By Paul Rincon
Science reporter, BBC News, Houston
The European and US space agencies are moving ahead on their next major missions to explore the Solar System.
What should follow the great success of Cassini-Huygens?
Nasa has begun choosing a destination for a "flagship" robotic venture along the lines of Cassini-Huygens, which has been exploring Saturn and its moons.
It is considering four targets: the Jupiter system, Jupiter's moon Europa, and Saturn's moons Enceladus and Titan.
The European Space Agency has called for proposals for one flagship mission and another medium-sized mission.
Europa, Titan and Enceladus are also among the destinations expected to be proposed under the European Space Agency's (Esa) "Cosmic Visions" programme of exploration.
Other proposals likely to be submitted include a mission to return soil from a near-Earth asteroid.
"There are some ambitious projects being proposed," said John Zarnecki, director of the Centre for Earth, Planetary Space and Astronomical Research (Cepsar) at the Open University, UK.
"We've been talking about them for months or even years. Teams are together and proposals are being prepared."
And many of the issues are topics of discussion here in Texas this week at the 38th Lunar and Planetary Science Conference.
Professor Zarnecki is part of an international consortium behind a mission to Europa proposal being prepared for submission to the European Space Agency.
He is also involved with a team hoping to mount a European return to Titan. This time, a probe might survey the saturnian moon's surface by balloon.
The American and European agencies are developing their space exploration programmes separately, but both say co-operation further down the line is a distinct possibility.
Nasa and Esa are looking at similar timescales for the launch of their respective missions. Nasa is planning for a launch date sometime after 2015; Esa plans its missions for the period between 2015 and 2018.
Some scientists would like to go back to Titan
The US space agency (Nasa) has drawn together four science definition teams to investigate one target each. They will scope out how the missions might be performed.
Curt Niebur, discipline scientist at Nasa's headquarters in Washington DC, told the BBC News website: "All three of these worlds hold our interest because of their unusual similarities to Earth and the knowledge they can give us about how life does, or does not, start.
"Chemically, Titan resembles a primordial Earth, and it can serve as a laboratory to show us how unusual and complex organic chemistry takes place."
Under the ice
A probe to Europa has been on the wish lists of planetary scientists for a decade.
Thought to host an ocean of water under its icy shell, this Jupiter moon is considered to be one of the best places in the Solar System to search for extraterrestrial life.
"It has the three ingredients that life needs: liquid water, energy and nutrients," said Dr Niebur.
Professor Ron Greeley, a planetary scientist at Arizona State University, has been appointed to co-chair the Nasa science definition team for a Europa mission. He told BBC News: "We fly one flagship roughly every decade; it takes a lot of resources to do that category of mission.
Europa may have oceans and micro-organisms under its ice
"We will determine what the major science goals are and how to do them. These include what instruments could do the job, what orbits are required, how we could operate in the harsh radiation environment of Europa."
Scientists have long wanted to burrow through Europa's ice to the ocean beneath, but John Zarnecki says that, for now, this may be too ambitious.
He favours a two-stage approach, in which an orbiter is sent first, possibly with ground-penetrating radar, followed later by a lander capable of penetrating the ice.
"Others say we should go for a lander and a penetrator straight away, but I don't know how feasible that is, financially and technically. Landing on Europa is very demanding, you have to brake very hard on approach," he said.
Saturn's moon Enceladus has recently surged up the list of priority targets for exploration.
When the Cassini-Huygens probe arrived at the Saturn system in 2004, it observed water vapour erupting in huge geysers from an active volcanic region at Enceladus' south pole.
This combination of heat and liquid water close to the surface makes it interesting to astrobiologists, scientists who study the origin and evolution of life in the Universe.
Nasa's science definition teams will report back in August.
An independent evaluation will take place over the following few months with a view to choosing a target for the flagship mission.
Scientists have to submit their letters of intent to the European Space Agency by 30 March. They will then need to submit fuller mission proposals by the end of June.
A previous joint Esa-Nasa mission to Europa was dropped following budgetary changes at the US space agency in 2006.