Scientists in the US have also been studying a Titan balloon concept
A mission could be launched before the end of the next decade to put a balloon on Titan, the hazy Saturnian moon.
The balloon is one of several ideas being considered by the European Space Agency as it sketches out where its science should be focussed in future.
Other proposals include an X-ray telescope that flies in two parts; and a sample-return mission to an asteroid.
All the ideas will be subjected to further study; and are likely to evolve as international partners get involved.
Eventually, two missions will be selected, one to fly no earlier than 2017 and the other no earlier than 2018.
Esa's future-scoping project is known as Cosmic Vision. It assesses the big questions currently in space science and then tries to find mission architectures that can best deliver the answers.
There are two categories: large (L-Class), which will cost Esa something in the region of 650m euros; and medium (M-Class), which is projected to cost the agency about 300m euros.
Xeus would fly two elements in formation
For the big missions, international partnerships are necessary because the costs involved are so great. A recent US space agency (Nasa) report found that no meaningful mission to the Saturnian system could be undertaken for less than $1bn (700m euros) and would in all events cost considerably more.
The Cosmic Vision 2015-2025 candidates will undergo an internal review in Esa before contracts are awarded to industry to carry out feasibility studies.
The L-Class candidates include:
- Laplace: This mission would go to Jupiter and its moons. A key target of interest would be the icy moon Europa which is thought to harbour an ocean under its icy crust. The mission would deploy three orbiting platforms to perform coordinated observations of Europa, the other Jovian satellites, Jupiter's magnetosphere and its atmosphere and interior.
- Tandem: The mission would explore both Titan and Enceladus, the other Saturnian moon currently fascinating scientists. The mission would carry two spacecraft - an orbiter and a carrier to deliver an instrument-carrying balloon and three probes on to Titan.
- Xeus: This next-generation telescope would study the X-ray Universe. It comes in two parts: a mirror satellite and a detector satellite which have to be flown in formation with extreme precision.
The M-Class candidates include:
- Cross-scale: A swarm of 12 spacecraft to make simultaneous measurements of plasma (charged gas) surrounding Earth.
- Marco Polo: A sample-return mission to a near-Earth object. It would consist of a mother satellite which would carry a lander, sampling devices, re-entry capsule as well as instruments.
- Dune and Space: These are two mission ideas before Esa that would tell us more about the mysterious "dark matter" and even stranger "dark energy" that seem to dominate our Universe but which have proven frustratingly difficult to explain with current observation technologies.
- Plato: A mission to find and study planets beyond our Solar System. It would be capable of observing rocky (similar to Earth) exoplanets around brighter and better characterised stars than its predecessors, such as the recently launched Corot mission.
- Spica: The Japanese are proposing a mission which would launch a telescope to study the cosmos at far infrared wavelengths. If Europe became involved, it would bring expertise and technology developed for its own Herschel telescope due to launch next year.
At the end of the assessment process, it is likely Esa will select just one L-Class and one M-Class to take forward to full development and launch.
One complicating factor is the desire to loft an observatory into space that can test a key prediction of Albert Einstein's Theory of General Relativity - by making a detection of gravitational waves.
Gravitational waves are an inevitable consequence of the Theory of General Relativity
They describe the gravity force as distortions made by matter in the fabric of space-time
Any moving mass will produce waves; they are expected to propagate at the speed of light
Detectable sources to include exploding stars, merging black holes and neutron stars
If Lisa is made to work, it would see remnant radiation from the Big Bang itself
Europe and the US are together developing a mission known as Lisa which would detect these "ripples" in the fabric of space-time - but it is proving an immense challenge from a technological standpoint.
Whether or not one of the L-Class missions listed above gets to be developed may depend on how well, or not, progress is made on Lisa (Laser Interferometer Space Antenna).