Nasa: Jupiter Europa Orbiter could launch on Atlas rocket 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
Nasa and the European Space Agency have decided to forge ahead with an ambitious plan to send probes to the Jupiter system and its icy moon Europa.
The proposal could be the agencies' next "flagship" endeavour, to follow on from the successful Cassini-Huygens mission to the Saturn system.
Officials had been considering the Jupiter mission along with a venture to Saturn's moons Titan and Enceladus.
But they will target an earlier flight opportunity for the Europa mission.
A Saturnian return will have to wait until later in the century, agency chiefs say.
The missions would cost several billion dollars/euros to build and execute and might never fly if other future space endeavours become higher research priorities.
The decision was made last week at a meeting in Washington DC.
Scientists and engineers from the space agencies carefully studied both potential missions, which resulted from merging separate Nasa and Esa mission concepts.
They decided it was most technically feasible to do the Europa Jupiter System Mission (EJSM) first.
Professor David Southwood, Esa's director of science and robotic exploration, said: "This joint endeavour is a wonderful new exploration challenge and will be a landmark of 21st Century planetary science."
But Esa's Solar System Working Group decided that, based on scientific merit, there was little to separate this venture from the Titan Saturn System Mission (TSSM).
They therefore recommended, with Nasa's agreement, that both missions move forward for further study and implementation.
TITAN SATURN SYSTEM MISSION
Mission would leave Earth 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
Ed Weiler, associate administrator for Nasa's science mission directorate in Washington DC, commented: "Although the Jupiter system mission has been chosen to proceed to an earlier flight opportunity, a Saturn system mission clearly remains a high priority for the science community."
EJSM proposes Nasa and Esa combine efforts in the Jovian system.
Major targets here would be the Galilean moons Europa and Ganymede. Scientists have long dreamed of visiting Europa with sophisticated instrumentation.
The icy moon's cracked surface is thought to hide a sub-surface ocean; and researchers want to start assessing the habitability of this strange world.
Louis Friedman, executive director of The Planetary Society, commented: "A mission to Jupiter's icy moon Europa will take us to one of the most likely habitats in the Solar System - other than Earth - where life might have evolved."
He added: "We are delighted that it is being organised as an international project - making the mission more affordable and increasing its support."
The EJSM assessment 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).
"It's a double integrated mission. What we want to do with two platforms is to contemplate the Jupiter system as a whole - each platform looking at specific objects; and the two platforms looking at the same objects from two different perspectives," said Michel Blanc, from the CESR (Centre d'Etude Spatiale des Rayonnements) in Toulouse, France.
A mission to Jupiter's icy moon, Europa, will take us to one of the most likely habitats in the Solar System - other than Earth - where life might have evolved
Louis Friedman, The Planetary Society
Although the two spacecraft would occasionally conduct joint observations, only Nasa's probe would spend time around Europa - which is known to have a severe radiation environment.
A previous Esa feasibility study into a Europa mission gave spacecraft orbiting the moon a lifetime of just 66 days. Nasa plans to use specific shielding to protect sensitive electronic systems.
"[The Americans] are confident now that they can operate for several months in orbit and do the mission which is complete coverage both of the surface and the interior," Dr Blanc told BBC News
The JEO and JGO would end their missions by crashing into their respective moons.
On the subject of the Galilean satellites, Dr Blanc added: "They are fascinating. They are coupled; they are in resonance.
"While Ganymede is doing one turn around Jupiter, Europa is doing two turns; and Io is doing four orbits. And in this motion they keep eccentricity; they excite each other. And there is a lot of tidal interaction which heats the internal bodies of those satellites."
My heart belongs to Titan
Professor John Zarnecki, Open University
Professor John Zarnecki, from the Open University in Milton Keynes, UK, told BBC News: "Europa will be absolutely brilliant when it happens."
But the planetary scientist, who is a principal investigator on the Cassini-Huygens mission to the Saturnian system, added: "My heart belongs to Titan.
"One gets used to long timescales, but when it came home that I will probably be in my Bath chair by the time [TSSM] happens, it was disappointing."
The TSSM concept envisages sending an orbiter to the moons Titan and Enceladus. It would build on the discoveries made by Cassini-Huygens, which arrived at Saturn in July 2004 and continues to operate.
Cassini has sent back data indicating that Titan resembles a primitive Earth - albeit one that is deep frozen. It has a thick, hazy atmosphere and is rich in organic (carbon-rich) molecules.
Recent revelations at Enceladus show that its southern polar region contains hot spots spewing huge jets of water-ice into space. And scientists think this moon may also host an ocean of liquid water beneath the surface.
British scientists and engineers will look to play key roles on the European aspects of the Europa and Jupiter system venture.
One group will put forward the idea of firing darts, or penetrators, at the icy surface of Europa to "taste" its chemistry.
The icy surface of Europa is riven with cracks
The UK Penetrators Consortium, led from the Mullard Space Science Laboratory, is working on a proposal that would see instrumented probes dropped into the dusty surface of Earth's moon.
But the researchers believe the technology could just as easily be deployed into a Jovian satellite.
"It allows you, for example, to implant a seismometer under the surface; or to do some simple organic chemical analysis," explained Dr Andrew Coates from MSSL.
Other small sensors could determine the temperature of the sub-surface, its mechanical properties and detect any movement; they could also probe the minerals present and assess the local magnetic and radiation fields.
Professor Zarnecki commented: "If you can't sample the surface directly, one option would be to look at the ejecta - the material thrown off the surface during impacts.
"[At the Open University] we would like to fly a spectrometer around Europa to find out whether we can see organics."
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