- Johannes Kepler (1571-1630)
- Born in Germany.
- He discovered that the planets move around the Sun in ellipses, not circles.
- He drew up the three laws of planetary motion.
- He also prepared a new set of planetary tables.
- Born in Holland.
- He believed that the planets were built up gradually from a cloud of dust and gas asssociated with the Sun.
- He discovered that Titan, a satellite of Saturn, has an atmosphere.
- He also discovered Miranda, a satellite of Uranus, and Nereid, a satellite of Neptune.
- He prepared a detailed lunar photographic atlas for space researchers.
- He determined correctly that the Martian atmosphere is mainly carbon dioxide.
- Born in Estonia.
- He studied the site of the 1908 devastation in Siberia, feeling certain that it was caused by a meteorite, but finding no evidence that it was.
Gerard Kuiper (1905-1973)
Leonid Kulik (1883-1942)