23 December
1973: On 23 December 1973, Gerard Peter Kuiper died.
Kuiper was a Dutch-born astronomer, who is noted for his contributions to knowledge of the Solar System and for his important role in connection with the United States space programme.
His achievements include measurement of the diameter of Pluto, the discovery of satellites of Uranus and Neptune, and the detection of carbon dioxide on Mars. In the 1960s Kuiper served as chief scientist for the Ranger missions to crash-land probes on the Moon. By analysing Ranger photographs, he predicted the safety of the lunar surface for manned landings and helped pinpoint the first landing sites.
1672: On 23 December 1672, astronomer Giovanni Cassini discovered Saturn's moon Rhea, the fifth major satellite of Saturn, which is one of the most heavily cratered satellites in the Solar System.
