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Is The Hubble Space Telescope Still Doing Good Science?

By Tim Hunter

 

Introduction- The Hubble Space Telescope (HST)

The Hubble Space Telescope (HST) is one of the most ambitious scientific and exploratory enterprises in human history.  The 2.4 meter Hubble Telescope was launched on April 24, 1990 from the Space Shuttle Discovery and deployed on April 25, 1990.  It cost 1.5 billion US dollars by the time of its launch, and it required four servicing missions (SM1, SM2, SM3A, SM3B) (1993, 1997, 1999, 2002) to correct its initial defective optics and to service and upgrade equipment for routine maintenance and for replacement of obsolete scientific instruments (Hubblesite, 2009; HST Overview, 2010). 

Servicing mission 4 (SM4) was launched May 11, 2009 after much study and no little controversy.  The original SM4 was cancelled following the loss of the Space Shuttle Columbia.  SM4 (also known as STS-125) was an intense mission with the installation of the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS).  The very important spare Science Instrument Command and Data Handling unit was also installed giving a spare system to replace the one that failed in late September 2008.  That failure delayed SM4 from October 2008 until May 2009 (Hubblesite 2010: servicing missions).  During STM4 there was on-site repair of the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS) which had both failed in previous years.  Six new gyroscopes and all of HST batteries were replaced along with a Fine Guidance Sensor. 

 

Figure 1 shows the changes in HST instrumentation from 1990 to 2010:

Hubble instrumentation

From Apai 2010.  See HST Overview for explanation of abbreviations.

 

Hubble was designed for a lifetime of 15 years, but STM4 significantly extended its working life with essentially new instruments and replacement or repair of vital telescope systems.  HST is now equipped with a Soft Capture Mechanism that will allow a robotic spacecraft to attach itself to the telescope to guide it to descent through the Earth’s atmosphere at the end of its life. In the next decade HST is to be complemented and then replaced by the James Webb Space Telescope which will have a 6.5 meter primary mirror and sit at the second Lagrange point (L2) (Hubblesite 2010).

HST is a large telescope, but its size is only modest compared to many considerably larger ground based telescopes, several of which are in the 6-10 meter range.  The Hubble Telescope’s main usefulness comes from it being above most of the Earth’s atmosphere allowing it to have a far better resolving power than ground based telescopes.  It is never bothered by an unsteady atmosphere and cloudy skies.

HST has a low Earth 96 minute orbit at an altitude of 569 km (353.6 miles) inclined 28.50 degrees to the equator.  The Sun is dangerously too bright for HST to observe, and HST requires a target-to-Sun angle of > 50 degrees, thereby avoiding Mercury and Venus which are closer to the Sun.  HST has many instruments, but these will not be discussed in this short essay other than to note they cover the ultraviolet through the infrared portions (115-2500 nm) of the electromagnetic spectrum.

The Hubble Space Telescope is controlled from the Space Telescope Science Institute (STScI) in Baltimore, MD. On a daily basis, the HST takes 3 to 5 GB of data. The STScI makes available to the scientific community on a daily basis between 10 to 15 GB of scientific data (HubbleSite, 2004).  As of October 2009 the HST Data Archive contained over 903,000 individual observations, comprising over 42 terabytes of data.  About 180 new observations are archived every day with the data stored on hard disk arrays and a collection of optical disks for off-site backup storage (Primer for Cycle 19, 2010).

 

What Has HST Produced?

HST’s original job description was:

* Explore the solar system.

* Measure the age and size of the universe.

* Search for our cosmic roots.

* Chart the evolution of the universe.

* Unlock the mysteries of galaxies, stars, planets, and life itself.

Hubble has fulfilled its job description. This is well stated by Livio (2003) who emphasizes “…the Hubble Space Telescope has made some of the most dramatic discoveries in the history of astronomy. From its vantage point 600 km above the Earth, Hubble is able to capture images and spectra that would be difficult or impossible to obtain from the ground.” According to Livio and others, HST has led to cutting edge discoveries in the study of Mars and Jupiter, stellar astrophysics, the interstellar medium, the Milky Way, galaxies, supermassive black holes, and in the determination of important cosmological parameters (Livio, 2003).

Hubble gave us the first close-up view of star birth and planet formation, and Hubble was the first telescope to resolve Cepheid variable stars in moderately distant galaxies (Gino, 2004).  In this way, HST helped derive a more precise value for the Hubble Constant, Ho. The Hubble Constant is a fundamental value that allows us to determine the age of the Universe and the expansion rate of the Universe (Freedman, 2003).

The Hubble Deep-Field (HDF) and the follow-up Hubble Ultra-Deep Field (HUDF) imaged hundreds of never before seen very early galaxies. The HDF and HUDF contain the faintest astronomical objects ever imaged. These two projects are amongst the most successful single scientific experiments of all time (Gino, Meylan, 2004).

According to Villard (July 2010), “…Hubble has made remarkable discoveries across astronomy, rewritten textbooks, been the source of more than 7000 science papers, and reawakened the public to the wonders of the universe.”  Villard lists Hubble’s top 5 science discoveries as the following:

1.  Galaxies evolved from smaller structures.

2.  Supermassive black holes are common in galaxies.

3.  Dark energy exists

4.  The universe’s expansion rate nailed down {see Freedman 2001, 2003; Riess 2009}

5.  Sampling the atmospheres of extrasolar planets.

 

The Controversy – Is HST worth it or what has it done lately?

The Hubble Space Telescope has been extraordinarily successful from a public relations and a scientific point of view, but is it worth the cost?  And what has it done lately?  Has it produced any good science in the last five years? 

In 2004 Gino stated “…by the time the HST is retired…the estimated total cost will be about 6 billion dollars. Spread out over its ten years of development and twenty years of operation, however, the cost is negligible…when put into the proper perspective, the HST would be a bargain at twice the price.” Nevertheless, what is a bargain to one person may seem like a financial disaster to another.  Fortunately, HST’s productivity can be examined by objective scientific standards.  

Publication of scientific results, especially in peer reviewed journals, has long been the established way for scientists to disseminate their work. Publication provides a forum for scientific ideas to be examined, confirmed, refuted, or modified. It is the way science works. The primary scientific product of modern astrophysical experiments and observations are the resulting peer review publications. 

In 2004 Meylan described methodology developed at STScI for estimating the scientific impact of HST. This methodology was developed for two reasons: 1) to monitor Hubble’s operations to maximize its scientific output, and 2) to report objective measures of HST’s output to the public, the scientific community, Congress, and other funding agencies. This evaluation process examines the scientific publications resulting from HST observations, and it examines the citations for papers using HST data.

According to Meylan (2004), “there are two straightforward and relevant measures of the effectiveness of a telescope: the number of refereed papers based on data obtained by the telescope, and the citation count for those papers.” Examining HST using these criteria, shows it has been a most productive resource, and it continues to be one (Apai, 2010). 

A personal review by this author of the monthly Publications of the Astronomical Society of the Pacific (PSAP) from January 2003 through June 2004 showed there were 98 peer reviewed scientific articles in these issues.  Eleven of these articles (11.2%) used data in part or entirely from HST observations.  The articles in PSAP studied by this author include original scientific articles, review articles, instrumentation articles, and theoretical articles.  Letters to the Editor and thesis review articles were not examined in this context. 

When the NASA ADS Data System was cursorily searched by this author on Abstract/Key Words using “HST” for the period of January through June 2004, 1709 listings were obtained.  For the period of January through June 2010, 498 listings were obtained.  When the search was performed using “Hubble Space Telescope,” 906 listings were obtained for the period of January through June 2004, and 13,424 were obtained for the period of January through June 2010.  The quality or range of these listings was not investigated.

The HST related articles in PSAP from January 2003 through June 2004 covered stellar astrophysics, infrared measurements, HST instrumentation, supernovae, the HST Key Project, planetary transits, open clusters, and data archiving.  The distribution of HST proposals across science topics for cycles 12 and 13 (July 2003-June 2005) was similar (Apai, 2010). 

The formal scientific study of HST publications by Meylan (2004) and colleagues showed 90% of HST refereed papers were published in the five refereed astronomical journals. These journals are the Astrophysical Journal (ApJ), the Astronomical Journal (AJ), Astronomy and Astrophysics (A&A), the Monthly Notices of the Royal Astronomical Society (MNRAS), and PSAP.  A significant number of papers using HST data are also published in the more general, but very prestigious, journals Nature and Science.  HST related papers represented a total of 7% of the articles in the peer reviewed astronomical journals in 2003. Interestingly, my superficial review of PSAP described above for the 2003-2004 time frame came up with a somewhat similar figure of 11.2%.

Apai (2010) and colleagues summarized HST’s scientific productivity between 1998 and 2008.  They counted more than 8,000 refereed articles from HST related observations with more than 300,000 citations to those articles.  Ninety-five per cent of the refereed articles were published in 12 journals: Advances in Space Research, Astrophysical Journal, Astronomical Journal, Astronomy and Astrophysics, Icarus, Publications of the Astronomical Society of the Pacific, Monthly Notices of the Royal Astronomical Society, Science, Nature, Astrophysics and Space Sciences, Astrophysical Journal Supplement, Journal of Geophysical Research (Apai, 2010).  In the journals studied by Apai 8 % of the articles were written directly using HST data, and another 13% were influenced in various ways by HST science. 

 

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