All quotes are from the Survey report.
Dropped to Maintain Balance
As I noted in the previous post (which was originally intended as just a sentence in this post), a key goal of the recommended program is maintain a balanced program. If a target or class of targets already had a Flagship mission recommended, then other good missions to the same or similar targets were not prioritized. Some examples:
Ganymede Orbiter—"This mission's primary science objectives are the characterization of the satellite's subsurface ocean, geology, magnetic field, and origin... Consideration of the Ganymede Orbiter is deferred to the decade following 2013-2022 because of its lower science return per dollar relative to the JEO [Jupiter Europa Orbiter] mission, and because EJSM [Europa Jupiter System Mission] as currently envisioned would include an ESA-provided spacecraft to study Ganymede, making this mission largely redundant."
Mars Geophysical Network—"The primary science objectives of this mission are to characterize the internal structure, thermal state, and meteorology of Mars... Consideration of the Mars Geophysical Network is deferred to the decade following 2013-2022 because of its lower scientific priority relative to the initiation of the Mars sample return campaign. "
Enceladus orbiter - "...because of the broad similarity of its science goals to those of JEO, NASA should consider flying the Enceladus Orbiter in the decade 2013-2022 only if JEO is not carried out in that decade."
Not Enough Bang for the Buck
Neptune/Triton/KBO Mission - "The flyby mission architectures were deemed to achieve significant science progress since Voyager 2's visit of Neptune and offer the potential for new KBO science. Even the simplest of the flyby missions exceeded the cost cap of a New Frontiers mission and offered low science return relative to its cost, it was deemed not compelling. More complex missions and orbiters provided a vast gain in science objectives unavailable to flyby missions, but at increased cost; the highest performance option yielded a modest increase in estimated science value for its higher cost. More detailed design work of a “sweet spot” mission design identified technical risks that make a Uranus mission more favorable for the coming decade. Technology development will increase the feasibility of a future Neptune orbiter mission."
Titan Lake Probe- "The exploration of Titan's hydrocarbon lakes has high scientific potential and the Titan lake lander concepts appear feasible. However, based on the costs and the relatively limited science scope of a stand-alone lake probe without the orbiter and balloon elements, the stand-alone lake probe concepts were judged to be lower priority than a lake probe which was an element of a flagship mission, or some of the other mission concepts studied."
Technical or Mission Maturity Issues
Titan Lake Probe - "The cryogenic environment and lack of heritage in lake probe design necessitates strategic investment in technology development, including cryogenic sample acquisition and handling."
Venus Tessera Lander - "The most significant challenges posed by this mission were related to the development of a high- TRL [technical readiness level for flight] level Raman/laser-induced breakdown spectroscopy (LIBS) system, safe landing, and testing at Venus environmental conditions. To reduce risk, advancements in two key technology areas are needed: first, verifying the Raman/LIBS implementation, calibrated operation, and sizing for the Venus surface environment, including high entry loads on the laser and second, additional analyses and testing to ensure safe landing in potentially rugged terrains (at lander scales)."
Mercury lander - "Because of the complex and challenging nature of this mission, a more detailed characterization study is recommended before moving forward with the Mercury Lander concept. Both SEP [solar electric propulsion] and ballistic trajectory approaches and concepts should be further explored with a more detailed mission design and concept definition to determine the preferred mission implementation approach. Currently each has benefits and risks that could not be fully characterized at this level of study."
Chiron orbiter - "Of the five propulsion options considered for trajectories into Chiron orbit, the all-chemical option did not deliver a viable payload. The two solar-electric and chemical propulsion options delivered useful masses with reduced science payloads. Finally, the two radioisotope-electric propulsion (REP) options delivered a viable payload capable of meeting all science requirements. However, the REP system will likely need more than the two ASRGs assumed available for this mission. This study demonstrated the need for continued investments in long-term communication infrastructure and propulsion technologies before such missions could be attempted."
Just Didn't Make the Cut
Lunar Polar Volatiles Explorer - "A lunar polar volatiles mission represents an important opportunity to study the nature, composition and dynamics of volatiles trapped in the frigid interiors of lunar polar impact craters. It also provides an opportunity to investigate polar volatiles, especially water ice, as a potential resource for future human exploration of the Moon and destinations beyond. Although such a mission retains a high science value, the polar crater environment presents a number of technical challenges, including rover survivability, sample collection and characterization, and navigation. Although some technical maturation is required, there remain no major impediments to such a mission within this decade. "
The loss of outer planet Flagship missions in the coming decade with new budget forecasts makes the choice to not prioritize the Ganymede Orbiter or an Enceladus mission unfortunate. The icy-ocean moons have been the top priority for outer planet exploration, and now there are no viable missions in the candidate list for the coming decade. If the Decadal Survey committees had known this ahead of time, they might have commissioned mission studies for highly focused flights to these worlds that might fit within the New Frontiers program. For example, the recommended candidate Comet Sample Return mission has a cost estimate of $1.6B, which is near the cost for the Enceladus Orbiter at $1.9B. There may have been ways to reduce missions costs for an Enceladus mission to come in within the range of the New Frontiers cost cap.
Note the technical issues mentioned for the Titan Lake Probe and the Venus Tessera Landers. Similar missions are currently proposed for the Discovery and New Frontiers programs, respectively. The proposers hopefully have good solutions to these issues; if not, this may not bode well for these proposals.