Compelling Planetary Science Missions: What Comes After the Top Five?

This completes a series of posts inspired by a similar set of posts at Future Planetary Exploration blog selecting the 5 most compelling missions from the Planetary Science Decadal Survey list.  This presents my point of view of the type of planetary science accomplishments possible through the next Decade's work beyond what I found to be the 5 most compelling missions from the Decadal Survey list.

First, let's review the missions I selected, and their estimated FY15 costs with reserves:

• Lunar Polar Volatiles Explorer - long-range rover with drill - $1,132M
• Enceladus Orbiter - Saturn tour followed by Enceladus orbit - $1,613M
• Mars Polar Climate Mission (2 selections from Decadal Survey options) - climate and weather orbiter and polar subsurface sampler lander - $613M + $860M = 1,473M
• Mars Geophysical Network - 2 geophysical landers - $1,015M
• Lunar Geophysical Network - 4 geophysical landers - $903M

This selection is consistent with this blog's theme of restoring the Vision for Space Exploration that was, with some exceptions like LRO and LCROSS, lost in 2005 with the onset of Constellation.  It covers the "Moon, Mars, and beyond" idea almost a bit too literally, since (in my view) Planetary Science should "lean a bit" in this direction while still being driven by science priorities, while a well-funded Robotic Precursor line on the astronaut exploration side should be the main robotic support line for the VSE's robotic needs.  The Lunar Polar Volatiles Explorer is a particularly important mission in support of the VSE goals, whether funded through Planetary Science or Robotic Precursors.  The Enceladus orbiter doesn't quite fit the specific outer planet wording from the Outer Planets section of the VSE, which emphasizes Jupiter's moons and then Titan, but I think it could be justified in the context of the VSE given knowledge gained by Cassini since the VSE was developed.

The total estimated cost in FY15 dollars of these 5 missions (including 8 landers and 2 orbiters) is in the ballpark of $6B.  I could assume the cost would be less, since the missions include significant reserves, and some have substantial heritage.  I could also assume partnerships lower the cost to NASA Planetary Science (e.g.: partnerships with international space agencies, synergy with NASA robotic precursor or technology development lines, commercial participation), but I'll instead assume that such partnerships tend to add capabilities rather than lower cost.  The budget for the Enceladus Orbiter includes operations spending that happens far past the timeframe of the Decadal Survey, so I might be able to overlook some of the budget needs of that mission.  However, I'm more comfortable leaving the estimate for the 5 missions at $6B.

I don't know what the NASA Planetary Science new mission budget will be for the next decade, but let's suppose it's $11.5B in FY15 dollars.  That would leave some money for other areas like Planetary Science technology development, instruments, research, operating long-duration missions, data systems, and so on.  I'll ignore issues like missions whose funding spans multiple Survey decades.  With those simplifying assumptions, with $6B or so for the top 5 missions, we'd still have a healthy $5.5B for other missions.

It would have been easy to have chosen 5 missions that together cost far more than $6B, since the Decadal Survey list concentrates on New Frontiers and Flagship missions (i.e. ambitious but expensive ones).  For example, here are the listed FY15 costs for 3 particularly capable, long-sought-after, and undeniably compelling missions that very well might be emphasized by the Decadal Survey:

• Jupiter Europa Orbiter - $3,897M ($1,200M of this is reserves)
• Mars 2018 MAX-C Caching Rover - $2,196M (and this requires 2 more large missions before the biggest reward - Mars sample return - happens)
• Titan Saturn System Mission - $3,456M

As you can see, if we go with those 3 missions, and assume a total Planetary Science budget of $11.5B for missions, we aren't left with much else for the other 2 of the top-5 missions, let alone other missions.  The Mars 2018 MAX-C mission assumes an earlier ESA/NASA ExoMars mission for telecommunications, so we have even less room for flexibility if we select these 3 missions.  Those would be 3 great missions, but I don't think I'd want Planetary Science to be limited almost entirely to these 3.  I'd also consider the impact to Planetary Science if any of these 3 suffered serious cost overruns or a major technical failure, considering how many eggs would be in so few baskets.

Since I happen to have picked some of the less-expensive missions from the Survey list, though, I now have a chance to provide some depth to what would be, if left to stand by itself, a fairly unbalanced set of missions in my top 5.  I seem to have emphasized Mars, the Moon, and geophysical networks at the cost of sample return, Venus, small bodies, and other priorities.  Can that be fixed?  What should we do with the "leftover" mission funding?

The first thing I'd do with that extra $5.5B is establish a funding block (for the sake of discussion, let's say $1B over the decade) for "frequent, very low cost missions".  This would be in addition to existing areas like flights of opportunity for instruments on non-NASA missions.  This might sound like the Discovery line of missions, but for various reasons, the Discovery line is getting a bit expensive.  The FY10 Discovery mission limit used in the Survey studies is $580M for FY2010; it's assumed to be $666M in FY2015.  That includes $155M (FY10) or $178M (FY15) for an assumed Atlas V 401 launch.  The new mission line that I'm proposing would be for lower cost Planetary Science missions than that.  You might think of some of the early Discovery missions, the new "Venture Class" Earth observation missions, or astrophysics and heliophysics Explorer, MIDEX, and SMEX missions.  This line would seek to take advantage of opportunities like

• potential lower-cost launchers, such as the Falcon 1e, Falcon 9, and Taurus II
• potential increased availability of secondary payload slots on launchers
• commercial data purchases, similar to NASA's Innovative Lunar Demonstrations Data contracts, but for planetary science data rather than engineering data
• other cooperative arrangements with non-NASA partners such as commercial vendors
• cooperative missions with other NASA areas like Space Technology, Exploration Technology Development and Demonstration, and Robotic Precursors, or even entire small space missions whose main purpose is to demonstrate products of the Planetary Science technology development budget
• focused, low-cost mission approaches (for example, penetrators like the Deep Space 2 Mars technology demonstrators)
• favorable trends in the small satellite field

Such missions would probably tend to go to some of the Planetary Science destinations that are easier to reach quickly, such as the Moon, Venus, Mars, comets, and NEOs.  If that happens, they could fill in some of the gaps of my original 5 mission list (Venus, NEOs, comets) while strengthening the Moon and Mars focus of that list.

If we used $1B for that, we would have $4.5B left.  I think we should have at least 3 Discovery missions (4 if you count the Mars Scout-like Mars Climate Orbiter I selected in the most compelling missions as a Discovery mission).  There are a lot of gaps left in my mission choices that these Discovery missions could fill in.  For example, even with my predisposition to favor missions with "astronaut robotic precursor" potential, I didn't select a single Near Earth object mission, even though Near Earth objects are often discussed as the first beyond-LEO destination in NASA's new Flexible Path plan for astronaut missions.  (Actually the first beyond-LEO mission destination in that plan is cislunar space - possibly lunar orbit or an Earth-Moon Lagrange point - but those early beyond-LEO missions are often overlooked when the new plans are discussed).

Why didn't I select any Near-Earth asteroid planetary science missions?  Well, for one thing, there weren't any on the Decadal Survey list.  I didn't allow the current batch of New Frontiers missions, including the Near-Earth asteroid sample return mission OSIRIS-REX, in my selection.  The Decadal Survey studies include an analysis of "Near Earth Asteroid Trajectory Opportunities", but I didn't consider that to be an actual mission.  The list also includes an affordable "Trojan Tour Concept", but Trojan asteroids are by definition the "cloud" estimated to include hundreds of thousands of objects (if we only count those greater than 1 km in diameter) around the L4 and L5 Jupiter-Sun Lagrange points.  Those are not candidates for early astronaut exploration missions, and in fact, it could be a couple of decades before the Trojan Tour robotic mission would get there if it was selected - with 1 decade for the actual trip.  Nevertheless, the Trojan Tour is affordable and would cover a set of bodies that has never been visited before, so it is certainly worth consideration.  There is also an affordable Chiron Orbiter mission in the Decadal Survey list, but this would take even longer to launch and to reach its destination.

So, with the most compelling missions I've selected, we probably have a coverage gap in the area of Near Earth objects, or at least primitive bodies in general.  One or two Discovery missions to fill that gap might be in order.  We also have coverage gaps at Venus and Jupiter.  So, if we add 3 Discovery missions, we might be able to let the Discovery mission selection process fill some of the gaps I left with my top 5 selections.

If we assume the Discovery missions cost $700M each, that leaves us with $2.4B.  What should we do with that?  There are a number of interesting possibilities:

• Upgrade the Enceladus Orbiter to a full-blown Titan Saturn System mission, or switch it to the Jupiter Europa Orbiter after all.  This would make a lot of the planetary science community and international partners happy, although I would still worry about mission cost until data starts coming in (upon which time I would undoubtably forget cost).
• Fly the Mars 2018 MAX-C Rover after all.  This would make a different, but also big, part of the planetary science community and international partners happy.  With $2.4B available and a mission estimate of about $2.2B, there would be a little bit of slack to also cover NASA's contributions to the earlier Mars Trace Gas mission (e.g.: the rocket, instrumentation, telecommunications), but one of the Discovery missions might need to be traded or postponed to fully cover that.  Because of the amount of planning and interconnected missions involved, this selection might make a great deal of sense, in spite of my serious worries about mission cost.
• Fly a variant of the Venus Climate Mission (which barely missed my 6th-place spot).  The basic mission should allow room for anther Discovery mission, which is the approach I would tend to take.  Alternately, the Venus Climate Mission could fill up the $2.4B by taking on some of the capabilities of the more ambitious Venus Climate Flagship reference mission.
• Assuming the current New Frontiers selection picks one out of MoonRise (lunar sample return), SAGE (Venus lander), and OSIRIS-Rex (Near-Earth asteroid sample return), we should be able to afford to fly the other 2 with the leftover $2.4B.  I find this to be a particularly attractive option, since these missions should be in a more well-developed state than some of the others in the Decadal Survey list, since they address sample return (which I've completely skipped in my selections), since they have significant "robotic precursor" and "exploration technology demonstration cooperation" potential, and since they partially address some of the content that I lost by letting the Venus Climate Mission slip into 6th place on my list.
• Fly 3 more Discovery missions, giving a total of 6 - a good decade for this class of missions.  I tend to think of Discovery missions as the "meat and potatoes" of Planetary Science, so I'd seriously consider this option.  One of those Discovery missions (or a mission with similar cost but selected and managed differently) might be a second Lunar Polar Volatiles Explorer, just like the first one but at another lunar location (e.g.: the other pole).
• All sorts of other possibilities.

We can't do everything, but picking one of these would at least fill in some of the gaps that I've left with my original 5 selections.