Correction to Cassini Costs and a Good Link

In my post yesterday on the potential loss of the last years of Cassini's extended mission at Saturn, I quoted the wrong costs for operating Cassini.  The correct figure is $60-65M per year, and not the $50-55M.  (The latter reflected a potential cut -- at the cost of a reduction in science conducted -- proposed for a past funding review.)

The following link provides a lot of detail on the current extended mission (through the end of Fiscal Year 2014) and the science results: http://www.lpi.usra.edu/opag/mar2012/presentations/OPAG_Thurs_morning/5_Cassini_Status_Spilker.pdf

Will We Lose Cassini’s “New” Mission at Saturn to Budget Cuts?

Update, August 2014.  NASA has now committed to fully funding the Cassini mission to it's planned end in 2017.

NASA’s shrinking budgets for planetary exploration may force it to decide between continued funding for the Saturn Cassini mission and the continued funding for its Mars missions.  Mars would seem to be the likely winner.  If so, we would lose a dramatic ending to the Cassini mission that would be essentially an entirely new mission to Saturn.  The ring and cloud-top grazing final orbits would produce entirely new science and would be as rich as the Juno mission to Jupiter.

Note: I've corrected this post to reflect Cassini extended mission costs of $60-65M per year instead of the $50-55M in the original post.

Comparison the possible end-of-mission orbits for Cassini and the orbits for Juno.  By dipping close the rings and atmosphere, Cassini can make highly detailed measurements of the rings, atmosphere, and Saturn's interior.  The Juno mission en route to Jupiter will provide similar measurements for Jupiter's atmosphere and interior.  Source: NASA's Outer Planets Assessment Group (OPAG).

Operating planetary missions is an expensive proposition.  This year, NASA will spend something like $215M or ~18% of is planetary science budget operating its fleet planetary spacecraft that have already launched.  (These numbers are approximate because NASA doesn’t separate out the costs of operating the Cassini and Lunar Reconnaissance Orbiter missions.)

The initial costs of operating a mission during its early phases, what is known as the prime mission, are included in the funding budgeted specifically for the mission.  Once a spacecraft continues operating past its prime mission (most do), it enters what is known as an extended mission.    NASA budgets money to operate extended missions.  The amount planned for the next several years is fixed and doesn’t go up with the number of extended missions.  As a result, there’s a process known as the  Senior Review that judges which missions should continue to be funded and at what levels to divvy up the pie. 

For the past several months, NASA has been warning the planetary community that there will be insufficient funds to operate its all extended missions at their current levels.

The Senior Review encourages mission teams to both be clear on what new science is enabled and to find ways to continue to operate on smaller budgets than were used for operations in the prime mission.  It’s not unusual for the science return in an extended mission to equal or exceed that of the preceding prime mission. 

In general, mission teams use three key justifications for funding extended missions:

• In the prime mission, it’s not unusual for new discoveries (e.g., the plumes of Enceladus) to be made, but scientists aren’t sure what they mean or what processes drive them.  Those discoveries become the subject of follow up observations (e.g., multiple passes through the plumes to conduct a variety of targeted measurements) that greatly deepen our understanding.
• Many processes simply require a long period of time to observe.  For example, the Mars Reconnaissance Obiter has imaged and re-imaged many seemingly blank spots on the surface of Mars.  At some of those spots, small meteor strikes revealed ice lying just below the surface.  The ice later sublimated away; only repeat imaging revealed the ice during the brief time it was visible.
• An extended mission may allow the spacecraft to move to a new location for entirely new observations.  The extended mission of the Opportunity rover has allowed it to essentially become a “new” mission at least twice by arriving at locations that were considerably different than those visited in the prime mission.  In another example, the lunar GRAIL orbiters used their extended mission to map the moon’s gravity field from a much lower (and riskier) orbit than the orbits of the prime mission, resulting in higher resolution maps.

The budget problem for NASA is that the demand for extended mission funding will jump in FY15 when Curiosity begins its extended mission.  At best, NASA’s budget for extended missions will be flat (news accounts quoting NASA managers) or be cut by eliminating funding for Cassini mission operations.  (The latter is suggested by NASA’s budget documents which show the budget that funds Cassini dropping to levels that don’t support the extended mission.)

Demand up, budget flat or down, something may have to go.

The Senior Review will look across all missions and could recommend funding Cassini instead of one or more Mars missions.  However, that seems unlikely to me.  The Curiosity mission is just beginning what is likely to be a decade of exploration.  The Mars Reconnaissance orbiter is required to help select landing sites and travel routes for the planned 2020 Mars rover as well as relay data from the rovers.  The Mars Odyssey  orbiter is an essential data relay for currently operating rovers.  And who would suggest turning off the venerable Opportunity rover?  (Neither of the last options provides much in the way of savings anyway; they are lean, mean extended missions.)

Cassini has had a long run at Saturn, having entered orbit in 2004 and been operated as an extended mission after the first four years.  Between the end of its current extended mission in late 2014 and the time the spacecraft is expected to run out of fuel in 2017, the spacecraft could continue to follow up on past discoveries (for example, with three final Enceladus flybys).  It also would continue to look for seasonal changes at Titan and Saturn. 

However, in its last year of possible operation before its fuel is exhausted (November 2016 to September 2017), Cassini could become an entirely new mission. 

Up until November 2016, Cassini will have stayed well away from Saturn’s rings (except during orbit insertion in 2004) because the risk of catastrophic collision with an ice particle or boulder would be too high.  With the fuel almost exhausted, though, Cassini’s managers want to bring it in close to the rings and Saturn itself.  Twenty orbits would carry the spacecraft just outside the rings for close up observations of their structure and mass.  (The latter measurements, for example, would help scientists determine how old the rings are.)

Following these orbits, the spacecraft would slip into the gap between the inner most ring and the top of Saturn’s atmosphere.  From these 22 close-in orbits, the Cassini mission would essentially replicate the measurements that will be made at the same time by the Juno mission to Jupiter:  detailed measurements of the interior of Saturn and of its atmosphere.  As a bonus, scientists can make more detailed measurements of the rings.  And for all of us who vicariously explore the solar system through these missions, think of how beautiful the images would be looking out at the rings and Saturn from just above the clouds. 

At the end of the mission, its fuel gone, Cassini would enter and burn up in Saturn’s atmosphere, an end that would prevent it from accidently impacting and contaminating Enceladus or Titan with micro-organisms from Earth.

All of the science in this end-of-mission scheme would be new, never done before science because extending the mission would allow Cassini to go to somewhere new.   NASA thought that these measurements were so valuable at Jupiter that they funded and are flying the ~$1B Juno mission, which will do similar science there. 

At one time, funding for Cassini’s full extended mission to the end of 2017 had been approved.  Then NASA’s planetary budget was cut and now the mission has to seek approval again to continue operating.

I suspect that the Cassini team is scrubbing its budgets to look for ways to reduce costs and still enable the close up orbits at the end of the mission.  That team has stated before, though, at a large portion of the budget goes to simply operating the spacecraft safely.  Cuts would have to come from reducing science operations.  (I don’t know if the orbital ballet enabling the close-in orbits would allow them to be done earlier to shorten the length of the extended mission.)

However, the real  solution is for NASA to receive more funding so that it doesn’t have to make choices that turn off a highly productive mission to save 4% of its planetary science budget.

If you would like to see Cassini make those final orbits, skimming the rings and Saturn’s atmosphere, I encourage you to join the Planetary Society’s campaign to have NASA’s budgeting program receive more funding.

Appendix: Budgets for extended missions

For those who like to see the numbers, I’ve gone through NASA’s Fiscal Year 2014 budget proposal to pull out the planned budget for extended missions and the budget for Curiosity’s last year of funding in its prime mission.

The budget doesn’t break out funding for the Cassini or Lunar Reconnaissance orbiter.  Public accounts have put the Cassini extended mission operations at $60M to $65M a year.  The MESSENGER orbiter at Mercury is expected to run out of fuel before the start of FY15, and will not be needing funds beyond FY14.

All extended missions currently are funded through the end of FY14.  The crunch begins in FY15 when the budget numbers show a funding cut and Curiosity needs new funding to begin its extended mission.

	FY14	FY15
Cassini	$50.0		Saturn
Curiosity	47.1*		Mars
Mars extended missions		$82.3	Mars
Mars Reconnaissance Orbiter	$30.5		Mars
Opportunity	$14.7		Mars
Mars Odyssey	$12.8		Mars
Messenger	$4.9		Mercury
Mars Express	$2.2		Mars
Lunar Reconnaisance Orbiter	?	?	Moon
Extended Missions	$115.1	$82.3
	* Prime mission funding

Funding identified in NASA FY14 budget proposal for extended missions

While NASA’s budget documents don’t show spending categories within the Outer Planets budget category, the chart below shows sharp cuts to this account.  By FY16, funding is too low to support the Cassini mission (and may represent NASA’s contribution to Europe’s JUICE Jupiter/Ganymede mission).   There may be some funding for Cassini in FY15.  Proposed budgets following FY16 show approximately flat funding.

	FY13	FY14	FY15	FY16
Outer Planets	$147.80	$79.00	$45.60	$24.40

Europe Bypasses the Solar System for Its Next Large Science Missions

In September I wrote about the European Space Agency’s competition to select its next two large (~€1B) science missions.  While many exciting solar system concepts were included, ESA’s management selected X-ray and gravity observatories as its missions to fly in 2028 and 2034.  Both missions promise to deepen our knowledge of the universe considerably.  (The journal Nature has a good overview of the missions.)

While it’s disappointing the ESA did not select a planetary mission, Europe will make strong contributions to planetary exploration in the next two decades with the Rosetta mission rendezvousing with a comet (2014), a Mars orbiter (2016), a Mars rover (2018), the Bepi-Colombo Mercury orbiter (arrives  2022), and the JUICE Jupiter and Ganymede orbiter (arrives 2030).  This is in addition to the currently operating Venus and Mars Express orbiters.

Europe can still select planetary missions for its Medium-scale program.  The Marco Polo-R asteroid sample return mission is currently in consideration for the next selection.  ESA’s Mars program is also funded separately and is looking at missions for the 2020s.

ESA's Rosetta spacecraft.  Credit: ESA

CubeSats to the Planets

I’ve seen the evolution of computers used as an analogy for the evolution of spacecraft.  Once, all computers were massive machines (called mainframes).  Then the evolution of electronics allowed the creation of what were known as mini-computers that expanded the number of organizations that could afford the computer.   Further evolution brought the personal computer to billions of people, and now the smart phone allows hundreds of millions of use to carry computers in our pockets.

Similarly, planetary spacecraft originally tended to be large missions (today called Flagships).  NASA’s New Frontiers and Discovery program enabled a series of lower cost missions that can be compared to mini-computers.  As I discussed in an August post, planetary SmallSats could become the equivalent of personal computers. 

Before the decade is out, I expect that we will have at least one planetary mission based on the equivalent of the smartphone, a CubeSat.

Comparison of masses and costs of missions with different classes of spacecraft.  Click on the image for a larger view.

CubeSats were invented a decade ago by researchers at the California Polytechnic State University (CalPoly) and Stanford University to create a standard for university-built spacecraft.  As the name suggests, the basic configuration is a cube.  At just 10 centimeters on a side (or 1unit (1U) in CubeSat parlance) and weighing approximately one kilogram, CubeSats depend on the electronics miniaturization that has enabled personal computers and smartphones.  If a research team needs a larger spacecraft, cubes can be stacked together to create larger volumes with standard configurations up to six units and even twelve units.  The small size of CubeSats allows them to be carried into orbit cheaply as secondary payloads on launches of larger Earth satellites.

Example of a 1U CubeSat.  Credit: Svobodat, Wikimedia Common

I wasn't able to find a definitive number for how many CubeSats have launched, but Wikipedia lists a number of satellites.  An industry has grown up that will supply standard parts that allow a CubeSat to be built as essentially a kit (see, for example, the CubeSat Kit website).

Given the success of Earth-orbiting CubeSats, it’s only natural that engineers and researchers want to use their design philosophy to explore beyond Earth.  There’s even a regular conference devoted to the topic.

I don’t doubt that CubeSats will study other worlds in the coming decade.  However, there are a number of challenges designers will have to overcome: Longer lifetimes (years instead of weeks or months), propulsion for trajectory changes and potentially entering orbit, power (especially further from the sun), communications from vastly further distances than Earth orbit, higher radiation outside Earth’s protective magnetosphere, and instruments to make meaningful measurements.  The technology to enable serious planetary CubeSat spacecraft is under development but not yet here.  .  (This presentation gives a good overview of technology in development while this presentation shows the vision for one approach to interplanetary CubeSats.)

The first CubeSat mission planned to leave Earth orbit is a NASA-funded project led by the Jet Propulsion Laboratory with several university partners.  Called INSPIRE (for Interplanetary NanoSpacecraft Pathfinder In a Relevant Environment), the twin spacecraft will each be 3.8 kg and 3U in size.  The mission goals are modest: travel 150,000,000 km from Earth (in an orbit near the Earth’s around the sun), enhance CubeSat design to successfully operate outside Earth orbit, and conduct scientific measurements with a magnetometer and simple camera, and communicate with the Deep Space network.  The spacecraft should be ready launch in 2014.

Inspire CubeSat design.  Acronyms: ACS - attitude control system; C&DH - command and data handling; EPS - electrical power system; UHF - ultra-high frequency.  Credit: JPL.

Each INSPIRE spacecraft will generate just 20 watts of power using body mounted solar arrays.  A downside to this approach is that at least half the spacecraft’s body is in shade at any given time, limiting power.  Several proposed designs would have more substantial arrays with pointing mechanisms that allow the arrays to be kept pointing at the sun.  One design would also use the solar panel as a communications reflector to boost data rates, helping to address the problem of deep space communication.  However, even with larger solar panels, power levels will still be modest.  One of the more capable proposed designs, by Tethers Unlimited, would generate 80 W in Earth orbit.  At Mars, the power levels would be ~40 W to power the spacecraft systems, instruments, and deep-space communication.  That would be a tight energy budget.

Perhaps the greatest creativity in the interplanetary CubeSat community is going into propulsion.  Even if a CubeSat simply flies by its target world, it would still need to perform several trajectory corrections along the way.  Entering orbit around another world or rendezvousing with an asteroid would require a more substantial propulsion system that would need to fit within a liter or two of space.  A challenge for engineers is that their fuel must be utterly safe to launch as an auxiliary payload with a primary spacecraft.  Carrying traditional rocket fuels that could become explosives wouldn’t be allowed.

At least two teams are proposing to carry water aloft and once in flight covert the water to hydrogen and oxygen, which are excellent traditional rocket fuels.  Other teams are developing ion engines that convert substances such as xenon into high speed plasma jets (the same basic technology on a much, much smaller scale as the Dawn spacecraft is using to reach the asteroids Vesta and Ceres).  A JPL team has proposed a 6×6 meter solar sail that would use the pressure of sunlight for propulsion.  Several teams are developing electrospray micropropulsion systems in which a liquid is drawn up tiny capillaries and electrically charged and expelled as high energy ions.  A version under development at JPL would simply melt tiny blocks of the element indium to provide the liquid, creating a system that is extremely simple and robust.

An example of one set of technologies and design for an interplanetary CubeSat.  Click on the image for a larger version.  Credit: JPL

Once an interplanetary CubeSat reaches its target, what measurements could it make?  Some instruments, such as magnetometers, already are small and can easily fit within a CubeSat spacecraft.  Other instruments, such as cameras can be made quite small.  The wide-angle JunoCam camera aboard the Juno spacecraft is 1.66 kg (not including the two kilograms of radiation shield for the Jovian environment).  (These were the best figures for these masses I could find, but may reflect early estimates.)

One proposed instrument would be a miniaturized version of an imaging spectrometer under development by JPL and Louisiana State University.  The design would both take pictures of the target world(s) and the spectral data collected would provide information on the surface composition.  While larger versions of the instrument exist, development is needed to reduce its size to fit within a CubeSat.

Where might independent interplanetary CubeSats go?  Given their limitation on fuel, power, and communications, I suspect most independent missions are likely to stay relatively close to Earth.  While some teams will undoubtedly do a dedicated mission to orbit Mars or Venus (to show it can be done if nothing else), many much more capable spacecraft have and will continue to visit those worlds.  However, CubeSats could be used to scout large numbers of near Earth asteroids both for science and to prospect for resources.  Tethers Unlimited has proposed a 6U, 9 kilogram CubeSat called HAMMERsat that could flyby and image 15 near Earth objects in a 2.5 year mission.  The company estimates that once the first HAMMERsat is designed and proven, subsequent copies could be flown and operated for approximately $3+M.    (The first spacecraft, however, could cost several times that figure I suspect.)

I think another likely destination will be our own moon.  While many larger spacecraft have and will visit this word, too, the moon is close by and well suited to CubeSat missions.  Just as many university teams have flown Earth-orbiting CubeSats, I suspect a number will also target our moon.

Microcosm's Hummingbird interplanetary SmallSat concept that would incorporate elements of CubeSat design in a larger (but by traditional planetary spacecraft standards, still quite small) spacecraft.  Credit: Microcosm. 

Another approach for independent missions would be to forego strict adherence to CubeSat form factors.  In this approach, CubeSat components would be used and augmented with larger, more capable components as appropriate.  Microcosm has taken this approach with their Hummingbird design.  It would carry a capable telescope that by itself would weigh over 3 kg (almost the weight of each INSPIRE CubeSat).  The attached instrument(s) could be up to 30 kg.  The spacecraft would carry sufficient fuel to enable complex missions.  (See my previous post for JPL and NASA concepts that also would scale up CubeSat technology to SmallSats.)

I believe, however, that the greatest potential for CubeSats may be as auxiliary spacecraft carried to their targets by larger, more traditional spacecraft.  These CubeSats can play any of several roles.  They could carry out measurements in locations where the mother craft can’t be.  Or they could work in coordination with the mother craft and/or possibly other CubeSats to make measurements in several or many locations at once.  And finally, they could be sent on “suicide” missions to locations too dangerous to send the mother craft.

As an example of the first possibility, consider a flyby mission to Uranus.  Each of Uranus’ five major moons appear to have diverse geologic histories and two are candidates for hosting their own internal oceans (see Oberon and  Titania) and Ariel has features like Saturn’s Dione suggesting internal processes that modified the surface.  As a mother craft transits the Uranus system, it could flyby only one moon closely enough to make detailed imaging, composition, magnetometer, and gravity measurements. To enable measurements in many places during the flyby, the mother craft could drop off multiple CubeSats that would perform their own close flybys of a moon and then relay their results back to the mother craft.

Uranus's moon Titania as seen in low resolution by the Voyager 2 spacecraft in 1986.  Credit: NASA

Researchers have proposed a number of possibilities for missions that would use Cubesats to make dispersed, coordinated measurements.  One proposal would have a mother craft and several Cubesats detect the radio waves characteristic of lightning at Venus and triangulate the location of the bolts.  If the lightning were located preferentially over volcanoes, that would be powerful evidence of active volcanism on Venus.  Another proposal, Atromos, would modify the CubeSat form factor to enable low cost weather stations on Mars to study processes such as dust storms from multiple locations simultaneously.  Another concept for Mars would use multiple CubeSats to form an orbital network for meteorological observation.  (This presentation briefly describes this concept and provides good of background on the challenges and possible solutions for planetary CubeSats.)  Multiple CubeSats could also deployed during a flyby of a moon, asteroid, or comet to, for example, image the world from multiple vantage points and enable high resolution gravity measurements via spacecraft-to-spacecraft radio tracking.

Many possibilities exist for CubeSats sent to locations too dangerous for a multi-hundred million dollar spacecraft.  One possibility would be for a Europa flyby mother craft to deploy CubeSats to crash onto the surface of Europa.  As the CubeSats make their plunge, they would gather high resolution images of potential future landing sites or areas of scientific interest that they would relay back to the mother craft.  Another possibility would be to have a CubeSat skim above Uranus on a flyby of that world for high precision gravity measurements, a trajectory that carries considerable risk because of that world’s rings.

While I believe that planetary CubeSats will play an important role in future exploration, it’s important to also recognize their limitations.  There are some missions where many spacecraft making simple measurements would enable studies a single spacecraft couldn’t do (see above).  With scientific payloads of a kilogram or two, however, the measurements CubeSats could make will be constrained.  Sending one or a hundred CubeSats to Europa, for example, would not equal the measurements that would be made by the 100+ kg of highly capable, synergistic instruments proposed for the Europa Clipper mission. 

Also, although CubeSats are cheap compared to traditional planetary spacecraft, their costs will still be meaningful.  If a CubeSat is intended as an integral part of a mission’s science goals, then it must have the same reliability as the science instruments on the mother craft.  Carrying a capable CubeSat plus its deployment device might add 15+ kg of mass to a mission (and multiply that figure for each additional CubeSat carried).  The CubeSat design, hardware, and testing would add several million dollars in cost.  The mother craft may have to be upgraded with a communications system and software to relay commands and data to and from the CubeSat at a cost of additional millions of dollars.  The entire system of CubeSat(s) and mother craft would have to be extensively tested.  In the end, deciding whether to carry a CubeSat as part of the core mission could come down to a tradeoff: carry an extra spectrometer or radar or another instrument on the main spacecraft or carry a CubeSat or two?

One possible role of planetary CubeSats, though, especially excites me.  In Earth orbit, CubeSats have opened space flight to university students around the world.  Perhaps future planetary missions could reserve the mass for one or two student-led CubeSats that would operate independently of the main spacecraft, limiting the cost impacts to the primary mission.  Imagine how the opportunity to build their own spacecraft to make measurements at Mars or a comet or wherever in the solar system could energize the next generation of planetary scientists and engineers.

Europe Plans Its Next Large Missions

Last spring, the European Space Agency (ESA) put out a call for concepts for its next two €1 billion science missions.  If history proves to be a guide, there’s a good chance that one of the selected concepts will be a solar system mission.  ESA’s managers will announce their selection this coming November.

These large European missions are particularly important because they have the resources both to reach targets throughout the solar system and to carry enough instruments to conduct wide ranging studies once there.  If one of the solar system concepts is selected, we may get our first orbiter for Uranus, a return to Titan, or an orbiter and balloon for Venus.   The range of concepts proposed shows that planetary exploration continues to have a wealth of possible missions.

However, don’t hold your breath.  ESA believes in long term planning and the launches of the selected missions are planned for 2028 and 2034.  Add in ten to sixteen years for, say, a flight to Uranus, and you may be looking at first science return in the 2040s or 2050s.  (Actuarial tables suggest I won’t be around then, so for purely selfish reasons, I like the concepts with much shorter flight times, like those to Venus.)

ESA breaks its science missions into three classes: Large (~€1B or $1.2B), medium (~€400M or $480M), and small (for which I couldn’t find a price target).  For comparison, NASA also has three classes of missions: Discovery ($425M likely to become $500M), New Frontiers ($750M likely to become $1B), and Flagship (>$2B for the last three Flagship missions). 

ESA and NASA account for mission costs differently, making direct currency conversion comparisons difficult.  For example, ESA includes the launch but not the instruments or much of the data analysis (which are paid for by its member states separately).  NASA does the opposite.  As a rough guide, I assume a Large ESA mission buys somewhere between the equivalents of up to $1.5B in terms of how NASA implements missions.  That mission cost target nicely fits between NASA’s New Frontiers and Flagship mission classes.  (ESA also conducts some planetary missions out of different accounts, such as the two ExoMars missions planned for 2016 and 2018.) 

Previously selected ESA Large missions (at one time called Cornerstone missions) show what can be done within the ESA budget.  The Rosetta mission will conduct humanity’s first rendezvous and landing on a comet.  The BepiColombo mission will be the equivalent of the Cassini mission to Mercury with a far more capable instrument suite than the Discovery-program MESSANGER currently at that planet.  The JUICE mission will carry out extensive studies of Jupiter, flybys of Europa and Callisto, and will orbit Ganymede.

ESA’s budget allows it to fly three Large missions every 20 years.  In the past, ESA has balanced its large missions between astronomy/astrophysics and planetary missions.  The first mission selection for the upcoming 20 year period, JUICE, is a solar system mission to launch in 2020.   A second solar system mission in a row for the 2028 slot seems to me unlikely.  A solar system mission for the 2034 mission seems likely, but ESA could pick a second astronomy mission in a row and restore the balance in the following 20 year period.  I’ve looked through the astronomy/astrophysics concepts, and they are stiff competition for the solar system concepts.

So no guarantees, but the list of solar system mission concepts is exciting, and I’m hopeful for one of the two slots going one of them.  I’ve listed the mission concepts in order from the sun.  None of the proposed concepts returns to the target of a previous ESA large mission.  One though, returns to Mars where ESA will send its two ExoMars missions and two would return to Titan where the joint NASA/ESA Cassini/Huygens mission is conducted a descent and landing and continues to make frequent flybys.

How may ESA decide among what are a number of exciting proposals?  My guess is that three criteria will be used.  First, would the mission fundamentally enrich our understanding of an important world or class of objects?  Second, would a broad spectrum of the European planetary science community be involved?  Third, would the mission be feasible within the budget target and with technology likely to be available?

A mission to Uranus, for example, would greatly deepen our understanding the ice giant worlds.  By studying the atmosphere, magnetosphere, and moons, it would involve a wide range of planetary science disciplines.  However, a Uranus mission would need a radioactive power supply to produce electric power for the spacecraft.  U.S. law prevents supplying plutonium 238 to other nations (and the supply is already critically low).   ESA has proposed developing power supplies based on another radioactive element, americium, but that would represent a bet on an unproven technology.

As you read through the list of proposals below, you might ask yourself how you would rank each according to these three criteria.  If you decide to read the original mission concept proposals (130MB download), be prepared for a lack of detail in some of them.  While NASA typically selects missions after detailed design studies and then launches in four to five years, ESA selects concepts far in advance of launch and fills in the details after selection.  In addition, based on cost estimates from previous mission studies, some of the concepts as presented may well bust the €1B price cap.  So it is possible that a selected concept would be scaled back as definition progresses.

With this background, here are the solar system concepts.  Where the concept title itself doesn’t summarize the goals, I’ve quoted a sentence or two from the proposal that captures the essence the concept’s goals.

Sun

SOLARIS: SOLAR sail Investigation of the Sun  “SOLARIS from its highly inclined orbit around the Sun, aims to combine helioseismic and magnetic observations, solar irradiance measurements and EUV images at various latitudes.”  A solar sail would be used to place a spacecraft into an orbit close to the sun and eventually over the solar poles.

Venus

Possible future missions to Venus have been extensively studied, and so it’s no surprise that the three Venus concepts propose addressing similar goals and similar approaches.  The main difference appears to be in the ambition of the concepts.  The following summary of goals is quoted from the first of the three concepts and summarizes the overall goals for all three (although the focus for each is somewhat different): “A common thread for Venus and Mars is that the atmospheres on both planets appear to have undergone catastrophic change change—Mars may have lost almost all of its atmosphere, while Venus may have driven off much of the water in a runaway greenhouse and perhaps increased its atmosphere… A major part of understanding how Venus evolved as a terrestrial planet is its  [internal] thermal evolution.”

Venus: A Natural Planetary Laboratory.    Implementation: Two orbiters for atmospheric, surface, and interior measurements, balloon or unmanned aerial vehicle, short lifespan landers.

Venus: Key to understanding the evolution of terrestrial planets.  Single orbiter to study the atmosphere and surface, a balloon, and an optional atmospheric descent probe to measure the atmosphere and image the surface.

Europe returns to Venus.  Implementation: Single orbiter to study the atmosphere and surface and a balloon or unmanned aerial vehicle.

Lunar

Lunar Science as a Window into the Early History of the Solar System.  Implementation: multiple penetrators to sample volatiles at the poles and a return of lunar samples.

Science from the Farside of the Moon. Implementation: Multiple landers to conduct radio astronomy measurements and to measure the composition of the surface and surface impact rates.

Mars

Master: A Mission to Return a Sample from Mars to Earth.  Implementation: A single unified mission to land, grab a 150 g surface and atmospheric sample, and return it to Earth

Asteroids

INSIDER - Interior of Primordial Asteroids and the Origins of Earths Water. “The scientific objectives of the proposed INSIDER mission require the exploration of diverse primordial asteroids - possibly the smallest surviving protoplanets of our Solar System - in order to constrain the earliest stages of planetesimal formation.”  Implementation: Spacecraft to orbit several >100 km diameter main belt asteroids and a lander/rover to explore the surface of a volatile-rich asteroid.

The Case for an ESA L-Class Mission to Volatile-Rich Asteroids.  Explore one of the class of asteroids known as a main belt comet, which are in the asteroid belt but which have been observed to emit volatile gases like a comet.  Determine if bodies like these could have been the source for Earth’s water.  Implementation: Spacecraft to orbit one or more bodies.  Possibly carry a lander or return a sample to Earth.

Saturn, Uranus, and/or Neptune

In situ exploration of the giant planets and an entry probe concept for Saturn.  “Comparative studies of the elemental enrichments and isotopic abundances measured on the four giant planets would provide unique insights into the processes at work within our planetary system at the time of giant planet formation.”  Implementation: Put a probe into the atmosphere of Saturn, Uranus, or Neptune, with Saturn suggested as the highest priority.

Titan/Enceladus

These two proposals would continue the exploration of these two moons following the Cassini mission.  Fairly little detail is provided on implementation.

The Exploration of Titan with an Orbiter and a Lake-Probe.  Implementation: Saturn/Titan orbiter and a probe to land one of the polar lakes.

The science goals and mission concept for a future exploration of Titan and Enceladus.  Spacecraft delivers balloon to Titan and then performs multiple flybys of Enceladus before entering orbit around Titan.

Uranus and/or Neptune

The Science Case for an Orbital Mission to Uranus: Exploring the Origins and Evolution of Ice Giant Planets.  “The Ice Giants (Uranus and Neptune) are fundamentally different from the Gas

Giants (Jupiter and Saturn) in a number of ways and Uranus in particular is the most challenging to our understanding of planetary formation and evolution.”  Implementation: Orbiter to observe Uranus remotely, explore its magnetosphere, and flyby all major moons.  An atmospheric probe would study the structure and composition of the atmosphere.

The ODINUS Mission Concept – The Scientific Case for a Mission to the Ice Giant Planets with Twin spacecraft to Unveil the History of our Solar System.  Implementation: Two relatively modest spacecraft that would orbit Uranus and Neptune to allow comparative studies of these two ice giants and their systems of moons.

Neptune and Triton: Essential Pieces of the Solar System Puzzle.  “Neptune and Triton hold the keys to paradigm-changing advances in multiple fields of planetary science: Solar System and planetary formation, exoplanetary systems, geology and geophysics, atmospheric science, magnetospheric physics, and astrobiology.”  Implementation: Neptune orbiter to observe that planet and perform multiple flybys of Neptune.

General Solar System

Solar System Debris Disk.  “The dynamical and compositional interrelations between dust, interplanetary meteoroids and their parent objects are still largely unknown… [This mission] will shed light on all these questions by mapping our solar system in dust, using the unique combination of in-situ dust measurements, analyses of returned samples, and a bird’s eye view for infrared observations of our outer “home” debris disk and beyond.”  Implementation: Infrared telescope and a spacecraft that will analyze dust in-situ and return samples to Earth.

Exploring Planetary Origins and Environments in the Infrared.  “We propose an observatory--‐class ESA mission to provide spatially resolved infrared spectroscopy of solar system and planetary objects in all their guises, from their origins (remaining debris in our solar system and planet--‐forming discs around other stars) to their present--‐day appearance (atmospheres, surfaces and interactions with their host stars for planets in our solar system and beyond).  Implementation: Space-based thermal infrared telescope.

ExoPlanets

Several concepts propose telescopes that could observe exoplanets in addition to other targets.  Only the concept below addresses exoplanet studies as its focus.

Exploring Habitable Worlds beyond our Solar System.  “Among the remarkable feats of the exoplanet community has been the ingenuity with which new observing techniques have been invented and put into successful use over the past twenty years. We now have a diverse set of tools at our disposal, with which we can explore different aspects of exoplanetary systems. A number of complementary approaches have been identified that can address habitability from different angles. Coronographs and infrared interferometers have been studied at some level of detail, and other more recent concepts (external occulters and integratedlight telescopes) also show considerable promise. While none of these is ready yet for flight, the rapid progress over the past few years in the development of the key enabling technologies gives confidence that an exoplanet exploration mission will become viable technically and financially in time for implementation in the middle of the next decade.”  Implementation: Several possible approaches are given.