The Titan Saturn System Mission (TSSM) was a plan created by NASA and the European Space Agency (ESA) to explore Saturn and its moons, Titan and Enceladus. These moons showed many complex events that were discovered by the Cassini spacecraft. TSSM was planned to launch in 2020. After launching, the spacecraft would use the gravity of Earth and Venus to help it travel faster, and it would reach the Saturn system in 2029. The main mission, which lasted four years, included a two-year tour of Saturn, a two-month phase to study Titan's atmosphere, and a 20-month period of orbiting Titan.

In 2009, a mission to Jupiter and its moons was chosen first, even though TSSM was still considered for possible development and launch in the future.

Origin and status

The Titan Saturn System Mission (TSSM) was officially created in January 2009 by combining the European Space Agency's (ESA) Titan and Enceladus Mission (TandEM) with NASA's Titan Explorer (2007) study. Plans to combine these ideas began as early as 2008. TSSM competed with the Europa Jupiter System Mission (EJSM) for funding. In February 2009, NASA and ESA announced that EJSM would receive priority over TSSM. TSSM was still studied for a future launch, planned for the 2020s. In February and October 2009, detailed reports about the mission were shared, along with a proposal for a lake-landing module called Titan Mare Explorer (TiME), which could become part of TSSM. In 2014, it was considered that TSSM might be revived for a launch using the SLS super-heavy-lift rocket.

Mission overview

The TSSM mission includes an orbiter and two Titan exploration probes: a hot air balloon that will float in Titan's clouds, and a lander that will splash down on one of Titan's methane seas.

Both probes will send their data to the orbiter. They will use tools to study Titan's features, including imaging, radar, and sampling of the surface and atmosphere. These tools will provide more detailed information than the Cassini–Huygens mission.

The spacecraft will use gravity assist flybys of other planets to reach Saturn. The original plan was to launch in September 2020, with four gravity assists (Earth–Venus–Earth–Earth), and arrive at Saturn in October 2029. This was one of several possible Earth-to-Saturn transfer options between 2018 and 2022. However, NASA has not prioritized the TSSM mission, and it is unlikely the proposed launch dates will be met.

After arriving at Saturn in October 2029, the orbiter’s chemical propulsion system will place the spacecraft into orbit around Saturn. A two-year mission phase will follow, during which the orbiter will deploy the probes. This phase includes at least seven close flybys of Enceladus and 16 flybys of Titan. Gravity assists and maneuvers will help reduce the energy needed to enter Titan’s orbit. During this time, the orbiter will study cryovolcanic plumes at Enceladus’s south pole.

The Montgolfière, a hot air balloon, will be released during the first Titan flyby to enter Titan’s atmosphere. It will operate for six Earth months, from April 2030 to October 2030. Based on findings from the Cassini–Huygens mission, the Montgolfière should travel around Titan at least once during its mission at a latitude of about 20°N, 10 kilometers above Titan’s surface.

Several ideas have been proposed for the lake-lander concept. One detailed plan is the Titan Mare Explorer (TiME), originally designed as a separate mission but possibly included in the TSSM. If approved, TiME would be released during the orbiter’s second Titan flyby. Because of Titan’s thick atmosphere and distance from the Sun, TiME cannot use solar panels for power. Instead, it will use the Advanced Stirling Radioisotope Generator (ASRG), a prototype designed to provide long-term power for planetary missions. TiME will target Ligeia Mare, a northern polar sea of liquid hydrocarbons at about 79°N. The probe will descend by parachute, similar to the Huygens probe in 2005. During its six-hour descent, it will study Titan’s atmosphere and splash down on the liquid surface. TiME’s main goal is to analyze organics on Titan’s surface for three to six months, marking the first floating exploration of an extraterrestrial sea. Additional instruments on TiME could study the chemical composition and physical properties of Titan’s lakes, including a spectrometer to analyze liquid hydrocarbons, a sonar to map the lakebed, and a weather station to monitor Titan’s meteorological conditions. Mission planners are also considering adding a submersible probe to TiME, which could explore beneath the surface of Ligeia Mare and provide insights into potential prebiotic chemistry in Titan’s seas.

Science goals and objectives

The main goals of the TSSM mission are divided into four categories:

• Study Titan as a complete system.
• Investigate Titan's organic materials and whether they could support life.
• Learn more about how Titan formed and changed over time.
• Gather information about Enceladus and Saturn's magnetic field.

At Titan, scientists want to study the surface's composition and how organic materials are spread across the moon. They will examine the methane cycle, including where methane is stored. They will also look at the age of surface features and whether ice volcanoes or tectonic activity are currently active or only remnants of past activity. Scientists will search for ammonia, a magnetic field, and a hidden ocean beneath the surface. They will study chemical processes in Titan's upper atmosphere and explore the atmosphere between 400–900 kilometers above the surface, which has not been studied in detail since the Cassini mission. Scientists also want to understand how Titan's atmosphere changes with the seasons and how gases escape into space over time.

The TiME lander will land on Ligeia Mare, a large methane sea in Titan's northern hemisphere. Titan's methane cycle is thought to work similarly to Earth's water cycle, with methane existing as both liquid and gas. TiME will study this cycle directly and compare it to Earth's water cycle. However, scientists still do not know where new methane comes from to replace what is lost to space. Studies suggest Titan has a balance of geological and atmospheric processes similar to Earth, making it the most Earth-like world in the solar system. Cassini discovered a deep ocean of liquid water beneath Titan's surface. The TSSM mission would be the first in 50 years of space exploration to conduct a detailed, multi-disciplinary study of Titan's active organic chemistry, climate, and environment on land, in the sea, and in the air.