On a clear day, you may think you can see forever, but even a clean night sky is a barrier to telescopes aimed at distant parts of our universe.
This was the obstacle that the Hubble Space Telescope was designed to overcome.
On April 25, 1990, the space shuttle Discovery placed the telescope into an orbit 381 miles above the surface of the earth, a particularly high orbit so the telescope would not encounter any trace of the Earth's atmosphere.
Scientists were excited. They calculated the 94.5-inch reflector telescope mirror would let the satellite see with 10 times the clarity of an earthbound telescope.
But when they began operating the telescope, they discovered a major problem -- Hubble was nearsighted. The mirror had been improperly curved, so the focus was only a third as sharp as planned.
Because of this carelessness, the telescope's usefulness will be limited until the problem can be fixed, which could take several years.
