This stunning spiral galaxy is Messier 100 in the constellation Coma Berenices, captured here by the NASA/ESA Hubble Space Telescope — not for the first time. Among Hubble’s most striking images of Messier 100 are a pair taken just over a month apart, before and after Servicing Mission 1, which took place 25 years ago in December 1993.

After Hubble was launched, the astronomers and engineers operating the telescope found that the images it returned were fuzzy, as if it were out of focus. In fact, that was exactly what was happening. Hubble’s primary mirror functions like a satellite dish; its curved surface reflects all the light falling on it to a single focal point. However, the mirror suffered from a defect known as a spherical aberration, meaning that the light striking the edges of the mirror was not travelling to the same point as the light from the centre. The result was blurry, unfocused images.

To correct this fault, a team of seven astronauts undertook the first Servicing Mission in December 1993. They installed a device named COSTAR (Corrective Optics Space Telescope Axial Replacement) on Hubble, which took account of this flaw of the mirror and allowed the scientific instruments to correct the images they received. The difference between the photos taken of Messier 100 before and after shows the remarkable effect this had, and the dramatic increase in image quality.