It’s the most famous telescope in the world, and April 2015 marks its 25th anniversary. Though it was launched with a flawed optical system, the Hubble Space Telescope has become the most successful telescope ever built. Because it operates in space, the HST’s images aren’t blurred by atmospheric turbulence, and it can observe in wavelengths that don’t reach ground-based telescopes. It is also the most expensive telescope ever built: $5 billions from NASA, plus several hundred million euros from the European Space Agency. But to astronomers who have gained new insights into the workings of the universe, and the general public who have looked in wonder at Hubble’s amazing images, it is money well spent. The Hubble Space Telescope has already exceeded its design life by five years: astronomers expect it to last past 2020.