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

• Use dual manifesting to reduce individual mission costs. Combining two missions with
complementary scientific objectives onto one launch vehicle reduces costs for each mission. Recent
examples of this approach are the Cassini/Huygens mission to Saturn and the Lunar Reconnaissance
Orbiter/LCROSS mission to the Moon. This approach, however, does entail additional technical and
managerial complications.
• Use dual manifesting for missions to different destinations. For example, a planetary mission
could be combined with an Earth observation mission. While significant savings may be possible, such a
combination of missions would bring substantial technical and management complications, for example
resulting from the schedule constraints imposed by planetary launch windows.
• Buy blocks of launch vehicles across all NASA users to reduce unit costs. Block procurement
of launch vehicles reduces unit costs because of increased production efficiencies both at the prime
launch vehicle contractor and at the vendors supplying components and subsystems. According to a 2010
study by the Center for Strategic and International Studies, inefficiency in the US production of launch
vehicles adds 30-40 percent to US launch costs. 15 NASA once procured the Delta II in blocks.
• Buy blocks of launch vehicles across organizations to reduce unit costs. At present, NASA
and the Department of Defense procure launch vehicles separately. Combined procurement across both
organizations would result in greater production efficiencies and reduced unit costs. Interagency
cooperation to bring about such block buys could be a significant challenge, however.
• Exploit technologies that allow use of smaller, less expensive launch vehicles. For some
orbital missions to planets with atmospheres, use of aerocapture can result in a substantial reduction
spacecraft mass, replacing propellants with a less massive heat shield. For other missions, low-thrust inspace
propulsion may enable trajectories that have less stringent launch performance requirements. In
both instances, of course, launch savings are partially offset by the cost of the necessary technology
development.
THE NEED FOR PLUTONIUM-238
Radioisotope Power Systems (RPSs) are necessary for powering spacecraft at large distances
from the Sun, in the extreme radiation environment of the inner Galilean satellites, in the low light levels
of high martian latitudes, dust storms, and night, for extended operations on the surface of Venus, and
during the long lunar night. With some 50 years of technology development, funded by over $1 billion,
and use of 46 such systems on 26 previous and currently flying spacecraft, the technology, safe handling,
and utility of these units are not in doubt.
While there are over 3,000 nuclides, few are acceptable for use as radioisotopes in power sources.
For robotic spacecraft missions, plutonium-238 stands out as the safest and easiest to procure isotope that
is compatible with launch vehicle lift capabilities.
Past NASA use of plutonium-238 in RPSs is well documented. Future requirement planning is
subject to periodic (ideally annual) updates to the Department of Energy. Such plans are complicated by
cross-agency budgetary expectations, changing NASA plans, and the competitively selected nature of
future NASA missions that may require this isotope.
Unfortunately, production of plutonium-238 in the U.S. ceased in 1988 with the shutdown of the
Savannah River Site K-reactor, and separation of the isotope from existing inventories stopped in about
1996. The remaining stock of plutonium-238, largely purchased from Russia, has continued to be drawn
down, most recently for the Multi Mission Radioisotope Thermoelectric Generator (MMRTG) on MSL
(~3.5 kg of plutonium). An additional potential lien against the remaining supply is the use of plutonium-
238 in two Advanced Stirling Radioisotope Generators (ASRGs) on the next Discovery mission (~1.8 kg
of plutonium). While an exact accounting of plutonium-238 in the U.S. is not publicly available, previous
estimates are consistent with a current supply of ~16.8 kg, not including 3.5 kg now on MSL. Recent
NASA requirements reported to DOE are given in Table 9.5.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
9-24