Complex sentence: R. Ibata, M. Irwin, and G. Gilmore discover the Sagittarius Dwarf Elliptical Galaxy, a satellite galaxy of the Milky Way, considered the closest galaxy to the Milky Way until 2003.
Split into simpler sentences: R. Ibata, M. Irwin, and G. Gilmore discover the Sagittarius Dwarf Elliptical Galaxy, a satellite galaxy of the Milky Way. It was considered the closest galaxy to the Milky Way from its discovery until 2003.