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?	Institution	Papers	Citations	Citations per paper
1	Institute for Advanced Study, Princeton	614	26,610	43.34
2	Princeton University	1,674	66,380	39.65
3	University of Chicago	1,401	50,254	35.87
4	University of Durham	1,119	39,263	35.09
5	Carnegie Institute for Science, Washington	1,139	38,535	33.83
6	University of Washington, Seattle	1,110	34,106	30.73
7	United States Navy	1,209	34,838	28.82
8	Space Telescope Science Institute, Baltimore	2,830	80,833	28.56
9	Pennsylvania State University State College	1,549	44,803	28.56
10	Australian National University, Canberra	1,029	29,122	28.30
11	University of California, Santa Cruz	1,576	44,184	28.04
12	University of Cambridge	2,879	78,415	.24
13	University of California, Berkeley	3,447	93,107	.01
14	Ohio State University	1,034	,746	26.83
15	University of Michigan	1,458	93,107	26.56
16	California Institute of Technology, Pasadena	4,989	129,863	26.03
17	University of Hawaii	1,761	45,795	26.01
18	Johns Hopkins University, Baltimore	2,882	73,996	25.68
19	Harvard-Smithsonian Center for Astrophysics	4,654	107,290	23.05
20	University of Arizona, Tucson	3,328	76,222	22.90

The data above were extracted from the Essential Science Indicators database of Thomson Reuters. This database, currently covering the period January 1998 through June 2008, surveys only journal articles (original research reports and review articles) indexed by Thomson Reuters. Articles are assigned to a category based on the journals in which they were published and Thomson Reuters¡¯ journal-to-category field definition scheme. Articles tabulated and citation counts to those articles are for the period indicated. Naturally, institutions publishing large numbers of papers have a greater likelihood of collecting more citations than those publishing fewer papers. This ranking is by citations per paper (impact), among those institutions that have collected 25,000 or more citations in space sciences (which here includes astronomy, astrophysics and atmospheric sciences). For papers with multiple institutional addresses, each institution receives full, not fractional, citation credit. Essential Science Indicators lists institutions ranked in the top 1 per cent for a field over a given period, based on total citations. For the current version, 121 institutions are listed in the field of space sciences, meaning that a total of 12,100 institutions were reviewed to obtain these results. Of the 121 institutions, 35 collected 25,000 or more citations. The ranking by citations per paper (impact) seeks to reveal ¡°heavy-hitters¡± based on per-paper influence, not mere output. For more information on Thomson Reuters¡¯ Essential Science Indicators, see .