Saturday, January 31, 2009
Some editorial comments:
NASA and ESA are talking about combining their efforts for a 2016 rover mission. However, one ESA official was quoted as saying that ESA would want to develop its own technologies for this mission. While I understand national pride, I don't think this makes best use of scarce funds. NASA has a tremendous investment in precision entry, desent, and landing. With the MER rovers, it was difficult to find interesting places that had large landing footprints and were suitable for airbags (the ExoMars planned landing system).
The United States has tough laws on technology export (and I personally think too tough). If Russia launches the mission, it may be difficult for NASA to contribute much.
ExoMars started as a roughly 800M euro mission, perhaps a bit less (as I recall). If you simply want to go somewhere with interesting geology, you could do a good rover mission for that amount, I suspect. I have a geologist friend who focuses on Mars and he would love to put a MER rover in a couple of interesting places to groundtruth the orbital data. (MER cost $650M several years ago, as I recall; in inflated dollars, $800M would probably be reasonable swag in today's dollars for a mission to launch in 4-6 years.) However, the focus of rover missions for both ESA and NASA has moved to exploring locations that may have been habitats for life. That requires sophisticated instruments and rovers that support them beyond what the MER rovers could do. ESA and NASA have both costed rover missions to do this type of work in the ~$1.5B/1.2B euro range. (NASA and ESA include different items in their budgets so direct comparisons are not simple.)
Because of the success of past missions, Mars is becoming an expensive place to explore. The missions that address the cutting edge questions seem to start at ~$1B (Mars Science Orbiter or network mission), go to ~$1.5B for a solar powered rover, and then jump to $3-5B for a sample return mission.
Friday, January 30, 2009
I'll start with the three roadmap slides from the NASA presentation (click on images for larger views).
here to go to their website for more information):
Rosetta will arrive at comet 67P/Churyumov-Gerasimenko in May 2014.
BepiColombo will launch in 2013 and arrive at Mercury in 2019.
I'm not sure why the ExoMars rover and network station (launch 2016) aren't on ESA's graphic; perhaps it has not been formally approved.
The Japanese Aerospace Exploration Agency has two approved planetary missions that I know about. Venus-C will launch in 2010 to study that planet's atmosphere. The Mercury Magnetospheric Orbiter (MMO) is part of the BepiColombo mission and shares that mission's timeline. Japan is also considering a Jupiter Magnetospheric Orbiter if Europa-Jupiter is chosen by NASA and ESA as the target of the next outer planet's Flagship mission.
So far as I know, the Russian and Chinese space agencies have a single joint mission, Phobos-Grunt planned. The Chinese craft will orbit Mars while the Russian craft will collect samples from the surface of Phobos and return them to Earth. The Finns are planning to provide two Mars landers for network science. Launch is scheduled for October of this year with Earth return in 2012.
The Indian space agency has announced plans for future lunar landers and Mars orbiters, but I don't know if they are funded or not.
All in all, the 2010s are shaping up to be as rich as the last decade for planetary exploration.
I'm sure that there are missions I've missed (and possibly contributions by entire nations). If you see errors or ommissions, please let me know and I'll update this post. vkane56[at]hotmail[dot]com.
Thursday, January 29, 2009
The last Surveys for both disciplines each had a major failure, with one mission ended up goobling the budget in each discipline. The James Web Space Telescope (JWST) was supposed to be $1B and is now $4.5B, and the Mars Science Laboratory (MSL) was supposed to be $650M and is now, per Science, around $2B. I actually judge both of these as cases not as costs run wild (although there was probably that, too) but failures in the process. The then NASA administrator, Goldin, claimed the JWST could be done for $1B because he could manage the agency to do things better, cheaper, and faster. Of course once the real issues were known, JWST's real costs became known and ate up NASA's astronomy budget. For MSL, the Decadal Survey called for a cost-capped rover that would be modestly more capable than the MERs. The Mars advisory committee, on the other hand, was calling for a much more capable mission that would never fit within that cap. NASA chose to go with the latter recommendation and in doing so made missions that the Decadal Survey thought were higher priority impossible to start. (See this blog entry for more on the history of MSL.)
Science gives the results of the last planetary Decadal Survey (released in 2003) mixed grades. For smaller missions, five Discovery missions have been approved and two New Frontiers missions. A regular program of these small, principle investigator (i.e., lead scientist) led missions was a high priority. However, neither of the two priority large missions, a Europa orbiter and a Mars sample return have been approved. Earliest launch for the former is now 2020 (but it is locked in a competition with a Titan mission) and for the latter probably the late 2020s.
New Decadal Surveys have been kicked off with the public release of the planetary report due in 2011. This time, both Surveys will put a heavy focus on developing real cost estimates instead of using management goals or best guesses. The good news for the readers of this blog is that the scientific community is very open in sharing the preparatory material. We will have a feast of proposed missions to think about, although only a few of them are likely to be high enough priority to make the final cut and even fewer may actually make into the budget and eventually to a launch pad.
You can download the previous Decadal survey here.
Wednesday, January 28, 2009
This was the first meeting of this group, and the presentations generally appear to be updates on missions, perhaps as a way of bringing everyone up to speed. There were some interesting tidbits that I'll comment on when I get further on revising a paper for publication (my real job). In the meantime, you can read:
An update to the New Horizon's mission that includes an a map showing expected imaging resolutions across the face of Pluto.
A nice update on the Dawn mission to the asteroids Vesta and Ceres with a summary of expected scientific measurements and results from a study that supports the theory that Ceres may be layered and one of the layers may contain significant ice.
A summary of the proposed Argo mission to Jupiter, Saturn, Neptune/Triton, and one or more Kuiper Belt Objects. This is hands down my favorite mission for selection launch in the late twenty teens, in part because Triton is one of my favorite worlds. (I didn't spot anything new in this presentation from past Argo presentations to other advisory boards.)
There are also several more updates on specific issues.
(I don't want the headaches of being a discussion board administrator, which is why this is a blog. I will post -- always with permission and anonymously if requested -- thoughtful commentaries sent to me at vkane56[at]hotmail[dot]com.)
I will self-identify, as you know, as a jovian proponent. Both my heart and brain are there, and I haven't felt myself budging.
This is despite the fact that I think that Titan is easily the most interesting world of the bunch in discussion. I really enjoyed reading the two proposals, but in reading them, I found my preference coming out in another way. I first read the EJSM proposal. Perfect? No. Io gets shortchanged, Ganymede gets overemphasized, and as Jason points out, the astrobiological motive is overplayed. But I did come away quite certain of what the combo mission is intended to do. It will leapfrog quite radically over the accomplishments of Galileo. I remember jovianists contemplating with envy what Cassini might have done at Jupiter. This combo proposal would radically surpass even that.
Having read it first, I found the TSSM proposal failed to connect with me, even given that, again, I think that world clearly exceeds the rest of the field in interest. But my gut reaction has a strong basis in logic -- Titan is a target-rich environment for science, to be sure. But the questions are, relative to the jovian system, comparatively formless. I'm sure we would see a fascinating world emerge from the haze, with a methanifer system laid out before our eyes, maybe itself the most interesting thing in the solar system. But it's all exploration, if you will, and little science. This would be Titan's Mariner 9 mission (Cassini is more of a Mariner 6+7), whereas the EJSM combo is more of a Viking orbiters mission to Jupiter. And this recapitulates what is obvious: the Jovian system is one step further along in its exploration and yet it is also "due", with its last dedicated [partial-success] mission having launched in 1989. It was built when Ronald Reagan was president! Titan is receiving fresh flybys from Cassini at present.
The jovian system has to win. The papers have yet to be written on what Cassini WILL HAVE found, and on which questions merit focus in the exploration architecture.
EJSM may effectively close the door on jovian system science in this epoch (with the exception of Io, which could merit its own dedicated missions), if Europa ends up, against appearances, to be a dead end for studies of the ocean-surface interface.
Titan, on the other hand, is not yet well enough known for the architecture to be optimized for its next mission. As rich a bounty as the current proposal would undoubtedly reap, there's just no excuse for leaping ahead with a possible gap between Titan as we see it now and Titan as we may see it before TSSM would arrive.
Right now, the voting on the blog is running 74% in favor of Titan-Saturn and 26% in favor of Europa-Jupiter.
My preference seems to change daily and it is always close. These are excellent proposals and both deserve to fly. They are also highly complimentary since they explore moons of gas giants. Titan's geology likely will be harder to decipher because its surfaces is modified by atmospheric processes (what lies beneath all those dunes, for example?). The Jovian moons lack an atmsphere to modify their surfaces and can provide examples of processes that are likely to operate on Titan but that might be obscured.
Much of the decision process that NASA and ESA will use will revolve around details of mission risk and technology readiness that the public will not have knowledge of. We therefore cannot handicap that portion of the decision making.
Here are some personal observations I keep in mind when trying to decide between the two missions:
As a mission to explore a gas giant system, I think that Europa-Jupiter gets the nod since it will include observations of 4 moons, the magnetosphere, and Jupiter itself. The Titan-Saturn mission appears to be more narrowly focused on just two moons although it will also include magenetosphere studies. The strength of the Europa-Jupiter mission as an exploration of the Jovian system would be enhanced if Japan flies its proposed magnetosphere orbiter.
As a mission to explore individual moons, I think that Titan-Saturn gets the nod since it can sample the interior material of Enceladus by flying through the plumes and can land on Titan and float in its atmosphere. (The Europa-Jupiter mission will sample material sputtered off the moons, but I think the concentrated and probably relatively unaltered material from Enceladus is a better sample.)
One of the goals of the Europa orbiter is to find one or more locations where a future lander could sample relatively pristine ocean material that lies at or near the surface. If no locations are identified, then the exploration of Europa may be stymied. If one or more locations are found, the next step of Europan exploration would require a sophisticated lander (and possibly radiation hardening technologies that don’t yet exist) that I suspect would be Flagship mission in its own right. Such a follow on mission would have to wait the results of the Europa orbiter, and so probably wouldn’t launch until the late 2030s or 2040s. Russia is exploring the idea of launching a Europan lander which would address at least part of this issue (how much depends on how capable the lander would be).
Based on previous studies of missions, it appears to be cheaper to make up a meaningful portion of the Europa-Jupiter mission with cheaper mission alternatives than with the Titan-Saturn mission. Titan and Saturn appear to require Flagship scale missions while the Jovian system can be explored (IN MUCH LESS DEPTH) with New Frontiers and maybe even Discovery missions.
The ESA contribution to either mission is not guaranteed. ESA will decide between participating in the the winning Flagship mission and other missions proposed for its Cosmic Vision program in 2011. It is always hazardous to second guess mission selection processes (my record would be no better than if I'd thrown darts), but I have a feeling that the Titan lander and balloon might fare better in the selection than a Ganymede orbiter. Titan builds on ESA success and expertise in Titan probes from Huyegens. The public appeal of a lander floating on a sea on another world and a balloon floating above an alien but strangely Earth-like world be tremendous.
I'm sure that there are considerations that I haven't thought of; feel free to list them in the comments. Whichever way it goes, an excellent proposal will be selected and an excellent proposal will be rejected.
Sunday, January 25, 2009
The cumulative impacts of these challenges are well understood in the planetary science community. The Planetary Science Subcommittee (PSS), which is a group of scientists that advises NASA on its planetary program, has called on NASA to formulate a new Mars exploration roadmap.
NASA’s Mars program had been focused on a Mars sample return (MSR) mission in the early to mid-2020s. Given the suspension of the development of key technologies, that date appears to be problematic. The estimated $3-5B price tag (to be split among several space agencies) is also problematic. (Editorial comment: MSR has been studied for the last 20 years at least. The technology challenges are significant. I suspect that costs at or greater than $5B are probably most realistic, but have no inside knowledge.)
It will take time for a new Mars program to gel. In the meantime, based on past exercises in developing Mars roadmaps, we can expect that the roadmap issues to revolve around several opportunities, challenges, and philosophical viewpoints.
What scientists would like to do at Mars after MSL and the MAVEN (upper atmosphere-oriented orbiter to launch in 2013) has been established in a number of planning exercises (read the results of the most recent here and here). The following paragraphs summarize the key opportunities that have been identified.
Mars has shown itself to have around a dozen distinct terrain types that show evidence of past water. It would be desirable to explore several of them with rovers. (Opportunity is exploring one type (with a limited instrument suite); MSL will explore another; ESA’s ExoMars (2016) will presumably explore another.) NASA and other space agencies could fruitfully spend a decade or more sending rovers to interesting spots on Mars. The rovers also could collect and cache samples for a future MSR.
For as long as MSR missions have been studied, a parallel goal has been to place a network of stations around the Martian surface. Continuous measurements of seismic activity, meteorology, and heat flow from multiple stations would address key questions about the interior and atmosphere of Mars.
Rovers and network stations require an orbiter to act as a telecommunications relay if significant amounts of data are to be returned. The Mars Odyssey and Mars Reconnaissance Orbiter (MRO) serve that role now. Mars Express and MAVEN can also acts as relays, but they have highly elliptical orbits that reduce their effectiveness for this role. NASA seems to believe that it needs to put a new, highly capable orbiter in place every 8-10 years to ensure a high probability of an operating orbiter being in place.
Once an orbiter is added to the roadmap, the question becomes which science instruments to also add. NASA has decided to focus the next large orbiter on climatology and atmospheric chemistry. Follow on orbiters could study the surface with a variety of instruments.
The ultimate goal of Mars missions for almost two decades, though, has been to return carefully selected samples from the Martian surface. That would allow the incredibly sensitive instruments in Earth laboratories (that couldn’t be replicated as spacecraft instruments) to be brought to bear in the studies of Mars. We have samples of Mars delivered as meteorites, but we don’t know where they came from and they don’t represent the high priority sites scientists want to sample.
Money is the obvious challenge. A mid-sized rover (larger than MER, smaller than MSL) could cost $1.6B, a network mission $1.2B, a Mars orbiter $1.1B, and the MSR mission $3.5B (although I’ve read estimates in articles that go up to $5B). (Cost estimates from this document and are real year, i.e., inflated dollars from this document.) All this adds up to $3.9B without MSR and $8.4-10.4B with MSR. The team that most recently examined options for NASA’s Mars roadmap assumed annual budgets in the $450-550M (current year dollars) range. (It appears that the budget was assumed to inflate by 3% a year.) Even with inflating budgets (and NASA’s budget in recent years has semi-regularly been frozen at previous year’s levels) this roadmap is a 15-20 year plan, or 8-10 Congresses, 4-5 Presidential elections, and probably an economic cycle or two.
NASA needs to keep its Mars entry, landing, and descent and rover technology teams together. That is tough to do without a major active program. To deal with this problem, the plan (it appears to have been a working plan rather than a formally approved plan) before the MSL slip was to launch a rover mission in 2016 and an orbiter in 2018. Post MSL slip, NASA cannot afford its own rover in 2016 and will do a joint rover mission with ESA. (NASA will be the junior partner, so I don’t know if this mission would serve to keep key technology teams together.) Doing a rover mission in 2018 may keep the technology teams together, but pushes the replacement for what will then be very old orbiters to 2020.
NASA’s previous roadmap had the goal of doing the MSR mission as soon as possible. That mission, though, requires new technologies be developed. That new technology development will be suspended to pay for the MSL cost overruns. That means that NASA will have to restart those efforts in 2-3 years and then allow the new technologies to reach maturity before it can fly MSR.
Every mission roadmap has certain assumptions built into it that I refer to here as a philosophy. One such assumption is that Mars will retain the high priority (and high proportion of NASA’s planetary budget) that it has now.
Another key assumption is that progress in understanding Mars is best served by flying MSR at the earliest opportunity. Another approach would be to wait an additional 5-10 years. In that time period, fly multiple rovers that increase our knowledge of the surface and ready-to-return caches of samples. That way, proponents of this view hold, we have a better chance of returning the most important sample. The technology for MSR could continue to mature in parallel. Those favoring MSR at the earliest date prevailed in the previous roadmap discussions.
Another key assumption is the level of international cooperation. ESA, JAXA, and Russia, for example, can clearly mount sophisticated missions to Mars. Other nations can or will be able to soon send missions to Mars. ESA and NASA are planning to cooperate on a 2016 rover mission. Discussions are underway to make MSR a multi-agency effort. International efforts, though, are complicated and can tangle up missions in the domestic politics and budgets of each partner. Cooperation will be done, but how much does a space agency want to make its Mars roadmap dependent on partners?
It’s clear that the teams developing NASA Mars roadmap and partner teams at other agencies will have a lot to ponder to try to put the puzzle pieces together into a coherent, feasible, and saleable roadmap.
Saturday, January 24, 2009
Wednesday, January 21, 2009
NASA and ESA announced today that the NASA-ESA Decision Board meeting for the Outer Planet Flagship mission selection has been delayed from January 30 to February 12, 2009. The NASA ESA Evaluation Board will still be meeting on January 28, 2009.
Thanks to Jason Perry for pointing this out.
Nature, in a separate editorial in their subscription-only issue comes down marginally in favor of Titan over Europa. For them the deciding factor is that the Titan mission can sample Titan directly with its lander and balloon. The Europa mission can identify possible sites for landers. Conclusive proof of life on Europa would have to come with yet another flagship mission that samples the surface or subsurface.
One piece of new information for me in the Nature article was that the mission choice may be announced by February 3, or a little more than a week away. Perhaps in days, one excellent proposal will be accepted and one excellent proposal will be rejected.
For the last two decades, NASA has launched a Flagship class (~$3B in today's dollars) about once a decade (Cassini in the late 1990s and Mars Science Laboratory in 2011 and the outer planets flagship in 2018-2020). That means the loser of this contest likely would get its next shot in the 2020s, when it will have to compete for selection against Mars, Venus, and possibly other destinations. Even if the losing outer planets Flagship mission is selected then, it would not reach its destination until the 2030s or even 2040s.
This blog entry looks at the question of what mission could fly at a fraction of the cost of the proposed flagship mission within the next 20 years and provide a meaningful fraction of the science for the full proposed mission. For this mental exercise, I will set a few rules. ESA's contribution must be less than 650M euros (~$850M), which is what it may spend on the flagship mission. NASA's contribution would have to be less than $650M (above what it will pay for the selected Flagship orbiter) including instruments but not including launch vehicle, which is the price of a New Frontiers class mission.
I'll make a disclaimer here. I am not a planetary mission planner, just an enthusiastic follower. What I suggest may in fact be folly on one or many points. Wherever I can, I draw use analogies to past mission proposals to make sure that the ideas have at least some basis in reality.
If Not Titan-Saturn
A study was done a few years ago was done to see what the cheapest meaningful Titan mission would be, and based on several mission concepts, the answer came back that it would be ~$1.5B (with a Titan balloon judged as having the greatest science value). That floor means that no New Frontiers-class mission alone could meaningfully add to our knowledge of Titan.
So, we need to look for ~$1.5B to mount a mission to Titan-Saturn. Let's suppose that instead of providing a Jupiter-Ganymede orbiter, ESA decided to go ahead an build the Titan lander and balloon that is part of the (in this scenario) not selected Titan-Saturn proposal. ESA's 650M euro appears to be enough to provide those two craft, but not a launch vehicle or a carrier craft to ferry the two in situ vehicles from Earth to Titan over a flight of nine years or so. NASA could provide another $650M plus a launch vehicle that would bring resources near $1.5B.
While many possible mission concepts could be done, in this case I'll suggest a mission based on the study mention above. In the version we'll look at here, the two ESA in-situ probes and and a carrier craft launch in 2020 and arrive at Titan in 2029. Both Titan probes are released as the carrier passes Titan without entering Saturn orbit. The balloon in this imagined mission would have to relay its data directly to Earth. Most of its instruments would use low data rates and might not be overly impacted. The number of pictures returned, however, would drop dramatically without a capable Saturn orbiter to act as relay. The Titan-Saturn Flagship mission's balloon would have returned ~1.3Tb of data. Back of the envelope calculations based on data rates stated for a standalone Titan balloon like that considered here suggest a data return of ~20Gb. Also lost in the scenario is the detailed global mapping of Titan as well as the loss of the Enceladus flybys the orbiter would have done.
Under this scenario, science is also lost at Jupiter. The ESA craft was to have done a campaign of Callisto encounters and the close up examination of Ganymede from orbit that the ESA orbiter would have done. The NASA Europa orbiter already is planned to carry out a handful of flybys of these moons (plus Io flybys). Perhaps some of this science loss in this scenario could be made up by having the NASA Europa orbiter do an extra year of Callisto and Ganymede flybys prior to entering Europa orbit.
If Not Europa-Jupiter
In this scenario, the full ESA funding goes to the Titan lander and balloon that are support by a NASA Saturn-Titan orbiter. Any Jupiter mission, therefore, would have to fit within the $650M New Frontiers budget. ESA estimates that it needs the equivalent of $850M to build and fly a Ganymede orbiter. To make our mission fit within the New Frontiers budget, we'll assume that it would be a Jupiter orbiter that conducts multiple flybys of the icy Jovian moons but orbits none. We'll also assume that the orbiter comes no closer to Jupiter than Europa's orbit to minimize radiation exposure and therefore the cost of radiation hardening. Based on past studies, I think solar cells could be used, but if not, we'll assume that a plutonium-based power system is available within the budget.
A mission similar to what I'll propose, a Ganymede observer, is already a high priority for the New Frontiers program. That mission is generally assumed to be a Jupiter orbiter that makes multiple passes by Ganymede. In this exercise, we'll assume that the craft is the same (with additional radiation shielding) but also includes Europa and Callisto as targets of flybys. I'll call this mission the icy moons observer.
Jovian and Saturnian icy moon missions have been studied a number of times, and similar instrument suites keep being proposed. I'll assume that our mission includes a narrow angle camera, an ice penetrating radar, instruments to sample the vapors sputtered off the moons, a magnetometer, and a visual-near infrared spectrometer. Several additional instruments, such as a thermal spectrometer and a wide angle camera, would be useful but may not fit the budget.
In the 1990s, missions to study Europa after the Galileo mission were done. They all assumed a Europa orbiter as the preferred mission, but analyzes of multiple flyby missions for Europa were also done. Such missions included up to 30 Europa flybys. I don't know if the radiation hardening available within our budget would permit that many flybys. (Jupiter's radiation field increases as you come closer to Jupiter. The radiation at Europa is more intense than at Ganymede which in turn is more intense than at Callisto). I'll simply assume that as many Europa flybys will be done as the radiation hardening permits.
In addition to Europa, our craft would perform flybys of Callisto and Ganymede. ESA's Jupiter-Ganymede orbiter would have done 19 Callisto flybys before orbiting Ganymede. In theory, our conceptual icy moons observer could do that many flybys of both moons each. However, mission operations are expensive, so perhaps 5 flybys of each of the three moons during the primary mission might be more reasonable. If the spacecraft is healthy at the end of its nominal mission, additional moon flybys coud be done in an extended mission. The craft also would make long distance observations of Jupiter and Io.
It is important to remember that the science returned by our conceptual mission would be much less than that which would have been done by orbiters of those moons. Several studies of icy moon flybys have concluded that 15-25Gbits of data could be returned per flyby – far more than returned on each moon by the Galileo missio with its crippled main antenna. That is a small fraction of the data that would have been returned by the Ganymede orbiter (~1.5Tbits) or the Europa orbiter (~4.5Tbits).
We need to think of each flyby as sampling a small portion of a moon with high resolution images and a transect of ice penetrating radar data immediately beneath the ground track at closest approach. This is a far cry from the high resolution coverage of the entire globes that would have been done by the orbiters. The science is far more than Galileo provided but far less than the dedicated orbiters. Still, if a Ganymede observer with these limitations has been ranked as a high priority mission, then a mission that observes three icy moons should be even higher priority.
This mission would also be a nice compliment to the Titan-Saturn mission. Titan is a an icy world, but one in which atmospheric haze and active surface processes will likely will make it hard to determine the history of internal geologic processes. The results of those processes are exposed on the surfaces of Jupiter's icy moons for easy study.
Editorial: If I Were King
If I were king, my preference for exploring the outer solar system would be to select the Titan-Saturn mission and a New Frontiers icy moons observer. The reduced Titan mission described in this post feels too compromised to me without the orbiter to relay the balloon's data. Also, we would lose all the close flybys of Enceladus that were planned for the Titan-Saturn mission. Exploring Titan after Cassini seems to need a flagship class mission. At the same time, Jupiter and its icy moons are fascinating targets in their own right. If my thought exercise has a basis in reality, it would seem that we could add significantly to our knowledge of those worlds (plus remote observations of Jupiter and Io) for something within or approaching the cost of a New Frontiers missions. And we launch New Frontiers missions two to three times a decades so there could be launch opportunities that would deliver the icy moons observer to Jupiter before or within a few years of the arrival of the Titan flotilla.
I welcome comments from professional mission planners correcting my mistakes and oversights.
Tuesday, January 20, 2009
You can download them at:
Europa Jupiter System Mission
Titan Saturn System Mission
Both summary reports begin with compelling cases for the science for each mission. The short extracts below give an idea of the flavor the cases made for these missions.
The Case for the Jovian System and Europa
"Jupiter is the archetype for the giant planets of the Solar System and for the numerous planets now known to orbit other stars. Jupiter’s diverse Galilean satellites—three of which are believed to harbor internal oceans— are the key to understanding the habitability of icy worlds. The Galilean satellites are quite distinct with respect to their geology, internal structure, evolution, and degree of past and present activity. To place Europa and its potential habitability in the right context, as well as to fully understand the Galilean satellites as a system, the two internally active ocean-bearing bodies — Europa and Ganymede — must be understood. Thus, the Europa Jupiter System Mission (EJSM) is guided by the overarching theme: The emergence of habitable worlds around gas giants.
"Europa is unique among the large icy satellites because its ocean is in direct contact with its rocky mantle beneath, where the conditions could be similar to those on Earth’s biologically rich sea floor. The discovery of hydrothermal fields on Earth’s sea floor suggests that such areas are excellent habitats, powered by energy and nutrients that result from reactions between the sea water and silicates. Consequently, Europa is the prime candidate in the search for habitablezones and life in the solar system. However, the details of the processes that shape Europa’s ice shell, and the fundamental question of its thickness, are not well understood."
The Case for the Saturn System and Titan
"Titan is a complex world more like theEarth than any other: it has a dense mostly nitrogen atmosphere, the only other place besides Earth to have one, it has an active climate and meteorological cycle where the working fluid—methane—behaves under Titan conditions the way that water does on Earth (Figure 2.2-1). And its geology—from lakes and seas to broad river valleys and mountains—while carved in ice is, in its balance of processes, again most like Earth. Beneath this panoply of Earth-like processes an ice crust floats atop what appears to be a liquid water ocean. Goal A seeks to understand Titan as a system, in the same way that one would ask this question about Venus, Mars, and the Earth...
"Titan is also rich in organic molecules— more so in its surface and atmosphere thananyplace in the solar system, including Earth ... Goal B is to understand the chemical cycles that generateand destroy organics and assess the likelihood that they can tell us something of life’s origins...
"For Goal C, two aspects of the Saturnian system must be studied. These are Enceladus, whose interior is exposed to analysis through an active plume-geyser system, and the Saturnianmagnetosphere which is a medium of exchange of matter and energy with Titan. Here the objectives divide into exploring those aspects of the Saturnian magnetosphere directly related to Titan, and exploring the composition of the Enceladus plumes and whether the source region is liquid water (with implications for the sources of heating). Exploring Enceladus, if liquid water exists in its interior, adds a second target of astrobiological importance to the mission: a “two for one” in the search for life and its origins in the solar system."
The reports also provide considerable detail on the mission implementations. The following chart, compiled by myself, gives a comparison of key facts pulled from the reports (click on the image for a larger view):
Here is an example of the detail from the Europa Jupiter System Mission report (click on the image for a larger view):
and an example of the detail from the Titan Saturn System Mission report:
Jason Perry has posted an article on his blog, Gish Bar Times, on imaging (think dots, not details) of the Galilean moons by the Juno Jupiter mission at http://gishbar.blogspot.com/2009/01/juno-io.html
Monday, January 19, 2009
This is (for now, see below) the last of the descriptions of Discovery mission concepts being studied that would use the new ASRG plutonium power sources. Previous entries looked at the Io Volcano Explorer, the Venus VALOR balloon, and the CHopper comet mission.
The Jovian Trojan asteroids share an orbit with Jupiter. Their exploration was ranked as a high priority by the last planetary Decadal Survey (2003):
"The Trojan Asteroid/Centaur Reconnaissance mission would send a KBP-like flyby reconnaissance spacecraft equipped with imaging, imaging spectroscopy, radio science, and, potentially, other instruments to make the first explorations of both a jovian Trojan asteroid and a Centaur. Beyond simply opening up these two new classes of primitive bodies to exploration, this mission has deep ties to understanding the origins of primitive bodies. In particular, the Trojan flyby would sample primitive material from the jovian accretion region of the nebula; it would also allow an important recalibration of the bombardment flux on objects in the jovian system and would offer new insights into space weathering and other processes affecting asteroids, particularly in the main belt."
While spacecraft have orbited and landed on near Earth asteroids and flown by main belt asteroids (and the Dawn spacecraft will orbit 2 of the 3 largest main belt asteroids in the next decade), no spacecraft has visited a Jovian Trojan asteroid. Ilion would do that by:
"The Ilion mission will flyby several Trojans and rendezvous and land on one of them. It carries remote sensing instruments to characterize the asteroid’s structure and landed instruments to measure its surface composition. Preliminary orbit calculations have shown that several of the Trojans can be reached by Discovery-class missions with reasonable travel times... Approximately the final 2 years of the cruise will be spent within the L5 Trojan cloud... After [orbit insertion], Ilion will observe the target asteroid for several months and a landing site will be identified. After landing, a variety of compositional and physical measurements can be made."
See this abstract for more information.
The mission concepts that have been summarized in this blog are only about a third of the concepts under study, but they are the only ones that have had information about the proposals released publicly. My understanding is that the final report from each team preparing a report will be released publicly after they have all been collected; however, this information is more than a year old and I don't know that it is still the plan. If the proposals are released, they will be discussed in this blog.
The concepts that have been released show that the small, lightweight (and plutonium frugal -- important with a limited supply) ASRG power systems open up a whole new class of small missions. The Venus VALOR and Io Volcano Observer could be done with batteries or solar cells, respectively, but the science return is much greater with the ASRGs. Missions that are combinations of orbiters and landers like CHopper and Ilion appear to be uniquely enabled by lightweight plutonium power supplies.
A bit over a year ago, I shared a beer with one of the members of the team preparing the proposal for the Titan Flagship mission. He told me that the current plutonium power systems limited the flexibility of mission designers. Because of the size and weight of these power systems, it just wasn't practical to design small Titan balloons and long-lived landers. He said that with smaller plutonium power supplies, a Titan Flagship mission could conceivably have more than one plutonium-power in situ craft. (The current proposal for Titan has a battery powered, short-lived lander and a single plutonium-powered balloon.) Perhaps the availability of ASRG power supplies would enable a Titan Flagship mission to have two balloons (each smaller than the one currently planned) or a balloon and a long-lived lander.
ASRGs also would enable another class of missions -- long lived networks of stations to monitor seismic activity, heat flow, weather and the like on the moon or Mars. That will be the subject of a future blog entry.
Saturday, January 17, 2009
The Space News article will only be available for a week, I believe. This presentation gives lots of detail on various options including a possible end of mission where Cassini flies inside the rings and just above the atmosphere to provide high resolution gravity and magnetosphere measurements. This mission end would replicate at Saturn a good portion of the science that will be done by Juno at Jupiter. (Cassini can't do Juno's deep atmospheric structure and composition sounding because it lacks a microwave radiometer that Juno will have.)
Friday, January 16, 2009
In these posts, I try to keep my opinions to a minimum (I'm no smarter or more knowledgeable than most of my readers) and focus on facts and leave the opinions to you. Here I will make an exception. Given the course spatial nature of the emissions of methane observed, I hope that MSL is not re-targeted with the goal of exploring a methane rich area. The area in question, Nili Fossae, is a geologically interesting area, but one that had been dropped from consideration. If further geological analysis raises its priority given the studies that MSL can conduct, then I'm all for it as a landing site.
However, we know very little about the sources of the methane. Are they coming from a broad area or from very small and localized vents? We could send MSL to Nili Fossae and find that we landed a hundred kilometers from any area venting methane.
I believe that the right strategy is to target MSL based on the instruments in its payload, which are tuned to geochemical analysis and the search for signs of past or present life on the surface. I believe that a follow on orbiter to nail down the precise locations of methane venting should fly as early as possible. After that, a rover with instruments tuned to the quest should investigate the methane emitting regions.
You can read the rankings of the proposed landing sites (on science and engineering grounds) in this letter. For extensive analysis of the science potential of Nili Fossae as an MSL landing site, look at the presentations on this site (look under Day 2's presentations).
Your comments telling me that I am nuts are welcome.
Thursday, January 15, 2009
Today brought exciting news of local concentrations of methane in the Martian atmosphere. From the press release:
"A team of NASA and university scientists has achieved the first definitive detection of methane in the atmosphere of Mars. This discovery indicates the planet is either biologically or geologically active... "Methane is quickly destroyed in the Martian atmosphere in a variety of ways, so our discovery of substantial plumes of methane in the northern hemisphere of Mars in 2003 indicates some ongoing process is releasing the gas," said Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md. "At northern mid-summer, methane is released at a rate comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, Calif." Mumma is lead author of a paper describing this research that will appear in Science Express on Thursday."
Look here for a longer explanation of the importance of the methane discovery.
Observations of trace gases from Earth observatories will always have problems with precisely pinpointing sources, amounts, and any possible seasonal variation. A mission, the Mars Science Orbiter (MSO), has been proposed to follow up on previous studies that suggested the presence of methane. Here's a summary of the proposed mission as presented to MEPAG (a group of Mars scientists that advises NASA about science and mission priorities) (click here for the entire presentation; this material is from slide 23):
Concept: Science Orbiter Providing Long-lived Atmospheric Remote Sensing and Telecom for Landed Assets
– Extend atmospheric and seasonal surface climate baseline through the next decade
– Provide improved and new (e.g., winds) profiling capabilities
– Provide extensive global, diurnal and seasonal survey of key trace gases, including carbon-bearing compounds with implications for interior bio/geochemical processes
Methane and higher order hydrocarbons
Photochemical products, isotopes (CO, NO, etc.)
– Particularly synergistic with Network for both relay and atmospheric science
– Scientifically synergistic (lower atmosphere) with 2013 Scout (upper atmosphere)
– Use low-cost sounders & wide angle imagers with new microwave/sub-mm profilers
– Provide high-resolution, high-sensitivity spectrometers for trace gas detection
– Long-life (≥ 4 Mars Yrs) extends climate records and relay capability for next decade
– Payload could accommodate international contributions
– ROM Cost: ~$1.1B (includes long-life components and possibly site imaging)
– Methane detection has been controversial; intervening landed rovers (MSL, ExoMars) might augment or dilute need for these particular measurements
– Could be paradigm shifting in that trace gas measurements could take program in a different direction or to different places than currently envisioned, but would diverge from the current path of geologic/geochemical landed missions leading to MSR
This presentation dates from last June; the presence of methane apparently is no longer controversial. In addition to studying the martian atmosphere, the orbiter would also act as a telecommunications relay for landed missions for a decade or more. A high resolution camera (~1 meter resolution) has also been discussed for this mission. If you'd like to read a lot more about the science and mission scenario, look at this report which examined possible orbital missions to the Red Planet for the next decade. (After this report was published in 2007, atmospheric focus was selected for MSO.)
At ~$1.1B, this mission would not be cheap. Right now, the Mars Science Laboratory (MSL) for 2011 and the MAVEN orbiter focused on the upper atmosphere for 2013 are firmly on the roadmap. After the MSL cost overruns, NASA can put only about $450M into a 2016 mission, which would presumably be used to make NASA a junior partner with in a joint rover mission for 2016. So the earliest MSO could fly would be 2018. However, it would be in competition with a NASA led rover. NASA is concerned that if it delays a follow on rover to MSL beyond 6-8 years, it will lose the engineering team that knows how to build these craft.
So the earliest we are likely to get a mission to Mars to follow up on this exciting discovery would be 2018 or possibly 2020. Such are the realities of living within a budget. Perhaps this discovery will be seen as exciting enough for the administration and Congress to provide NASA with additional money. Even in this case, 2016 would probably the the earliest launch date for MSO given realistic planning, engineering, and testing leadtimes.
Wednesday, January 14, 2009
My take on the NASA administrator is that I'm less concerned about a space background than I am on managerial competence and political effectiveness. NASA has very talented engineers, scientists, and managers throughout its ranks. The role of any senior manager (I've been an almost senior manager in a large company and seen up close the workings of senior management) is to set broad goals, give the troops the resources to achieve them, and then make sure that due diligence is done. Where I've frequently seen senior managers fail is when they decide they should make decisions about the details. (At one company, for example, I was told that the CEO of a multi-billion dollar a year firm was reading the technical manuals for a pet project and sending the writers notes on improvements they should make. I knew then that the project was doomed (and it turned out to be) because the managers running the project no longer had the freedom to manage it using their detailed knowledge.)
Goldin, one NASA administrator, managed the details of the Mars program to ensure that it was done "faster, cheaper, better." Managers within the Mars program were sounding alarms (I was surprised by how frankly one of them discussed their worries publicly at a conference) . Goldin got his cheaper missions and NASA paid the price with two Mars missions lost in a single year.
O'Keefe decided that plans for a Europa mission weren't grand enough, and instead proposed JIMO which would have been a Battlestar Gallactica of a mission to orbit three Jovian moons. Details such as a humongous cost, no launch vehicle big enough to launch the thing, and many technologies yet to be invented were ignored.
Goldin and O'Keefe also were responsible for the current shoot out between Jupiter-Europa and Saturn-Titan for the next Flagship mission slot. Goldin was convinced that if he pushed hard enough, a Europa mission could be done for less than $1B. O'Keefe wanted to hold out for the grand mission. As a result, the Europa mission didn't get its start in the late 1990s or early 2000s when (in my opinion) it should have based on science and technical readiness. If it had, we could be looking forward to its launch in the near future and a Titan mission launching a decade from now.
In my opinion, the adminstrator should set broad policies such as what proportion of of NASA budget should go to planetary exploration or what should be the date for a return to the moon. The rest should be left to the engineers and managers with the expertise, with tough due diligence reviews to make sure that the lower level processes are working correctly.
I also want a NASA administrator with political savvy and connections. NASA has to compete within the executive branch for support for budget proposals and then within Congress for actual appropriations of funds. I want an administrator who is very effective at working both processes and winning and holding support for NASA's programs.
On the subject of the budget, we won't learn the new administration's true priorities for months to a year. A new budget will be presented to Congress in mid February (for the 2010 fiscal year). However, almost all the details of that budget were worked by the exiting Bush administration. The Obama team will only be able to work the margins of the proposal in the 2-3 weeks between them entering their jobs and the budget submission. Given the economic problems facing the country, I think that NASA's budget is likely to receive little attention from the Obama team in the coming month. The first full budget developed by the Obama team, reflecting its priorities, won't come until next winter. There is a chance that there may be a mid-year budget submission this spring to tweak the current year's budget, so that may give us some insight into the new administration's priorities.
In writing this blog entry, I come to re-appreciate how bizarre American politics and budgeting can be. I almost feel sorry for citizens of other countries where the budgeting process seems (from the outside, anyway) almost sane and perhaps a little boring :> .
On a completely different topic, the Washington Post has a great article about big telescopes and the budets they require.
Monday, January 12, 2009
"They [the transition team] have also asked for an estimate of the cost of carrying out all 15 missions that were recommended in a recent review of the agency’s Earth-science programme, which looks at things like the planet’s climate. At the moment, there is no money in the kitty for these missions, nor is much progress expected before 2020. The unstated implication of these questions is that someone is considering moving these missions up NASA’s priority list."
There's a general update of the planetary program which states that the selection of the next Flagship target (Saturn-Titan or Jupiter-Europa) remains on track for this February. The official release of the next New Frontiers announcement of opportunity is on hold pending final resolution of the MSL budget replan (but based on the proposed plan, it doesn't sound like the delay will be substantial).
There are several presentations summarizing the overall status of the MSL project, technical status, and budget issues. There's also a short and very nice summary of the MSL mission and science; I recommend this presentation for anyone not familiar with this ambitious and exciting mission..
The most interesting presentation (in my opinion) is the one that details the plan to handle the impacts of the MSL slip on the rest of the planetary program. There's no additional revelations that weren't in my previous summaries (here and here). For those with limited time, I've reproduced the three key slides here (clip on them for larger versions).
Sunday, January 11, 2009
The Comet Hopper (CHopper) is a study proposal led by Dr. Jessica Sunshine of the University of Maryland. (Perhaps not the best name for an investigator proposing to use a plutonium power supply instead of panels that utilize sunshine. :> ). This proposal addresses two key issues regarding comets: (1) the nature of their surface changes across the comet nucleus and (2) the rates (and perhaps, types) of activity they display changes as they move to different positions in their orbit around the sun. The ESA Rosetta mission will partially address these issues. It, however, will place a lander only at a single location on the nucleus and will follow its comet for only a portion of its solar orbit (1.5 years out of a 6.5 year orbit).
Because it uses plutonium as its power source, CHopper would have two advantages over Rosetta. First, a single craft can be both a lander and an orbital craft. If the mission used solar panels, it would be virtually impossible for it to do both. (Think about managing repeated landings on a comet with Rosetta's 14 m solar panels extending from either side of the craft!) Second, because it doesn't need solar power, it can operate when the craft and comet are far from the sun.
The CHopper team has published a short summary of their mission. I'm reproducing the introduction here (using their words minimizes my mistakes), but I encourage you to read the entire one page summary.
"The Comet Hopper (CHopper) mission explores the compositional and morphologic heterogeneity of a comet. Recent cometary flybys (Deep Impact at P/Tempel 1, Stardust at 81P/Wild 2, and Deep Space 1 at 19/P Borrelly) have revealed great diversity among comets as well as significant variation within individual bodies. Understanding the inherentdiversity of a comet nucleus and the origins thereof are now a clear objective of future cometary exploration. CHopper is an instrumented lander that will build upon the results of these recent missions. With a 2012-2013 launch Chopper will examine in detail the inner coma and surface of comet P/McNaught 2 (P/2004 R1).
"CHopper observes the comet while formation flying over one full orbital period obtaining measurements during the descent to, and on the surface of, the nucleus. CHopper takes advantage of the low cometary gravity field to take off and land (“hop”) multiple times during each descent to the surface. ... six “sorties” to the surface are envisaged to investigate changes with heliocentric distance."
My take on this proposal is that it is a clever design that is enabled by an ASGR power source. I would like to see this mission fly. How well it does in the proposal competition (if NASA opens the next Discovery competition to ASRG designs) will depend on the details of the design: What is the design risk? Do the instruments address the key science questions? Can the mission be implemented within a Discovery program budget?
Friday, January 9, 2009
It was pointed out that by cutting these projects, the Mars program will be impacted for years (15 was mentioned) into the future. NASA simply won't have the technologies it was counting on ready in time for the post-MSL generation of (at least landed) Mars missions.
MSL will be a hell of a mission, and one that I think will be good science for the dollar. We just need to realize that the price of making those dollars available will be felt for a long time
- The MSL mission slipped from a 2009 to a 2011 launch. This will cause a ~$400M cost increase in this mission that must be absorbed by NASA in its 2010 and 2011 budgets by taking money from other places in the budget.
- NASA has a number of standing committees of outside scientists who provide recommendations for various aspects of its program. This is the mechanism by which the larger science community can be informed of the options NASA is investigating and to provide a single voice of input back to NASA on those options. The PSS looks at the entire planetary program. (Other committees look at smaller portions of the program such as Mars, Venus, small bodies, and outer planets.)
Much of the disucssion was done with the audience in attendence being able to see slides that presented complicated budget informaiton. I could not see these. So that I minimize the chance of getting the facts wrong, I will keep these notes at a high level. The PSS traditionally posts presentations made to it on the web within a few days of meetings, so I presume that this will happen again and we can all see those details.
In making it's recommendations on how to handle the MSL budget increase, NASA made certain ground rules:
1) The other major planetary mission in development (Juno Jupiter mission (2011 launch), lunar GRAIL mission (2011 launch), and the Mars MAVEN mission(2013 launch)) would proceed as planned. NASA emphasized that slipping the Juno mission would be a huge hit to post 2011 budgets. MAVEN will not be slipped because NASA wants to ensure that it will be in place as a communications relay mission if necessary. (NASA expects both Mars Odysey and MRO to still be operating, but both will be in extended missions at that time frame. At current fuel consumption rates, MRO could still be operating, it was said, into the 2020s.)
2) The planetary research and analysis budget would not be touched. This is the budget that funds the science community and cuts here would have the impact of causing scientists to leave the field (as well as mean that the science results returned from previous missions would not be analyzed).
3) The New Frontier mission selection in progress will continue as planned. A number of universities and industry teams have committed significant funds to prepare proposals, and NASA felt that it would be unfair to slip the selection process.
4) The outer planet Flagship mission will launch around 2020 (a decision made independent of MSL to align NASA and ESA budgets), which slips the need for large amounts of money to carry out preparatory analysis to post 2011.
5) NASA and ESA will merge their 2016 Mars missions into a single mission.
The good news is that NASA found ways to fund approximately $353M of the cost increase by moving money around in budgets that don't impact the rest of the planetary program (beyond the Mars program). The other ~$47M would come from one of the following (1) reducing reserve funds for Juno, which increases the risks to that mission, (2) delay the start of the selection of the next Discovery mission from 2009 to 2010 (which would possibly also delay the first use of the the new ASRG power sources, which apparently NASA would prefer not to do (although it still hasn't formally decided to use ASRGs -- still working through technical and other issues)), or (3) delay the international lunar network (which would also use ASRGs, apparently). The PSS was asked to rank these three options. If the MSL budget increases beyond what is currently planned, then NASA would have to execute more than one of these options.
The net impact of all of these slips, NASA said, was to reduced the funding available for a 2016 Mars mission from a large (unstated) amount to something in line with a Scout mission (~$450). By combining efforts with ESA (which has budgeted ~$800M as a recall), the two space agencies can still fund a substantial mission. In addition, technology development for future Mars missions will be cut. Either the Discovery or lunar network may also be slipped.
On another, non budget issue, slipping the MSL launch creates a conflict with the Juno launch. Both will use the same launch pad, and by the rules currently used for pad preparation, the two missions cannot both launch in 2011. NASA is looking at two solutions (1) using a Type I trajectory for MSL instead of the currently planned Type II which moves out its launch by a couple of months but has some technical issues relating to the entry and landing that sounded workable or (2) finding ways to reduce the preparation time needed so both Juno and MSL can launch in their preferred launch windows.
I've done my best to capture the options and impacts and will let you know when the actual slides become available, but there may be some errors in my reporting. As an editorial aside, I will say that I fully agree with the approach that NASA is taking to this problem. The priorities are the same as I would have selected. Based on my management exerience (in high tech and not in aerospace), I suspect that the movement of funds to cover the first $353 mission will increase the risk that new budget problems will be found. NASA had previously budgeted these funds to other uses for good reasons. There may be additional budget impacts in future from spending this money on MSL instead.
Tuesday, January 6, 2009
Monday, January 5, 2009
Kevin Baines of JPL has proposed a number of Venus Discovery missions including an orbiter to study the atmosphere (whose goals were more than fulfilled by the Venus Express mission), and two balloon missions. The earlier balloon proposal, the VEVA (Venus Exploration of Volcanoes and Atmosphere) would have had been the most complex with a number of elements, while the second would have had just two balloon platforms. At least twice, to my recolleciton, Baines has been a finalist in the Discovery selection process, and both times other missions were selected.
The previous proposals suffered the limiation of being battery powered (solar cells are of questionable to no use within Venus' clouded atmosphere). Baines' current proposal would use an ASRG power source to enable much longer operation (a month instead of hours to days) and over ten times the data return. Baines and his collaborator, Tibor Balint of JPL, were kind enough to send me a copy of a poster on the mission they presented at a recent conference, and I've reproduced the abstract here:
"In situ exploration of Venus is expected to answer high priority
science questions about the planet’s origin, evolution,
chemistry, and dynamics as identified in the NRC Decadal
Survey and in the VEXAG White Paper. Furthermore,
exploration of the polar regions of Venus is key to
understanding its climate and global circulation, as well as
providing insight into the circulation, chemistry, and
climatological processes on Earth. In this paper we discuss
our proposed Nuclear Polar VALOR mission, which would
target one of the polar regions of Venus, while building on
design heritage from the Discovery class VALOR concept,
proposed in 2004 and 2006. Riding the strong zonal winds at
55 km altitude and drifting poleward from mid-latitude this
balloon-borne aerial science station (aerostat) would
circumnavigate the planet multiple times over its one-month
operation, extensively investigating polar dynamics,
meteorology, and chemistry. Rising and descending over 1
km altitude in planetary waves – similar to the two VEGA
balloons in 1985 – onboard instrumentation would accurately
and constantly sample and measure other meteorological and
chemical parameters, such as atmospheric temperature and
pressure, cloud particle sizes and their local column
abundances, the vertical wind component, and the chemical
composition of cloud-forming trace gases. As well, when
viewed with terrestrial radio telescopes on the Earth-facing
side of Venus, both zonal and meridional winds would be
measured to high accuracy (better than 10 cm/sec averaged
over an hour). Due to three factors: the lack of sunlight near
the poles; severe limitations on the floating mass-fraction
available for a power source; and the science requirements for
intensive and continuous measurements of the balloon’s
environment and movement, a long-duration polar balloon
mission would require a long-lived internal power source in a
relatively lightweight package. For our concept we assumed
an Advanced Stirling Radioisotope Generator (ASRG). In
return, this mission would provide two orders of magnitude
more science data than expected from the original batterypowered
VALOR concept, and could reduce measurement
uncertainties by a factor of five. In addition to the science
return, the secondary objective of this proposed mission would
be to space qualify ASRGs through all mission phases and in
various operating environments. Lifetime testing would be
demonstrated using a second ASRG on the carrier that would
keep operating after the in-situ element is delivered. Based on
the results of this and another eight ongoing NASA funded
studies, NASA will make a decision about the inclusion of
ASRGs in the next Discovery AO, due in the summer of 2009."
Friday, January 2, 2009
NASA has been aware of this problem and been working on a solution. As I understand it, thermoelectric generators work by creating an electron flow from a hot material (heated by the plutonium) to a cooler material. An alternative approach uses a Stirling Engine, dubbed the advanced Stirling radioisotope generator (ASRG). The heat source now is used to power a piston that generates the electricity. The cool part of this (if that term can be applied to a fuel source that literally glows red hot!) is that only about a quarter of the plutonium is needed to generate the same power as NASA's most recent RTG, the MMRTG being used by the Mars Science Lab. Until recently, NASA wanted a flight demonstration of the ASRG before committing it to a major mission such as an outer planets Flagship craft. In the most recently posted documents for the Saturn-Titan Flagship mission, however, ASRG's were the stated solution. ASRG's were an alternative to MMRTGs in the Jupiter-Europa presentation. The reason for the switch wasn't stated in the presentations. It could be, however, that the launch delay (for programmatic funding reasons) by 2-4 years gives NASA the additional testing time it feels it needs to commit a major program to a new technology.
ASRGs do create some design challenges for the spacecraft. Because they have moving parts, they create vibrations that could affect instruments requiring stability such as cameras. For this reason, the units are supplied as pairs that time the movement of their pistons to dampen vibrations. I had a hallway conversation with a NASA engineer at a recent conference, and he felt that the dampening would be sufficient to not interfere with cameras. He expressed concerns, however, about whether or not the vibrations could be dampened enough to work on a station with a seismometer.
If the next Flagship mission uses ASRGs, the plutonium supply problem seems to be solved for the next 10-20 years. (Eventually, the plutonium will decay sufficiently that it will not be useful. The half life is 87.7 years.) In fact, it would appear that NASA would have more plutonium than it it needs for identified missions (assuming all missions after MSL use ASRGs).
There are many places in the solar system where missions using solar panels are impossible -- Neptune or the permanently shadowed craters at the lunar south pole -- or inconvenient -- exploring the moons of Jupiter or floating in the clouds of Venus. What types of missions could be enabled if NASA had small plutonium power supplies available? Each ASRG pair produces ~280 W of power or ~6700 W hours each day. The MER rovers on Mars with freshly cleaned solar panels produce about 650 W hours each day. Even a single ASRG (they apparently don't have to used in pairs if vibrations aren't a problem) still produces far more power than the MER rovers have available on a good day.
About a year ago, NASA funded a number of studies of small Discovery and Mars Scout missions (~$450M) that would use ASRGs as an enabling technology and provide long life tests (ASRGs are designed for 14 year lifetimes). I'm told that eventually the results of all the concept studies will be made available. In the meantime, four of the teams have made details of their proposals public. One, the Io Volcano Observer, has already been discussed in this blog and more thoroughly by Jason Perry. In the next few weeks, I'll summarize the information available on the other three missions.