and with more recent cosmic-ray measurements at higher ener gies (open circles). The close agreement between cosmic-ray and solar system abun dances or the major elements provided the first proof that cosmic-ray material is pro duced by normal processes or stellar nucleosynthesis. The large excesses or the rare ele ments Li through B and Sc through Mn have been used to measure the amount or interstellar matter encountered during the 10 million year confinement time or cosmic rays in the galaxy.
33 V. CONSIDERATIONS FOR AN EFFECTIVE PROGRAM The long list of accomplishments of the Explorer Program largely resulted from the program philosophy of frequent flight opportunity, rapid response to scientific objectives, and subor dination of other goals to the pursuit of first-rate scientific research. The increase of costs, which has come about for a variety of reasons, has undermined that philosophy and can diminish the scientific yield of the program. The original philo sophy of the Explorer Program should be reaffirmed, and active steps need to be taken to allow the program to carry out a wide variety of scientific missions in space at a modest cost and on a relatively short time scale. SPECIFIC COST DRIVERS Increases in the costs of carrying out Explorer projects have escalated significantly over the past decade, resulting, in many cases, in long delays in carrying out missions. For the Explorer Program to continue to provide the maximum scien tific return, the factors driving these cost increases must be understood and controlled. Significant cost drivers are present in four major areas: instrument complexity, spacecraft com plexity, project engineering and management, and the interface with other NASA programs.
