This week, I'll continue to look at possible missions for my personal selection of the five most compelling missions from the list of Decadal Survey mission concepts. I've already selected the first four candidates and am now looking at options for my fifth selection. Last week, I wrote about the Mars and lunar geophysical network missions. This week, I'll look at Neptune missions.
My criteria for inclusion on my list is that a mission must offer the chance to significantly advance our understanding of the solar system. Humans have flown many missions to the terrestrial planets and a number to the hydrogen-helium giants, Jupiter and Saturn. The methane-rich gas giants have been visited just one time each by the Voyager 2 spacecraft. That craft -- state of the art for the 1970s -- carried instruments that by today's standard would be antiquated. A new mission to Neptune would offer the chance to investigate a key class of worlds with modern instruments and much higher telemetry rates. As a bonus, the spacecraft would also investigate another likely ice-ocean moon, Triton. Depending on the mission options chosen, the craft also could continue to flyby a Kuiper Belt Object (KBO) and continue the reconnaissance that New Horizons will begin at Pluto.
Many of the Decadal Survey concept studies looked at specific mission designs. The two network missions last week were good examples. Specific spacecraft configurations were chosen and examined in some depth. The charge given to the Neptune study group was to evaluate a number of mission concepts ranging from simple flybys to Cassini-class orbiters. The depth of analysis for any particular concept necessary was limited.
The missions break into two cost classes: Small flagship (~$1.5B [all costs in FY15 dollars]) flyby missions and very simple Neptune orbiters and larger flagship (>$2B) orbiters. The report notes that the flyby missions might be able to be cost optimized to fit within the New Frontiers program (~$1B). In this case, the mission would come to resemble the proposed Argo mission that I've written about before.
My description will focus on the small flagship mission class because it seems unlikely to me that the budget for the coming decade would fund a larger flagship mission. Within that group are seven flyby concepts that would all investigate Neptune, Triton, and a KBO:
- Four of the options differ in which world would receive priority and one "best compromise." (These options seemed to vary primarily on the design of the encounter geometry at Neptune/Triton. A geometry that optimizes Neptune observations, for example, would limit the quality of the Triton encounter and rule out a range of potential KBO encounters.)
- One option would include multiple free flying magnetometers to study Neptune's unusual magnetosphere from multiple locations.
- Another concept would add a single free flying magnetometer/transponder to enhance the KBO encounter.
- A seventh option would include a small atmospheric probe that would sample the upper atmosphere (to 5 times Earth surface pressure).
- Yet another option would strip the spacecraft of approximately half its instruments to fund a simple orbiter mission that would provide a number of encounters with Triton (but then could not go on to a KBO).
The study assessed the relative science merits of the different mission options. The simple flyby missions and the free flying magnetometers/transponder were judged to have 1.7-2.0 times the science value of the Voyager 2 flyby. The flyby mission with the atmospheric probe was estimated to have a relative science value of 2.3. The simple small flagship class orbiter was given a science value of 1.6. By comparison, the larger flagship mission concepts ranged from 2.3-3 (although the report emphasized that these values were probably understated).
A mission to Neptune an beyond necessarily involves a long flight. In one representative flyby mission, launch occurs in 2018, followed by a Jupiter gravity assist in 2021, and the Neptune/Triton encounter in 2029. The KBO encounter would occur sometime after that with the timing dictated by the target chosen. The Argo proposal lists possible KBO encounters as late as 2041.
The requirement for a gravity assist limits possible launch dates. Ideally, the mission would launch by 2018 to enable a Jupiter flyby. The Argo proposal includes a Saturn flyby with either a preceding Jupiter flyby or a Trojan asteroid flyby. Forgoing Jupiter for a Trojan, however, adds three years of flight time to reach Neptune.
Editorial thoughts: The arm chair explorer in me really likes the Neptune-Triton-KBO flyby missions. The science would be excellent, and a chance to return to fascinating worlds after a wait of 40 years would be exploration at its finest. This mission must either launch by 2018 (with 2019 and 2020 as poorer opportunities) or wait another decade for a favorable planetary line up. This represents the last reasonable chance in my lifetime (yes, I'm in the boomer generation) to visit these worlds. The mission might offer the chance for an international partner to provide either the atmospheric probe or free flying magnetometers.
The key issue I see is that this mission must compete with compelling missions to Jupiter-Europa-Ganymede and Titan-Enceladus for selection. Among these three, which would you select?
Neptune-Triton-KBO Decadal Survey concept report
Argo mission presentation
Appendix: Here is a list of instruments proposed for the flyby options; those with an asterisk would be dropped for the small flagship class orbiter.
Wide angle imager
Near-IR imaging spectrometer
RSCM link (the report doesn't spell out the acronym)
Narrow angle imager*
UV imaging spectrometer*