Hidden from the view of a stargazer on Earth, the x-rays that reveal the most violent events in the cosmos find sharp focus with Chandra. The Chandra X-ray Observatory, named for renowned astrophysicist Subrahmanyan Chandrasekhar, is the third of NASA's four Great Observatories (with the Hubble Space Telescope, the now defunct Compton Gamma Ray Observatory, and the soon-to-launch Space Infrared Telescope Facility). Sent into orbit in July 1999, Chandra has electrified astronomy with discoveries about the nature of black holes, the formation of galaxies, and the life cycles of stars.

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Unlike satellites such as Hubble that circle the planet every 90 minutes, Chandra orbits Earth every 2.6 days. It travels a third of the way to the moon to allow longer observation time and to slip beyond Earth's Van Allen radiation belts, which can disrupt imaging.

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What could be emitting x-rays way out there? To find out, astronomers pointed Chandra into the southern hemisphere at a tiny patch of sky that appeared blank. For a total of 11 days over the course of more than a year, Chandra soaked up the faintest x-rays from this remote spot. Like a bore hole into deep space and the distant past, the resulting image, called Deep Field South, clearly showed a great variety of x-ray sources, including luminous quasars and active galactic nuclei powered by supermassive black holes. Chandra is giving scientists a peek at galaxies at a much earlier stage of formation. "That's the excitement," says Harvey Tananbaum, director of the Chandra X-ray Center at the Smithsonian Astrophysical Observatory. "Chandra shows us how spectacular the universe is."

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By the 1950s we had been observing the sky with optical telescopes for 350 years, but the x-ray universe was still an enigma. Earth's atmosphere absorbs x-rays, thus no observations of the high-energy objects that emit them can be done with ground-based telescopes. Young physicists stretched their intellects and imaginations to build detectors that could transcend the obstacles—human or earthly—and observe the elusive particles theorized to zip through space and bombard the upper atmosphere. The post World War II era gave these physicists an opportunity to place their nascent detectors on leftover sounding rockets and launch them a hundred miles (160 kilometers) above Earth.

The first celestial x-ray source was discovered in 1962. It was an unusual bright blue star located in the direction of the constellation Scorpius. The observation lasted all of 350 seconds. Scientists persevered, sending their instruments up on rockets and balloons whenever they could. In 1970 the first Earth-orbiting satellite dedicated to x-ray astronomy—named Uhuru—was launched and operated for three years. A couple dozen missions have flown since, and right now two other orbiting x-ray observatories—NASA's Rossi X-ray Timing Explorer and the European Space Agency's XMM-Newton Observatory—complement the discoveries Chandra is making. X-ray astronomy is giving us spectacular vision of an explosive and turbulent universe. Ultimate recognition of the significance of revealing the x-ray sky came in October 2002 when Riccardo Giacconi, a stalwart x-ray astronomer, was awarded the Nobel Prize in physics.

— Barbara W. McConnell