Two decades ago, on Dec. 10, 1999, an Ariane 5 rocket climbed into the morning sky from Kourou, French Guiana. It carried into orbit the X-ray Multi-Mirror Mission (XMM-Newton), the largest scientific spacecraft yet built by ESA (European Space Agency) and a pioneering satellite for studying the universe with different kinds of light. XMM-Newton has studied over half a million X-ray sources, including supernovae, star-shredding black holes and superdense neutron stars.
Source:
https://phys.org/news/2019-12-x-ray-satellite-xmm-newton-celebrates-years.html