The data above were extracted from the Essential Science Indicators^SM database from Thomson Reuters. This database, currently covering the period January 1999 through February 28, 2009, 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’s journal-to-category field definition scheme. Both articles tabulated and citation counts to those articles are for the period indicated.

Here our ranking in space science is by citations per paper—among nations that collected 10,000 or more citations during the period—to reveal weighted impact. For articles with multiple authors from different nations, each nation receives full, not fractional, citation credit. Essential Science Indicators lists nations ranked in the top 50% for a field over a given period, based on total citations. In space science, 68 nations are listed, meaning 136 were surveyed. 32 nations collected at least 10,000 citations during the period.