Cassini-Huygens is an unmanned spacecraft sent to the planet Saturn. It is a Flagship-class NASA-ESA-ASI robotic spacecraft. Cassini is the fourth space probe to visit Saturn and the first to enter orbit. It has studied the planet and its many natural satellites since arriving there in 2004. Development started in the 1980s. Its design includes a Saturn orbiter (Cassini) and a lander (Huygens) for the moon Titan.
The mission consists of two main elements: the ASI/NASA Cassini orbiter, named for the Italian-French astronomer Giovanni Domenico Cassini, discoverer of Saturn's ring divisions and four of its satellites; and the ESA-developed Huygens probe, named for the Dutch astronomer, mathematician and physicist Christiaan Huygens, discoverer of Titan. The mission was commonly called Saturn Orbiter Titan Probe (SOTP) during gestation, both as a Mariner Mark II mission and generically.
Cassini was conceived to be the second spacecraft in the “Mariner Mark II Series.” NASA was planning a long list of planetary Mariners for the nineties and hoped to dig into planetary explorations. This was not to be completely, however, because of budget limitations. Cassini’s original mission to Saturn was axed, along with the CRAF (Comet Rendezvous Asteroid Flyby) mission, both using the Mariner Mark II Bus.
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, and it landed on Saturn's moon Titan on January 14, 2005. It successfully returned data to Earth, using the orbiter as a relay. This was the first landing ever accomplished in the outer Solar System.
The total cost of this scientific exploration mission is about US$3.26 billion, including $1.4 billion for pre-launch development, $704 million for mission operations, $54 million for tracking and $422 million for the launch vehicle. The United States contributed $2.6 billion (80%), the ESA $500 million (15%), and the ASI $160 million (5%).
Cassini continued to study the Saturn system in the following years. However, since November 30, 2016, due to the spacecraft's dwindling fuel resources for further orbital corrections, Cassini entered the final phases of the project. Cassini passed close to the outer ring of Saturn 22 times, once every seven days, getting the closest look ever at Saturn's outer rings. The first pass of the rings took place on December 4, 2016.
On April 22, 2017, a final flyby of Titan changed the orbit again, sending Cassini through the gap between Saturn and the inner ring; it survived the first pass through the gap on April 26, 2017. After a further 21 orbits through the gap, Cassini will be destroyed by diving into Saturn's atmosphere on September 15, 2017. The decision to end Cassini's mission in 2017 was made in 2010, to prevent the spacecraft from crashing into one of Saturn's moons notably Enceladus or Titan, with possible biological contamination.