On this day in 1999 - December 10th - the European Space Agency (ESA) launched the Newton X-Ray Multi-Mirror Telescope (XMM-Newton).

XMM-Newton, named after Sir Isaac Newton, investigates interstellar X-ray sources, uses narrow- and broad-range spectroscopy, and performed the first simultaneous imaging of objects in both X-ray and optical (visible and ultraviolet) wavelengths. Although planned to be operational for 10 years, the telescope is still in operation.