EOS is the centerpiece of NASA’s Mission to Planet Earth, whose overall goal is to understand the total earth system (air, water, land, life, and their interactions) and the effects of natural and human-induced changes on the global environment. EOS has three major components: (1) a constellation of satellites designed to collect at least 15 years of key climate-related data; (2) a data and information system designed to operate the satellites and process, archive, and distribute the data; and (3) teams of scientists who develop algorithms for converting sensor data into useful information and conduct basic research using the information. The satellites, and data and information system, which will absorb most of the program’s funding, provide the researchers with measurements that will enable them to address established research priorities. EOS is designed to make 24 types of long-term measurements of solar irradiance and the earth’s atmosphere, land cover, ice sheets, and oceans from orbiting spacecraft. By 2002, when the full constellation will be in orbit, EOS will be generating data from 25 instruments on at least 10 spacecraft. Over the 20-year EOS data-collection phase, about 80 instruments will be launched on more than 30 satellites. As currently planned, the last EOS satellite will cease operations in 2020. EOS measurements will support researchers’ efforts to address Mission to Planet Earth’s research priorities: (1) determine the causes and consequences of changes in atmospheric ozone; (2) improve seasonal-to-interannual climate prediction; (3) determine the mechanisms of long-term climate variability; (4) document changes in land cover, biodiversity, and global productivity; and (5) understand earth processes that can lead to natural disasters and develop risk assessment capabilities for vulnerable regions. Mission to Planet Earth is NASA’s contribution to the governmentwide U.S. Global Change Research Program. An important goal of these interconnected efforts is to improve the predictive capability of numerical earth system models, especially global climate models that investigate and predict the general circulation of the atmosphere and ocean. NASA has identified a potentially large and diverse “user community” for EOS-related information. Members of this community could be, for example, educators, businessmen, and public policymakers. The focus of our analysis, however, is the EOS basic research community, by which we mean NASA’s currently funded EOS interdisciplinary science and instrument investigations. In our June 1995 report, we estimated that funding requirements of the EOS baseline program would total about $33 billion for fiscal years 1991 to 2022. This estimate was developed for the program described in NASA’s 1995 EOS reference handbook and included costs for satellites, launch services, data systems, science, construction of facilities, and civil service personnel. However, NASA later recognized that this program was not affordable in an environment of declining budgets and began studying ways to cut costs by using advanced technology and increasing collaboration with other agencies, international partners, and the commercial sector. NASA intended to use these future savings to fund more science under EOS and to reduce the program’s total cost. Over the past several years, the Congress has progressively reduced NASA’s planned spending on EOS for fiscal years 1990 to 2000 from $17 billion to $7.25 billion. In response, NASA changed EOS in 1991 and 1992 from a complete earth system measuring program that would have supported a wide array of global change investigations to a measurement program that will primarily support investigations of global changes to the earth’s climate. For example, NASA dropped the measurement of upper atmospheric chemistry and solid earth processes. Other changes followed in order to further adjust EOS to its progressively lower budget profile through 2000. NASA officials stated the current planned spending for EOS through 2000 is about $6.8 billion. The administration’s fiscal year 1997 request for Mission to Planet Earth is $1.402 billion, of which $846.8 million is for development of EOS’ data and information system, spacecraft, instruments, and algorithms. NASA’s request includes $47.5 million for EOS interdisciplinary science. According to NASA’s 5-year plan based on its fiscal year 1996 budget submission, NASA intends to increase spending on EOS interdisciplinary science to $73.2 million per year in fiscal year 2000. Like EOS-related space systems and information systems, the development of the EOS basic research community that will conduct interdisciplinary global climate change research requires planning. The current number of EOS investigations funded by NASA is relatively small, and NASA recognizes that it needs to increase their number, broaden the membership of EOS science teams, and take other steps to develop and sustain an EOS-era research community. NASA’s strategy for developing the EOS research community is partly based on increased funding. In 1995, it began efforts to fund additional investigations and to reevaluate the current investigations. NASA’s ability to add more investigations is uncertain within its expected future budgets, especially if it must depend on savings from improved technology and increased collaboration with others. The EOS program is currently funding 29 interdisciplinary science investigations that were selected in 1989 and 1990 to use data from EOS instruments in more than one earth science discipline, such as geology, oceanography, meteorology, and climatology. Scientists associated with these investigations serve as members of the Investigator Working Group, developing detailed science plans and assisting NASA in optimizing the scientific return of the EOS mission. Currently, these 29 investigations are led by 31 interdisciplinary principal investigators (2 of the interdisciplinary science investigations have coprincipal investigators). There are 354 coinvestigators associated with the 29 interdisciplinary science investigations, as well as 20 instrument principal investigators/team leaders and 197 other instrument team members. The number of EOS investigations is relatively small when compared with (1) the number of currently funded investigations associated with two pre-EOS missions—the Upper Atmosphere Research Satellite (UARS) and the U.S.-French Oceanography Satellite Ocean Topography Experiment (TOPEX/Poseidon)—to their EOS-era counterparts and (2) the ratio of the number of investigations to the raw data acquisition rate expected from instruments on EOS spacecraft to the number of investigations and raw data acquisition rates of UARS and TOPEX. The comparison is based on the following EOS spacecraft and instruments: AM; PM; Chemistry mission (CHEM); Landsat-7; Radar ALT; Laser ALT; Stratospheric Aerosol and Gas Experiment (SAGE) III on space station; and Solar Stellar Irradiance Comparison Experiment (SOLSTICE), Active Cavity Radiometer Irradiance Monitor (ACRIM), and Clouds and Earth’s Radiant Energy System (CERES) on flights of opportunity. The data rates of the EOS spacecraft and UARS/TOPEX are not strictly comparable because the instruments on the latter satellites do not directly observe the Earth. Imaging instruments are more data intensive than nonimaging instruments. However, data rate comparisons can serve as a rough indicator of the magnitude of potential research opportunities afforded by EOS and two pre-EOS-era missions. The National Aeronautics and Space Administration (NASA) used similar comparisons in its 1993 and 1995 editions of the EOS reference handbook. In the 1995 edition, NASA graphically compared the combined data rates of EOS-era satellites with the combined data rates of numerous pre-EOS-era (including UARS and TOPEX) and foreign satellites to demonstrate that the magnitude of potential research opportunities for EOS is much greater than for other combinations of Earth-sensing satellites. In its handbooks, NASA depicted the data streams flowing from the two groups of satellites to “10,000 users” in the 1993 edition and a more vaguely defined “user community” in the 1995 edition. In place of the broadly defined “users” and user community, we used the actual number of currently funded EOS, UARS, and TOPEX investigations to illustrate (1) that the magnitude of potential EOS basic research opportunities is much greater than those afforded by UARS and TOPEX (as indicated by their respective data rates) and (2) that the number of currently funded EOS investigations is small compared to the number of currently funded UARS and TOPEX investigations. UARS, launched in September 1991, consists of 10 instruments that are measuring the composition and temperature of the upper atmosphere, atmospheric winds, and energy from the sun. The UARS science investigations are led by 22 teams. NASA broadened the UARS science investigations in 1994 by selecting 40 additional teams led by “guest” investigators. It is also funding correlative measurement investigations led by 38 teams to develop an independent database to validate and complement measurements made by UARS’ instruments. In the EOS era, solar energy and atmospheric chemistry measurements will be made principally by the ACRIM, SAGE, and SOLSTICE instruments and the CHEM spacecraft. Currently, only 12 instrument and interdisciplinary science investigations are associated with these instruments and the CHEM spacecraft. In contrast, UARS supports research conducted by 62 instrument and science teams. TOPEX was launched in August 1992 to study the circulation of the world’s oceans. The primary instrument is an altimeter that measures the height of the satellite above the ocean, wind speed, and wave height. NASA and its French partner, Centre National d’Etudes Spatiales, selected 38 science investigations. The 38 TOPEX-related science teams have about 200 members, and NASA plans to solicit additional investigations. In the EOS era, the follow-on mission to TOPEX is Radar-ALT. An instrument team has not yet been selected, but only 7 of the 29 interdisciplinary science investigations currently plan to use Radar-ALT data. There is a large difference between the number of (1) currently funded EOS investigations and the expected volume of data from EOS and (2) the currently funded UARS and TOPEX investigations and volume of data of these two pre-EOS missions.