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Monthly Discussion
After a decade of operations, observations, and publications, the scientific effectiveness of the Hubble Space Telescope (HST) is being questioned. The telescope’s days had always been numbered. NASA has long planned to end Hubble's spectacular run and bring it down in 2010 to make way in the budget for the James Webb Space Telescope (JWST), scheduled to be launched in 2011.
Still, some astronomers are urging that Hubble's life be extended. They argue that the telescope has grown even more productive in its years in orbit, thanks to periodic service calls by astronauts. These astronomers say that killing Hubble in its prime makes little sense, either scientifically or from the standpoint of public relations. "Hubble is by far the best news NASA has now," a senior astronomer said.
Steven Beckwith, director of the Space Telescope Science Institute, has gathered lots of statistics around the telescope’s successes in an effort to extend its life.* I have retained only a couple of graphs from his reports and tried to see whether the telescope’s planned end makes sense from the point of view of a natural process that approaches completion.
Exhibit 3 shows the evolution of the cumulative number of referred papers based on HST data. (“Referred” papers are publications that have undergone a critical review and selection by a group of scientists). On the same graph I also show the S-curve that fits the process and the delimitations of the various growth phases, “seasons”, as describe in Conquering Uncertainty.
We see that presently the telescope is just coming out of “summer” its most productive season.
Exhibit 3. The growth of scientific publications based on data provided by the HST.
The rate of growth of the purple curve
gives us the life cycle of the process and is shown in Exhibit 4. It depicts
rather eloquently that the HST is at turning point of its productivity
curve.
Exhibit 4. The life cycle of the HST as measured by the number of annual scientific publications.
There is independent corroboration of
the fact that the HST is presently at its turning point. Exhibit 5 shows the
natural-growth life cycle describing the evolution of the percentage of
publications based on HST in the 5 major astronomical journals (The
Astrophysical Journal, The Astronomical Journal, Astronomy and Astrophysics,
Monthly Notices of the Royal Astronomical Society, and Publications of
the Astronomical Society of the Pacific). This metric is different from the
one in Exhibit 4 because it directly takes into account the competition with
other sources of astronomical data.
Exhibit 5. The life cycle of the HST as measured by the evolution of its share in the 5 major astronomy journals.
The fact that the data point for year 2002 is above
the trend in both Exhibits 4 and 5 is not very significant. Such a fluctuation
could be due to the numerous recent discussions generated around the HST in
efforts to extend its life. The conclusion that the HST is entering its fall
season seems sound.
If this is indeed the case, the provisional date of
2010 for brining the HST down appears rather timely. By 2010 the HST will have
completed 97% of its growth potential (see Exhibit 3) and the launching of the
JWST in 2011 can be seen as just-in-time replacement.
Drawing on the seasons metaphor we can say that
during the next five years—HST’s fall—its reason for existence, besides
continuing to collect data, should be to “teach”, i.e. provide knowledge
essential for the success of the follow-up JWST. Granted the winter-like
replacement season toward the end of the decade will be trying for data-hungry
astronomers. But this is the nature of transitional periods. The argument to
extend HST’s life to cover that period does not conform to the natural aspect
of competitive growth.
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* http://sco.stsci.edu/newsletter/PDF/2003/spring_03.pdf
http://www.spaceref.com/news/viewsr.html?pid=9910