VOL 20 ISSUE 02 Space Telescope Science Institute Credit: NASA; K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory, University of Arizona, Tucson, Ariz.) Hubble Science Metrics Georges Meylan, [email protected],Juan Madrid, and Duccio Macchetto fter a decade of Hubble operations, observations, and We could apply a number of parameters to evaluate the impact of Hubble publications, the Institute decided it was pertinent to measure observations on astronomical research. The two most objective parameters the scientific effectiveness of the Hubble observing programs. are: (i) the numbers of refereed papers based on Hubble observations, which 2003 To this end, we developed a methodology and a set of can be measured per year or totaled and correlated with specific observing software tools to measure—quantitatively and objectively— programs or not, and (ii) the number of citations generated by each paper. the impact of Hubble observations on astrophysical research. Figure 1 (see page 3) shows the histogram of the number of refereed We gathered Phase I and Phase II information on the observing programs papers, by year, based on Hubble data that appeared in the main A professional journals: Astrophysical Journal, Astronomical Journal, from existing Institute databases, among them MAST (Multimission Archive at Space Telescope). We gathered numbers of refereed papers and their Astronomy and Astrophysics, Monthly Notices of the Royal Astronomical citations from the Library, the Institute for Scientific Information (ISI), and the Society, Publications of the Astronomical Society of the Pacific, and Nature. NASA Astrophysics Data System (ADS), cross-checking information and Following a strong and regular increase of publications during the first eight verifying that our information was complete and reliable. We organized a years of Hubble, the number of papers keeps increasing, although at a unified database with links connecting any specific set of observations to slower pace, during the last four years, to reach a value of 499 for the year 2002. Continued one or more scientific publications. We used this system to evaluate the page 3 scientific outcomes of Hubble observations according to type and time. SPRING DIRECTOR’S PERSPECTIVE Courage and Purpose Steven Beckwith We had almost come to take for granted the precision with which NASA launches astronauts into orbit and brings them safely back home. The fiery breakup of Columbia over Texas showed us how daring each mission really is. On April 24, 1990, STS-31 carried the Hubble Space Telescope aloft to begin its mission of astronomical discovery. The flawed mirror, discovered only after it was in orbit, would have doomed any telescope that could not be serviced. Yet in December 1993, astronauts aboard STS-61 went back to fix the observatory and make it the superlative the first detection of the atmosphere of an extrasolar planet. We could not instrument that it is today. visualize the awesome Eagle Nebula, the auroras on Jupiter and Saturn, nor Not only has Hubble delivered images of the universe that captivate the the rings around SN1987a, and we would not know the value of the Hubble entire world; it has kept getting better as NASA has installed new constant from Cepheids in Virgo galaxies. Without the shuttle astronauts, instruments in subsequent servicing missions. In February 1997, STS-82 we would not have had this window on the universe at all. brought up the infrared camera, NICMOS, and the advanced spectrograph, But—thanks to STS-31, STS-61, STS-82, STS-103, and STS-109— STIS. In December 1999, STS-103 refurbished Hubble’s guidance system Hubble exists, and all it has accomplished is real. Today, Hubble is the and computers. In March 2002, STS-109 installed the Advanced Camera for world’s most recognized and admired scientific instrument. It has helped Surveys (ACS) and revived NICMOS with a novel cooling system. This feat astronomy become the foremost science in the public interest, with a gift for recalled memories of the historic mission in 1993 to fix the telescope itself. educating and inspiring. And people worldwide are excited to be learning Hubble has become an icon of scientific progress and technical where the universe came from and what it is all about. innovation, and its scientific impact continues to grow at an even faster pace Hubble has accomplished so much—and will accomplish so much than in the past. more—because astronauts are willing to risk their lives for science, for a Where would astronomy be without a shuttle program and astronauts to telescope. We are grateful that they go into space to improve our lives and repair and upgrade Hubble? We would not have the Hubble Deep Field. Nor our culture by enabling the discovery of new knowledge. We are both the distances to high-redshift supernovae making the case for dark energy. proud of and humbled by their courage and purpose, and we will always Nor the images of circumstellar disks making planet formation credible. Nor remember their sacrifices when we think of Columbia’s last crew. Ω 2 The current total is over 3570 refereed papers. During the last four years, the number of refereed papers based on Hubble data constitutes about 7% of all refereed papers published in astrophysics, as represented by these journals. # of Refereed Papers Based on HST Data Per Year 499 500 Total # of Refereed 467 Papers = 3577 453 450 431 434 400 360 350 304 300 243 250 200 179 150 109 100 57 50 41 0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Publication Year Figure 1: Histogram of the number of refereed papers, by year, based on Hubble data that appeared in the main professional journals. Figure 2 shows the histogram by year of the mean number of citations of refereed papers based on Hubble data. For comparison, the yellow curve shows the mean number of citations for all refereed astrophysics papers. This shows the impact of Hubble science clearly: the average paper based on Hubble data received twice the citations of the average astronomy paper. Mean # of Citations of Refereed HST Papers by Year 45.0 42.0 40.0 HST 35.0 All Astrophysics 30.0 30.0 30.0 28.0 28.0 26.0 25.0 20.0 19.0 18.0 15.0 12.0 10.0 8.0 5.0 4.0 1.0 0.0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Publication Year Figure 2: Histogram by year of the mean number of citations of refereed papers based on Hubble data. Figure 3 (see page 4) displays the histogram by year of the number of refereed papers based on Hubble data that have not been cited. Understandably, there is a time delay between the publication of a scientific paper and the first paper citing it. After allowing a few years time, only about 2% of the Hubble based papers have no citations, whereas about one third of all refereed papers in astrophysics are never cited. Figure 4 (found on page 4) shows the histogram by year of the number of refereed papers generated by programs in the early Cycles 4 and 5. The peak number of publications occurred about 3 years after the cycle start, delayed by data acquisition, data reduction, analysis, and the publication process. To study the impact of Hubble programs as a function of the number of allocated orbits, we divided them in bins of small (1 to 50 orbits, with sub-bins 1 to 10, 11 to 20, and 21 to 50 orbits), medium (51 to 100 orbits) and large programs (over 100 orbits). Continued page 4 3 Science Metrics Percentage of Refereed HST Papers without Citations from page 3 90% 80% HST 70% All Astrophysics 60% 48.7% 50% 40% 30% 20% 14.6% 8.2% 10% 7.4% 3.0% 3.0% 0% 1.8% 1.8% 2.5% 2.5% 1.4% 0% 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Publication Year Figure 3: Histogram by year of the number of refereed papers based on Hubble data that have not been cited. Distribution of Refereed HST Papers per Year for Cycle 4 & 5 180 166 169 171 160 160 Maximum reached 151149 3 years after the beginning 138 140 of the cycle 123 119 120 95 100 90 Cycle 4 80 71 Cycle 5 60 43 40 22 20 10 7 0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Publication Year Figure 4: Histogram by year of the number of refereed papers generated by programs in the early Cycles 4 and 5. Figure 5 shows the histogram of the mean number of refereed papers generated per program versus program size. On average, large programs have generated five times more papers than small programs. Figure 6 shows the histogram of the mean number of refereed papers generated per orbit. On Mean # of Papers per Program by Program Size 5.00 4.50 4.44 4.00 3.50 3.24 3.24 3.00 2.50 2.00 1.85 1.50 1.26 1.00 0.75 0.83 0.50 0.00 1-10 11-20 21-50 51-100 1-50 51-100 >100 Program Size (orbits) Figure 5: Histogram of the mean number of refereed papers generated per program versus program size. 4 average, large programs have generated three times fewer papers per orbit than small programs. However, it must be pointed out that large programs typically observe significantly fainter targets, which requires more orbits.