Wednesday, December 14, 2011

Titan on the Installment Plan



Without planning it, almost all my posts for the last several weeks have looked at options for modest Flagship-scale missions, whether from NASA (Europa), ESA (Ganymede), or both jointly (Mars).   That's largely by default -- there's little news for either small or medium-cost missions.  For lower cost missions (<$500M), NASA will not select its next Discovery mission for several months and the next competition is likely two years away, ESA's next selection for a medium class science mission (roughly equivalent to a NASA Discovery mission) is years away, and Japan, India, and China continue developing their next missions.  For medium scale missions(~$1B), I've not heard a date for the next NASA New Frontiers competition, and I believe that the agency is waiting to see the budget projections from the FY13 budget proposal before deciding when to select the next medium scale mission.

That leaves the modest cost ($1.2-1.5B) Flagship missions as the current topic of news.  Scientists have identified three high priority worlds for future intensive investigations: Mars, Europa, and Titan.  Fulfilling the identified high priority science goals for each will not come cheaply: $8.5B for a Mars sample return mission, $3-4B for Europa, and ~$4B for Titan (per Decadal Survey estimates).  To help make these costs more manageable, engineers have found ways to explore Mars and Europa on what would be essentially the installment plan. 

The Mars sample return program would be split across four missions and the Europa program would be split across three missions (including an eventual lander(s)).  The four Mars missions would be a 2016 orbiter that provides a communications relay, the 2018 rover that would collect samples, a mission to retrieve and launch the samples into Martian orbit, and a mission to collect the samples from orbit and return them to Earth. 

The Europa missions would be a multiple-flyby spacecraft that would collect high volume remote sensing data, an orbiter that would carry the minimum instrument set for measurements that could only be done from orbit, and eventually one or more landers.

So could Titan be explored in a series of smaller missions?  The Titan Saturn System Mission Flagship proposal called for a highly sophisticated orbiter, a balloon platform for aerial surveys, and a probe to sample the atmosphere and a northern lake.  As many of you already know, mission teams have already taken advantage of this natural division to propose three low cost missions.

Before looking into those missions, it’s useful to look at the advantages and disadvantages of Titan from a mission designer’s point of view (it’s scientific and exploration advantages are well known to readers of this blog).  On the plus side, Titan poses a thick atmosphere that makes entry, flight, and landing easier than on any world except possibly our own.  It is frigidly cold, but for either a short-lived probe or a long-lived probe with a nuclear power supply, that is likely not a problem.  (In fact, the AVIATR plane depends on constant movement to bring cool air past its power units to keep from overheating.)  Unlike Europa, there are no harsh radiation belts to fry electronics at Titan.  And there are no Richter-scale technology developments needed to continue exploring Titan as would eventually be needed for a Mars sample return to launch the samples from the surface and retrieve them in Martian orbit.

On the negative side, Titan is far from Earth.  This results in long cruises, typically around seven years, with mission operations costing $7-10M a year during cruise.  On a $425M Discovery mission budget, that’s a significant piece of change. 

Perhaps more difficult is that at those distances, data rates either must be low if a small antenna that could be carried by a probe or plane is used, or the spacecraft must have the power supply to sustain high bandwidth communications to Earth.  It is for this reason that an attempt to define a lower cost Titan orbiter in 2007 determined that the lowest practical cost for an orbiter would be ~$1.5B.  A less expensive spacecraft couldn’t support the data rate to return a high resolution map of Titan.

Despite these challenges, teams have proposed two Discovery class missions and one that fits between the Discovery and New Frontiers ($1B) mission classes.  The inherent tradeoff for all these missions is that they accept low data rates to fit within allowable mission budgets among other tradeoffs that include smaller instrument compliments.

For the TiME Discovery proposal, which would land a long-lived probe on a Titan lake, the low data rate probably does not incur significant science tradeoffs.  Its composition and physical properties instruments are inherently low data rate.  The probe will carry a camera, but the number of pictures would likely be limited.  (I also suspect that the view from the middle of an arctic lake under a hazy sky far from the sun is likely to be low contrast, which should enable efficient data compression of images.)

The following list compares quoted data return (some calculated on the back of an envelope from data in published documents, so all assumptions may not be the same) for one year of operation for different mission proposals.  I’ve included a Martian orbiter for comparison.

AVIATR Titan airplane (~$700M) - ~1 Gbyte
JET Saturn orbiter with multiple Titan and Enceladus flybys ($425M) – 120 Gbytes
2007 Titan orbiter concept (~$425M)– ~300Gbytes
2009 Titan Saturn System Flagship Titan orbiter (several $Bs)- ~600Gbytes
Mars Reconnaissance Orbiter (MRO) ($720M) - ~1,000Gbytes

In one sense, these numbers may be misleading.  The AVIATR mission, for example, would employ elaborate procedures to allow scientists to determine which data to return.  Each of its bits may be 10X to 100X as impactful as a bit from the Mars Reconnaissance Orbiter.  However, the high data rates of MRO have allowed extensive coverage of terrain types and monitoring that the 1GB of the AVIATR mission would not allow.

The message is that mission costs can be reduced, but there are few magic bullets.  Reductions in cost come at the expense of capabilities, with the amount of data returned a key tradeoff.  Since these missions are justified by the data they collect, this is a significant tradeoff. 

If data rates are a challenge for Titan exploration, the relatively benign environment allows for missions to fly at cheaper incremental rates than for Mars sample return or Europa.  For approximately $1.5B (if the proposer’s cost estimates are correct), NASA or another space agency could fly three missions to Titan.  That is approximately NASA’s contribution for the 2016 and 2018 Mars missions or one of the new lower cost proposed Europa spacecraft.  There would probably be some cost savings if one or more of the Titan missions were combined.  The AVIATR plane mission would be greatly enhanced if it flew with an orbiter such as JET that could relay data back to Earth.  The amount of high resolution imaging from the plane would increase many times over.

Two important caveats must be considered before getting too excited by visions of Titan missions.  First, we haven’t seen independent cost reviews for these missions, and proposers have been known to be too optimistic.  Second, we don’t know how these missions would rank scientifically in a mission selection competition.  Would 1Gbyte of high resolution imaging from a Titan plane provide better return on the dollar than a sample return from a comet or a lunar geophysical network, for example?

NASA currently is committed to supporting the priorities of the Decadal Survey, which after funding the Discovery and New Frontiers programs, prioritizes the 2016 and 2018 Mars missions and if they cannot be flown, a Europa mission, and as third priority a Uranus orbiter.  However, we are seeing a new level of creativity from NASA and the planetary science community in finding ways to continue exploration of the Solar System’s highest priorities.  With the proposals on the table, Titan is in play as a target, either through individual missions or through a relatively inexpensive program for several low cost missions.

2 comments:

Anonymous said...

Interesting post as always!

I'll only make a comment on the general appearance of the blog, and is that the black background makes reading the blog really hard sometimes. Don't you consider to change that?

Regards.

Anonymous said...

I myself love the black background.