One of the most important contributions to science by Galileo was the invention of the telescope and its improvement.
Galileo was a man with a lot of courage and strong drive for knowledge and exploration, and he had troubles with the authorities (the church) on multiple occasions and he was always on a risk of getting imprisoned or get killed because of his explorations and inventions which were forbidden at the time. Still he managed to secretly construct the telescope and afterwards do some improvements on it, and he used it to explore the space.