By Jonathan Amos
Science reporter, BBC News
An impression of the TSSM orbiter set against a classic Cassini image
Ambitious plans to send probes to the outer planets are being considered by US and European space officials.
One proposal envisages sending an orbiter to Saturn which would also drop a lander and a balloon on to the haze-shrouded moon Titan.
The other sees two separate orbiters despatched to investigate Jupiter and its icy moons - Europa and Ganymede.
Space agency officials will meet next week to decide which of the two plans should go forward for further study.
The respective space agencies' two top science executives, Ed Weiler (Nasa) and David Southwood (Esa), are expected to announce a "winner" in February.
TITAN SATURN SYSTEM MISSION
Mission leaves Earth in 2020 on Atlas heavy-lift rocket
Would take 9 years to reach Saturn with Venus fly-by
Nasa: responsible for 1.6-tonne instrumented orbiter
Esa: would provide balloon and hydrocarbon lake lander (above)
Orbiter to tour Saturn system before entering Titan orbit
Tour allows further studies of Enceladus and its plumes
Will need to raise the high scientific bar set by Cassini
The mission, which would cost several billion dollars/euros to build and execute, would not get to the launch pad before 2020; and may never fly if the agencies decide there are other space missions in their future portfolios that they consider to be a higher research priority.
Reports from the two competing definition teams were published online this week by Esa.
The documents provide detailed descriptions of the science rationale and goals of the different mission concepts, and how Nasa and Esa would dovetail their participation.
The Titan Saturn System Mission (TSSM), as it is currently known, would follow up the remarkable discoveries made by the Nasa/Esa Cassini-Huygens mission which continues to operate at the ringed planet.
The concept envisages another multi-instrumented orbiter that would make the moons Titan and Enceladus its chief targets.
Cassini has sent back data that indicates Titan is akin to a primitive - albeit frozen - Earth. It has a thick atmosphere and is rich in organic (carbon-rich) molecules. Recent revelations at Enceladus include the discovery that its southern polar region has hot spots that spew huge jets of water-ice into space. Scientists think there may be an ocean of liquid water beneath the moon's surface.
The TSSM orbiter would dip into Titan's atmosphere and the plumes at Enceladus to "taste" their chemistry. The orbiter would also drop a lander on to Titan to float on one of moon's lakes of liquid ethane and methane. In addition, a balloon would be injected into the atmosphere to take pictures and sample the "air" as it drifted with the wind.
The Europa Jupiter System Mission (EJSM) proposes Nasa and Esa combine efforts in the Jovian system. Major targets here would be the Galilean moons Europa and Ganymede. Europa in particular has long been at the top of scientists' wish lists to visit with sophisticated instrumentation.
The ice moon's crack-riven surface is also thought to hide a sub-glacial ocean (but on a larger scale to Enceladus). Researchers would love to get close enough to start to assess the habitability of this strange world.
EUROPA JUPITER SYSTEM MISSION
Nasa: Jupiter Europa Orbiter could launch on an Atlas in 2020
Esa: Jupiter Ganymede Orbiter (above) lofted by an Ariane
Probes use Venus gravity assist to arrive 6 years later
Orbiters conduct joint observations at other Jupiter moons
Would finally settle into orbits around dedicated targets
Studies will focus on Europa's and Ganymede's interiors
End destructions will allow unique measurement opportunities
The EJSM team suggests the US and Europe both send orbiters. Nasa would despatch the Jupiter Europa Orbiter (JEO); Esa would send the Jupiter Ganymede Orbiter (JGO).
The two spacecraft would conduct joint observations on occasions but only Nasa's probe would spend time around Europa which is known to have a severe radiation environment. The spacecraft will need specific shielding to protect sensitive electronic systems.
The JEO and JGO would end their missions by crashing into their respective moons.
The concepts have risen out of several years of discussion on both sides of the Atlantic.
US inspirations have been channelled through Nasa's Outer Planets Assessment Group (Opag).
The European initiative comes under Esa's Cosmic Vision programme which seeks to map out space science endeavours through to 2025.
Initial ideas evolved under titles known as Tandem (now incorporated into TSSM) and Laplace (now in EJSM).
The Paris-based agency has set aside 650m euros (at 2007 prices) for a large class mission. The TSSM and EJSM concepts would fall into that category.
But whichever is chosen to go forward for further feasibility work will ultimately have to compete with concepts in astrophysics.
There are joint Esa/Nasa proposals on the table for a next-generation X-ray telescope, known as the International X-ray Observatory (Ixo); and for a mission to study gravitational waves in space, known as the Laser Interferometer Space Antenna (Lisa).
These concepts are trying to win the same funding opportunity.
Peter Falkner, who leads the planetary exploration studies section at Esa, told BBC News: "The [planetary missions] will go to down-selection with Ixo and Lisa; and then - under the current plan - two will be selected for definition phase in parallel, still in competition, and out of that will emerge a winner that will go forward to implementation."