Since its launch in 1999, NASA’s Chandra X-ray Observatory has been studying the heavens through short-wavelength x-ray light, the best window for sighting colossal black holes, galaxy clusters and the remnants of violent supernovae. The telescope captures the position, energy and arrival time of each x-ray photon that reaches its detector. That ability, in combination with its uniquely sharp imaging quality and capacity to see x-ray light over a broad range of energies, has revolutionized our view of the x-ray universe. It has changed our understanding of big mysteries such as dark matter, the birth of stars and even the properties of the planets in our solar system.

Chandra was designed to solve a key question in x-ray astronomy: What is the makeup of the diffuse x-ray light that appears to be present in every direction of the cosmos—the so-called x-ray background? It was also designed to be a “general observatory,” with most of the telescope time awarded to scientists around the world working on diverse projects, chosen after an annual call for proposals. Even after two decades of operation, Chandra receives around 500 to 650 proposals every year, which amounts to about 5.5 times more observing time requested than we have to grant—the process is highly competitive.

Chandra has been extraordinarily productive. It achieved its original goal by revealing that nearly all of the mysterious x-ray background light comes from thousands of individual supermassive black holes at the centers of other galaxies. It also revealed new secrets from a host of celestial objects: strong x-ray emission from jets of material flying out of supermassive black holes in the process of gobbling up matter; shining aurorae in the atmosphere around Jupiter; light from colliding neutron stars that were also detected through gravitational waves; and extremely bright star-sized black holes aptly named ultraluminous x-ray sources. Science papers based on Chandra observations number more than 8,000, and our user community numbers more than 4,000 scientists worldwide.

I joined the mission three years before launch as deputy group leader for user support. I was involved with building the Web site and documents to provide information for our scientist users, the first call for proposals and peer review, and the calibration of the telescope as it was prepared for launch at NASA’s Marshall Space Flight Center in Alabama. Although this time was busy and stressful as we pulled everything together for liftoff, it was nothing compared with the first few months after launch.

Chandra has reached its 20th anniversary year, and the observatory is still going strong. I serve as director of the Chandra X-ray Center in Cambridge, Mass., where we run the telescope’s operations. With telescopes coming online now and in the future, such as the Event Horizon Telescope, the James Webb Space Telescope, and many more, we expect Chandra to continue to forge new ground and further expand our knowledge of the hottest and most violent places in the universe for years to come.

*Editor’s Note (1/24/20): This sentence was edited after posting to avoid incorrectly implying that the supernova occurred in the year 1054. It was first observed on Earth at that time but had occurred thousands of years earlier.