Before that happened, however, the space telescope did help scientists test and verify many existing astronomical theories. They used Hubble to follow the string of impacts—each more powerful than all this world's combined nuclear warheads—of the disintegrating comet Shoemaker-Levy 9 into the upper atmosphere of the giant planet Jupiter in 1994, a sobering spectacle that helped build a political consensus that NASA ought to inventory asteroids that might one day strike Earth (an effort that is itself threatened by budget constraints, even though most of the potentially dangerous asteroids remain uncharted). Astronomers produced striking images showing the astonishing and unique beauty of planetary nebulae—the shells of gas ejected by unstable, dying stars—which continue to refine astrophysical accounts of how stars evolve in the late stages of their colorful careers. They captured protoplanetary disks in the Orion Nebula and other star-forming regions, confirming that planets begin as disks of dust and gas, as had been theorized. They discovered several of the now more than 200 known planets orbiting other stars and obtained a spectrum for one of them, the first to show the atmospheric composition of an extrasolar planet. They verified the existence of black holes squatting at the centers of galaxies and nailed down a theoretical link between such black holes and the brilliant beacons called quasars. They confirmed that the mysterious high-energy flashes of light called gamma-ray bursts arrive from all over the universe, and that one class of bursts results from the implosion of massive stars.
But the strangest and least expected discovery, the one that really would "modify profoundly our basic concepts of space and time" as Spitzer had predicted, came the year after he died.
Two teams of astronomers were using Hubble to investigate supernovae—exploding stars—in galaxies long ago and far away. Their prey was a particular class of supernovae whose intrinsic brightness makes them suitable "standard candles" to help determine the change in the rate of the universe's expansion since light left the distant explosions. They were expecting to find that the rate of expansion has slowed over the eons. The idea was that cosmic expansion ought to be braked by the combined gravitational attraction exerted by all the galaxies on one another—in much the same way that a ball, thrown into the air, is slowed down by Earth's gravity. If the cosmic deceleration rate was greater than a certain quantity, the universe would eventually stop expanding and collapse, like a ball falling back to Earth; if lower, the universe was destined to expand forever.