Flagships of the future augustus 2009 BBC Sky at Night

Picture a hot air balloon drifting around in the thick atmosphere of Saturn's moon Titan, or a space probe splashing down in one of its murky hydrocarbon lakes. While such amazing feats of space exploration won't happen for at least a quarter of a century, plans are starting to come together. Earlier htis year, there were even hopes that new missions to both Saturn and Jupiter would both get the green light. However, on 18 February, NASA and ESA decided to set out first on a new flagship mission to the giant planet Jupiter and its enigmatic moons, known as the Europa Jupiter System Mission (EJSM).

For now, scientists involved with the Titan Saturn System Mission (TSSM) will have to await their turn. ‘It’s disappointing,’ says Jonathan Lunine, a planetary researcher at the University of Arizona, who co-chaired the joint science definition team for the Saturn mission. Even in January 2009, at a big meeting of the American Astronomical Society, Lunine was still lobbying on behalf of TSSM.

The two largest planets in our solar system haven’t nearly been scrutinized as vigorously as tiny Mars or cloudy Venus. Between 1973 and 1981, they were briefly visited by the Pioneer and Voyager spacecraft. Galileo toured the Jupiter system between 1995 and 2003, and Cassini is orbiting Saturn since the summer of 2004. Probes were released into the atmospheres of Jupiter and Titan, respectively, and both missions produced stunning results. But if the Pioneers and Voyagers were the hors d’oeuvres of outer solar system exploration, Galileo and Cassini were the scientific starters. Now, it’s time for the main course.

But space research is like a five-star restaurant, in the sense that quality comes with a price tag. Neither the American space agency NASA nor its European counterpart ESA could afford the chef’s special on their own. So in a novel approach, after having developed and evaluated their own individual plans and proposals, the two agencies decided to join forces in two truly international mission concepts. ‘This joint endeavour will be a landmark of 21st-century planetary science,’ according to David Southwood, ESA’s director of science. ‘It’s a win, win situation for all parties involved,’ said Southwood’s American colleague Ed Weiler when the deal was announced.

French planetary scientist Michel Blanc, who is lead scientist for the ESA study of the joint Jupiter mission, thinks it makes sense that EJSM comes first. ‘In fact, we know less about the Jupiter system than about Saturn,’ he says. ‘Moreover, technologically we’re ready to go.’ Lunine concedes that there was a lot of incentive for NASA to do the Jupiter mission first. Still, he says, Titan and the smaller Saturnian moon Enceladus are extraordinary objects that deserve further study ‘as quickly as possible. We now have young, talented researchers in our teams who might drift to other fields if new projects disappear beyond their horizon. We don’t want to lose those people.’

Complimentarity
The Europa Jupiter System Mission is in fact a misnomer, since it will not only target Jupiter’s icy moon Europa, but will also focus heavily on giant Ganymede, the largest planetary satellite in the solar system. NASA’s Jupiter Europa Orbiter and ESA’s Jupiter Ganymede Orbiter will launch and operate separately, but according to Blanc, the two spacecraft will essentially constitute a single mission. One of the challenges, he says, is to get as much complimentarity out of the two craft as possible. There’s even a chance that Japan will contribute a third space probe, which would focus on Jupiter’s magnetosphere.

Before settling in their orbits around Europa and Ganymede, respectively, the two spacecraft will first tour the Jovian system, with multiple flybys of the other two Galilean satellites, Io and Callisto. Once in orbit, their main task is to characterize the subsurface oceans of Europa and Ganymede, and to study the interaction between these oceans and the outer ice shells of the two worlds. ‘Depending on this interaction,’ says Blanc, ‘the surface of Europa might even reveal indirect chemical signs of biological activity in the subsurface ocean.’ A very detailed comparison of both moons will help understand the formation and evolution of the Jovian miniature solar system.

By far the biggest challenge of the missions is keeping the two spacecraft alive and healthy in the harsh radiation environment of Jupiter’s magnetosphere, especially the Europa orbiter, which will operate closest to the planet. Even then, they won’t live forever, since their low orbits (at an altitude of only some 200 kilometers) are unstable. When the two craft run out of the fuel they need for small trajectory corrections, they will eventually crash into the surface – an event that might produce interesting science in itself. And no, there’s no need to worry: both orbiters will be thoroughly sterilized, to prevent contamination of a potential microbial ecology.

But those terminal impacts won’t happen until twenty years from now or so. Launch for the two spacecraft is now scheduled for early 2020, with subsequent arrival at Jupiter six years later. Only in the spring or summer of 2028, the two probes will start orbiting Europa and Ganymedes, with an expected operational lifetime of about nine months. ‘I will be retired before the end of the mission,’ says Blanc, ‘so most of the data analysis will be carried out by a younger generation of scientists.’

Technological ambition
In the case of the Titan Saturn System Mission (TSSM), the wait will be even longer. By the time the Europa and Ganymede orbiters run out of fuel, TSSM is still cruising the solar system, provided it will launch in 2024, which is the earliest possible opportunity, according to Lunine. The craft will arrive at Saturn no earlier than in 2033, for a nominal four-year mission. Does it even make sense to start designing something that’s so far in the future? ‘There’s no choice,’ says Lunine. ‘If you wait for new technologies to be developed, the mission will never happen at all.’

Not that the Saturn mission lacks technological ambition. In this case, there will be only one spacecraft, which will first tour the Saturnian system and study the small icy moon Enceladus and its water geisers up close and personal. Meanwhile, at least fifteen flybys of Titan are being planned, before the craft slips into orbit around the shrouded mystery world. Using high-resolution infrared cameras and radar, Titan’s surface will be mapped in three dimensions in unprecedented detail, providing planetary scientists with clues about the weird geology and hydrology of the strangely Earth-like body, where ice plays the role of rock and liquid methane substitutes water.

But by then, the most challenging parts of the mission will be completed. For TSSM is really three space missions wrapped up in one package. During its first Titan flyby, the probe will release a French-built hot-air balloon that is going to sample the dense, nitrogen-rich atmosphere, map weather and climate on a regional scale, and study the structure and composition of the surface. And at one of the subsequent flybys, the second European contribution to the mission sets sail: a device that will splash down in one of the many liquid hydrocarbon lakes close to Titan’s north pole. ‘No one has ever flown a hot-air balloon or taken a dive on another solar system body,’ says Lunine.

Wildest dreams
As for the expected science return, both EJSM and TSSM will build heavily on the results that have already been collected by the Galileo and Cassini orbiters and by the European Huygens probe that soft-landed on Titan in January 2005. According to the U.S. Outer Planetary Flagship Study Team, ‘we know enough to ask the key questions, yet we anticipate being surprised by scientific discoveries.’ Lunine appears fully ready to be surprised. ‘In my wildest dreams,’ he says, ‘I look forward to discover some form of life in Titan’s hydrocarbon lakes. The lander will be able to test the occurrence of organized chemistry’ that might be a reliable signpost to exobiology.

Big planets, big missions, big science. And incredibly long timelines, unfortunately. Jonathan Lunine realizes that the Saturn mission runs the risk of being postponed well beyond 2024, or being scrubbed altogether. And although he says there are plenty of other interesting fields in astronomy and planetary science, he sounds a bit saddened when he discusses the possibility that TSSM won’t happen in his lifetime. ‘We’ve learned so many exciting things… Sometimes I’m puzzled that we’re not following up on our discoveries. Maybe we’ve become too much involved and absorbed in our own problems… Could this be the ageing of Western civilization? I hope not.’

Even EJSM is not as secure as scientists would hope. In Europe, the mission still has to compete for budget with other big projects within ESA’s Cosmic Vision 2020 program, like the space-born LISA gravitational-wave observatory. Says Michel Blanc: ‘If ESA doesn’t go, there will still be a great NASA mission focusing on Europa. But the Jupiter system is absolutely fascinating. We really need to improve on the Galileo heritage.’

Box - The hardware
Jupiter Europa Orbiter, to be built by NASA, is a big 4-ton spacecraft (including 2.6 tons of fuel) featuring two high-resolution cameras, a laser altimeter and an ice-penetrating radar to map Europa’s surface – and sub-surface! – in three dimensions. Infrared and ultraviolet spectrometers will provide information on surface composition, while magnetometers and particle instruments shed light on the harsh radiation environment at Europa’s location deep inside Jupiter’s magnetosphere. The craft features thick aluminum shielding and specially hardened electronics to protect the delicate instruments from radiation. A 3-meter high-gain antenna enables communication with distant Earth. Radioisotope thermonuclear generators (RTGs) provide 540 watts of power.

Jupiter Ganymede Orbiter, to be built by ESA, is a bit smaller than its American companion, with a launch mass of 3.5 tons (including 2.6 tons of fuel). The most eye-catching elements are the huge solar panels with a total surface area of 51 square meters that will provide power. Perpendicular to the solar panels are the magnetometer and radar sounder booms. Among the spacecraft’s other science instruments are a camera, two imaging spectrometers, a thermal infrared mapper and a micro-laser altimeter. While the Europa orbiter will launch on a heavy Atlas V551 rocket, the Ganymede orbiter will be put into space by a European Ariane 5.

Box - The destinations
Europa Jupiter’s fourth-largest moon Europa has a rocky core, a thick mantle of water and a surprisingly youthful-looking outer shell of ice. Tidal energy, generated by Jupiter, keeps the interior warm, the water liquid and the ice somewhat flexible. But no one knows how thick the ice shell is, how it interacts with the subsurface ocean, and if life might thrive beneath.
Ganymede As the largest moon in the solar system (even larger than Mercury), Ganymede is as interesting and complicated as a full-blown planet. It’s the only moon with a self-generated magnetic field and possible signs of plate tectonics. Scientists want to know if Ganymede has a subsurface ocean, possibly sandwiched between an icy core and crust.
Io is the innermost of Jupiter’s large moons. It’s also the most volcanic world in the solar system, powered by tidal heating. Coordinated observations of Io by both Jupiter orbiters should reveal details of the interaction of the volcanoes with Jupiter’s magnetosphere: sulphur particles become electrically charged and trapped by magnetic field lines.
Enceladus Measuring a mere 500 kilometers across, Enceladus is a scaled-down, icy-in-stead-of-rocky version of Io. Tidal flexing by Saturn keeps the mantle warm and produces powerful geisers of water vapor, ice crystals and dust particles in the south polar region that feed Saturn’s tenuous E-ring. Some scientists believe micro-organisms could live beneath the frozen crust of Enceladus.
Titan Titan is the dream of any SF aficionado, with mountains of ice, dunes of sticky hydrocarbon particles, lakes of liquid methane gas, and dark, tarry marshes. It’s the only moon in the solar system with a thick atmosphere, and it probably has a deep subsurface ocean. The big question: does Titan really resemble the primitive Earth, and could it sport life?

© Govert Schilling

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