Hubble Space Telescope

The Hubble Space Telescope is the descendant of a planned Large Space Telescope, but during the 1980's it was downsized in planning both for psychological reasons, so the word "Large" wouldn't go in the proposal to Congress, and to allow it to fit in the payload bay of a space shuttle.  Lyman Spitzer of Princeton University and John Bahcall of the Institute for Advanced Study, also located in Princeton, were principal scientists who worked not only scientifically but also politically to see the project advance.

The mirror was made by the Perkin-Elmer Company in Connecticut, and was said to be the most perfectly and smoothly shaped telescope mirror ever.  The telescope mirror was completed and the telescope almost ready to be launched when the space-shuttle Challenger exploded in 1984.  With space-shuttle launches suspended, the telescope was put into storage.

The telescope was launched on April 24, 1990, from Cape Canaveral on a space shuttle.  It was named after Edwin P. Hubble, the astronomer at the Palomar Observatory who had discovered the important cosmological law about the expansion of the Universe, linking its rate of expansion linearly with distance.  It received the name in recognition of the prospectively important cosmological work the telescope could do, and in the hope that it could refine Hubble's Law—in particular, Hubble's constant, the constant of proportionality between redshift and distance—to a higher accuracy than had been previously possible.

Over the next months, the systems were turned on and tested.  It soon turned out that no good focus could be achieved, and there was worldwide negative publicity about NASA's supposed failure.  It turned out that though the mirror was made very smooth and passed its tests for shape very well, it was made to slightly wrong specifications, the result of an error in how technicians adjusted the testing apparatus.  Some of the blame has been assessed on the security requirements in the factory that resulted from mirrors for spy satellites being made there, and probably limiting enough access from the variety of astronomers and others who might have found the problem before launch.

In any case, the error turned out to be spherical aberration in the telescope, one of the standard optical aberrations that can be fixed with the equivalent of a person's wearing eyeglasses.

The Hubble Space Telescope's science and public activities are controlled by the Space Telescope Science Institute, which is located in Baltimore, Maryland.

Astronauts from the space-shuttle Endeavour, launched in December 1993, carried instruments that would compensate for the spherical aberration of the main mirror.  The main camera, Wide Field/Planetary Camera (the latter part of its name came because its field of view would be appropriate for imaging Jupiter and Saturn), had always been known to be crucial, so a spare had been available on the ground.  The astronauts, Jeff Hoffman and Story Musgrave, brought up this Wide Field and Planetary Camera 2 (WFPC2), a grand-piano-sized object, and installed it.  It fit into the side of the telescope in the space vacated by its predecessor.  For the other instruments, they were in the axis of the telescope, and NASA had prepared the Corrective Optics Space Telescope Axial Replacement (COSTAR), which fit into a slot from which the least-planned-usage existing instrument had been installed.  COSTAR deployed little mirrors, almost like dental mirrors, on stalks that brought correcting optics in front of the other axial instruments.

The astronauts also brought up new gyroscopes and, to limit the jitter that had been caused by the large, flexible solar panels, they brought up and installed new, more rigid, solar panels.

This servicing mission was carried out by astronauts who were launched in February 1997 on the space-shuttle Discovery.  They installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which carried along a finite amount of coolant.  They also installed the Space Telescope Imaging Spectrograph (STIS).  Both instruments carried their own internal correcting optics; they replaced the Goddard High Resolution Spectrograph and the Faint Object Spectrograph.  They also installed a refurbished fine guidance sensor, a data recorder, a reaction-wheel assembly, and other items.

This servicing mission was carried out by astronauts who were launched in December 1999 on the space-shuttle Discovery.  Since it was urgent to replace some of the gyroscopes, to make sure that control of the telescope was not launched, Servicing Mission 3 was split into two parts.  The astronauts not only replaced all the gyroscopes but they also replaced a fine-guidance sensor, a central computer, and other basic items.

This servicing mission was carried out by astronauts who were launched on the space shuttle Columbia in March 2002.  They installed the Advanced Camera for Surveys.  It has a wider field of view and enhanced sensitivity over the Wide Field and Planetary Camera 2.  They installed a new cooling system for NICMOS (Near Infrared Camera/Multi-Object Spectrograph).  They also put on new, more compact solar arrays, and replaced one of the reaction-wheel assemblies that are used to control steady Hubble's pointing at astronomical objects.

With the declaration that the space shuttles were to be retired in 2010, and that all future missions were to be devoted up until then for completing the International Space Station, NASA Administrator Sean O'Keefe declared a cancellation of the planned Servicing Mission 4.  Following a public outcry and much political maneuvering, a new NASA Administrator, Michael Griffin, was appointed.  He announced the reinstatement of Servicing Mission 4, planned for late 2008.

The mission is to install Wide Field Camera 3, an upgrade in spectral coverage (especially in the infrared), sensitivity, and other matters from WFPC2, to install the Cosmic Origins Spectrograph, to install new gyroscopes, and to make several other repairs and upgrades.  It is also necessary to boost the Hubble Space Telescope in its orbit every few years, to keep it from spiraling slowly down far enough that it would run into enough atmospheric drag to bring it crashing down.

Also, both the Advanced Camera for Surveys and the Space Telescope Imaging Spectrograph have been broken for some time, ACS in 2007 and STIS in 2004.  The astronauts are to try to fix these instruments, though they were not designed for on-orbit repair and so they require the astronaut John Grunsfeld, in spite of his thick gloves, to remove over 100 tiny screws (while taking care that they don't get loose and find their way into the telescope itself).  Grunsfeld will be on his third visit to repair Hubble, having carried out repairs in 1999 (Servicing Mission 3A) and 2002 (Servicing Mission 3B). He will be joined by Michael Massimino, on his second visit to Hubble, having also been there in 2002.  The missions Commander, Scott Altman, also commanded the 1999 mission.

Shortly before the scheduled October 2008 launch of the Servicing Mission 4, a main electrical system, a router, failed on Hubble.  Though a backup was on board, it had not been activated since launch 18 years earlier, and it was thought advisable to postpone the repair mission until a backup system that was available on the ground could be tested and prepared for launch.  This backup system started to work as of late October, bringing the telescope to full usage again.  As of this writing, Servicing Mission 4 is scheduled not earlier than May 2009.

The Hubble Space Telescope has been used by astronomers to study objects in the Universe near and far, excepting only the Sun (which is too bright, but which was indeed detected through the back of the mirror!).  A main reason for the launch of Hubble is that from its perch outside Earth's atmosphere, it can have resolution about 7 times finer than normal ground-based resolution from good telescope sites; that is, it can see detail about 7 times finer.  It had often been loosely said that it could therefore see 7 times farther into space, but large ground-based telescopes had already been observing objects so far in the outer solar system that it was impossible to see 7 times farther.

In the years since Hubble's launch, ground-based capabilities have advanced, and Active Optics has allowed Hubble's resolution to be achieved in certain limited but growing circumstances from ground-based telescopes.  Still, Hubble can attain its high resolution for all objects it observes without complicated post-processing.

Note, however, that Hubble is only one 2.4-m telescope, not large by today's standards. The twin Keck telescopes in Hawaii, for example, have mirrors each 10 m in diameter, about 4 times in diameter and 16 times in area compared with Hubble's.  Thus many astronomical projects, particularly those that require collecting as much light as possible, are better carried out with this new generation of large, mountain-based telescopes, still leaving many, many projects best achieved with the high-resolution of Hubble.

Hubble has been used to observe all types of solar-system objects, including the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Eris, Makemake, Haumea, asteroids, and comets.  It has detected auroras on Jupiter and Saturn, for example, and allowed delicate changes in the atmospheres of the planets to be studied.  It has provided the only images showing structure on Pluto that will be available before NASA's New Horizons mission arrives there in 2015.

Hubble has been used to observe a wide variety of stars, both individual and in groups.  It has even imaged supergiant stars like Betelgeuse with sufficiently high resolution that structure shows on a disk of the stars, which are ordinarily merely points even to large telescopes.

Hubble has been used especially to follow stars in the late stages of their evolution.  Stars with masses like that of the sun become white dwarfs, casting off beautiful structures known as planetary nebulae.  Stars with somewhat higher masses wind up as neutron stars, with the densities of giant nuclei; they are mainly detected in the radio region of the spectrum as pulsars.  Stars that wind up with much more mass have such high gravity (in terms of Einstein's theory of general relativity, have warped space so much) that they wind up as black holes.  The Space Telescope Imaging Spectrograph has recorded motions so fast, and so close in to the center of what seems to be a giant black hole at the center of a galaxy, that only the presence of a giant black hole there can account for the observations.

Hubble also has observed exploding massive stars, supernovae, in many stages, both as individual brightenings in galaxies and as supernova remnants of gas in our own galaxy.

Hubble can observe gas and dust between the stars.  Many of its images are displayed in false color, to show the chemical or molecular constituents of the gas and dust.

Galaxies are basic building blocks of the Universe.  Edwin Hubble not only found his law relating velocity (measured as redshift) and distance but also classified galaxies.  He found classes Sa, Sb, and Sc in order of increasingly open arms for spiral galaxies; SBa, SBb, and SBc in order of increasingly open arms for barred spiral galaxies; and E0 through E7 for elliptical galaxies.  It was his use of the Mount Wilson 100-inch reflector (and later the Palomar 200-inch reflector) that enabled him to pick out individual stars in the closest galaxies to us, still far enough away to enable him to find their distances (using especially Cepheid variable stars, a type of star whose period of variation is linked to its absolute brightness).

Galaxies exist in clusters, surrounding giant voids.  The nearest cluster to us in the direction on of the constellation Virgo (and beyond it), so it is known as the Virgo Cluster.  Studies of the galaxies in clusters allowed Edwin Hubble and more recent astronomers to refine our knowledge of the size and scale of the Universe.

Studies of distant supernovae starting in the 1990s revealed that the most distant ones were fainter than had been expected, indicating that they are farther away than expected.  This idea led to the discovery that the uniform expansion of the Universe is accelerating.  The work has been verified by observations from other spacecraft, such as the Wilkinson Microwave Anisotropy Probe.

The observations of the Hubble Space Telescope are carried out in parallel to many other astronomical observations.  In particular, NASA's Chandra X-ray Observatory and NASA's Spitzer Space Telescope (which works in the infrared) join Hubble as NASA's series of "Great Observatories."  (The gamma-ray Great Observatory, Compton Gamma-Ray Observatory, has long been defunct, but it was replaced in 2008 with the Gamma-Ray Large-Area Space Telescope, renamed after launch after Enrico Fermi, making it the Fermi Gamma-Ray Space Telescope.)

Jay M. Pasachoff and Alex Filippenko, The Cosmos: Astronomy in the New Millennium, 2nd ed. (Cengage Publishing, 2007). http://www.solarcorona.net

Jay M. Pasachoff, A Field Guide to the Stars and Planets, 4th ed. (Houghton Mifflin Co., 2006) http://www.williams.edu/astronomy/fieldguide

Robert Zimmerman, The Universe in a Mirror: The Saga of the Hubble Space Telescope and the Visionaries Who Built It (Princeton University Press, 2008)

Lars Lindberg Christensen, Robert A. Fosbury, and M. Kornmesser, Hubble: 15 Years of Discovery (Springer/European Space Agency, 2006)

Robin Kerrod, Carole Stott, and David S. Leckrone, Hubble: The Mirror on the Universe (Firefly, 2007)

http://hubblesite.org

http://heritage.stsci.edu