Monday, August 25, 2014

Europa: How Less Can Be More

Three factors make exploring Europa hard.  First, we want to explore an entire complex world, and mapping its features requires acquiring vast amounts of data.  Second, Europa lies far from the Earth, which necessitates capable communications and power systems (read, “expensive”) to return the data to Earth.  Third, Europa lies well within the harsh radiation fields surrounding Europa, which both requires significant radiation hardening (again, read, “expensive”) and limits the life of any spacecraft that explores this world.  These factors can make a mission concept that seems like less actually be more.

The limiting factor on science for most planetary orbiters is not the time the instruments can make observations.  Rather it is the time available to return data to Earth because many instruments can gather data far faster than the communications system can transmit it to antennas on Earth.  (There also are a limited number of antennas to listen to planetary spacecraft, so few missions receive continuous coverage, and spacecraft often cannot continuously transmit either because they must turn to observe the planet or the planet itself blocks communication.) 

To get a sense of the challenges, compare the problems of exploring Mercury and Europa.  A mission to Mercury must deal with the intense heat coming from both the sun and the planet surface.  However, a spacecraft designed to overcome that challenge can continue to function until its fuel is exhausted.  As a result of the luxury of spending years in orbit around Mercury and the fact that Earth is never more than 222 million miles away, NASA’s MESSENGER mission has been able to return terabytes of data to Earth.  Between its orbital insertion in March 2011 and March 2012, the spacecraft generated 2.3 TB of data to be archived by NASA.  The mission continues to operate today, so the total returned to date should be substantially more.  The maximum data rate for this $446M (2008 dollars) mission is 104 kilobits per second.

By comparison, it’s cold at Jupiter, but it is the intense radiation around Europa that limits spacecraft life.  Different mission studies have assumed lifetimes in orbit between one ($1.6B estimated cost, 2015 dollars) and nine months ($4.7B estimated cost), many times shorter than the approximately four Earth years MESSENGER will have at Mercury.  While Jupiter is never closer than approximately 2.7 times as far from Earth as Mercury, more capable spacecraft systems would allow data rates of around 135 kilobits per second.  With a lifetime of one month in orbit, the data return would be around 334 gigabits, and with a lifetime of nine months, around 4.5 TB.  (Different mission designs made different assumptions about data return, so nine month mission data return isn’t a simple multiple of the one month mission data return.)

Comparison of data return for different Europa mission concepts with the data archived from NASA's Mercury MESSENGER orbiter from its first terrestrial year in orbit.  Credit: Kane

These challenges for exploring Europa have been well known since the Galileo Jupiter orbiter in the 1990s all but proved that Europa likely has a vast ocean that could harbor life that lies under a relatively thin icy shell.  As mission planners and budget directors have wrestled with this problem, we’ve been through at least five distinct eras of Europa mission planning.  (There have also been various proposals by independent teams for simpler and cheaper missions, which may or may not have been feasible for their proposed costs.)

Immediately following the Galileo spacecraft’s discoveries, JPL conducted preliminary mission studies that envisioned a capable spacecraft using conventional technology to orbit Europa.

In the late 1990’s, NASA’s then Administrator redirected efforts to a mission concept that would use yet-to-be-developed technologies (the X-2000 project) to dramatically lower mission costs to the neighborhood of MESSENGER’s cost.  By the time the program was cancelled in 2002, mission estimates had shot from around $190M to $1.4B (early 2000’s dollars). 

Not to be outdone, the next NASA administrator proposed the Battlestar Galactica of missions, the Jupiter Icy Moons Orbiter (JIMO) that would orbit the moons Callisto and Ganymede in addition to Europa.  This mission depended on the development of radically new capabilities such as space-rated nuclear fission reactors to power the spacecraft.  This $16B concept died quietly when the administrator left NASA.

NASA's concept for the Jupiter Icy Moons Orbiter.  Credit: JPL/NASA.

If the previous two efforts were perhaps fanciful, the next effort, the 2008 Jupiter Europa Orbiter (JEO) concept was based solidly on feasible technology.  This highly capable spacecraft would have conducted extensive studies of the Jovian system before beginning nine months in orbit around Europa with a highly capable instrument suite.  This was the mission any fan of Europa really wanted.  Unfortunately, an estimated $4.7B price tag doomed the concept.

The bulk of the data that would have been returned by the 2008 Jupiter Europa Orbiter concept would have been produced by the ice penetrating radar, infrared spectrometer, and a trio of cameras.  Credit: Kane

Following the JEO studies, NASA conducted studies of three missions that each would have a firm cap of $2B: an orbiter, a multi-flyby spacecraft, and a lander.  It was quickly realized that the latter would not be feasible until a previous mission had better studied Europa’s surface to find the best combinations of most scientifically while still safe landing sites.

That left the choice of an orbiter that would spend 30 days circling the moon and a multi-flyby spacecraft that would spend less than a cumulative 6 days close to Europa during 34 flybys.  The scientists who reviewed the two missions solidly backed the multi-flyby concept (that has evolved into the current Europa Clipper concept).

So how can 6 days of science be better than 30?  For the comparison that follows, I’ll use the assumptions of the 2012 studies.  Since that time, the capabilities of the multi-flyby concept have been substantially enhanced into the Europa Clipper concept.  Because the orbiter concept didn’t have the additional two years of fine-tuning of the multi-flyby craft, comparing their 2012 conceptions allows comparison of equally developed concepts.

Comparison of data that would be returned by instrument for the 2012 multi-flyby and orbiter mission concepts.  Credit: Kane

Between each of the flybys, the multi-flyby spacecraft would have seven to ten days to transmit data stored during each brief encounter back to Earth.  That would let the multi-flyby craft have up to a year of time to transmit its data compared to just 30 days for the orbiter.  The result would be almost three times as much data returned to Earth.  (Differing assumptions about how much of the time antennas would listen to the spacecraft mean that the amount of data returned is not a multiple of time.)

The larger data return of the multi-flyby spacecraft would enable the spacecraft to carry two high priority instruments that generate large amounts of data.  The more data hungry of these, the ice penetrating radar, would study the structure of the icy shell beneath the surface.  This would allow scientists to study whether bodies of water are trapped within the ice between the surface and the ocean below and fracturing of the shell.  The radar might penetrate through the shell to the top of the ocean to measure the total depth of the icy shell.  These measurements will help scientists understand how material is transported between the ocean and the surface.

The second instrument, a short-wave infrared spectrometer, can identify materials exposed on Europa’s surface and map their distribution.  Scientists believe that Europa’s surface exposes materials transported from the ocean below, where we can easily see it and eventually study it with a lander.  The interaction of materials on the surface with Jupiter’s radiation field creates chemicals that may be transported to the ocean below to be available for use by any life.  This spectrometer would map the presence and distribution of these materials across Europa’s globe.

Both the 2012 orbiter and the multi-flyby spacecraft would carry a third data-hungry instrument, a topographic imager that would map the surface.

 Not all potential instruments require high data rates.  The orbiter would have carried a trio of instruments that required measurements from around the globe: a laser altimeter to measure surface tides to enable estimates of the thickness of the icy shell and a magnetometer and plasma instruments that would have enabled estimates of the volume and salinity of the underlying ocean.  Unfortunately, the measurements of these instruments are lower priority than those for a radar and shortwave infrared spectrometer.  (In the 2012 study, the multi-flyby spacecraft also would have carried a heavy, power-intensive, but low data rate mass spectrometer that would directly sample material sputtered from the surface.)

The importance of the ice penetrating radar and mid-IR spectrometer tipped the weight of opinion in favor of the multi-flyby concept.  Given a limited number of encounters that would fly over just a tiny fraction of Europa’s surface, they key was to distribute those flybys to fly over key locations. 

With two years of further study, the multi-flyby concept has evolved into the Europa Clipper concept has added an additional eleven flybys (for a total of 45) and several instruments compared to the 2012 concept.  By balancing the placement and number of encounters with many months to return data, the Europa Clipper concept would enable a $2B mission that conducts the most crucial measurements of the $4.3B JEO concept.  The $1.6B orbiter concept couldn’t match this feat.

However, the Europa Clipper is not NASA’s plan for a Europa mission.  White House budget analysts and NASA’s senior management are looking for a $1B concept that wouldn’t do the job of the Europa Clipper but would still do significant science.  Earlier this summer, they reportedly received six proposals that target this cost cap.  NASA ’s managers are examining the proposals to ensure that they are both fiscally and technically feasible within the budget.  In the meantime, they are not releasing any information about the types of missions proposed.

From what I understand, much of the scientific community and many NASA managers are skeptical that a meaningful mission can be done within a $1B budget.  Sometime in the coming months we will learn whether NASA thinks any of the proposals have merit.  If they do, then the broader scientific community will weigh in with its assessment. 

I’ve argued in a previous post that a $1B mission is likely technically possible, but I have doubts about whether it could address enough high priority science to be worth the expenditure.  The coming months will see if I’m proved wrong or not.

In the meantime, NASA continues to refine the Europa Clipper concept, which so far has shown the best balance between doing more with less to perform the critical science for the next step in exploring this world.

Current concept for the Europa Clipper mission, which is an evolved version of the 2012 multi-flyby concept.  Credit: JPL


Wednesday, August 6, 2014

Mars 2020 Instruments – A Plan for Sample Return

Last week, NASA’s managers announced the selection of seven instruments for its 2020 Mars rover from a pool of 58 proposals submitted by teams of scientists.  Reading through the capabilities of the instruments makes them seem like technology from science fiction, complete with lasers and x-rays.  However, the types of instruments that weren’t selected say almost as much about the goals and expectations for the mission as those that were.  This mission will be optimized for finding the best samples to return to Earth rather than carrying out the most sophisticated science that could have been sent to Mars.

The Mars 2020 rover will be based on the design of the Curiosity rover but will have a new instrument suite and hardware for collecting and caching samples for possible return to Earth.  Credit: NASA

For Mars, the key questions are about the earliest environments present on Mars, whether they could have enabled the development of life, and whether life or its precursors arose.  Answering these questions can require devilishly subtle measurements.  On Earth with the best instruments available (far, far more capable than those that could be flown to another planet), concrete answers are hard to come by and debates rage about the earliest conditions on Earth.  (It doesn’t help that the active surface of the Earth has erased all but a few traces of the earliest surface, atmosphere, and ocean.)

The Mars scientific community has collectively decided that the best and perhaps only way to answer these questions is to return carefully collected samples to Earth for study in terrestrial laboratories.  The primary goal the science community laid out for the 2020 rover was to enable the efficient selection of the most compelling sample set possible – so compelling that Congress will spend the additional few billions of dollars for missions to retrieve and return them to Earth.

Placement of the just announced instruments on the 2020 rover.  Credit: NASA

To see how the instruments selected will work together towards this goal, imagine that you were sent to Mars and given the assignment to select a small set of samples to return to Earth.  Because you can return only a few samples, you are under pressure to find those few special samples that can best reveal insights into the earliest history of Mars.

The first thing you are likely to do is to look to see what types of terrain and rock formations surround you.  The 2020 rover will carry two Mastcam-Z cameras for this task.  These cameras were originally intended to fly on the Curiosity rover currently on Mars, but weren't completed in time.  Unlike Curiosity’s cameras, these will have the ability to zoom from wide angle to moderate zoom (28 mm to 100 mm, 35 mm film equivalent) and to take movies.  (If still photos from Mars are cool, imagine movies.)  These cameras will take color images, but unlike our eyes they will also be able to take images in twelve carefully selected bands (“colors”) in the visible and near infrared spectrum to help map subtle distinctions in composition. 

The 2020 rover also will have the ability to assess the area around it in ways that our eyes never could.  Like the Curiosity rover, the 2020 rover will zap rocks and soils with a laser to determine their composition.  Curiosity’s ChemCam laser heats its targets sufficiently that a tiny amount vaporizes.  The instrument analyzes the glow of the plasma cloud to measure the elements present. 

However, if the laser hits a target with specific wavelengths of light at a lower energy, the target will “glow” in characteristic ways that reveal the mineralogy and the presence of organic molecules. (In technical terms, these are Raman and time-resolved fluorescence spectroscopy.) 

(An analogy helps explain the difference between elemental and mineralogical composition.  French bread, Indian naan flat bread, and tortillas, for example, have similar ingredients (they are much more similar to each other than to, say, a steak or a Greek salad).  In this analogy, the ingredients in the recipes are the elemental composition, while the specific type of baked good reflecting both the proportion of ingredients and method of cooking is the mineralogy.)

The 2020 rover will carry an advanced version of ChemCam called SuperCam that will use all three types of laser analysis to provide both elemental and mineralogical analysis.  In addition, it will have capabilities for mapping composition using visible and infrared spectroscopy, although no details were provided (such as whether this capability will be just for the spots targeted by the lasers or will be full images of the scenes around the rover).

The rocks and formations that Mastcam-Z and SuperCam can study, however, are only those at the surface.  Geological formations often continue beneath the surface and the rocky outcrop in front of the rover may be the same or different than the outcrop viewed a hundred meters earlier in the rover’s drive.  A Norwegian-supplied ground penetrating radar, RIMFAX, will map soil and rock layers up to a half kilometer below the surface with a resolution of 5 to 20 centimeters.

Artist’s concept of how the RIMFAX surface penetrating radar will study rock formations below the surface.  Credit: NASA
To return to our analogy of you as Mars geologist, once you survey a location, you would go to specific soils or rocks that look interesting for closer examination.  Similarly, the 2020 rover will carry two instruments to study small patches (approximately the size of postage stamps) in detail. Both will be contact instruments that operate once the rover’s arm has placed them against a patch of soil or a rock.  (It is likely that the 2020 rover, like NASA’s previous Martian rovers, will be able to brush dust and the outer surface of rocks off to allow instruments to sample the more pristine internal rock.)

The Curiosity rover carried two contact instruments, a microscopic imager and the Alpha Particle X-Ray Spectrometer to measure elemental composition.  The 2020 rover will carry two much more capable contact instruments.  The PIXL instrument will measure elements using X-ray lithochemistry while the SHERLOC instrument will measure minerals using laser Raman and fluorescence spectroscopy.  Both of these instruments will have their own microscopic cameras, and the SHERLOC instrument carry a near copy of Curiosity’s MAHLI microscopic imager.  (MAHLI operates as both a normal camera as well as a microscopic camera.  This camera, mounted on the rover’s arm, has taken the selfie pictures that show the rover on the Martian surface as well as images of the wheels and beneath the rover.)

While Curiosity’s Alpha Particle-X-Ray spectrometer could measure only the average composition of the surface in front of it, both PIXL and SHERLOC will make hundreds to thousands of measurements across each surface.  Each measurement point will be approximately the size of a grain of sand.

An example of how the PIXL instrument will map elements (identified by their chemical symbol in each panel) at a fine resolution across rock surfaces.  The SHERLOC instrument will map mineral and organic composition at similar resolution.  Credit: NASA
The new capability to measure composition at near microscopic resolution will be revolutionary.  If you look at soils and the interiors of most rocks, you’ll find that they are composed of many smaller rocks and inclusions.  By taking many fine-scale measurements, each rock or patch of soil becomes a rich story of many rock fragments that together provide clues to their individual formation and that of their larger rock or soil type.

SuperCam and SHERLOC’s laser spectroscopy will have an important capability that Curiosity lacks – they can easily identify and map the presence of organic materials.  While many processes other than life can produce organic chemicals, life as we understand it requires a rich abundance of organic material.  A key goal for the 2020 rover is to find biosignatures to indicate pre-biotic chemistry or life itself.

The Curiosity rover can detect organic materials through its mass spectrometer, but preparing samples for and using this instrument is a laborious process and has only been done rarely in the mission to date.  In addition, the way the Curiosity’s instrument works, it must heat samples, which triggers chemical reactions with the perchlorates found in the soils, destroying the organic materials.  Careful measurements have allowed scientists to conclude that the samples taken by Curiosity contain some organic materials, but we aren’t sure how much or what types.  (Curiosity’s instrument has a mechanism to avoid the “perchlorate trap,” but it can be used only seven times and hasn’t been so far.)

By using lasers, the 2020 rover can find organics quickly and won’t be skunked by perchlorates, key advantages over Curiosity.

Two other instruments round out the 2020 rover’s manifest.  The MEDA instrument, supplied by Spain, will monitor the weather and study the airborne dust.  MOXIE will demonstrate the extraction of oxygen from the predominantly carbon dioxide atmosphere at Mars.  Missions (manned or unmanned) that are to return to Earth could substantially reduce their launch weight if they could manufacture the oxidizer portion of their rocket fuel form the Martian air.  The same applies to the oxygen supply to breath for any future astronauts.

How does the 2020 rover’s scientific instrument suite (MOXIE is an engineering demonstration) compare to that of the Curiosity rovers?  The 2020 rover will have far superior remote sensing instruments (Mastcam-Z, SuperCam, and RIMFAX) and contact instruments (PIXL and SHERLOC) than Curiosity.  This will allow this new rover to much more quickly find important samples to study and potentially cache.  This is especially true for finding any rich deposits of organic material.

To locate two to three dozen samples within the mission’s lifetime on Mars, the 2020 rover will need to operate much more efficiently than the Curiosity rover has.  The scientific team that defined the requirements that NASA used to select this instrument suite specifically asked for a suite of instruments simpler than Curiosity’s to speed operations.  Because almost a decade has passed since Curiosity’s instruments were selected, the march of technology allows the new rover’s instruments to be considerably more capable than Curiosity’s. 

So what’s left off?  Ignoring the miniature greenhouse and the solar-powered helicopter proposals (either likely would have been media sensations), the 2020 mission will not have the class of laboratory instruments included in the both Curiosity and the ExoMars 2018 payloads.  Performing the most sensitive measurements requires larger instruments than can fit on the robotic arm.  To address this, both the Curiosity and ExoMars rovers have instrument laboratories housed within their bodies.  For example, the Curiosity and ExoMars mass spectrometers can identify the specific composition of organic molecules.  This is useful to separate organics created from non-biotic processes from those created from possible biotic processes.  The laser instruments to be carried by the 2020 rover will be limited to more general identification of the presence of broader groups of organic molecules. 

The mass spectrometer instruments proposed but not selected for the 2020 rover could have been more sensitive still than Curiosity and ExoMars’.  The proposed CODEXinstrument, which would have had to be located as a laboratory instrument within the body of the rover, would have used lasers to vaporize minute quantities of material across the sample to be fed into a mass spectrometer.  (By vaporizing samples, the instrument would have avoided the perchlorate problem.)  The resulting measurements would have provided detailed maps of the chemistry of samples, the types of organics within it, and the age of the rock from which it came.  Achieving both of the latter goals have been two of the justifications for returning samples to Earth.  CODEX would have made progress towards both on Mars, although measurements made in terrestrial laboratories would be much more precise. 

NASA doesn't discuss why particular instruments aren't chosen for a mission.  CODEX and its kin may not have made the cut because the team of scientists that laid down the mission requirements specifically requested a simplified instrument suite.  Or the reviewers may have concluded that the more sensitive measurements would not have been sensitive enough to answer critical questions about Mars.  Or it could be that there wouldn't have been room in the rover or in the budget for them.  The 2020 rover program has a tight budget, and the instrument suite selected will cost $130M, more than the $100M NASA had originally hoped to spend.  (Curiosity’s instruments cost $180M.)

The instruments will be half of the 2020 rover’s payload.  Still to come are details on the sample collection and caching system.  Based on work done to date, it appears that the rover will collect and store two to three dozen sample cores that each will be about as wide as a pencil and about half as long as a new one. 

The 2020 rover will carry an instrument suite optimized for efficiently finding the best sample suite at its landing site for a possible return to Earth.  If those samples do make it to our world, we likely will have a revolution in our understanding of the Red Planet.  If they do not, the scientific community may come to wish they had asked for a more capable instrument complement to do more sophisticated science on Mars.  But life is about choices, and NASA and the scientific community have bet that the samples collected will be so compelling that funds will be made available for their return to Earth.

Either way, the instruments of the 2020 rover will be marvels much more advanced than their counterparts on Curiosity.  We will get great science.

Tuesday, August 5, 2014

ExoMars 2018 Status

Elizabeth Gibney provides an update on the Mars 2018 mission development at the blog site of the journal Nature.  She reports that schedules are tight, but that managers are working to keep the mission on track for a 2018 launch.  She reports, however, that at least some members of the project team believe a slip to a 2020 launch is a possibility.  Funding for the 2018 mission also is a couple of hundred million Euros short of what is needed, and this will be addressed at a meeting of ESA's governing council in December.

Gibney's report provides more hope that the launch will occur on schedule than does the report on the website that I summarized in a previous post.

Sunday, August 3, 2014

Possible Delays for Lunar and Martian Missions

Anatoly Zak, author for the well-respected site, reports on delays for Russia's lunar and Martian missions.

He reports that a three year slip in Russia's lunar missions has been officially announced.  These missions will deliver spacecraft to orbit and land on our moon.  (See this page for a list of the currently planned missions (without launch dates) as well as my earlier post on Russian lunar and planetary plans.)

Zak also reports that the joint European and Russian 2018 ExoMars rover and surface station is facing a likely two year delay to 2020.  A series of delays caused by coordinating the efforts of the two agencies to develop the missions is the apparent cause.  Separately, Europe's portion of the mission's cost reportedly will exceed the currently approved funding limits.  If this occurs, the European Space Agency will need to find and approve additional funds to complete the mission.

The joint 2016 Mars orbiter mission appears to be proceeding well and is expected to launch on time.