Vision and Voyages for Planetary Science in the - Solar System ...

the majority of the Jupiter Europa Orbiter science goals are achieved independently of the Jupiter
Ganymede Orbiter. In contrast, many key science objectives of the Titan mission rely on the balloon and
lander elements, which are in an early stage of development. The use of three spacecraft elements at
Titan (orbiter, lander, and balloon) also increases the complexity of spacecraft integration and mission
operations, and thus the associated risk.
For these and other reasons, the independent evaluation of the 2008 Flagship mission studies
rated the Europa mission as having lower mission implementation and cost risk than the Titan flagship
mission. Estimated costs to NASA as estimated by the Decadal Survey (Appendix C) were lower for the
Jupiter Europa Orbiter ($4.7 billion in FY2015 dollars) than for the Titan Saturn System Mission (at least
$5.7 billion, after subtraction of the estimated $1 billion cost of the ESA-supplied balloon and lake lander
from the $6.7 billion estimate for an all-NASA mission, and addition of any potential costs associated
with dividing the mission between NASA and ESA). Finally, the outer planets community, as
represented by the Outer Planets Assessment Group (OPAG), ranked the Europa mission as their highest
priority flagship mission, followed by Titan. JEO is also given higher priority than the Enceladus Orbiter
for two prime reasons. JEO’s flagship-class payload will return a greater breadth and volume of science
data than the more focused payload of the Enceladus Orbiter (see the discussion of mission size above).
Also the severe limitations of the Galileo data set, due to Galileo’s low data rate and the older technology
of its instrument payload, mean that our knowledge of Europa and the Jupiter system is now poorer than
our knowledge of Enceladus and the Saturn system, giving a particularly high potential for new
discoveries by JEO at Europa and throughout the Jupiter system.
Among the smaller missions studied by the panel, the Enceladus Orbiter was given highest
priority because of the breadth of science questions that it can address (with the potential for major
contributions to understanding the chemistry, active geology and geophysics, and astrobiological potential
of Enceladus), coupled with its relatively simple implementation, requiring little new technology. The Io
Observer was chosen as a New Frontiers candidate because of its compelling science and because it was
the only outer planet satellite mission studied for which cost estimates placed it plausibly within the New
Frontiers cost cap. Of the other satellite missions studied (Appendix D) the stand-alone Titan Lake
Lander was rated lower priority because of its relatively narrow science focus and relatively challenging
technology requirements. The Ganymede Orbiter was rated as lower priority for a NASA mission
because of the probability that the planned European Jupiter Ganymede Orbiter will achieve most of the
same science goals.
Other stand-alone Titan mission concepts that could achieve a subset of the goals of the TSSM
mission are also possible. However, implementation of such stand-alone missions is challenging, as
evidenced by the fact that only one additional mission that could replace an element of TSSM was
proposed in any of the community white papers submitted to the Decadal Survey: a stand-alone Titan
airplane. 79 This concept is intriguing, and is noted above as a possible alternative to a balloon as an
element of a flagship mission. However, high data rates are required to obtain full benefit from the
remote sensing that would be a key measurement goal of an aircraft or balloon. High data rates are
difficult to achieve without use of a relay spacecraft, making aircraft or balloons less attractive as standalone
mission candidates than the lake lander chosen for detailed study. One additional stand-alone
mission, the Titan Geophysical Network was proposed in a white paper (Lorenz et al. 2009), but was not
chosen for detailed study because the science goals, which go beyond those of TSSM, were judged to be
of lower priority, and the required low-power radioisotope power supplies would require significant
additional development. A stand-alone Titan orbiter, without the in situ elements might also be
considered, but was not chosen for study because it was not proposed by community white papers, and
because of the advantages of an integrated orbiter and in situ elements both for delivery to Saturn and for
data relay.
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