INTERNATIONAL COOPERATION ON THE ASCA PROGRAM
University of Tokyo
ASCA, meaning “flying bird” in Japanese, and also the acronym for Advanced Satellite for Cosmology and Astrophysics, is the fourth Japanese cosmic X-ray satellite, launched in February 1993. This mission includes a significant contribution from the United States supported by the National Aeronautics and Space Administration (NASA). The scientific objective of ASCA is to perform high-sensitivity imaging spectroscopic studies of cosmic high-energy phenomena, covering a broad energy band of 0.5-10 keV. In particular, ASCA is the world's first satellite that can take X-ray images of celestial objects in energies above 4 keV. Furthermore, the ASCA instruments have much better spectral resolution than most of the previously flown cosmic X-ray instruments.
The Japanese-U.S. collaboration on ASCA has been implemented in the following way. The X-ray instruments of ASCA have been developed in close collaboration with U.S. scientists based on the agreement between the Institute for Space and Astronautical Science (ISAS) and NASA. These instruments are designed, fabricated, tested, and calibrated as a joint effort of the Japanese and U.S. scientists involved. Also, the U.S side gives a major contribution in the software development and data archiving, and NASA provides partial support to the data acquisition utilizing the Deep Space Network (DSN).
The cooperation has been based on three principles. The first is “no exchange of funds,” for obvious practical reasons. The second is to conduct the collaboration in a grassroots manner based on scientist-to-scientist contact, with the least amount of bureaucratic formality possible. Finally, there should be no “black box”; an instrument fabricated in either Japan or the United States should have collaborating groups in both countries, so that its performance is fully understood by both parties.
The entire ASCA team has been led by the Project Manager Y. Tanaka (ISAS) and Deputy Project Manager H. Inoue (ISAS), while the U.S. participants are represented by S.S. Holt (Goddard Space Flight Center (GSFC)). Japanese institutions participating in the ASCA mission include ISAS, University of Tokyo, Tokyo Metropolitan University, Nagoya University, Osaka University, Kyoto University, RIKEN (The Institute for Physical and Chemical Research), and several other smaller groups. U.S. participation involves NASA/GSFC, Massachusetts Institute of Technology (MIT), and Pennsylvania State University. In addition, the ASCA team also included the international experiment advisors.
2.0 Historical Background
The Japanese-U.S. collaboration in the ASCA mission, formerly called Astro-D, was initiated in the mid-1980s. NASA had the “Great Observatories ” program but also maintained strong interest in international collaborations in the missions of other countries. Since the mid-1980s, Y. Tanaka (then director, Space Astrophysics Division, ISAS) and C. Pellerin (then director, Astronomy and Space Science, NASA) had maintained close contact and had actively discussed a possible NASA contribution in the ISAS missions. (U.S. participation in the Yohkoh mission was one of the outcomes.) Both sides agreed to pursue the collaboration in the Astro-D mission.
From the NASA side, continuity of research with frequent launch opportunities provided by the ISAS M-3SII launcher was appreciated, and participation of the U.S. scientists in the ISAS missions was considered to contribute to excellent science in specific fields.
From the Japanese side, international collaboration was considered important to maximize scientific return within the limited resources. It was believed that by joining the expertise and technology of both sides, the most advanced scientific capability could be realized. Also, the Challenger accident had caused a long hiatus in space research in the United States, with no U.S. X-ray astronomy missions since the Einstein Observatory launched in 1979. As a result, participation in the Astro-D mission received strong support from the U.S. X-ray astronomy community.
2.1 Previous Japanese Cosmic X-Ray Missions
It may be helpful to briefly review the Japanese X-ray astronomy missions preceding ASCA. The first mission, Hakucho (called CORSA-b before launch), weighing only 96 kg, was launched in February 1979 by the M-3C-4 launcher as a purely Japanese project. It provided the community with valuable lessons as to the satellite project in general, although the observation was limited to galactic objects.
The second mission, Tenma (called Astro-B before launch), which was about twice as heavy as Hakucho, was launched in February 1983 using the M-3S-3 launcher. It was also an entirely domestic project. Though rather short lived, Tenma produced a number of fine spectroscopic results, carrying an on-board gas scintillation proportional counter newly developed at ISAS. A number of galactic sources (X-ray binaries, supernova remnants, and so on), as well as a limited number of extragalactic objects (active galactic nuclei and clusters of galaxies), became the research targets.
The third X-ray satellite project, Astro-C, was initiated in the early 1980s. It was expected to use the newly developed M-3SII launcher and to become a 400-kg-class spacecraft. The satellite was planned to use the increased capacity for a large-area proportional counter (LAC) array in order to obtain much higher photon-collection capability than before. Then, in 1981, a proposal came from the United Kingdom to collaborate in this mission. The Japanese community decided to collaborate with the U.K. groups in the preparation of the LAC instrument. In addition, a small gamma-ray burst detector was prepared jointly with a U.S. group. The cooperation evolved quite successfully, and Astro-C was launched by M-3SII-3 on February 5, 1987, and was renamed Ginga. The Ginga LAC achieved superior sensitivity, producing many important results on both galactic and extragalactic objects.
2.2 Planning Phase of Astro-D (ASCA)
In 1984, when Astro-C (Ginga) was still under construction, a working group (WG) was formed in Japan to plan the fourth X-ray mission. Involving virtually the entire Japanese community working on cosmic X-ray research, the WG considered launching the fourth cosmic X-ray satellite using the next-generation M-V launcher, which was then in the planning phase. Meanwhile, however, it became apparent that M-V development would take longer than originally anticipated. Although M-V allows a much larger payload than M-3SII, the WG proposed launching the fourth X-ray satellite in the early 1990s using the operational M-3SII launcher so as not to break the research continuity. This mission was called Spectroscopic X-Ray Observatory (SXO) in the planning stage, was later renamed Astro-D, and was nicknamed ASCA after launch.
The WG agreed that SXO should far exceed Astro-C in sensitivity and exceed Tenma in spectral resolution. To realize such ambitious requirements within payload capability that is essentially the same as Astro-C, SXO was to carry on-board X-ray focusing mirrors, together with imaging spectroscopic X-ray detectors. For the focusing optics, the spacecraft length is too short for an acceptable focal length. An extensible optical bench that is folded during launch and extended in orbit was proposed for a new development program at ISAS.
As to the focal plane instruments to measure the position and energy of each incoming X-ray photon, the WG agreed to put on board several, perhaps two, types of instruments with somewhat different characteristics.
2.3 Collaboration on the X-Ray Mirrors
The mirrors on board SXO were to be of the thin-foil optics type, rather than high-precision polished X-ray optics such as were flown on board the Einstein Observatory and would be flown on the Roentgen Satellite (ROSAT; a German mission in collaboration with the United Kingdom and United States). This choice was almost unique for the mass of the satellite (400 kg) but was needed to achieve a high throughput over a sufficiently wide energy range, which is essential for X-ray spectroscopy.
At that time there were at least two candidate technologies for such X-ray optics. One was the multinested conical X-ray reflectors using gold-coated thin aluminum foils, which had been developed at NASA GSFC by P. Serlemitsos. The mirrors with this technology provided a key element of the Broadband X-Ray Telescope (BBXRT) experiment, which was one of the Astro-1 payloads about to be flown on board the Space Shuttle in 1990. The other technology was similar conical thin-foil reflectors using plastic substrate, developed in Japan at Nagoya University by K. Yamashita, H. Kunieda, Y. Tawara, and their collaborators. The groups representing these two technologies had actually been collaborating for a few years, including participating in mutual exchanges of scientists.
After a series of discussions, both within Japan and between Japan and the United States, it was agreed that the X-ray mirrors on board Astro-D should be a joint U.S.-Japanese project, with P. Serlemitsos (GSFC) being the principal investigator (PI) and H. Kunieda (Nagoya) the co-PI. This collaboration became the XRT (X-Ray Telescope) experiment.
In March 1987, C. Pellerin, then director of the NASA Astronomy and Space Science Division, sent to M. Oda, then the ISAS director-general, a letter of intent expressing the willingness of NASA to collaborate with ISAS on SXO, through the production of X-ray mirrors and related activities. The precedent for this type of collaboration on the experiment level was the successful, ongoing ISAS/NASA cooperation on the Solar-A mission (launched in August 1991 and renamed Yohkoh).
2.4 Approval of the Astro-D Project
The SXO project was proposed to the ISAS Space Science Committee in 1986 as the fourth X-ray astronomy mission including a significant contribution from NASA and was supported by the space scientist community. The mission was then renamed Astro-D and proposed to the Space Activities Commission (SAC) of Japan for the ISAS mission to be launched in early 1993. The proposal was officially approved in 1987 by SAC, and funding started in April 1988. By that time, Astro-C (Ginga) had already been launched into orbit and was producing numerous important results, including the X-ray detection of the supernova SN1987A.
In order to implement the Astro-D mission, an international Astro-D science working group (SWG) was formed. It consisted of those scientists in Japan and the United States who were working directly on Astro-D, including hardware development, software development, spacecraft design/production, and mission operation. The SWG met roughly once per year either in Japan or the United States. The SWG activity lasted not only until Astro-D was launched, but also long after the launch to promote the observational activity.
2.5 Collaboration on the CCD Cameras
As to the focal plane instruments on board Astro-D, two different types of detectors were selected. One is the gas scintillation proportional counter, which was developed in Japan and was used successfully in Tenma. This is the gas imaging spectrometer (GIS) instrument, some details of which are given in Section 3.2.
The other is a solid-state device, which in general has much better energy resolution than the gas detectors but suffers from smaller collecting areas. When the funding started, there were two options: a
silicon PIN-type device or X-ray charge-coupled device (CCD). The former was being studied at ISAS and was thought to be available in Japan, but the expected position resolution was rather inadequate. The latter, under development at Osaka University, was thought to have by far the better position resolution and a significantly better energy resolution than the PIN device then available, but high-quality X-ray-sensitive CCD chips were not expected to become available domestically in time.
At that time the first space-borne X-ray CCD was chosen as the focal plane detector for the Soft X-Ray Telescope experiment, a joint U.S.-Japanese effort, on board Solar-A (Yohkoh). The Yohkoh CCD was chosen from the commercially produced types (specially fabricated by Texas Instruments, Japan), but these CCDs were operated in the ordinary flux integration mode like the optical CCDs. By then, none of the commercially available CCDs could be used for X-rays in the photon-counting mode (i.e., measuring the charge produced by a single X-ray photon).
Meanwhile, efforts to develop X-ray photon-counting CCDs were being carried out by several groups in the United States and Europe. Among them, the MIT group, led by G. Ricker, was making significant progress in the development of the AXAF focal plane detector. That group' s advantage was that high-quality developmental CCDs were produced at the MIT Lincoln Laboratory. The MIT group was able to convince us that their device and related technology was ready for X-ray spectroscopy application in space. NASA also supported the MIT group's involvement.
Accordingly, an agreement was established between NASA and ISAS that the solid-state focal plane instrument would be implemented with two sets of CCD cameras prepared by MIT, as a part of the U.S.-Japanese collaboration, with G. Ricker of MIT serving as the PI and H. Tsunemi of Osaka University as the co-PI. This instrument was called the solid-state imaging spectrometer (SIS). The entire SIS system was completed with frequent exchange of scientists and was thoroughly tested at ISAS by the SIS team.
The Astro-D spacecraft was designed, constructed, and tested in Japan under the strong leadership of Y. Tanaka and H. Inoue. The spacecraft was launched successfully into orbit by ISAS on February 20, 1993, using the M-3SII-7 rocket. The launch was entirely a Japanese task. In orbit, the spacecraft was renamed ASCA.
3.2 Scientific Instruments
As mentioned in Sections 2.3 and 2.5, ASCA carries on board three scientific instruments: XRT, the SIS, and the GIS. The XRT provides the X-ray optics, while the SIS and the GIS serve as focal plane imaging spectrometers with complementary characteristics. The SIS and GIS observe the same target and acquire data simultaneously. An ASCA observer generally uses the GIS and SIS data together.
The XRT consists of four identical multifoil X-ray mirrors and has been developed under U.S.-Japanese cooperation as already described. The four mirrors were fabricated one by one at GSFC under U.S. responsibility and then shipped to Japan, where prelaunch X-ray calibration and environmental tests were carried out under the responsibility of ISAS and Nagoya University. The XRT has quite complicated angular and spectral responses, so that extensive in-orbit calibrations have been conducted as a joint U.S.-Japanese program.
The SIS, a joint U.S.-Japanese instrument, uses two X-ray CCD cameras, which occupy focal planes of two of the four XRTs. As mentioned in Section 2.5, the CCD chips were produced at the MIT Lincoln Laboratory. The CCD cameras and the analog electronics were integrated at MIT under U.S. responsibility. The Japanese collaborators at ISAS and Osaka University took responsibility for
fabricating the digital data processing electronics, as well as the cryogenic system including radiation cooling and heat pipes. The prelaunch tests of the entire SIS system and in-orbit calibration have been conducted as a joint U.S.-Japanese effort.
The remaining two XRTs are coupled to the GIS instrument, which is a Japanese experiment led by T. Ohashi of Tokyo Metropolitan University and K. Makishima of Tokyo University, with collaborators at ISAS and several other Japanese institutions. GIS is a position-sensitive gas scintillation proportional counter, newly developed as an extension of the technology previously developed for the Tenma satellite. All the GIS components have been designed, produced, tested, and calibrated in Japan. However, since the ASCA launch, U.S. involvement in the GIS in-orbit calibration has been extensive.
3.3 Spacecraft Operation
The uplink to ASCA is available only from Kagoshima Space Center, southern Japan, where transmission of all the necessary commands is accomplished during five ground contacts every day. Usually two Japanese duty scientists are attending at Kagoshima and two more at Sagamihara, the ISAS headquarters for ASCA daily operations. About 40 staff scientists, about 10 postdoctorates, and about 80 graduate students make up the available human resources. In addition one or two U.S. scientists are stationed at ISAS to assist the general ASCA program. In principle there is no direct U.S. involvement in the daily spacecraft operation except the data receiving at NASA Deep Space Network (DSN) stations.
The downlink from ASCA is available at Kagoshima, as well as at NASA DSN stations at Canberra, Madrid, Goldstone, and Wallops. The stored data are transmitted to the Kagoshima ground station by a real-time command, whereas data transmission to the DSN stations is automatically done by preloaded programmed commands.
3.4 Data Sharing in the Performance Verification Phase
Following the first 2 months of spacecraft run-up and instrument check-out, the next 6 months were used as the performance verification (PV) phase of ASCA. The strategy during the PV phase features one of the most important aspects of the Japanese-U.S. cooperation on ASCA.
For the purpose of joint observation, the ASCA team was defined as an assembly of about 100 Japanese, about 30 U.S. scientists, and 1 U.K. scientist, who contributed in hardware development, software development, spacecraft construction, observation planning, or spacecraft operation. Essentially, the ASCA team, which includes many graduate students, is an enlarged version of the Astro-D SWG. Then, the observation plan and the target list were created based in principle on discussions among the entire ASCA team. In practice the targets to be observed were divided into the following categories: (1) instrumental calibration targets, (2) stars and cataclysmic variables, (3) X-ray binaries, (4) supernova remnants and rotation-powered pulsars, (5) normal galaxies, (6) active galactic nuclei, (7) clusters of galaxies, and (8) diffuse X-ray background. For each category, one Japanese and one U.S. scientist were assigned as coordinators. After discussing with each other the possible PV-phase targets in their category, as well as gathering ideas and proposals from the entire team, the coordinators came up with a baseline plan for the relevant category. The final PV-phase observation plan was then generated by adjusting these baseline plans from all the categories.
A still more important feature of the PV phase was that all the ASCA data acquired during this time period were made a common property of the ASCA team, that is, they were accessible to any team member. When the time came to analyze the data and write papers, any team member was allowed to sign up for any number of PV-phase targets in which he or she was interested. The author list of a specific publication included practically all the team members who signed up for that particular object and contributed to the paper. Normally the category coordinators assigned one principal member for each object, who coordinated the publication but did not necessarily become the top author. This scheme,
although it appeared somewhat awkward at first sight and forced one-to-one correspondence between targets and publications, in fact worked amazingly well. Further evaluation of this scheme is given in Section 4.0.
3.5 Data Sharing in the Guest Observation Phase
When the PV phase ended, all the ASCA observing time became open to guest observations based on competitive proposals. The available observing time, after reserving about 5 percent for the spacecraft and hardware maintenance, was divided into three sectors: 60 percent for the Japanese investigations, 15 percent for the U.S. investigations, and 25 percent for joint Japanese-U.S. investigations. Of the 60 percent Japanese time, 10 percent was allocated for joint European-Japanese investigations.
The announcement of opportunity for the ASCA guest observation has been issued semi-regularly every year, through NASA and ISAS simultaneously. Basically, proposals from Japanese scientists are sent to ISAS and are evaluated in Japan, while those from U.S. researchers go through the NASA channel and are evaluated in the United States. For European-Japanese proposals, the European Space Agency offers evaluation, and the result is sent to ISAS. The proposals successfully selected via these channels are then submitted to the merging committee consisting of several Japanese and U.S. representatives. The merging committee makes a necessary adjustment of the time share, taking into account the priority of the proposals. In some cases the committee makes recommendations for merging proposals on the same target or moving one into the 25 percent joint Japanese-U.S. time (except European-Japanese proposals). This joint time provides an implicit way of encouraging joint efforts between guest observations from the two countries, beyond the confines of the ASCA team.
3. 6 Data Archiving
Data archiving, which is an important U.S. contribution in the ASCA program, is handled by the High-Energy Astrophysics Center (HEASARC) at NASA/GSFC. All the ASCA data become publicly available after a certain length of time (1 to 1.5 years depending on the condition), and any scientist from any country can have online access to these data by contacting HEASARC. A mirror site exists at ISAS, which is useful for Japanese investigators. HEASARC also provides various services for the convenience of the archival data users worldwide. The assistance of HEASARC is highly appreciated in Japan, because the resources available for these archiving tasks are extremely limited in Japan.
4.0 Lessons Learned
The collaboration on ASCA has been highly successful; it enabled putting into orbit the most advanced cosmic X-ray instruments available at that time, despite severe limitations of the spacecraft resources. It has also enabled maximum use, on a worldwide scale, of this high-performance observatory.
No major problems or fundamental difficulties have occurred in the course of the collaboration. The best way to illustrate the successful aspects of the ASCA collaboration is that it is now being used, with minor modifications of course, as an ideal template for a similar Japanese-U.S. collaboration on the fifth Japanese cosmic X-ray satellite, Astro-E, to be launched in February 2000. A significant portion of the Astro-E team, again composed of Japanese and U.S. scientists, collaborated on ASCA.
The ASCA collaboration has greatly expanded the frontier of X-ray astronomy. As of September 1998, 450 scientific papers on ASCA results have been published in refereed journals. Of these, about 150 have Japanese primary authors, another 150 have non-Japanese primary authors but have Japanese co-authors, and the remaining 150 have no Japanese co-authors. The number of Ph.D. theses written on
the ASCA data now exceeds 45, and of these about 30 are in Japan. A quantitative way of evaluating the scientific outcome of ASCA was provided by the U.S. Senior Review in 1996, which awarded ASCA the second ranking, after the newly launched ISO mission, among various astrophysics missions in which NASA was involved.
One particular benefit brought about by the cooperation is mutual exchange of scientific cultures between Japan and the United States. Obviously, the high-energy astrophysics communities in the two countries have experienced a number of subtle differences in their experiences, methods, attitudes, and mentalities toward solving the same scientific issues. By analyzing the data together and writing a joint paper, people from the two countries became aware of these interesting differences. In particular, many U.S. scientists expressed that the style of the PV-phase investigation (Section 3.4), in which a good balance was achieved between competition and cooperation, was a completely new experience. There will be a similar PV phase for Astro-E, because its merit has been highly evaluated by the communities in the two countries.
Finally, it would be unfair not to mention the very successful Japanese-U.K. collaboration on the preceding Ginga (Astro-C) mission. This collaboration greatly helped the Japanese X-ray community to become international, and the positive experience of this international collaboration encouraged the Japanese scientists to commence a still more extensive international cooperation on ASCA. The Ginga joint effort is also highly evaluated in the United Kingdom, where a strong interest is being expressed as to future U.K.-Japanese collaborations in this research field.
PERSPECTIVES ON ASTRO-D/ASCA
1.0 Historical Background
1.1 Timeline of Major U.S. Events on Astro-D
Fall 1987-Spring 1988—
Presentations made to various National Aeronautics and Space Administration (NASA) advisory committees (e.g., the HEAMOWG) to generate support for U.S. involvement in the Astro-D mission
Committees generally supportive but wanted (1) U.S. costs limited to roughly $10 million, (2) a significant share (a minimum of 15 percent) of the observing time for U.S. astronomers, and (3) all data eventually made available for archival analysis
Astro-D project approved as a mission of opportunity, funded under the International Projects program
January 1989—Charge-coupled device (CCD) detector contract let to Massachusetts Institute of Technology (MIT)
October 20, 1989—NASA agreement letter to Institute of Space and Astronautical Science (ISAS) suggesting the terms and conditions acceptable to the U.S. side
March 15, 1990—ISAS acceptance letter for U.S.-Japanese collaboration on Astro-D
February 1991—Astro-D NASA Technical Plan published
June 1991—U.S. Astro-D users group constituted to provide input and guidance to NASA to help ensure optimum scientific return from the Astro-D mission
February 1993—Astro-D launch, renamed Advanced Satellite for Cosmology and Astrophysics (ASCA)
Spring 1993—First NASA announcement of opportunity for ASCA general observers (GOs)
October 1993—Beginning of ASCA GO phase
1.2 Important Players in the ASCA Project on the U.S. Side
Alan Bunner, Chief, High Energy Astrophysics Branch
Steve Holt, U.S. Astro-D Project Scientist
Nick White, Deputy Project Scientist
Peter Serlemitsos, Principal Investigator (PI), foil mirrors (Goddard Space Flight Center (GSFC))
George Ricker, PI, CCD detectors (MIT)
U.S. members of the International Astro-D Science Advisory Committee:
1.3 U.S. Contributions to Astro-D
Two single photon counting X-ray CCD cameras including analog electronics and thermoelectric coolers
Four multinested thin-foil conical X-ray mirror assemblies (Wolter-I type)
Use of NASA Deep Space Network tracking station in Australia for additional telemetry downlink contacts to increase overall mission efficiency
Development of data analysis and reduction software, maintenance of an ASCA archive, and dissemination of data to U.S. PIs
1.4 History of CCD Development at MIT (Ricker)
1984-1987—NASA supporting research and technology (SR&T) funds ($120,000 per year) to evaluate commercial CCDs (mostly from TI)
1985—Teamed with Penn State (PI) on successful Advanced X-Ray Astronomy Facility (AXAF) proposal for CCD imaging spectrometer. The proposed AXAF devices were fairly conservative but were greatly improved based on the Astro-D experience. AXAF funding was low during Astro-D development.
Late 1980s—Began working with Lincoln Labs
Summer 1987—Ricker convinced Tanaka that CCDs provided considerably better performance than the PIN diodes he was considering at the time for Astro-D. Furthermore, Ricker and his collaborators had built and tested X-ray CCDs, demonstrating both their technical superiority and flight readiness.
1.5 History of Thin-Foil Conical Mirror Development at GSFC (Serlemitsos)
Late 1970s—NASA SR&T funding led to the development of a lightweight X-ray mirror using thin plastic reflectors in the conical approximation. First test of this type of mirror done in 1978 at the X-ray calibration facility of Marshall Space Flight Center
Early 1980s—Successful proposal for a shuttle attached payload experiment called the Broadband X-ray Telescope (BBXRT) using thin metal (aluminum) mirrors with a cryogenic nonimaging solid-state detector at the focus
Early 1988—Rocket launch of a thin-foil mirror telescope to detect X-ray emission from SN1987A (not detected). Experiment performed well, demonstrating flight readiness
1990—Ten-day shuttle flight of BBXRT. Many X-ray sources were detected and spectra from them were accumulated. This flight dramatically demonstrated the richness of X-ray spectroscopy and pointed toward an exciting future for Astro-D.
2.0 Cooperation: Net Benefits of Collaboration
Access to data for U.S. astronomical community—ASCA filled the gap between the Einstein observatory (1978-1981) and Chandra (formerly AXAF) (1999- ). The ASCA mission has resulted in many scientific publications. In addition, pioneering ASCA studies now allow well-focused follow-up observations with more powerful upcoming U.S./European missions. Finally, the development of new models and analyses led to identification of problems in the basic atomic physics of our spectral emission models.
Experience in software development—The only previous GO experience was with the Einstein observatory by the Smithsonian Astrophysical Observatory. In the late 1980s the same group was developing software for the Roentgen Satellite (ROSAT) emphasizing imaging analysis. Largely under the ASCA program, GSFC developed a software system for X-ray data analysis based on the multimission concept involving generic software tools that can be used by different missions. In addition, standards for data formats were established that are now in use virtually worldwide by X-ray astronomy missions.
In-flight experience—ASCA gave the United States the opportunity to incorporate new features and improvements in upcoming missions. The Chandra CCD project has greatly benefited from experience learned from the ASCA mission, which provided the proof of concept for reducing background (from cosmic rays) using charge distribution morphology and verified the model for proton-induced radiation damage on orbit, which allowed better determination of the amount of shielding required for Chandra. The ASCA experience also convincingly showed the need for full bias maps, an on-board gain calibrator, and extensive preflight CCD calibration. The bottom line is that the ASCA experience was worth considerably more than $4.6 million to the Chandra project. Thin-foil mirror technology also benefited from ASCA. Flight improvements growing out of ASCA resulted in significant reduction in surface micro-roughness and better overall optical figure, leading to a factor-of-two improvement in spatial resolution for Astro-E.
3.0 Lessons Learned, Concerns, and Issues for Future Collaborations
U.S. hardware contributions, guest observer facilities, software development, and mission scientist roles must be competed freely through scientific peer reviews. On ASCA and now Astro-E the hardware contributions were awarded to MIT and GSFC through unsolicited proposals to NASA. In these situations good cases could be made that the groups proposing had a unique capability to provide the required hardware. This will not necessarily be the case in the future. In particular, U.S. contributions to Astro-G should be handled through competitive scientific peer review. This will force groups to produce realistic cost estimates, schedule hardware delivery milestones, and strive to provide the best instruments for the allocated funding, which therefore will be in the best interests of NASA and the U.S. government. Moreover, support for the project among the U.S. X-ray astronomy community will be severely weakened if the hardware contributions are not competitively awarded. On ASCA the U.S. science advisors were chosen by Dr. Tanaka. Astro-E science advisors were selected though a competition in the United States and then approved by the Japanese. This policy, or a similar one, needs to be followed in future collaborations.
On the U.S. side, a mechanism needs to be worked out that allows these international missions of opportunity to be peer reviewed in the context of current U.S. missions in a similar price range. This mechanism needs to consider the potential loss of a U.S.-led mission, due simply to lack of funds (zero-sum game). Also, the United States may decline to participate in an international mission of opportunity if it risks a current or planned U.S. mission.
On the Japanese side, it might be helpful if U.S. involvement could be brought in at an earlier stage. As it appears now, planning for new missions on the Japanese side is done entirely in-house and the international community is presented with a rather advanced mission concept. It seems reasonable to suggest that early involvement might result in more and better collaborations.
Personalities and egos are involved so great care must be taken that proper recognition and credit are given for the important contributions made by both sides. ASCA would not have been the great success it was without both the U.S. and Japanese contributions. The organizations and individuals involved on both sides must strive to always highlight the collaborative nature of the mission in their press releases, Web sites, promotional materials, and so on.
ROSAT, AXAF/Chandra, and X-ray Multi-Mirror Mission have all devoted considerable effort and money to carry out extensive preflight ground calibration and end-to-end testing. As Japanese missions become more complex and powerful, it will be expected by the international community that they attain a similar level of calibration accuracy. This might require the development of more extensive ground calibration facilities.
I would like to acknowledge useful discussions with Pete Serlemitsos, George Ricker, Steve Kahn, and Pat Henry on various aspects of the ASCA mission.