Cassini–Huygens is an unmanned spacecraft sent to the planet Saturn. It is a flagship-class NASA–ESA–ASI robotic spacecraft sent to the Saturn system. It has studied the planet and its many natural satellites since arriving there on June 30, 2004, also observing Jupiter and the heliosphere, and testing the theory of relativity. Launched in 1997 after nearly two decades of development, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan called Huygens, which entered and landed on Titan in 2005. Cassini is the fourth space probe to visit Saturn and the first to enter orbit, and its mission is ongoing as of 2014. The two-part spacecraft is named after astronomers Giovanni Cassini and Christiaan Huygens. The spacecraft launched on October 15, 1997 aboard a Titan IVB/Centaur and entered orbit around Saturn on July 1, 2004, after an interplanetary voyage that included flybys of Earth, Venus, and Jupiter. On December 25, 2004, Huygens separated from the orbiter at approximately 02:00 UTC. It reached Saturn's moon Titan on January 14, 2005, when it entered Titan's atmosphere and descended to the surface.