. On this day in 1920 – April 26th – the Shapley-Curtis debate took place on the nature and distance of "spiral nebulae".
Even in 1920, we didn't know what the "spiral nebulae" were. Were they objects in our Milky Way galaxy and therefore comparatively close? Or were they "island universes", i.e., other galaxies, and therefore at a great distance?
. On this day in 2017 – April 26th – the Cassini spacecraft made the first dive of the Grande Finale of its 13-year study of the Saturnian system.
It was the first of a series of close orbits that involved Cassini diving between Saturn and its rings. Astronomers collected data to learn more about the origins, mass, and age of Saturn's rings, as well as the mysteries of the gas giant's interior.
. On this day in 20103 – April 28th – NASA's Galaxy Evolution Explorer (GALEX) was launched
GALEX, which operated until early 2012, was an orbiting space telescope observing galaxies in ultraviolet light. Its observations provided data about how galaxies - the basic structures of our Universe - evolve and change.
. On this day in 1900 – April 28th – Dutch astronomer Jan Oort was born.
Oort made major contributions to the understanding of the Milky Way and was a pioneer in radio astronomy. He realized that the orbits of comets "implied there was a lot more solar system than the region occupied by the planets." The Oort cloud of comets that surrounds the Solar System was named for him.
. On this day in 2013 – April 29th – ESA's Herschel Space Observatory mission ended.
The European Space Agency's infrared observatory was named after William Herschel, who discovered infrared radiation, and his sister and collaborator Caroline Herschel. The observatory was able to see the coldest and dustiest objects in space, but it needed liquid helium to cool its instruments. When that ran out, the mission ended.
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