Explorer of Enceladus and Titan

The search for another world with life in our solar system has arguably become the most powerful theme in planetary exploration.  While microbial life – the most likely form of life elsewhere in the solar system – itself is likely to be hard to prove with a unified series of past, current, and planned missions to Mars.  NASA is developing an orbiter to investigate the habitability of Europa and studying a follow on lander to directly search for life.  And for the third time in less than a decade, scientists have proposed a multiple-flyby mission to explore the habitability of Saturn’s ocean moons Titan and Enceladus.

The latest proposal, led by European scientists, is called Explorer of Enceladus and Titan (E²T).  It builds on the experience gained from two less ambitious previous proposals, led by American scientists, the Journey to Enceladus (JET) in 2010 and the Enceladus Life Finder (ELF) in 2014.

I met the principle investigator for the E²T proposal, Giuseppe Mitri with the University of Nantes, at a conference in December.  We discussed his team’s proposal and he has subsequently provided me we a comprehensive look at the goals and the mission implementation.  In an email to me, he explained the motivation for the mission: “Enceladus and Titan are two unique worlds in the Solar System not only in terms of their geology and evolution but also for their habitability. Flyby missions such as E2T provide an unprecedented opportunity to explore in detail, surpassing that of Cassini, two worlds in a single relatively low cost mission”

Credit: Guiseppe Mitri and the Explorer of Enceladus and Titan team

 

For scientists interested in habitability and life, Titan and Enceladus are obvious targets for exploration.  The Cassini spacecraft, which is nearing the end of its thirteen year exploration of the Saturn system, discovered that both worlds have salty water oceans beneath icy crusts.  Titan also has a rich stew of organic molecules in its atmosphere that are deposited on its surface and into its methane-ethane surface seas.  Enceladus conveniently has plumes jetting samples of its ocean into space. Within the plumes, Cassini’s instruments have found organic molecules and trace minerals suggestive of hydrothermal water-rock interactions that could provide a habitat for microbes.

The three proposed missions to Enceladus and Titan would employ relatively simple spacecraft with just two to three instruments.  They stand in contrast to NASA’s planned mission to explore the habitability of Europa, another ocean world.  The Europa mission will bristle with nine instruments, several of which will produce floods of data that require a high-powered communications system to return the data to Earth.

A key difference between the Europa and the Saturn moons missions is the prior history of exploration.  Europa was explored by the Galileo spacecraft built with 1970s technology that had a crippled communications system.  As a result, NASA’s new Europa mission must conduct its own comprehensive global study of this world.  At Saturn, this initial investigation of Enceladus and Titan has been conducted by the highly capable Cassini spacecraft built with 1990s technology.  As a result, the next mission to these moons can focus on a few specific questions answerable with two to three instruments.

(For those of you who recall that the Galileo spacecraft explored Jupiter’s system in the 1990s, the completed spacecraft sat on the ground for almost a decade due to launch delays.)

Two of the E²T spacecraft’s instrument would focus on in situ composition measurements.  At Enceladus, the spacecraft can fly through the plumes and directly taste the ocean’s contents.  At Titan, complex organic molecules are carried to the outermost fringes of the atmosphere where the spacecraft can sample the atmosphere’s chemistry. 

The E²T spacecraft’s two mass spectrometers would sample material during each passage and determine their composition by “weighing” their constituent molecules.  The distribution of the weights of the different molecules can be interpreted to determine the composition of the original material.  The spacecraft’s ion and neutral gas mass spectrometer would determine the composition of gasses while the Enceladus Icy Jet Analyzer would determine the composition of ice, salt, and dust particles in Enceladus’ plumes.  The two mass spectrometers will measure the nature of the organic chemicals, the pattern of carbon isotopes, the relative abundances of noble gases, and search for amino acids and abnormal isotope rations in organic molecules that suggest a biological origin.

The Cassini spacecraft carried versions of both these instruments, but the E²T instruments would have a forty to fifty times improvement in resolution (the ability to distinguish similar molecules) and sensitivity (the ability to measure minute amounts of a substance) over their predecessors.  (NASA’s Europa mission would carry similar instruments to E²T’s as will Europe’s JUICE mission to Jupiter and its moon Ganymede).

The E²T team has several key questions that the two mass spectrometers would address.  For Enceladus, are the materials in the plume most likely from its formation or from current geological or biological processes?   What does the composition reveal about the nature of the liquid reservoir (currently believed to be a global ocean beneath an icy cap) and its potential as a habitat for life?  For Titan, what are the sources of its volatiles and how they have been subsequently processed?  Does the atmospheric composition suggest that the current atmosphere is refreshed by material reaching the surface from the deep water ocean?  

The E²T’s spacecraft’s third instrument would image Titan’s surface and the sources of Enceladus’ plumes (the so called ‘tiger stripes’) in the near- and short-wave infrared.  (The instrument’s name spells out TIGER for the Titan Imaging and Geology, Enceladus Reconnaissance camera.)   Titan is perpetually shrouded in atmospheric haze, hiding the surface from cameras that image in most wavelengths.  However, spectral windows at 1.3, 2, and 5 microns allow a camera to image the surface at several times finer resolution than a similar instrument on the Cassini spacecraft (and also at better resolution than Cassini’s radar images).  The images returned by the camera will address several key questions such as: To what degree are sediments produced and transported by fluvial and aeolian processes?  How have the rivers and seas of liquid methane and ethane modified the surface?  How does the composition of the surface, revealed by the three ‘colors’ of the spectral bands, vary?

If the E²T mission is selected as a finalist in ESA’s medium class competition, the team will investigate whether a radio science experiment to study the two moons’ gravitational field could be added.  If it is, this experiment will investigate the thickness and mechanical properties of the ice shell at Enceladus’ southern pole where the plumes originate.  The gravity measurements would also be used to investigate Titan’s ice shell and the properties of its internal ocean.

The E²T team hasn't release a description of its planned imaging coverage of Titan's surface or flights through the plumes of Enceladus.  However, the similar JET proposal would have imaged large portions of Titan's surface at several times the resolution of Cassini's instruments and flown through the plumes at different altitudes.  Credit: JPL/Caltech

 

	JET	ELF	E2T
Neutral and ion mass spectrometer	X	X	X
Particulate mass spectrometer		X	X
Infrared camera	X		X

Comparison of instruments proposed for the three proposed Saturn orbiters that would perform multiple flybys of Enceladus and Titan.  The E²T team is proposing a more ambitious mission than did the preceding teams.

If Titan and Enceladus are obvious targets for further exploration, what is surprising about E²T and its predecessor proposals is that they were put forward for the lowest-cost class of missions flown by the European Space Agency (ESA) and NASA to explore the planets. 

Both ESA and NASA have mission categories for similarly priced low cost planetary exploration.  For the Europeans, this is the Medium class mission program where missions can be proposed for planetary science, astrophysics, or heliophysics.  For the Americans, this is the Discovery program that is dedicated to the exploration of the solar system.  These programs cap the cost for costs directly managed by the mission’s principle investigator at €550 million for ESA and $450M for NASA.  (A Euro spent in Europe and a dollar spent in the United States have similar purchasing power.)  For the ESA missions, the cap must cover the spacecraft, mission operations, and launch but doesn’t include the costs of instruments, science teams, or data analysis, which are paid for separately by individual nations.  For NASA missions, the cap must cover the spacecraft, instruments and the science team (except those contributed by foreign nations) but doesn’t include launch or mission operations, which are paid for separately by NASA.  (The difference in the way instrument costs are accounted for by ESA and NASA explains why European mission proposals typically include more instruments than similar American proposals.  For European missions, instruments are off the PI’s budget.  US proposers like to include foreign instruments paid for by their governments because they again are off the PI budget.)

Once all costs are added up, these missions typically cost somewhere in the neighborhood of 600 to 700 million Euros or dollars. However, this is still only about 60% of the cost of the cheapest outer planets mission to date, the Juno Jupiter orbiter, at $1.1 billion.  (The Juno spacecraft has to survive Jupiter’s intense radiation levels, which are absent at Saturn, accounting for a portion of its greater costs.) 

Developing a winning proposal for a mission to Saturn for less than €700 million is challenging.  Per the abstract for an upcoming conference, the JET and ELF missions “were rejected [by NASA] for too-high cost risk.” 

The E²T proposal, however, incorporates the lessons learned from those earlier proposals, and scientists from the earlier efforts are members of the E²T team.  We aren’t privy to all the details of how the E²T team proposes to fit within the budget cap.  This information is a key part of the proposal’s competitive edge.  (The presentation on the mission supplied to me by Dr. Mitri, for example, has a number of figures removed because they would reveal too much about key details of the proposal.)  However, the team has shared enough information about the mission to reveal some of the key strategies.

As described above, the E²T would have limited, focused science goals allowing for a simple, low-cost spacecraft.  Once at Saturn, the spacecraft would orbit Saturn with six flybys of Enceladus and seventeen flybys of Titan over 3.5 years.  In the weeks between encounters, the spacecraft would leisurely return each flyby’s data to Earth.  This enables a lower peak data return rate, which ripples through the spacecraft’s design, lowering mission costs.  Having just three instruments and substantial time between encounters also reduces the number of mission controllers needed, again reducing costs.

The E²T team hasn't release a description of its planned orbits around Saturn to explore Enceladus and Titan.  However, the similar JET proposal would have used orbits to encounter Titan in multiple locations to allow imaging of different hemispheres.  A series of orbits would have provided flights through Enceladus' plumes.  Credit: JPL/Caltech

 

The E²T team proposes to use a shared launch to a geostationary transfer orbit with another spacecraft such as a communications satellite.  Splitting the launch cost would save the E²T up to 60 million Euros, or enough to pay for much of the cost of mission operations on the way to Saturn.  This strategy is enabled by using solar electric propulsion, which allows the spacecraft to propel itself out of Earth orbit and reach Saturn in just six years.  (The JET mission would have required 7 to 8.5 years and ELF ~10 years to reach the ringed world.)  Once at Saturn, the spacecraft would use chemical propulsion to enter orbit and set up its encounters.

Further cost savings would come from using solar power to generate electrical power instead of the more costly plutonium-powered radioisotope generators.  (Although part of the reason for this choice is that ESA lacks the technology for radioisotope generators and US law prohibits NASA from supplying one for a foreign spacecraft.)  By using massive solar arrays of 160 square meters (compared to around 70 square meters for the Juno spacecraft), the spacecraft can create a comfortable 620 Watts of power at Saturn.  While a portion of this power will need to go to heaters to keep the spacecraft and its instruments warm (around half of Juno’s power is reserved for this), the E²T mission would have ample power margins.  

The E²T spacecraft would use massive solar arrays to gather enough sunlight at Saturn to produce electrical power. Credit: Guiseppe Mitri and the Explorer of Enceladus and Titan team

 

It is these large solar arrays that enable the spacecraft to return the data from mapping Titan’s surface.  The ELF mission, by comparison, would have had smaller solar arrays and would have lacked the power to return imaging data.  (The JET mission would have used radioisotope generators, which would have supplied enough power, and spare heat, to enable the mapping of Titan.)

The E²T proposal also has another advantage over its predecessors.  If the mission is selected by ESA, the team would have almost five years to refine the design before the actual implementation begins.  This is more time to fine tune cost savings and to take advantage of advances in spacecraft technology that have the potential to reduce costs.  NASA’s Discovery missions, by contrast, have to be ready to begin implementation immediately after their selection.  As a result, the proposing teams must have resolved all key design issues at the time of the proposal. 

The most encouraging sign for me, though, about the E²T concept is that the community of scientists interested in exploring Titan and Enceladus continue to put forth similar if more refined proposals.  Preparing for these mission competitions is time consuming and expensive.  If the budget math wasn’t close, I don’t believe they would keep coming back.  Another fact to consider is that in NASA’s Discovery program, teams often propose missions several times, incorporating the lessons learned each time their proposals are passed over.  The E²T team builds on the experience of two previous proposals.

If the E²T proposal is passed over, the Discovery program may hold another lesson.  The team behind the OSIRIS-REx mission on its way to do a sample return from the asteroid Bennu twice unsuccessfully proposed their ideas to the Discovery program.  Their efforts finally succeeded when they proposed to NASA’s New Frontiers program, which at the time allowed PI costs of around $800M.  NASA has begun the competition for its fourth New Frontiers mission, and proposals for missions to Titan and Enceladus are requested (along with proposals for missions to five other solar system destinations).  I expect that at least one mission similar to E²T, perhaps with more instruments, will be proposed.  (We may also see teams used the additional funding available to propose a sample return from Enceladus’ plumes or a Titan lander.  At the time of the JET proposal, a Titan lake lander also was proposed for the Discovery program but not selected.)

ESA will announce the list of finalists for its competition this June, NASA will announce its list in November.  If selected, an ESA mission to Titan and Enceladus would launch around 2030 while a NASA mission would launch around 2025.  These two competitions likely represent the best hope for fans of these two worlds to see a spacecraft arrive at Saturn by the mid-2030s.

Lucy and Psyche Asteroid Missions

Earlier this week, Santa in the guise of NASA managers brought the solar system small bodies science community a sack full of belated Christmas presents.  The Venus science community was unfortunately left with no presents under the tree.

As I’m sure almost all of the readers of this blog are aware, the space agency announced that it selected the Lucy multiple asteroid flyby mission and the Psyche asteroid orbital mission as its thirteenth and fourteenth missions in its low cost Discovery program.  In addition, the NEOCam space telescope mission to discover and map large numbers of asteroids was awarded an additional year’s funding for its team to mature its design.

An artist’s conception of the Lucy spacecraft flying by the Trojan Eurybates and the Psyche spacecraft in orbit around asteroid 16 Psyche.  Credit: SwRI and SSL/Peter Rubin

 

The losers were a Venus mapping mission and Venus atmospheric probe mission.  Their rejection will continue a drought in NASA launches to our sister world that followed the Magellan mission’s launch in 1989.  When asked why neither Venus mission was selected, the head of NASA Planetary Science division, Jim Green, answered that the competition for selection was among mission proposals and not between destinations.  He said that the review teams found that the proposals for the Venus missions scored less well than the proposals for the selected missions.

Dates for key events in the Lucy and Psyche missions.

Lucy Mission	Encounter date	Location	Dia-meter (km)	Spectral type
Launch	Oct. 2021
DonaldJohanson	April 2025	Main belt	3.9	C
Eurybates	Aug. 2027	Greeks	64	C
Polymele	Sept. 2027	Greeks	21	P
Leucus	April 2028	Greeks	34	D
Orus	Nov. 2028	Greeks	51	D
Patroclus/Menoetius	March 2033	Trojans	113/ 104	P
Psyche Mission
Launch	Oct. 2023
16 Psyche	2030	Main belt	210	M

The Lucy mission, named after the famous humanoid fossil, will survey two asteroid fossil beds for clues to the early history of the solar system.  It will study the Trojan asteroids that share Jupiter’s orbit, either preceding (the “Greek” camp in L4 Lagrangian orbits) or trailing (the “Trojan” camp in L5 Lagrangian orbits) the giant planet.  Telescope observations suggest these bodies have primitive compositions, several of which don’t appear to be represented in our meteorite collections and that haven’t yet been visited by spacecraft.

The planned orbits and asteroid encounters for the Lucy mission.  Credit: SwRI

The origin of these asteroid populations is a mystery, and its solution would tell scientists much about the dynamics of the young solar system.  Planetary scientists now believe that the orbits of the giant planets migrated in toward the sun and then out again soon after their formation.  In the process, they scattered asteroids and comets hither and thither.  Jupiter’s Lagrangian orbits may have been sticky gravitational traps that caught a diverse sample of bodies that originated from throughout the outer solar system to its fringes.  Another theory suggests that the Trojans originated in the same region as Jupiter and followed it in its movements and are therefore samples of conditions where Jupiter formed.  Either way – and it’s possible that the present population represents a mixture of sources – these bodies hold clues to conditions and processes from the infancy of our solar system. 

The most recent planetary decadal survey emphasized the importance of these bodies by prioritizing a mission to explore them.  “Trojan asteroids, at the boundary between the inner and outer solar system, are one of the keys to understanding solar system formation.  Originally thought to have been captured from the outer parts of the asteroid belt, Trojan asteroids are proposed in new theories to have been captured instead from the Kuiper belt during a phase of extreme mixing of the small bodies of the solar system.  In-depth study of these objects will provide the opportunity to understand the degree of mixing in the solar system and to determine the composition and physical characteristics of bodies that are among the most primitive in the solar system.”

The report listed three key questions related to the study of Trojan asteroids in context with other bodies throughout the outer solar system:

• What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated? Important objects for study: comets, asteroids, Trojans, and Kuiper belt objects.
• “What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play? Important objects for study: Mars, the Moon, Trojans, Venus, asteroids, and comets.
• “What were the primordial sources of organic matter, and where does organic synthesis continue today? Important objects for study: comets, asteroids, Trojans, Kuiper belt objects, Enceladus, Europa, Mars, Titan, and Uranian satellites.”

The Lucy looks to the New Horizon Pluto mission for two of its instruments with near copies of that mission’s LORRI high resolution camera and the RALPH color camera and imaging spectrometer.   The third instrument is a thermal emission spectrometer derived from an instrument on the OSIRIX-REx asteroid mission.  Data from these instruments will provide information on the processes that shaped these worlds, their composition, and physical properties of the surface material such as the average size of particles.  Tracking of the spacecraft’s radio signal will provide information on each asteroids mass and therefore density which provides clues to their composition and to whether they are solid objects or rubble piles.

The creativity behind the Lucy mission is that its proposers found a trajectory that over 12 years encounters seven asteroids (two in a binary system).  The Lucy mission will encounter its targets using two large solar orbits that take it out to the orbit of Jupiter to encounter the Trojan swarms.  In the first of these orbits, it will fly by a tiny main belt asteroid (DonaldJohanson, named after the paleontologist who led the team that found the Lucy fossil) and then four diverse asteroids in the Greek population.  The next orbit takes it into the Trojan population for a single encounter with a binary asteroid system whose characteristics are similar to those of comets suggesting they may be refugees from the distant outer solar system.  After this second long orbit, the spacecraft should have sufficient fuel for further encounters with main belt and Trojan asteroids in a third orbit if NASA approves funding for an extended mission.  (Each of these extended orbits appear to take approximately six years, so any encounters from an extended mission seem likely to occur in the late 2030s.)

The Lucy mission will study a variety of asteroids through brief, but intense flybys.  It will be something like photographing boulders along the roadside while speeding by on a freeway for later analysis.  The second Discovery mission selected, by comparison, will be like parking your car next to one especially intriguing boulder for a nearly yearlong examination.

The single destination for the Psyche spacecraft will be the relatively large asteroid of the same name.  This world is the largest of the rare (type M) metallic asteroids.  Psyche could be unique remnant of a class of asteroids that formed so close to the sun that only metals could condense out of the early solar nebula and was later flung into the main belt of the asteroids.  Or it could be the inner, metallic core of a once larger protoplanet that had its overlying layers of rock and possibly ice blasted off by impacts with other asteroids. 

Telescopic observations reveal that Psyche’s surface is 90% metallic and 10% silicate rock.  The spacecraft’s instruments should distinguish between these scenarios by measuring the composition in detail and looking at the arrangement of the silicate material.  The mission’s principal investigator wrote me, “If the silicate material is primarily high-magnesian pyroxene or olivine, then these silicates are likely the remnants of a crystallizing magma ocean, and indicate that Psyche started as a differentiated planetesimal and had its mantle stripped, validating the mission’s prime hypothesis for this body. If the silicates are all primitive chondritic material, then they were likely added as later impacts, and Psyche may have started life as a highly reduced metallic body without a significant silicate mantle, or, the nature of impact flux and its consequences are far more significant than our current models indicate. The numbers and shapes of craters on Psyche’s surface may help decipher that story.”  The spacecraft’s gamma-ray and neutron spectrometer (derived from an instrument on the MESSENGER Mercury orbiter) will help determine the asteroid’s bulk elemental composition.

Psyche the asteroid won’t be an unchanged relic.  Its original surface will have been battered by numerous impacts over the subsequent billions of years. The hydrated materials recently discovered on its surface with telescopic studies, for example, are likely to have been delivered by impacts of other asteroids.  It’s possible that by now, the body is a jumbled rubble pile.  The cameras on the spacecraft (near copies of the cameras that the Mars 2020 rover will carry) will be tasked with taking the images that will allow geologists to reconstruct its history.  By using filters tuned to specific wavelengths of visible and near-infrared light, the camera’s images also will help map the surface’s fine-scale composition. 

The planned orbits for the Psyche spacecraft around its namesake.  Credit: Psyche mission team.

The Psyche asteroid’s sits deep within the asteroid belt at 3.3 times the Earth’s distance from the sun.  (By comparison Vesta is at 2.6, the asteroid Ceres is at 3, and the Trojan asteroids average 5.5 times the Earth’s distance from the sun.)  To reach this world, the Psyche spacecraft, like the Dawn spacecraft that has explored Vesta and Ceres, will use solar electric propulsion to slowly but methodically reach its namesake world.  The gentle thrust of its engines will deliver the spacecraft to Psyche approximately seven years after launch and will allow it to spiral down to progressively lower orbits.  The mission’s planners expect the spacecraft to orbit as close as 105 kilometers from the surface where the cameras will have a resolution of 5 meters.

While not yet selected as an approved mission, the NEOCam telescope was awarded an additional year’s funding to mature its design.  For the team proposing this mission, this is the third time it has vied for selection.  It was originally proposed in 2006 and not selected as a finalist and reproposed in 2010 when it was awarded funding to mature the technology of its sensors.  If the mission eventually is funded by NASA, it would have two goals.  The first would be oriented toward protecting our planet by discovering a large number of the small (from a few tens of meters across up to a kilometer), near Earth asteroids that have evaded detection by other means.  The second goal would be more scientifically oriented with the NEOCam telescope expected to also observe more than a million main belt asteroids and about a thousand new comets.  The resulting database would allow sophisticated analyses on the sizes, compositions, and orbital dynamics of the population of small worlds.

So far as I can recall, this is the first time that a Discovery mission finalist has been awarded additional funds to mature its design to be ready for a future funding opportunity.  (Two other missions from the 2010 competition were also given funding to mature instrument technologies, but neither were finalists.)  NEOCam’s (this is an acronym for Near Earth Object Camera) focus on small bodies whose orbits lie close to and often cross that of Earth’s places it at the junction of several of NASA’s programs.  From finding and mapping the location of these objects, there is good science, there is planetary protection, and there is finding potential worlds for future human exploration or mining.  As a result, says NASA’s Green, the additional funding awarded to mature NEOCam’s design is seen as a strategic investment.

Unfortunately, missions to Venus are not seen as a strategic investment and both finalists for this planet are simply left as unselected.  I was very disappointed to see that neither was selected.  (I had hoped for the selection of one Venus and one asteroid mission.)  I believe that this world can tell us much about the evolution of terrestrial planets in our solar system and represents what is likely to be a relatively common class of larger rocky worlds around other stars.

So for fans of Venus and for all the other solar system destinations, what are the opportunities for selection of future missions?  The European Space Agency is currently reviewing proposals for its fifth medium class science mission, which would enable planetary missions roughly the same capability as NASA’s Discovery program.  I know that there is a proposal for a Venus mapping mission and a Saturn orbiter to study the moons Titan and Enceladus.  Based on proposals for the last competition, there are likely to be other missions proposed to study other solar system bodies including orbiting main belt asteroids.  The planetary mission proposals are in competition not only with each other but also with astrophysics and heliophysics missions.  The selection of finalists for this competition is expected by June, the selection of the final winner is expected around 2019 with a launch around 2029.

NASA has just begun the process to select its next New Frontiers mission, which will have a total budget (likely $1.2 billion or more) 80-100% larger than the Discovery missions (likely $675 million or more).  These missions are selected from a pre-approved list of high priority missions.  For this competition, this list is:

Comet Surface Sample Return

Lunar South Pole-Aitken Basin Sample Return

Titan and/or Enceladus

Saturn Atmospheric Probe

Venus atmospheric probe and lander

Trojan Asteroid Tour and Rendezvous

We don’t know what the selection of the Discovery Lucy mission, which will study Trojan asteroids, will have on the chances for the selection of a New Frontiers Trojan mission.  The selection of the finalist proposals is expected in November, the final selection in mid-2019, and launch by the end of 2025.

Finally, NASA plans – subject to the generosity of the President and Congress with future funding – to launch Discovery missions approximately every three years.  With the launch for the Psyche mission in 2023, the 15^th mission in this series should launch around 2026.  Working backwards from that date, we might see the start of the next completion late this decade and selection of the next mission(s) in the early 2020s.  There were many exciting missions proposed for this just completed competition; many are likely to be re-proposed.  And we are likely to see new ideas put forth.

As the selection of Lucy and Psyche shows, these competitions among scientists result in creative and scientifically rich missions.  By the mid-2020’s we should have another two or more new missions to look forward to.