Cassini-Huygens Mission to Saturn (1997-2017): Rings and Titan

The Cassini-Huygens mission was a joint project of NASA, the European Space Agency (ESA) and the Italian Space Agency to explore Saturn, launched on 15 October 1997. The Cassini orbiter circled the planet for 13 years, while the European Huygens probe made the first landing in the outer Solar System on the moon Titan in 2005. The mission ended on 15 September 2017, when Cassini was deliberately plunged into Saturn’s atmosphere.

Background

Saturn had been visited only briefly before, by Pioneer 11 in 1979 and the two Voyager spacecraft in 1980 and 1981. These flybys revealed complex rings and showed that Titan, Saturn’s largest moon, has a thick, hazy atmosphere hiding its surface.

Planning for an orbiter with a Titan probe began in the 1980s. The mission was named after two seventeenth-century astronomers: Christiaan Huygens, who discovered Titan in 1655, and Giovanni Domenico Cassini, who discovered four more moons and, in 1675, the dark gap in the rings now called the Cassini Division.

Launch and journey

Cassini-Huygens was one of the largest and most complex interplanetary spacecraft ever built, weighing about 5.7 tonnes at launch with fuel. Because sunlight is weak at Saturn, it was powered by three radioisotope thermoelectric generators, which produce electricity from the heat of decaying plutonium-238. Some groups protested before launch, arguing that an accident could spread radioactive material, but NASA judged the risk to be very small.

A Titan IVB rocket launched the spacecraft from Cape Canaveral on 15 October 1997. It was too heavy to fly directly to Saturn, so it used gravity assists from Venus (twice), Earth and Jupiter to gain speed. Cassini entered orbit around Saturn on 1 July 2004 (30 June in US time zones), after a journey of nearly seven years.

Huygens lands on Titan

On 25 December 2004 Cassini released the Huygens probe, which coasted towards Titan for three weeks. On 14 January 2005 it entered Titan’s atmosphere and descended by parachute for about two and a half hours, photographing river-like channels and hills as it fell.

Huygens landed on a damp, sand-like surface strewn with rounded pebbles of water ice. It transmitted data from the surface for over an hour before Cassini moved out of range. It remains the most distant landing ever made by a human-built spacecraft.

Discoveries

Cassini’s radar and cameras later showed that Titan has lakes and seas of liquid methane and ethane near its poles, such as Kraken Mare and Ligeia Mare. Titan is the only world apart from Earth known to have stable bodies of liquid on its surface, with a weather cycle of methane clouds and rain.

In 2005 Cassini discovered jets of water vapour and ice particles spraying from long cracks, nicknamed “tiger stripes”, near the south pole of the small moon Enceladus. Later flybys showed that the jets come from a global salty ocean beneath the ice crust, and detected molecular hydrogen that could provide energy for microbes. Enceladus became one of the most promising places to search for life beyond Earth.

Cassini also studied a giant storm that circled Saturn’s northern hemisphere in 2010-2011, imaged the hexagon-shaped jet stream around the north pole, discovered several small moons and tracked how the rings change over time. In 2013 it photographed Earth as a tiny dot beneath Saturn’s rings.

Close flybys of other moons revealed their variety. Iapetus has one side as dark as coal and the other bright as snow, and a ridge running around its equator. Hyperion looks like a sponge, and Phoebe, a captured outer moon, appears to have formed in the more distant Solar System. According to NASA, the whole mission cost about 3.3 billion US dollars, most of it paid by the United States, with Europe and Italy contributing the probe, the antenna and other major parts.

The Grand Finale

The mission was extended twice, as the Equinox Mission (2008-2010) and the Solstice Mission (2010-2017). By 2017 Cassini was low on fuel. To avoid any risk of contaminating Titan or Enceladus with Earth microbes, NASA chose to end the mission inside Saturn.

From April 2017 the spacecraft made 22 dives through the narrow gap between Saturn and its innermost rings, a region never explored before, measuring the planet’s gravity and magnetic field and the mass of the rings. On 15 September 2017 it entered Saturn’s atmosphere, sending data until it lost contact and broke up.

By the numbers

Timeline

• 1997, 15 October: Cassini-Huygens is launched from Cape Canaveral.
• 1999, August: The spacecraft flies past Earth for a gravity assist.
• 2000, December: It flies past Jupiter.
• 2004, 1 July: Cassini enters orbit around Saturn.
• 2005, 14 January: Huygens lands on Titan.
• 2005: Cassini discovers plumes erupting from Enceladus.
• 2017, April: The Grand Finale dives begin.
• 2017, 15 September: Cassini plunges into Saturn, ending the mission.

Why it matters

Cassini-Huygens transformed knowledge of Saturn and turned Titan and Enceladus into leading targets in the search for life. It was a model of international cooperation, and its data will be studied for decades, following the path opened by earlier probes such as Pioneer 10.