With 4,000 published (refereed) scientific papers, Hubble is by far the most successful astronomical project ever. Typically, more than 1000 proposals to use Hubble are received each year, of which some 250 are finally selected. It is a measure of Hubble's continuing relevance that so many proposals continue to come from scientists all over the world.
In its 13 years of surveying the heavens (as of 2003), the NASA/ESA Hubble Space Telescope has made about 553 000 exposures and probed nearly 19 000 celestial targets.
Hubble's list of discoveries is long and it seems unfair to mention just a few. However, some of the most interesting discoveries in the first 13 years of Hubble's life have been:
- The Accelerating Universe: The discovery that the expansion of our Universe is in fact speeding up rather than slowing down was a scientific shock that rocked the world in 1998. Hubble worked together with many different ground-based telescopes, but its contribution was crucial.
- Supermassive black holes: Hubble has penetrated the dense environment of galactic centres and has confirmed previous suspicions by providing decisive evidence showing that supermassive black holes reside at the centres of many galaxies.
- Gamma Ray Bursts: Hubble has provided images that showed unambiguously that gamma-ray bursts are found in galaxies that are forming stars at high rates.
- The Expansion of the Universe: Hubble found the speed of the expansion of the Universe (the Hubble constant) to 70 km/s/Mpc with an uncertainty of about 10 percent by measuring the distances to 18 galaxies using Cepheid variables. This project would have been absolutely impossible without Hubble's resolution and depth of vision.
- Quasars: Hubble unambiguously confirmed that quasars reside in galaxies.
- Jupiter Impact!: Hubble had a grandstand view of the impact of comet Shoemaker-Levy 9 on Jupiter in 1994. The high-resolution images provided exquisite details of plume geometry, of the growth and dispersion of impact features and of atmospheric waves expanding around the impact sites.
- Stars: Hubble has gone beyond the achievements of other observatories by linking together studies of the complete lifecycle of individual stars from birth to death with theories of stellar evolution. In particular, Hubble's ability to probe stars in other galaxies enables scientists to investigate the influence of different environments on the lives of stars. This is crucial in order to be able to complement our understanding of the Milky Way galaxy with that of other galaxies. Hubble was the first telescope to observe white dwarfs directly in globular star clusters.
- Deep Fields: The Hubble Deep Fields are some of the images that have made the greatest impact on observational cosmology so far. These impressive dips into the depths of space and time have allowed astronomers to glimpse the first steps of galaxy formation more than 10 billion years ago and are without doubt some of the great legacies of the Hubble Space Telescope.