The Hubble Space Telescope
M. Colleen Gino
During the last decade of the 20th century a handful of telescopes were constructed with the common goal of peering deeper into space than was previously possible. Arguably, the best known of these instruments are the Hubble Space Telescope and the twin Keck telescopes on Mauna Kea, Hawaii. While both the Hubble and the Kecks accomplished many of their scientific goals and are making valuable contributions to astrophysics, the one that has been the most successful at both contributing to our understanding of astrophysical phenomena and bringing the science of astronomy into the publics awareness is the Hubble Space Telescope (HST).
The Hubble Space Telescope.
Image courtesy NASA.
The Hubble Space Telescope, developed and operated by NASA, was carried into space by the space shuttle in 1990. The HST is capable of observing in the near-infrared, electromagnetic radiation with wavelengths longer than those of visible light, through the visible spectrum, to the ultra-violet, electromagnetic radiation with wavelengths shorter than those of visible light. The Hubble immediately suffered a major setback when it was discovered that its 2.4-meter primary mirror was defective and could not bring light to a focus. Within three years the problem was corrected, however, and since that time the HST has been conducting successful science observations while in orbit at a distance of 600 kilometers above the Earth (1).
The Keck I and Keck II telescopes, completed in 1993 and 1996, respectively, are the largest fully-steerable optical and infrared telescopes currently in operation. The telescopes were funded through grants received from the W. M. Keck Foundation, and are operated by Caltech and the University of California. Each telescope has a 10-meter primary mirror consisting of 36 individually controlled 1.8-meter hexagonal segments that act as a single mirror. The next phase of the Keck Observatory will be to combine the light of the two 10-meter telescopes to produce an optical interferometer, the largest to date (2).
Twin Keck Telescopes atop Mauna Kea.
Image courtesy W.M. Keck Foundation.
Simply stated, the primary function of a telescope is to collect light. The larger the telescope, the more light it can collect. The more light collected, the fainter and more distant the objects that can be observed. With this in mind, how can a telescope that is one-quarter the size of its behemoth brothers ever hope to rise above the competition? By rising above literally.
Artist conception of Hubble Space Telescope in orbit.
Photo courtesy NASA.
The key to Hubbles advantage is that by being in orbit around the Earth, the telescope is above the Earths atmosphere. The light from celestial objects travels through the vacuum of space relatively unhindered. As this light passes through our atmosphere, it is refracted by various amounts and in different directions, depending upon the density of the air it encounters. These air pockets of different temperatures and therefore densities are constantly in motion, resulting in the phenomenon referred to as atmospheric turbulence. The end result of light being diffracted by its passage through the turbulent atmosphere is its apparent twinkling when viewed from Earth. Moreover, when an object is viewed through a telescope at high magnification the object seems to dance wildly about the field of view. This phenomenon, referred to as image motion, results in the photographically or digitally recorded image to appear blurred, and the telescopes resolution, the amount of detail that it can discern in an image measured in arcseconds, is greatly compromised. Since the HST is above the atmosphere, its images do not suffer from this problem as do images taken from ground-based telescopes. However, even ground-based telescopes have developed a way to get around this problem.
Adaptive Optics (AO) is a means by which the incoming planar wavefront of light which becomes corrugated by its passage through the atmosphere, can be once again made planar (3). An AO system employs a wavefront reconstructor to accomplish this task. The wavefront reconstructor consists of a high speed camera that is capable of analyzing the structure of the distorted wavefront. This information is used to provide a list of electronic corrections that are passed on to a deformable mirror. The deformable mirror is then commanded into a shape that matches that of the corrugated wavefront. In this manner the distortions in the wavefront are cancelled out, and the wavefront is returned to the planar shape it had before passing through the atmosphere. The Keck telescopes are equipped with such a system, which when operating at its full capacity, can cancel out most of the effects of atmospheric turbulence. The angular resolution of the resulting images is comparable to those obtained with the HST, but there is one big difference. Images obtained by Keck using the AO system have an extremely small field of view, while those obtained with the HST have an extremely large field of view. This is an important consideration when it is necessary to obtain high resolution images of large areas of the sky.
The HST offers one clear advantage to the astronomical community. Astronomers from any institution worldwide can apply for and obtain Hubble observing time. This is very different from a privately operated facility such as Keck. Keck serves a much smaller user community, that of its funding agents. The University of California and Caltech share the bulk of the time because they paid for construction and most of the operating costs. NASA is currently supplying some operating funds so also shares a portion of the observing time. Finally, the University of Hawaii gets a share of time on both telescopes because the mountain is in Hawaii. Anyone outside of this limited user community must form a collaboration with a member of the community to gain access to the telescopes (4).
HST offers a further advantage to the astronomical community. One need not acquire time on the telescope to have access to its data. All of the scientific data collected by the HST is considered public domain after one year, and therefore is available for anyone to study, including the ambitious student or amateur astronomer. This is not the case with the data obtained with the Keck telescopes.
If publicity is any measure of success, then the HST has been and continues to be immensely successful. The Hubble has been in the public eye since before it pointed its own eyes to the skies. We were told how far it could see and what groundbreaking science it would be capable of while it was still being assembled. Hubbles release from the space shuttle was covered in newspapers, magazines and TV broadcast news. Unfortunately, the HSTs problems were widely publicized as well.
HST images of the spiral galaxy M100 before (right) and after (left) the
correcting optics were installed.
Image courtesy NASA.
Upon taking its first images the unimaginable happened Hubble had opened its eyes only to discover that it was nearsighted. Its primary mirror suffered from spherical aberration and could only bring 20% of the light it collected to a sharp focus with a resolution of 0.1 arcseconds. The remaining 80% of the light formed a hazy halo with a resolution of 1.0 arcseconds around the faint focused image. Such images were no better than what could be produced by a ground-based telescope. Fervent cries rang out, claiming that the $1.5 billion of taxpayer money used to build the HST had gone down the drain. However, NASA rose to the challenge and came up with a solution. True, the Hubble was nearsighted, but its vision could be easily improved with a set of correcting optics. The HST was fitted with the most expensive set of "eyeglasses" ever developed, a set of small secondary mirrors whose curvature cancelled out the error of the curvature in the primary mirror.
When the servicing mission to install the corrective optics was successfully completed, the HST was restored to 97% of its promised operational capabilities. Soon after, not more than a few weeks would pass without another incredible image of star birth, star death or some distant galaxy being plastered on the front page of a newspaper or magazine. We were bombarded with press releases about Hubbles extraordinary accomplishments. Photographs, educational materials and lesson plans were supplied to teachers of all grade-levels. Thanks to NASA and the HST, astronomy became an accessible science and was thrust into the forefront of the publics attention. NASAs efforts in the public arena continue today. Their extensive web site contains hundreds of pretty pictures of celestial objects and their associated explanatory information, up-to-the-minute progress reports on Hubbles activities and observations, a live interactive webcam to observe instrument construction, and even more materials for teachers and students to bring the heavens down to earth.
The HST has many "firsts" under its belt (5). HST gave us our first close-up view of starbirth and planet formation. Direct observations of infalling gas and dust coalescing into new stars, called protostars, were made. Rotating disks of material and energetic jets associated with these new stars were also observered. Protoplanetary disks of gas and dust surrounding new stars were resolved for the first time, and it was discovered that these protoplanetary disks are a common occurrence.
The HST was the first telescope to resolve Cepheid variable stars in moderately distant galaxies. The period of variability of a Cepheid is directly related to its intrinsic brightness, and with that information the distance to the Cepheid can be accurately determined. These distance measurements were then used to recalibrate existing measurements of more distant galaxies. The improvement in distance measurements resulted in a more accurate determination of the Hubble Constant. Determination of this constant, the rate of the expansion of the universe, will enable scientists to better gauge the age of our universe.
The HST imaged several hundred never before seen galaxies in the deepest-ever view of the universe, called the Hubble Deep Field (HDF). Spiral and elliptical shaped galaxies are visible in the image, along with a number of other unusually shaped galaxies exhibiting a variety of colors. Scientists believe that some of these galaxies may have formed less than one billion years after the Big Bang, the event from which our universe was formed.
The HDF reveals an amazing variety of differently
colored and shaped galaxies.
Image courtesy NASA.
Hubble observations have given us insight into objects in our own solar system as well. The HST discovered the disappearance of the Great Dark Spot on Neptune and the subsequent appearance of a new but similar storm in the planets northern hemisphere. The Hubble has imaged auroral activity on Jupiter and Saturn, and mapped the movement of the ice chunks in Saturns rings.
The Keck telescopes have had their own accomplishments. They have conducted spectral analysis of brown dwarfs and other stars that had previously been unobserved due to their small size and low brightness, and have made progress in determining the cause of gamma-ray bursters. They have been responsible for the discovery of planets around distant stars, and determined the orbits of these planets by the gravitational wobble of the parent stars. Many Jupiter-mass planets were found to orbit their stars in days rather than years, a result that has made scientists rethink the current theory of planetary system formation. With the help of their AO system, the Kecks have been able to resolve stars around the central mass of our galaxy and determine their orbital rate, adding to the body of evidence that suggests there is a 2.5 million solar mass black hole in the heart of our galaxy.
Science at What Cost
Compared to the Keck telescopes which cost $94 million (US dollars) apiece, the HST seems to be extraordinarily costly with its initial price tag of $1.5 billion (US dollars). Moreover, the cost did not stop there, but continued to balloon throughout the next several years. By 1992 costs had increased to $2.5 billion (6). By 1999, approximately $3.8 billion had been invested (7). By the time the HST is retired in another ten years, the estimated total cost will be about $6 billion. Spread out over its ten years of development and twenty years of operation, however, the cost is negligible. According to Ed Weiler, the HST space science chief, that amount "...equates to about two cents per week per American taxpayer over that period of time" (8). When put into the proper perspective, the HST would be a bargain at twice the price.
If the Hubble Space Telescopes first ten years of operation are any indication of what we can expect in its next ten years of operation before its planned retirement, then we have much to look forward to. Between 1990 and 2000 Hubble observed over 25,000 astronomical objects, completed more than 330,000 separate observations, and provided the data for more than 2660 scientific papers. For the sheer number of observations completed, objects studied, scientific papers published, educational materials created, and the ability to incite interest and excitement over astronomy in general public, the HST outshines its competition and will likely continue to do so.
(1) The Hubble Project, http://hubble.nasa.gov/overview/
(2) The W.M. Keck Observatory, http://www2.keck.hawaii.edu:3636/
(3) Teare, Scott, Adaptive Optics Wavefront Reconstruction http://www.astro.uiuc.edu/~teare/recon.htm
(4) Schaffer, Barbara, Keck Observing Support Coordinator, April 2002, Personal correspondence
(5) NASA Facts, FS-2000-03-002-GSFC
(6) Carreau, Mark, 1992, Getting Ready to Help the Hubble,
(7) Carreau, Mark, 1999, Astronauts to Make Emergency Flight to Fix Hubble Telescope, http://www.chron.com/content/interactive/space/missions/sts-103/hubble/archive/990311.html
(8) Halverson, Todd, 1999, Hubble Aint Cheap But Cost Effective,