The impact of research in the Space Life and Physical Sciences (SLPS) is compared to that of similar ground-based research over a 20-year period. As SLPS publications are cited initially much lower than similar ones of ground-based research, use of a short-citation window (1--4 years) is detrimental in impact assessment of SLPS in comparison with ground-based research. The citation peak of space research is reached much later than that of ground-based research. During the first 10 years after publication of space research, its field-normalized impact is way behind comparable ground-based research, but it catches up during the next 10 years, although not completely. According to experts, the intermittent character of SLPS research may have contributed to this pattern. Bibliometric indicators should be calibrated to provide optimal monitoring of the citation impact of SLPS research, and of topics that face similar conditions. Copyright The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please email: firstname.lastname@example.org, Oxford University Press.