The earliest savants of rocketry and space travel, beginning with Russian Konstantin Tsiolkovsky in the nineteenth century, envisioned astronomical telescopes in orbit. Shortly after World War II, Lyman Spitzer of Princeton University advanced persuasive arguments for a space telescope with a 400-inch diameter mirror, twice as large as the world's greatest telescope at Mt. Palomar in California. After the National Aeronautics and Space Administration was created, Spitzer's proposal received serious attention but the mirror could be only 94 inches in diameter to enable the instrument to fit into the shuttle. The idea appealed to NASA. It would require astronauts to handle massive objects on space walks and exchange large instruments from time to time, skills useful eventually in the assembly of a space station.
Ground-based telescopes suffer from man-made light pollution and moonlight scattered by dust and aerosols. Even on moonless nights, airglow from the interactions of' electrons and ionospheric ions colors the night sky. On the darkest nights the sky shines as if there were a star of 25th magnitude in every square arc second. Finally, atmospheric turbulence-which makes stars "twinkle"-causes a "shimmering" of the image. In space, such interference would be virtually non-existent.
The telescope, named Hubble in honor of astronomer Edwin Hubble, would be NASA's finest technological achievement and the fulfillment of astronomers' dreams-attaining 10 to 20 times the best resolution possible on Earth and over a broader spectrum from ultraviolet to near infrared. It would also be the most expensive scientific payload ever placed in orbit.
Expectations came to grief. A gross design error in the main mirror was discovered immediately after launch in April 1990. The mirror was too flat by a few percent of the thickness of this sheet of paper but it was derided as the most scandalous mistake in the history of telescopes. NASA and astronomers everywhere were stunned. It worked only about as well as telescopes on Earth, an order of magnitude short of design goals. Various rescue options were frantically considered. Hands-on astronaut repair was chosen at a cost of $700 million.
To fix the Hubble, a device named COSTAR (Corrective Optics Space Telescope Axial Replacement) with more than 5000 parts was designed. Because of the Hubble's modular construction, COSTAR could be substituted for the seldom-used High Speed Photometer. COSTAR contained an array of small mirrors the size of coins with motors to adjust them. In December 1993 the astronauts, in effect, fitted the telescope with "spectacles" to correct its vision. It was a great success. The Hubble images are now remarkably detailed and almost magical in their beauty.
The telescope requires "house calls" by astronauts every few years to install improved instruments and repair damages to elements such as solar power panels. In the February 1997 mission the robotic arm captured the Hubble and secured it in the cargo bay. While orbiting 400 miles above the Earth at 22,000 miles per hour, astronauts replaced the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) with a $125 million Space Telescope Imaging Spectrograph (STIS) and a $105 million Near Infrared Camera and Multi-Object Spectrometer (NICMOS)-each roughly the size of a household refrigerator. STIS can gather spectra simultaneously from dozens of celestial objects, permitting an enormous increase in efficiency. These instruments will attempt to answer questions such as how stars and galaxies age and how many galaxies have black holes in their cores.
The Hubble provides a tenfold greater reach than was possible before in searching the early universe, analyzing its composition, gauging its rate of expansion, and estimating its age. In hunting for evidence of planets outside our solar system, it has focused on the Orion Nebula, a nursery of newborn stars, and found disks of dust clouds enveloping young stars in much the way astronomers believe solar system planets were formed. There is strong consensus among astronomers that quasars derive their almost incredible luminosities from super-massive black holes-millions to billions of suns collapsed into infinitesimal volumes of space in the nuclei of galaxies. Hubble observations may provide clinical data to support current astrophysical models.
While the Hubble's sensational success is appreciated by the public, astronomers using other space platforms are probing the extremes of the electromagnetic spectrum through X-rays, gamma rays, ultraviolet, infrared, and radio wavelengths with comparable success and reaping great rewards from a new generation of ground-based optical telescopes. These new-era instruments evade the degradation of the image by shimmering through the ingenious application of computer technology. For example, "speckle interferometry" stores thousands of digital exposures very rapidly. The computer compares the out-of-register images and superimposes them into a single sharp image. Unfortunately, public fascination and support for these successes falls well short of the euphoria shown for the Hubble.
HERBERT
FRIEDMAN ('64) has been affiliated with the Naval Research Laboratory for 57
years. He now holds the position of Chief Scientist Emeritus of Hulburt Center
for Space Research. He led the way in developing the field of rocket astronomy
and followed by space research using satellites. He has published more than
300 scientific papers and several books. Among his honors: the National Medal
of Science and the Cosmos Club Award.
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