Links for ESA Europa Mission Studies

A reader pointed out that I forgot to provide links to the European Space Agency studies of possible contributions to NASA's Europa mission.  You can find a press article on NASA's original offer to ESA here and a link to the studies here.

I've also put a poll in the upper right corner of the blog page where you can show your preference for which of NASA's Discovery missions you would like to see selected.

A European Spacecraft to Accompany NASA’s Europa Spacecraft?

Last year, NASA’s managers invited the European Space Agency (ESA) to propose a small spacecraft to explore the Jovian system.  The small craft would be carried to Jupiter by NASA’s own, large Europa multi-flyby spacecraft.  This daughter mission could add to the exploration of Europa or study another target within the Jovian system.

ESA has recently posted the results of studies for two possible spacecraft that might be carried by NASA’s Europa spacecraft to the Jupiter system.  One would land on Europa and the other would fly by the volcanic moon Io.  While these were concept studies and not an actual proposal from ESA to NASA, they give an idea of the possible capabilities and limitations on an ESA contribution. 

In the coming year, Europe’s scientists can make actual proposals for spacecraft to be added to NASA’s mission.  They can do so through ESA’s competition to select it’s fifth medium sized (~550M Euros or somewhat more in dollars at the current exchange rate) science mission.  Proposals for the Europa mission will be pitted against other planetary and astrophysics missions.  However, because NASA would cover the costs of launch and delivery to the Jupiter system, proposals for the Europa mission could have a leg up in the competition.

A small ESA spacecraft would need to find a scientific angle not already taken by its larger cousins.  Next year, NASA’s Juno spacecraft will study the Jupiter itself from just a few thousand kilometers above the top of the cloud deck.  ESA’s JUICE spacecraft will arrive in the late 2020s to study Jupiter from afar, flyby Europa and Callisto multiple times, and then orbit Ganymede.  NASA Europa Mission (apparently no longer called the Europa Clipper) will flyby Europa 45 times as well as flyby Ganymede and Callisto.  These missions will carry suites of extensive and highly capable instruments.

Under NASA’s proposal, the American space agency is reserving space and 250 kg of mass to host the European spacecraft.  (NASA is also separately reserving mass for the equipment to connect the two spacecraft.)

One concept for an ESA daughter probe (in blue) shown attached the NASA’s Europa Mission spacecraft.  Credit: ESA

So if you had a ride to the Jupiter system for 250 kg, what could you do with it?

Two obvious possibilities were mentioned at the time that the offer was announced: build a small lander for Europa or a small spacecraft that would fly through any plumes erupting from Europa’s surface.  The ESA team looked at both.

For a lander, the European study group considered a type of hard lander known as a penetrator.  Shaped roughly like a cannon shell, penetrators smack into the surface and then travel a few meters into it before coming to a stop.  (Think of a bullet shot into the ground.  The friction between the bullet’s surface with the soil slows and eventually stops the bullet.)

Concept illustration of a Europa penetrator (blue) embedded in the icy surface.  An after body (green) remains on the surface with the antenna to communicate with NASA’s Europa mission spacecraft.  A cable would connect the two sections.  Credit: ESA

Penetrators have been used on the Earth to deploy sensors from planes into remote locations such as ice shelves.  Penetrators have the advantage of not requiring expensive landing systems – the penetration into the surface supplies the braking.  A unique advantage on Europa is that once buried, the surrounding ice protects the probe from the radiation fields around Europa.

The concept studied would use the penetrator to deliver two sets of instruments in to the ice.  The first set would study the chemistry of the ice and materials within it.  A habitability package would use chemical reactions to look for conditions such as pH consistent with possible life, a mass spectrometer would analysis the composition, and a microscope would image the sample.  A seismometer would record Europa-quakes to study the level of activity within the icy crust and to gather clues about its structure.

The penetrator with its deployment stage.  Credit: ESA

The penetrator would be delivered by a deployment stage that would essentially be a small spacecraft with a substantial retrorocket.  The main NASA spacecraft would target the daughter craft to pass just 35 km above the landing zone.  Just before that distance, the European spacecraft would fire its rocket, reducing its speed to zero relative to the Europan surface below.  The delivery spacecraft with the attached penetrator then begin their free fall to the surface.  The delivery craft would have 231 seconds to reorient itself so that the penetrator points straight down and then release it.  NASA’s Europa spacecraft passing overhead would have a short few minutes to listen for a radio confirmation that the landing succeeded.

The penetrator deployment.  SRM burn is the rocket burn that removes all velocity between the penetrator and the surface of Europa.  The diagram uses the older name ‘Clipper’ for the NASA spacecraft.  Credit: ESA

Approximately ten days later, the NASA spacecraft would return to Europa and receive the data collected by the penetrator’s instruments.

While the penetrator concept is exciting, the devils are in the details.  Planetary penetrator missions have been studied for many potential missions.  Russia launched two on a mission intended for Mars but which never left Earth orbit.  NASA delivered two tiny penetrators to Mars, but they were never heard from after they were released by their carrier spacecraft.  Japan spent years developing penetrators for the moon but eventually cancelled the project because of development problems. 

A key problem with penetrators is that they need relatively flat landing sites for successful landings.  Europa’s surface is covered in slopes and rough terrain.  Also, Penetrators are built to tolerate high vertical velocities, but any lateral velocity can destroy the payload inside.  (Put another way, engineers can design for high G’s in one direction, but it’s hard to design for all directions.)  This means that the retro rocket must successfully kill all but the smallest lateral movement so the penetrator moves only vertically during its descent.

Another problem with penetrators is that the space inside is small and any instruments must be built to withstand high impact forces.  As a result, there’s usually significant instrument development required.  The penetrator concept report states that the instruments to study the chemistry of the ice are at a low state of technical readiness for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has never been matured enough to become a reliable tool for planetary exploration.

My take on the report descripting the penetrator concept is that delivering a penetrator for Europa appears to be a high risk possibility both for completing the development in time for a launch and for actual delivery.  Another significant problem is that the concept craft would have a mass greater than 300 kg, well above the 250 kg NASA is offering.

The other concept studied by ESA’s engineers would be a daughter spacecraft that would be a straightforward use of existing technologies.  The original idea was for a small spacecraft that could fly through plumes erupting from Europa.  This idea seems to have lost its appeal.  First, diligent searches have failed to confirm the original observation of a possible plume (which was made at the limits of detectability).  Second, NASA’s Europa spacecraft is highly capable with cutting edge instruments, and it could fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter system is already targeted for detailed study.  An exception, though, is the extremely volcanic moon Io that sits deep within Jupiter’s radiation field.  That moon became the target for the second study.

In this concept, NASA’s craft would release the European orbiter shortly after the two jointly enter Jovian orbit.  The ESA craft then would fire its own engine to lower the perijove of its orbit to encounter Io at least twice.

The Io flyby spacecraft would be so small that this top view engineering diagram uses millimeters for its length units.  The triangular main spacecraft body would be below the center main antenna dish.  Credit ESA.

The Io flyby spacecraft would carry four instruments.  A multi-color camera would image the surface at resolutions ranging from 2.2 km to 18 m per pixel, a thermal mapper would identify and measure the temperature of hotspots at resolutions ranging from 30 km to 50 m per pixel, a mass spectrometer would measure the composition of ions and particles ejected from Io, and a magnetometer would study the magnetic field around Io.

The trajectory for each of the two Io flybys (second flyby shown here) would be chosen so that the ground tracks pass over several areas of known volcanic activity so that these areas could be studied in high resolution.  The first flyby would pass 500 km above the surface at closest approach, the second flyby 100 km.  Credit ESA

Jupiter’s radiation is strongest in the plane of the equator where its major moons, including Io, orbit.  Since the ESA craft would be released from the NASA craft in an equatorial orbit, the ESA mission would receive the full radiation baking.  ESA’s spacecraft would have to traverse this radiation on both the inbound and outbound legs of its passes.  (A larger, fully dedicated Io mission such as the proposed Io Volcano Observer proposed for NASA’s Discovery program, would use a polar Jovian orbit instead, limiting its radiation exposure.)  The tiny ESA spacecraft potentially could perform more than two flybys if the harsh radiation close to Jupiter degrades the craft’s electronics more slowly than expected. 

The Io spacecraft study report does suggest that if the idea of an Io spacecraft is pursued, that the option of releasing it before the Jupiter orbit insertion burn is done should be considered.  That way, the Io spacecraft could do its own insertion burn and enter a Jovian polar orbit to reduce radiation exposure.

The Io flyby concept studies would come in on the heavy side by missing the 250 kg target mass by 17 kg.  To get that close, the concept design had accept a “more risky operational scheme”, that is, reduce backup systems and capabilities to minimize weight.

Either of these missions would be an exciting addition to the already planned JUICE and Europa missions.  The Io flyby craft seems to be less risky both to design and to fly.

When NASA first announced that it would offer room and mass for an ESA probe, ESA’s managers said they would let scientific teams propose which concepts would be considered as part of the next Medium science mission competition.  These two proposals are proof of concept studies that the proposing teams can use to inform their proposals.  We may be seeing even more interesting proposals from the science teams.  I can think of several alternative probes, and I’m sure the professionals can think of even more creative ones.

If ESA does contribute a small probe to NASA’s mission, the exploration of the Jupiter system may be more interesting than it already promises to be.

Finalists for the Next NASA Discovery Mission

Over the last few months, NASA’s managers have had the tough job of selecting a handful of proposals for new missions from an outstanding set of 27 proposals.  Proposed targets ranged from Venus, our moon, Mars and its moons, the comets and asteroids, Jupiter’s moon Io, Saturn’s moon Enceladus, to space telescopes to observe solar system bodies. 

In the end, two Venus and three asteroid missions received the nod to receive $3 million each for further study pending final selections in a year.  Launch of the selected mission or missions is likely for the early 2020s.

The current competition is to select the 13^th and possibly the 14^th missions in NASA’s Discovery mission program.  In the past, missions in this program have orbited Mercury, orbited the moon, landed on Mars, flown by comets, orbited three asteroids (landing on one), and searched for exoplanets.  This program lets teams of scientists propose and lead the missions (in partnership with a NASA or industry partner for engineering expertise).  Neat fact about the current finalists: Four of the five teams are led women.

Costs for the current competition are capped at $500 million for the spacecraft and instruments with NASA separately paying for other costs such as the launch.  For comparison, this is more or less half the cost of the New Frontiers Pluto mission, a fifth the cost of the Curiosity Mars rover, and a quarter the cost of NASA’s planned Europa mission. 

In the early years of the program from approximately the mid-1990s into the early2000s, NASA regularly selected Discovery missions every two to three years and would often select two missions at once.  Then budgets became squeezed and the last two mission selections were stretched out to every five years with a single selection each.  (The GRAIL orbiters selected in 2007 studied the moon for a year beginning in late 2011, and the Mars InSight geophysical mission selected in 2012 will launch next year).

In this current competition, NASA believes it may be able to again select two missions, which would be staggered in their launch and development costs.  NASA’s managers haven’t stated why they now hope to select two missions instead of the originally planned one.  Their budget forecasts may look rosier than previously expected.  It may be because the cost of the Discovery competitions to NASA and the proposal teams is high enough that the agency’s managers want to limit their frequency by delaying the next one and selecting two at once.  Or the estimated cost of some of the missions selected as finalists could be implemented under the cost cap. 

NASA evaluates the proposals based on two sets of criteria, and the competition is tough.  Teams of scientists rank each of the proposals based on its scientific potential to help us understand the solar system.  Separately, teams of engineers and budget analysts scrutinize the implementation details to determine whether each proposal likely could be implemented within the budget and schedule.  The finalists are selected from among the proposals that rank highest on both sets of criteria. 

The primary goal for the DAVINCI mission would be to make high priority measurements of the composition of Venus’ atmosphere.  Credit: NASA

The DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) proposal would drop an instrumented probe into Venus’ atmosphere.  During its descent to the surface, the DAVINCI probe would measure the composition of the atmosphere’s gases and image the surface from below the clouds. 

The team proposing DAVINCI was one of the quietest during the competition; while many other teams presented their proposals in some detail, not even the name of this proposal leaked.   A brief post on the Unmanned Spacelight message board reports that the instruments would include a mass spectrometer, a tunable laser spectrometer, an atmospheric structure package, and a visible and near-infrared descent camera.

By looking at papers and conference proceedings that include the proposal’s Principal Investigator, Lori Glaze with NASA’s Goddard Spaceflight Center, we can get some ideas about the mission’s scientific questions.

The composition of a planet’s atmosphere can reveal much about the planet’s formation, its evolution, and current geological processes such as surface weathering and volcanic eruptions.  A recent conference abstract that included Dr. Glaze, stated, “A key issue that remains after more than 50 years of planetary exploration is the formation and evolution of the atmosphere, particularly in the context of the other terrestrial planets. Comparing noble gas mixing ratios and isotopes of Venus, Earth, Mars, Jupiter, and the sun will help determine the timing and extent of atmospheric escape on Venus, a central process in planetary evolution.”  Several research papers that include Dr. Glaze also discuss how volcanoes on Venus would release gases such as sulfur dioxide into the atmosphere that would indicate whether or not Venus has currently active volcanism.

According to a blog post on the journal Science’s site, the probe would descend over one of the planet’s tesserae and would image the terrain below as it fell.  These crumpled highlands may be remnants of ancient crust on Venus.  Images as the probe falls below Venus’ clouds could provide clues about the origins of these mysterious regions and the evolution of the planet’s surface.

Both the DAVINCI and the VERITAS missions would search for current volcanic activity on Venus.  Credit: ESA - AOES Medialab

 DAVINCI is an example of a mission in which a few key measurements focus on selected critical science questions.  The entire descent would likely take less than an hour.  The data from the atmospheric composition measurements might be just a few megabytes of data.  (A study of an atmospheric Saturn probe to study composition listed the total data as less than 2M bytes, less than the size of a high resolution image from my personal camera.)  The images collected by the probe’s camera might be a few megabytes to gigabytes.  By comparison, orbiter missions at planets can return terabytes of data.

However, detailed composition measurements of planetary atmospheres is a high priority for planetary research because they can reveal details about the formation of each planet and its subsequent evolution.  The Pioneer Venus probe from the 1970s lacked the resolution for key measurements.  We have high resolution measurements of Mars’ atmosphere from various landers and from Jupiter from the Galileo atmospheric probe.  Obtaining high resolution composition measurements from Venus (as well as Saturn, Uranus, and Neptune) has been a high priority for planetary scientists for decades.  Each high resolution set of measurements for a new world provides a new piece of the puzzle to help us understand how the solar system formed.

Where the DAVINCI mission would focus on specific scientific questions and gather a small amount of critical data, the other finalist Venus mission takes the opposite approach.  The VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) mission would remap Venus’s surface with radar and conduct the first global mapping of its surface composition.  Venus’s surface was previously mapped in low to moderate resolution by NASA’s Magellan mission in the early 1990s. 

The VERITAS mission would map with radar and infrared spectroscopy.  Credit: NASA/JPL-CALTECH

The mission’s VISAR (Venus Interferometric Synthetic Aperture Radar) would map the surface in three ways.  First, it would create images of the surface at 30 m resolution globally and 15 m in selected regions compared to Magellan’s 280 m to 120 m resolution.  Second, it would measure elevations to create a topographic map at 250 m resolution compared to Magellan’s 15 to 27 km resolution.  And third, it would make repeat measurements in what’s known as an interferometric mode to spot tiny changes in relative elevations that could indicate surface movement from a seismic event or the swelling of a volcano.

VERTITAS could greatly improve the resolution of images of Venus surface (top) and topography (bottom) as shown by these simulations based on terrestrial data (Hawaii and Iceland; the topographic image is from a paper discussing general improvements possible by a new mission and VERITAS’s actual resolution may be different).  Credit: NASA/JPL-CALTECH; Decadal Survey White Paper, NASA 

A second instrument, the German Venus Emissivity Mapper (VEM), would study the planet’s thermal emissions for composition studies. The Galileo and Venus Express missions’ instruments discovered narrow spectral windows where thermal emissions can be transmitted through the otherwise opaque global cloud cover. These few windows would give the VEM instrument the ability to map the surface in six spectral bands to identify thermal hotspots that could indicate areas of current volcanic activity, map differences in the surface composition, and detect changes in key atmospheric gases that could indicate the eruption of a volcano. Because Venus’ thick atmosphere would scatter the light, the surface resolution of VEM would be low, perhaps around 50 km. The recently completed Venus Express mission carried out some measurements using this technique, but its instrument wasn’t optimized for measurements using these spectral bands and covered only the southern hemisphere.

While the DIVINCI mission would focus on a few critical measurements and would produce a relatively small data volume, the VERITAS mission would make multiple and repeated measurements over the surface of a large world.  A proposal for a similar European mission said that it would return hundreds of terabytes of data; VERITAS likely would do the same.  Researchers could use this database to enable hundreds of studies.  A poster on the VERITAS mission (unfortunately no longer available on the web) listed a few:

Origin and Evolution: How did Venus diverge from Earth?

• Determine if tesserae are remnants of an earlier wetter past

• Search for past tectonic or cratered surface beneath the plains

Venus as a Terrestrial Planet: What processes shape the planet?

• Determine how and when Venus was resurfaced

• Estimate lithospheric thickness variations with time

• Identify sources and rates of recent and active volcanism

NASA also has the option for a technology demonstration for the VERITAS mission that would partially address the DAVINCI composition measurement goals.  If funded, the tiny Cupid’s Arrow cubesat would be released by the main spacecraft and would skim the edges of the outer atmosphere to reach below the homopause where the atmospheric gases are well mixed.  A miniaturized mass spectrometer would measure ratios of key noble gases that provide clues to the formation and evolution of Venus.

These two Venus missions illustrate the different types of missions needed to explore the solar system.  The study of Venus requires both and eventually both will fly. 

The three finalist proposals to study asteroids provide another example of the complementary types of missions needed study the solar system.  Asteroids are remnants of small proto-worlds from the early formation of the solar system and differ in location and composition.  Our spacecraft will never visit more than a few of the millions of these bodies believed to orbit the sun.  Scientists instead use telescopes to gather a few facts on many bodies to enable statistical studies, make brief flybys of a small number to flesh out the statistics, and make prolonged visits at a very few for in-depth studies (and to return samples from a few).

The NEOCam space telescope.  Credit: NASA/JPL-CALTECH

The Near-Earth Object Camera (NEOCam) mission would launch the first space telescope dedicated to observing asteroids.  Its focus would be on the population of asteroids that, as its name states, approach near to our own world.  By making measurements in two infrared channels for each of the tens of thousands of near-Earth asteroids, the science team will be able estimate sizes, shapes, composition, orbit about the sun, and rotation for each body.  While the information on any one body will be limited, the statistical analysis made possible on a data set of tens of thousands of bodies would enable scientists to explore the dynamics, origins, and fate of these populations.  (Past or future observations of many of the same bodies in other wavelengths of light, particularly the visible, will add valuable complimentary information.)  During its survey, NEOCam also would observe approximately a million main belt asteroids and discover perhaps a thousand new comets, extending the usefulness of the statistics derived from its data.

However, the scientific study of these asteroids are only a part of the mission’s justification.  Some proportion of near-Earth asteroids will eventually strike our world.  Finding even one that threatens the Earth in the next few decades would justify the mission by itself.  Some of the objects discovered also could become targets of future robotic or human missions.

Summary of the Lucy mission from the proposal’s factsheet.  Credit: SwRI

The Lucy mission would follow the second strategy for asteroid exploration, brief flybys of a number of asteroids.  The mission’s proposers have reused the name of one of the most famous fossils from human paleontology to emphasize that the spacecraft would focus on a fossil population of asteroids that may hold the potential to illuminate the ancient history of the solar system.  It would 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 origin of this asteroid population is a mystery, and its solution would tell scientists much about the dynamics of the young solar system.  We 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 the tiny asteroids and comets hither and thither.  One set of theories holds that the migration brought in groups of asteroids from throughout the outer solar system into Trojan orbits with Jupiter.  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 creativity of the Lucy mission is that its proposers found a set of trajectories that over 11 years allow flybys of two Trojan asteroids in the L4 swarm and a binary Trojan system in the L5 swarm with a bonus flyby of a main belt asteroid.  The three Trojan encounters would sample a diversity of compositions, the C-, P-, and D-types. 

This mission looks to the New Horizon Pluto mission for two of its instruments with copies of that mission’s LORRI high resolution camera and the RALPH color camera and imaging spectrometer.  Another infrared spectrometer would draw on instrument heritage from Mars orbiters and the upcoming OSIRIX-REx asteroid sample return.  Tracking of the spacecraft’s radio signal would 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 Psyche spacecraft above an artist’s concept of what the surface of a metallic asteroid might look like.

The third asteroid mission would make an extended study of a single asteroid.  The asteroid16-Psyche is unique among the larger asteroids in having a composition that appears to be largely metallic.  Understanding how this world came to be is one of the goals for this mission. Psyche could be an asteroid in which repeated collisions chipped off the crust and mantle, leaving the core a naked body.  It could be the remnant of the collision of two protoplanets that shattered and expelled the core of the smaller body to become Psyche.   Or Psyche could have formed so close to the early sun that only metals (and some silicates) could have condensed from the nebula; the later migration of the giant planets could have moved it to its present location in the asteroid belt.  In either of the first two cases, we’d get our first look at material from one of the most inaccessible locations in the solar system – the deep core of a rocky world.  In the third case, we’d see the result of a new class of worlds that formed very close to the sun.

In its implementation, the Psyche mission would be much like the current Dawn mission to the larger asteroids Vesta and Ceres.  Solar electric ion engines would slowly propel it to its destination where the spacecraft would orbit the asteroid for long term studies.  A combination of cameras and spectrometers would image the surface and map its composition while radio tracking would reveal its interior structure. 

From the original 27 proposals, these five are the ones that NASA’s managers determined have the best combination of scientific appeal and low implementation risk.  For the next several months, the proposal teams will be consumed with fleshing out the design of their missions.  Then the space agency scrutinize the enhanced proposals to select one or two to fly.

If either DAVINCI or Lucy isn’t selected, scientists interested in their studies will get a quick chance to try again.  NASA has another program for scientist-led missions, the New Frontiers program, which flies missions costing approximately twice what Discovery missions cost.  For this program, NASA accepts proposals from a list of pre-selected, high priority concepts.  One of those concepts is for a Venus atmospheric probe and lander that would replicate the DAVINCI atmospheric studies and also provide measurements studies from the surface.  A second concept would be for a mission that would orbit a Trojan asteroid and possibly fly by one or two others.  (The other candidate concepts are for a lunar sample return, a comet sample return, and a Saturn atmospheric probe.)  The competition to select the next New Frontiers mission is scheduled to begin immediately after the selection of the next Discovery mission(s) next September.

I’m personally hoping that the agency selects both a Venus and an asteroid mission from the current Discovery competition.  The greatest strength of the Discovery program has been missions to a diversity of worlds.

Discovery Mission Finalists

For now, here's the press release on the five proposals chosen for further study.  Full blog post later tonight or tomorrow.

What I learned in describing so many of the proposed missions is how creative and scientifically compelling the many (28 in total of which there was some public information on perhaps 20) proposals were.  I hope that we will see the missions not chosen in the next Discovery competition planned to start in two or three years.

Sept. 30, 2015

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NASA Selects Investigations for Future Key Planetary Mission

NASA has selected five science investigations for refinement during the next year as a first step in choosing one or two missions for flight opportunities as early as 2020. The submitted proposals would study Venus, near-Earth objects and a variety of asteroids. 

Each investigation team will receive $3 million to conduct concept design studies and analyses. After a detailed review and evaluation of the concept studies, NASA will make the final selections by September 2016 for continued development leading up to launch. Any selected mission will cost approximately $500 million, not including launch vehicle funding or the cost of post-launch operations. 

"The selected investigations have the potential to reveal much about the formation of our solar system and its dynamic processes,” said John Grunsfeld, astronaut and associate administrator for NASA’s Science Mission Directorate in Washington. “Dynamic and exciting missions like these hold promise to unravel the mysteries of our solar system and inspire future generations of explorers. It’s an incredible time for science, and NASA is leading the way.”

NASA's Discovery Program requested proposals for spaceflight investigations in November 2014. A panel of NASA and other scientists and engineers reviewed 27 submissions. 

The planetary missions selected to pursue concept design studies are:

Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) 
DAVINCI would study the chemical composition of Venus’ atmosphere during a 63-minute descent. It would answer scientific questions that have been considered high priorities for many years, such as whether there are volcanoes active today on the surface of Venus and how the surface interacts with the atmosphere of the planet. Lori Glaze of NASA's Goddard Space Flight Center in Greenbelt, Maryland, is the principal investigator. Goddard would manage the project.

The Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy mission (VERITAS)
VERITAS would produce global, high-resolution topography and imaging of Venus’ surface and produce the first maps of deformation and global surface composition. Suzanne Smrekar of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California is the principal investigator. JPL would manage the project.

Psyche
Psyche would explore the origin of planetary cores by studying the metallic asteroid Psyche. This asteroid is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet. Linda Elkins-Tanton of Arizona State University in Tempe, Arizona is the principal investigator. JPL would manage the project.

Near Earth Object Camera (NEOCam)
NEOCAM would discover ten times more near-Earth objects than all NEOs discovered to date. It would also begin to characterize them. Amy Mainzer of JPL is the principal investigator, and JPL would manage the project.

Lucy
Lucy would perform the first reconnaissance of the Jupiter Trojan asteroids, objects thought to hold vital clues to deciphering the history of the solar system. Harold Levison of the Southwest Research Institute in Boulder, Colorado is the principal investigator. Goddard would manage the project.

Created in 1992, the Discovery Program sponsors frequent, cost-capped solar system exploration missions with highly focused scientific goals. The program has funded and developed 12 missions to date, including MESSENGER, Dawn, Stardust, Deep Impact, Genesis and GRAIL, and is currently completing development of InSight. The Planetary Missions Program Office at NASA's Marshall Space Flight Center in Huntsville, Alabama manages the program for the agency's Science Mission Directorate. 

For more information about NASA’s Discovery Program, visit: 

http://discovery.nasa.gov

-end-

Dwayne C. Brown / Laurie Cantillo
Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown@nasa.gov / laura.l.cantillo@nasa.gov

Last Updated: Sept. 30, 2015

Editor: Karen Northon

Searching for the Origins of Earth’s Water

Sometimes planetary exploration is drama and new vistas: Seeing the most distant planet for the first time, a harrowing landing on Mars, the fight for survival of a probe lost on a comet.  Much of planetary science, though, builds up from many quiet measurements that provide crucial insights that can change our understanding of the solar system and our world’s place in it.  Today’s post is a about a group of proposed missions that fall into that latter category.

(Oh, and before anyone reminds me that Pluto is no longer a “planet”, there are scientific classifications and personal ones.  Pluto emotionally will always be the planet on the distant edge of the solar system for me.)

One of the knottier problems in planetary science is understanding the origins of Earth’s water.  In the infant solar system, a temperature gradient existed extending from our star to the outer solar system.  Close to the sun, rocky materials with high melting points could have condensed.  Somewhere, possibly in the region of the mid-asteroid belt, it became cool enough for water ice to condense.  In the far outer solar system, even materials that are gases on our world would have frozen out to create, among other features, the plains of frozen nitrogen found on Pluto by the New Horizon’s spacecraft.

In this model, our early planet fell on the wrong side of the dividing line – referred as the snow line – for water to have condensed and become part of our world.  In one sense the Earth remained a dry world with much less than 1% of its mass as water.  By comparison, the asteroid Ceres on the other side of the snow line may have as much as 50% of its mass in water, liquid or frozen.

Yet on the Earth’s surface, water is abundant in our atmosphere, soils, and oceans with perhaps several times as much water again contained in the rocks of our planet’s crust and mantel.  How was this critical substance delivered to our world?

Scientists have long suspected that the Earth’s water was delivered from somewhere in the outer solar system by icy bodies colliding with the proto-Earth.  But from which region of the outer solar system?  And what dynamic processes perturbed those bodies into collision courses with our rocky inner world?

Karen Meech from the University of Hawaii puts the importance of this science this way.  “The compelling question to learning how Earth got its water isn’t just about the Earth.  If our solar system was somehow special and required a special architecture to deliver water to our world, what does that say about the thousands of worlds around other stars?  Would planets in the habitable zones around these stars have water to support life?  This question impacts the big question of ‘is there anybody else out there?’” 

We won’t be able to answer this question until we have analyzed the chemistry at bodies that represent several potential sources of water and volatiles for our own solar system.  At least three teams of scientists are proposing missions to explore these bodies.

Water is abundant in our solar system especially in the worlds that formed beyond the snow line.  Ceres is one example of asteroids on the far side of the line with substantial water.  Many of the moons of Jupiter, Saturn, Uranus, and Neptune have vast quantities of water.  Comets and the worlds beyond Neptune, of which Pluto is the largest example, have substantial water.  The Earth’s water could have come from any of these regions, or perhaps a mixture.  There’s also the possibility that the rocky grains that condensed to form the Earth absorbed some of the water vapor surrounding them, or that water was made on Earth’s surface when hydrogen in our planet forming disk combined with molten rocks on Earth’s surface.

If this sounds like a messy problem, it is.  The solution to finding the sources of the Earth’s water lies in chemistry.  Water itself varies in its isotope composition between normal water with hydrogen atoms containing no neutrons and the much rarer tone neutron hydrogen, deuterium. The ratio of these two water types, abbreviated to D/H, varies among the bodies of the solar system.  (You can read about D/H ratios in two Wikepedia articles here and here.)  The smallest ratios are found at Jupiter and Saturn, followed by Uranus and Neptune, and then primitive meteorites.  The Earth, Mars, a few comets, and interestingly Saturn’s moon Titan have similar D/H ratios.  Saturn’s moon Enceladus, however, has a higher ratio.  Some comets have D/H ratios similar to Earth while others have higher ratios.

Examples of D/H ratios in water for a number of solar system bodies. References to the Oort cloud and Jupiter family refer to different groups of comets. Credit: ESA.

Finding an icy body with D/H ratio similar to Earth’s suggests that it could have formed in the same region as the bodies that supplied water to Earth because the amount of deuterium that gets captured in molecules is a sensitive function of temperature.  That, though, is just the beginning of the detective work. Early in the solar system, the orbits of the giant planets migrated in and then out again.  In the process, they scattered many of the smaller bodies around the solar system (and ejected many from the solar system altogether).  The goal is not just to find bodies with similar chemistry to the Earth’s water but to also look for clues as to where these bodies might have originally formed before they were scattered.

Chemistry again should provide the critical clues.  Isotope ratios such as those of ^17,18O/¹⁶O[xygen] and ¹⁵N/¹⁴N[itrogen] along with the amounts of noble gases such a krypton and argon can reveal clues such as the temperature at which a body formed or the  distance from the sun where it formed.  Comparing different groups of isotopes and elements will allow scientists to explore multiple lines of evidence.

To explore the origins of Earth’s water, scientists need to deliver a capable chemistry lab to different worlds that are the remaining representatives of early reservoirs of water and volatiles.  Three teams, two American and one European are proposing missions to do this to their respective low cost planetary programs (the Discovery program for NASA and the Medium class program for ESA).

The core instrument for all three proposed missions would be a mass spectrometer.  These instruments sniff in gases or vaporized grains of ice and then “weigh” the distribution of their molecules and isotopes.  The two NASA missions propose to use a cutting edge advanced mass spectrometer, MASPEX, that is both highly sensitive to trace amounts of material and can measure large complex molecules.  MASPEX has been selected for NASA’s Europa mission and is proposed to be used by at least three Discovery missions (the two discussed in this post plus one that would study the gasses emitted by Saturn’s moon Enceladus).  The specific mass spectrometer for the European mission isn’t specified.  (For a brief and easy to read overview of mass spectrometers, check out this document from JOEL USA, which manufactures commercial instruments.  You can read a technical overview of the MASPEX instrument here.)

Comet Harley 2 as imaged during a flyby by the Deep Imact/EPOXI spacecraft.  Credit: NASA/JPL-Caltech

The Primitive Materials Explorer (PriME) Discovery mission would return to comet Hartley 2, which was briefly studied during a flyby with the Deep Impact/EPOXI spacecraft, for an extended study.  Measurements by a European space telescope, Herschel, measured the D/H ratio for this comet and found that it is similar to Earth’s oceans. 

The other two proposed missions would travel to an entirely different potential source of Earth’s water.  Several asteroids, including Ceres where Dawn is now exploring, have been observed by telescopes to emit dust and water vapor.   Five asteroids emit so much dust, forming a comet-like tail, that they are called main belt comets (MBCs).  Because the dust comes off the MBCs every time they make their closest approach to the sun, it is clear that heat is causing water to escape, pushing dust from the surface. 

Artist conception of asteroid 24 Themis and two small fragments of this dynamical family, which resulted from a large impact more than one billion years ago. Note that one of the small fragments is inert (as most asteroids are) and the other has a comet-like tail, produced by the sublimation of water ice from its surface.  Credit: Gabriel Pérez, Servicio Multimedia/IAC

Telescope measurements of one of the larger (~200 km diameter) and more distant main belt asteroids, 24 Themis, suggest that there may be water ice with some organic compounds on its surface (see the press release here).  Conveniently, past collisions have chipped off pieces of the parent asteroid, potentially allowing scientists to examine material that potentially previously lay beneath the surface.  One of those chips, tiny (estimated 0.6 km diameter) asteroid 238P/Read has been observed to have strong, comet-like outgassing, suggesting that a recent impact may have uncovered an ice layer or pocket. 

The proposed Discovery Proteus mission would explore the 24 Themis asteroid family to study it as a source of Earth’s water.  The spacecraft would first flyby Themis itself, studying its surface composition and structure with copies of the Dawn mission’s cameras.  It would also measure the composition of any gases being emitted from the comet with its MASPEX mass spectrometer.  Later during a months-long rendezvous with Read, the Proteus spacecraft would measure the composition of emitted water vapor and any organics with its MASPEX mass spectrometer in exquisite detail.

The third proposed mission that would search for the origins of Earth’s water, Castalia, was proposed for Europe’s fourth medium-class mission, but not selected.  The mission may be re-proposed for the upcoming fifth mission competition.  In its original form, the proposed mission would have visited a different main belt comet, asteroid 133P/Elst-Pizarro.  If the mission goes forward, it may change its target to another main belt comet.

 

Dr. Meech, who is also the Principal Investigator for the team proposing Proteus, makes the case that flying to a main belt comet rather than to a standard comet would better help us understand how Earth got its water. Scientists develop dynamical models to understand the process of planetary growth.  Many models predict that as the giant planets formed, they moved, tossing the smaller icy planetesimals that would become comets and water-rich asteroids and main belt comets both inward and outward. “These models make specific predictions about where asteroids with ices formed and where comets formed.  The distances over which the icy asteroids formed are much better constrained; comets on the other hand could have formed over a wide range of distances making them less diagnostic of a formation location.” The models suggest that it would have been easier for asteroids to have delivered the quantity of water on the Earth than standard comets.

 

Reading through the summaries of the missions reminded me of the incredible discipline scientists proposing Discovery missions must exhibit.  The Castalia mission was proposed to fly with ten instruments.  In the European system, the ESA budget doesn’t cover the costs of instruments, which are instead paid for separately by the nations that develop them.  As a result, low-cost European planetary mission proposals tend to be instrument rich. 

In the NASA system, instruments developed by US teams must be paid for out of the mission’s budget (although any instruments contributed by another nation are paid by that nation’s government).  As a result, the Proteus mission would carry just its multi-color cameras and the MASPEX instrument.  The PriME mission would carry these two instruments plus a duplicate copy of the JIRAM imaging infrared spectrometer developed originally for the Juno Jupiter mission and contributed by Italy.  Compared to the imaging infrared spectrometer that the OSIRIS-REx asteroid mission will carry (as an example of an optimized instrument for a small body mission), the JIRAM instrument is both lower resolution (for the technically minded, an instantaneous field of view of 240 mrad versus 4 mrad) with a narrower spectral range (2.0-5.0 mm versus 0.4 – 4.3 µm).  For the PriME mission team, however, the JIRAM copy is free.

An imaging infrared spectrometer would also be useful on the Proteus mission, especially to study the surface of asteroid Themis during the flyby.  I asked Dr. Meech why her team didn’t add this instrument.  “We are putting state of the art technology into the mission with the mass spectrometer, and that means it is expensive.  Also because of the long transit times for solar electric propulsion missions the mission operations are very long compared to other comet missions - which increases cost.  The key science requires the mass spectrometer.  An imaging spectrometer would be very nice but that would not make the breakthrough science we hope to achieve.”

However, NASA does allow some leeway on instruments, and teams can propose optional instruments that NASA could chose fund to add to enhance the mission’s science and also demonstrate technologies or educate students.  The PriME mission could also carry a student-led plasma instrument.  For the Proteus mission, JPL is proposing a mission enhancement that would use a CubeSat to make infrared spectral measurements at Themis.  (Cubesats are spacecraft that for planetary missions can be as small as a loaf of bread.)  The scientific measurements would be valuable in their own right, but this enhancement would also demonstrate the use of a CubeSat to work with a mother craft and the design of the spectrometer sized for these tiny spacecraft. 

A summary of the possible enhancement to the Proteus mission that a CubeSat daughter craft might make.  The spectrometer would make measurement in visible and near infrared (0.5 – 2 µm) wavelengths. Credit: JPL/Caltech

As I said at the beginning of this post, the question of how water was delivered to the Earth is one of the most important in planetary science, both to understand the history of our world and to understand the mixing bowl that was the earliest solar system.  Eventually, we will fly missions to comets that have D/H ratios similar to Earth’s such as Hartley 2 and to main belt comets to see if they could be the sources of our water and to explore these bodies as fascinating objects in their own right. 

However, if either PriME or Proteus is selected, we won’t see millions of people flooding the internet to learn what the D/H and other isotope ratios are.  These are examples of the less media-star missions that perform critical science to help us understand our world and the solar system.

Given 28 Discovery proposals, the odds of any one have the right match of compelling science, low engineering risk, and low budget risk and being the one selected is small.  If it came down to these two and I had a vote (not that NASA would ever be crazy enough to do such a thing), I’d lean towards Proteus because I’d like to see a new class of world.  However, the scientific case for both are compelling to me.

If you are interested, an article from the March 2015 Scientific American magazine discusses the science behind the questions of the origins of the Earth’s water in more detail.

Credit: Proteus Team