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.
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. ( BBC )