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History of Cosmic Discovery: Opening New Windows
An old saying goes: Those who don't remember the past are condemned to repeat it. But research astronomers are quite content to repeat the history of their field, and for good reason. If there's one theme that has played itself out over and over in astronomy, it's the fact that whenever scientists open a new window on our Universe, or bring about a major upgrade in instrumentation, a period of remarkable discoveries is bound to follow. We can expect the same for GLAST.

A case in point dates back to dawn of the telescopic era. Although the telescope was invented in Holland, the great Italian Renaissance scientist Galileo Galilei was the first to systematically train this newfangled device on the heavens. In a short period of time starting in 1609, he revolutionized human understanding of our Universe. He discovered the four largest moons of Jupiter and the phases of Venus, which bolstered the Copernican view of a Sun-centered solar system. Galileo saw sunspots, mountains on the Moon, and the rings of Saturn. With just a tiny, crude telescope he resolved the Milky Way into "congeries of innumerable stars distributed in clusters."

image of Galileo Galilei Image Left: Galileo Galilei (1564-1642). Credit: ESA

As astronomers built bigger and better telescopes during the 17th, 18th, and 19th centuries, they discovered new planets (Uranus and Neptune), new moons, asteroids, and entirely new classes of objects, including star-forming nebulae, planetary nebulae, "spiral nebulae," and binary stars. With improved optics, astronomers of the mid-1800s used the parallax method to make the first accurate measurements of the distances to other stars, proving that other celestial objects lie at immense distances from Earth. The development of spectroscopes in the late 19th century began to reveal the composition of stars.

In the early 20th century, researchers developed a powerful one-two combination: photographic film coupled to the first truly large telescopes. Both advances enabled astronomers to see much fainter and deeper objects than ever before. In 1923-24, Edwin Hubble used the 100-inch telescope on California's Mount Wilson to prove that spiral nebulae were actually "island universes" - external galaxies unto themselves. Just five years later, Hubble discovered that our Universe is expanding, laying the foundations of modern Big Bang cosmology.

Arguably the most profound achievement of the 20th century, however, was the opening up of the entire electromagnetic spectrum, some of which is accessible only from space. Discoveries in radio, X-ray, and gamma-ray astronomy shattered the illusions of a sedate, slowly changing cosmos. We live in a Universe of explosions, collisions, and processes involving mind-boggling temperatures and energies that would have been incomprehensible to the astronomers of yesteryear.

Following in the footsteps of the pioneering work of Karl Jansky and Grote Reber in the 1930s and 40s, astronomers came to realize that the radio sky contained a wealth of information about the solar system, stars, galaxies, and even more exotic objects. Thanks to the opening of the radio window, astronomers discovered quasars, pulsars, the cosmic microwave background, the spiral structure of our Milky Way, dark matter, and incredible jets shooting away from the centers of large galaxies. NASA missions such as COBE and WMAP have bolstered leading theories about the origin and possible future of our Universe.

In 1962, Riccardo Giacconi and several colleagues opened up yet another window when they launched a sounding rocket above the atmosphere. An X-ray detector picked up the powerful source Scorpius X-1 and a diffuse background glow of X rays. Subsequent balloon and rocket experiments, followed by satellites such as Uhuru, Einstein, ROSAT, Chandra, XMM-Newton, and Suzaku, have studied black holes, supernovae, relativistic jets, and active galaxies. For pioneering a new branch of astrophysics, Giacconi was honored with the 2002 Nobel Prize for Physics.

Also toward the end of the 20th century, scientists opened up the gamma-ray window, eventually leading to the discovery of gamma-ray bursts, gamma-ray blazars, cosmic-ray interactions, and other phenomena. Infrared astronomy is penetrating star-forming regions and yielding information about extrasolar planets and the disks that give birth to planets.

In recent times, the advent of precision spectrometers has enabled the discovery of more than 200 extrasolar planets. CCDs coupled to the new generation of 8- to 10-meter telescopes, and orbiting observatories such as the Hubble Space Telescope, are seeing back to an epoch when our Universe was just 1 billion years old. By catching supernovae in distant galaxies, these observatories have shown that our Universe’s expansion is accelerating.

Various orbiting and ground-based observatories have opened up almost the entire electromagnetic spectrum, with unexpected discoveries coming with almost every step. But the gamma-ray spectrum from 10 to 100 gigaelectronvolts (GeV) is virtually unexplored. GLAST's primary science instrument, the Large Area Telescope (LAT), will fill in that gap. The EGRET instrument on NASA’s Compton Gamma-ray Observatory saw hints of interesting and unexpected phenomena in that high-energy range, but GLAST will provide our first detailed look at that window. Expect a wealth of discoveries, and some big surprises, from GLAST!

By Robert Naeye