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Dark and light streaks on crater walls, Moon. (click to enlarge)

Although the Moon’s gravity is low, only about 0.165 of the Earth, rock and soil move down slope over time.  In geology, such processes are called mass wasting and is one of the principal sources of erosion on the Moon (the other being meteorite bombardment).  Mass wasting includes both gradual, infinitesimally slow soil creep on slopes and rapid, catastrophic mass movements, called landslides.  Long trains of rock debris can form scree slopes, loose fragments lying precariously at the critical angle beyond which they move, the angle of repose.  Because impact craters make steep walls and the larger ones bring up peaks in their centers, most mass wasting on the Moon is found in and around impact craters of all sizes.

As the number of high resolution images taken from the LRO mission continues to proliferate, several interesting and underappreciated lunar surface phenomena are becoming more apparent.  Among the fresh craters of the Moon, we find light and dark steaks on the walls of the ubiquitous craters of the Moon.  Although it is not surprising that material might move or flow down steep slopes on the Moon, the appearance of these flows can be startlingly similar to those seen on other planets, particularly Mars, where such streaks have been cited as evidence for the presence of subsurface water.

The new narrow angle LRO camera can see objects on the surface smaller than one meter (typically, 50 cm per pixel resolution).  These new views have shown us a wide diversity of new features within impact craters and have given us a new appreciation for mass wasting.  Larger crater walls are slumped, with stair step-like wall terraces, concentrically arranged around the crater between rim and floor.  In detail, these terraces show ponds of dark material that seem to collect in low areas.  Most of this material looks like it was once molten but now congealed; it is probably solidified impact melt.  Flows of melt may cascade down and over the walls of fresh craters.

However, many “flows” of both dark and light material on the Moon seem to consist of loose fragments of rock debris lying on steep slopes.  These debris flows show a variety of morphologies, including simple flow shapes, cascades, ponding, and fan-like termini.  Sometimes the dark and light flows intermingle within a single crater while others show only one type.  These debris flows can usually be traced back to outcrops of bedrock in the upper portions of the crater wall.  As the bedrock erodes (usually by meteorite erosion and disaggregation due to the intense fracturing induced by the original impact that formed the crater), it sheds small fragments that train down slope, forming flow-like landforms.

Because crater walls are uneven, undulating surfaces, the rates of down slope movement can vary widely over small distances.  This sometimes results in multiple, overlapping flows of debris.  Factors that control the albedo (reflectivity) of the debris flows are not well understood.  It could be related to composition (for example, dark, iron-rich mare basalt vs. white, anorthositic highland rocks).  Another factor might be particle size; small pebble-sized rock flows could be bright as new, fresh surfaces are constantly exposed.  Flows that contain mixed soil might be darker than normal, as this soil could cover the fragments and reduce its average reflectivity.  But while all these factors may be of significance to one degree or another, the brightness of a streak is not particularly indicative of origin.

Dark streaks on crater walls, Mars. (click to enlarge)

On Mars, many dark streaks are evident on crater walls and, as on the Moon, come in a wide variety of forms and occurrences.  Martian dark streaks have been variously interpreted as being caused by compositional and particle size differences, but the most popular idea is that the dark streaks are wet soil, i.e., they represent areas where liquid water is seeping out from the planet’s subsurface and moistening the surface.  One observation supporting this idea is an apparent correlation of some of the dark streaks with surface temperature, with warmer slopes showing more.  As liquid water is not stable on the martian surface, salt-rich brines (which would have much lower melting points than pure water) have been invoked as the possible liquid phase.

The dark streaks on the crater walls of the Moon call water-related interpretations of similar features on Mars into question.  The nature of down slope movement on Mars is likely to be controlled by even more diverse factors than the lunar case.  For example, large landslides partly cover the floor of the Valles Marineris, the large canyon system on Mars.  These landslides can extend tens of kilometers across the valley floor and the mass flow might have been lubricated by trapped atmospheric gas; this “cushioning” effect occurs within some landslides on the Earth.  Such a process would not occur on the Moon.  The diversity of geological processes on Mars suggests that explanations for dark wall streaks could encompass many more possibilities than simple wetting of the surface.

Although the existence of dark lunar streaks does not negate water-related interpretations of similar features on Mars, they do call attention to the need to keep alternative hypotheses in mind.  For many years (and with some success), planetary geologists have extrapolated landforms and processes (thought to be understood) on Earth, to similar appearing features on the planets.  In the case of the dark streaks, terrestrial water seepages in the desert can be darker than surrounding desiccated terrain.  A wide variety of evidence indicates that water is present in the subsurface on Mars but sometimes other effects such as rock composition or particle size are responsible for the streaks and alternatives to seepage should always be kept in mind.

Professor Ronald Greeley, 1939-2011

Yet another lunar and planetary scientist has departed this world.  My former teacher, dissertation advisor and mentor Professor Ronald Greeley passed away this week at the age of 72.  The news of his death came as a true bolt from the blue – Ron was in apparent good shape, good humor and active in his scientific research.  And so sadly, I say an untimely goodbye to another friend and colleague.

Ron became involved in planetary geology while fulfilling his military service requirement at NASA Ames Research Center.  Ames needed a geologist, and though trained as a paleontologist, Ron was assigned the task of examining images of the Moon to study volcanic landforms.  He quickly became interested in lava tubes (large horizontal conduits that transport lava from the eruptive vent outward as flows).  After an eruption, lava tubes sometimes drain out, leaving behind an empty cave.  Lava tubes can be many kilometers in length and tens of meters in cross section.

Sinuous channels wind their way across the relatively flat smooth surface of the lunar maria.  Some workers noted the similarity of these features to terrestrial lava tubes and postulated that sinuous rilles were remnants of lava tubes and channels on the Moon.  Ron examined this idea in detail by mapping and studying lava tubes on terrestrial volcanoes and by analyzing the images returned by orbiting lunar spacecraft.  He wrote a key paper on Hadley Rille (a large sinuous rille at the base of the Apennine Mountains) the outer ring of Imbrium basin and the largest impact feature on the lunar near side.  This area had been chosen as the landing site for the future Apollo 15 mission and understanding the origin of sinuous rilles was one of the mission objectives.  Ron detailed the evidence that Hadley Rille is a collapsed lava tube.  He noted the rille originated in an elongate, volcanic depression, had slightly raised edges and trended generally down slope to the north.  Parts of the rille were still roofed, raising the possibility that caves could exist on the Moon.  Years later, I had the honor to be a co-author with Ron and Gordon Swann (Principal Investigator of the Apollo 15 Field Geology Experiment) on another paper about Hadley Rille, modifying and extending the model Ron had developed in 1971.

While taking an undergraduate course at ASU in meteoritics, I wrote a term paper on the geology of Hadley Rille.  I was just getting into lunar science and as a big fan of the Apollo 15 mission, I had read Greeley’s paper with interest.   In a strange coincidence, Ron came to ASU that semester to give a talk on planetary geology and I arranged to meet with him after his seminar.   We ended up talking for a couple of hours and he offered me a job for the summer at NASA-Ames.

For a starry-eyed space cadet, this offer was almost too good to be true.  I worked the summer of 1976 on a Mars mapping project for an advanced mission study.  That was the summer of the Viking landings on Mars, and I spent part of my time in Pasadena as a JPL intern.  Ron was a team member of the Orbiter imaging team and arranged for me to work with him and John Guest on certifying the landing site for Viking 2.  It was a memorable and exciting introduction to planetary exploration and I will always be in Ron’s debt for having given me that opportunity.

After studying for my Master’s degree at Brown, I returned to work with Ron at Ames.  When he moved to ASU, I applied there to get my Ph.D.  Ron agreed to take me on and I became one of his first doctoral students.  Ron was a great mentor and a role model for a modern working scientist.  Even as his academic group grew to where he needed to assign work and follow up later with discussion, I was always welcomed into his office to discuss science or other concerns.  Besides showing his students how to do science, Ron also taught us how to survive scientifically.  Science is a social activity.  Navigating the treacherous political shoals of science is a learned and acquired skill and Ron generously passed those valuable lessons on to his students.

One of Ron’s best qualities as an academic mentor was assuming the role of what most graduate students desperately need, yet few ever get – a merciless and persistent editor.  I never learned how to write until I worked for Ron.  Hopefully I would turn in drafts of papers only to have them handed back to me in (almost literally) shreds.  (This was before the days of word processing – we typed our papers and then literally cut-and-pasted the text into some kind of readable form.)  Being told that your prose “stinks” is an infuriating rite of passage if you hope to become an acceptable writer.  Working with Ron all those years convinced me of an uncomfortable truth – there is only one way to learn how to write and that is to write often and be edited heavily.  Many do the first part, but few are fortunate enough to have a good editor for the second.  Of course, I didn’t see it that way at the time; getting a copy of your work covered in red ink is annoying as hell.  But an edit from Ron always improved the text, regardless of what it did to my blood pressure.  Again, I am in his debt.

Ron never let scientific grass grow under his feet.  He developed an interest in the geological effects of wind and was the first to determine the wind speeds needed to start sandstorms on Mars.  He made geological maps of every planet and was involved as an investigator on most of the robotic planetary missions of the last 30 years.  He served the scientific community through numerous committee memberships and chairmanships.  If Ron was asked to study an issue and write a report on it, you could be sure that his report would encompass the best thinking on a subject – lucidly and concisely presented.  He was a superb speaker and presenter of scientific results, always fluent, interesting and engaging.  Beyond science, Ron’s students learned how to write and speak, two critical skills for a working scientist.

In addition to being a good scientist, Ron was a fine man.  He cared deeply for his family and spent as much time with them as he could, taking his lovely and gracious wife Cindy and children Randy and Vanessa with him on many of his national and international travels.  He was a role model for his students both personally and professionally.  If one wants to be remembered as living a productive and valuable life, emulate Ron Greeley.

The lunar feature Ina, an extremely young, unusual depression that may represent a gas eruption site on the Moon. LROC narrow angle camera images.

There are times when seemingly unrelated discoveries about other planets come forward to enlighten us about the history and processes of the Moon. A recent paper, using data from the orbiting MESSENGER mission mapping Mercury, describes a number of newly discovered rimless pits and depressions.  These pits (called hollows by the mission team) are difficult to explain by impact processes and are hypothesized to be the products of outgassing from the planet’s interior.  They are often associated with color anomalies (which implies compositional differences from the surrounding terrain) and frequently found on the floors of impact craters and basins.

Impact craters come in a wide variety of sizes, but within selected size ranges, they all appear more or less similar.  Small craters are nearly perfectly round and bowl-shaped with smooth rims that are raised above the surrounding terrain.  Craters with irregular shapes and no raised rims suggest that processes other than impact might be at work.  It has been suggested that on Mercury, these “hollows” were created by the violent release of volatile substances.  Such a release of gas under pressure accompanies volcanic eruptions called pyroclastic, meaning “fire-broken” (fine liquid rock (magma) fragments spewed into space and cooled during flight).

We’ve known about pyroclastic eruptions on the Moon for many years, evidenced by the green glass of the Apollo 15 site and the orange-black glass from Apollo 17.  Careful search of the images taken from lunar orbit reveal the rimless pits that served as vents for the pyroclastic eruptions that produced these Apollo glasses.  They are distinct from impact craters and often are found on the floors of craters and basins along fractures, the conduit by which volcanic magma travels to the lunar surface.

Sometimes pit craters or “hollows,” found across the surface of the Moon, take unusual form.  The kidney-shaped feature shown above is named Ina; after its discovery in one of the Apollo orbital images, it was informally named the “D-caldera” after its shape and the interpretation that it represented a volcanic collapse feature.  Ina is about 3 km across and consists of a series of small platforms, mounds and holes within a larger irregular depression.  Other similar pits and hollows occur elsewhere on the Moon (e.g., on the floor of Rima Hyginis).  And while not major features, they have been found often enough to bother many lunar scientists, who had no good explanation for their origin.

About five years ago, we got a clue as to the possible origins of these features.  Pete Schultz and associates from Brown University published a paper showing Ina displayed unusual spectral reflectance characteristics.  The slow micrometeorite bombardment of the Moon adds craters to the surface and also makes small iron-rich glass particles that darken and redden the surface.  As these glass particles build up in the soil, a soil is said to “mature.”  Fresh surfaces are more “blue” in color (actually, less red) and become redder with time as the soil matures.  Most lunar features show age or “become mature” on timescales of millions of years.  Ina shows very few impact craters on top of it, meaning that geologically, it is very young.  Moreover, the soils associated with Ina are much bluer than surrounding areas.  Both of these observations suggest that Ina is young with immature surfaces.

How are these features created?  Significant volcanism on the Moon largely stopped at least a couple of billion years ago.  The Brown team thought that the combination of young age, low maturity and unusual morphology suggested a relatively uncommon pit-forming process.  They proposed that the explosive release of volatile substances from the lunar interior would have disrupted the surface, created a chaotic mixture of rock and soil, exposed fresh surfaces (creating the immature spectral signature), and formed a collapse depression caused by the instantaneous removal of mass from below.

Now we can see that the new Mercurian hollows have morphologies displaying spectral anomalies similar to the lunar collapse pits such as Ina.  The new data suggest that Mercury contains significant volatile substances.  These volatiles must be present at some depth, accumulated under high pressure until crustal failure ensues and a massive gas release results in an “eruption.”  This explosive event leaves behind a chaotic, disrupted surface (“immature,” with fresh bedrock and deep regolith “newly” exposed to space).

In the case of Ina on the Moon, its extreme youth is suggested both by the lack of overlying impact craters of almost any size, as well as the sharp preservation of topography in its cliff and pit interior morphology.  This extreme youth may be on the order of thousands to hundreds of thousands of years, not the millions and billions of years that typify most lunar landforms.  Such youth and the widespread distribution of Ina-like collapse pits across the lunar surface implies that outgassing events are occurring on the Moon now; it is highly unlikely that we were just lucky enough to find a singular or unique occurrence.

What might these volatile substances be?  Before the recent lunar missions flew, it was common to declare that water was not a possibility.  However, we recently discovered from study of the lunar samples that water was present in the deep interior of the Moon during the epoch of mare volcanism three billion years ago; water could still be present in the subsurface.  There are many other volatile substances that could be responsible as well, including carbon monoxide, hydrogen sulfide, gaseous sulfur, as well as other more exotic gases.  Because the compositions on Mercury are poorly known, the possibilities for exotic materials there are even more extensive.

The explosive release of gas from the deep interior (without the eruption of magma) appears to be an ongoing lunar process.  This gas release could provide at least a partial answer to two vexing lunar problems: the accumulation of volatiles at the poles of the Moon (discussed in my blogging many times, most recently HERE) and the infamous phenomena of Lunar Transient Phenomena (LTP), described as glowing reddish “clouds” hovering over the lunar surface that mysteriously appear and disappear.  Telescopic observers have reported seeing LTP for many years.  Unfortunately, we have not been able to verify and document these events, largely because they are transient.  Now we have direct morphological evidence for the venting of gas from both planets, making it possible that at least some LTP might be related to gas release from inside the Moon.  Stay tuned – the book of the Moon continues to be rewritten and expanded with new and interesting discoveries.

NOTE: The latest version of the paper Tony Lavoie and I wrote on using lunar resources to create a cislunar space faring system has been published in the Proceedings of the AIAA Space 2011 Conference.  A copy is available for download HERE.

People at an asteroid: What will they do there?

Part II:  Scientific Considerations

In my last post, I examined some of the operational considerations associated with a human mission to a near Earth asteroid and how it contrasted with the simpler, easier operations of lunar return.  Here, I want to consider what we might do at this destination by focusing on the scientific activities and possible return we could expect from such a mission.  Some of the operational constraints mentioned in the previous post will impact the scientific return we expect from a human NEO mission.

Asteroids are the left over debris from the formation of the Solar System.  Solid pieces of refractory (high melting temperature) elements and minerals that make up the rocky planets have their precursors in the asteroids.  We actually have many pieces of these objects now – as meteorites.  The rocks that fall from the sky are overwhelmingly from the small asteroids that orbit the Sun (the exception is that in meteorite collections, some come from larger bodies, including the Moon and Mars).

Moreover, we have flown by almost a dozen small bodies, orbited two, impacted one and “landed” on two others.  Thousands of images and spectra have been obtained for these rocky objects.  The chemical composition of the asteroids Eros and Vesta have been obtained remotely.  We have catalogued the craters, cracks, scarps, grooves and pits that make up the surface features of these objects.  We have seen that some are highly fragmental aggregates of smaller rocks, while others seem to be more solid and denser.  In addition to these spacecraft data, thousands of asteroids have been catalogued, mapped and spectrally characterized from telescopes on the Earth.  We have recognized the compositional variety, the various shapes, spin rates and orbits of these small planetoids.  We now know for certain that the most common type of meteorite (chondrite) is derived from the most spectally common type of asteroid (S-type) as a result from the Hayabusa mission, the world’s first asteroid sample return.

In short, we know quite a bit about the asteroids.  What new knowledge would we gain from a human mission to one?

Although we have (literally) tons of meteorites, extraterrestrial samples without geological context have much less scientific value than those collected from planetary units with regional extent and clear origins.  Many different processes have affected the surfaces of the planets and understanding the precise location and geological setting of a rock is essential to reconstructing the history and processes responsible for its formation and by inference, the history and processes of its host planet.

Most asteroids are made up of primitive, undifferentiated planetary matter.  They have been destroyed and re-assembled by collision and impact over the last 4.5 billion years of Solar System history.  The surface has been ground-up and fragmented by the creation of regolith and some details of this process remain poorly understood.  But in general terms, we pretty much know what asteroids are made of, how they are put together, and what processes operate upon their surfaces.  True enough, the details are not fully understood, but there is no reason to suspect that we are missing a major piece of the asteroid story.  In contrast, planetary bodies such as the Moon have whole epochs and processes that we are just now uncovering – in the case of the Moon, water has been recently found to be present inside, outside and in significant quantity at the poles, relations that have enormous implications for lunar history and about which we were nearly totally ignorant only a couple of years ago.

Most NEOs will be simple ordinary chondrites – we know this because ordinary chondrites make up about 85% of all meteorite falls (an observed fall of a rock from the sky).  This class of meteorite is remarkable, not for its diversity but for its uniformity.  Chondrites are used as a chemical standard in the analysis of planetary rocks and soils to measure the amounts of differentiation or chemical change during geological processing.  In themselves, chondrites do not vary (much) except that they show different degrees of heating subsequent to their formation, but not enough heating to significantly change their chemical composition.

Some NEO asteroids are pieces of bigger objects that experienced chemical and mineral change or differentiation.  Vesta (not a NEO, but a main belt asteroid) has reflection spectra similar to known, evolved meteorites, the eucrite group.  These rocks suggest that some asteroids are small, differentiated planetoids, having volcanic activity that dates from the very beginning of Solar System history.  Moreover, since we have pieces of the Moon and Mars as meteorite fragments, some NEOs may consist of material blasted off these planets.  However, given that most NEOs are inaccessible to human missions, the likelihood that we could visit one of planetary derivation is small (curious that the most interesting of the NEOs appear to be those derived from some bigger (planet-sized) object.)  In broad terms of meteorite science, multiple small samples from a variety of asteroid types are preferable to many bigger samples of a single specimen, exactly the opposite of what a human mission will provide.

What specifically would a crew do during a NEO visit?  An astronaut on a planet typically would explore the surface, map geological relations where possible, collect representative samples of the units and rock types that can be discerned, and collect as much mapping and compositional data as possible to aid in the interpretation of the returned samples.  In the case of a NEO, many of these activities would not be particularly fruitful.  The asteroid is either a pile of rubble or a single huge boulder.  Chondritic meteorites are uniform in composition, so geological setting is not particularly instructive.  We do have questions about the processes of space weathering, the changes that occur in rocks as a result of their exposure to space for varying lengths of time.   Such questions could be addressed by a simple robotic sample collector, as the recently approved OSIRIS mission plans to do.

One question that could be addressed by human visitors to asteroids is their internal make-up and structure.  Some appear to be rubble piles while others are nearly solid – why such different fates in different asteroids?  By using active seismometry (acoustic sounding), a human crew could lay out instruments and sensors to decipher the density profile of an asteroid.  Understanding the internal structure of an asteroid is important for learning how strong such objects are; this could be an important factor in devising mitigation strategies in case we ever have to divert a NEO away from a collision trajectory with the Earth.  As mentioned in my preceding post, the crew had better work quickly – loiter times at the asteroid will probably be short, on the order of a few days at most.

Although we can explore asteroids with human missions, it seems likely that few significant insights into the origins and processes of the early Solar System will result from such exploration.  Such study is already a very active field, using the samples that nature has provided us – the meteorites.  Sample collection from an asteroid will yield more samples of meteorites, only without the melted fusion crusts that passage through the Earth’s atmosphere creates.  In other words, from this mission, scientific progress will be incremental, not revolutionary.

In contrast, because they yield information on geological histories and processes at planet-wide scales, sample collection and return from a large planetary body such as the Moon or Mars could revolutionize our knowledge of these objects in particular and the Solar System in general.  Many years prior to the Moon missions, we had meteorites that showed impact metamorphic effects but the idea of impact-caused mass extinctions of life on Earth only came after we had fully comprehended the impact process recorded in the Apollo samples from the Moon.  The significance of impact-related mineral and chemical features were not appreciated until we had collected samples with geological context to understand what the lunar samples were telling us.

Of course, science being unpredictable, some major surprise that could revolutionize our knowledge may await us on some distant asteroid.  But such surprises doubtless await us in many places throughout the Solar System and the best way to assure ourselves that we will eventually find them is to develop the capability to go anywhere in space at any time.  That means developing and using the resources of space to create new capabilities.  I will consider that in my next post.

Destination: Moon or Asteroid?

Part I:  Operational Considerations

Part III: Resource Utilization Considerations

Lockheed-Martin's Plymouth Rock mission concept

Part I:  Operational Considerations

The current controversy over the direction of our national space program has many dimensions but most of the discourse has focused on the means (government vs. commercial launch vehicles) not the ends (destinations and activities).  Near-Earth objects (NEO, i.e., asteroids) became the next destination for human exploration as an alternative to the Moon when the Augustine committee advocated a “flexible path” in their 2009 report.  The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander whereas asteroids, having extremely low surface gravity, don’t require one.  The administration embraced and supported this change in direction and since then, the agency has been studying possible NEO missions and how to conduct them.

On the surface, it might seem that NEO missions answer the requirements for future human destinations.  NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and (wait for it)… we’ve never visited one with people.  However, detailed consideration indicates that NEOs are not the best choice as our next destination in space.  In this post and two additional ones to come, I will consider some of the operational, scientific and resource utilization issues that arise in planning NEO missions and exploration activities and compare them to the lunar alternative.

Most asteroids reside not near the Earth but in a zone between the orbits of Mars and Jupiter, the asteroid belt.  The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner Solar System, where they usually hit the Sun or one of the inner planets.  Between those two events, they orbit the Sun, sometimes coming close to the Earth.  Such asteroids are called near-Earth objects and can be any of a variety of different types of asteroids.  Typically, they are small, on the order of tens of meters to a few kilometers in size.  As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with it in deep space.  Think “formation flying” with the International Space Station (ISS) without the option to dock.

The moniker “near Earth” is a relative descriptor.  These objects orbit the Sun just as the Earth does and vary in distance to the Earth from a few million km to hundreds of millions of km, depending upon the time of year.  Getting to one has nothing to do with getting to another, so multiple NEO destinations in one trip are unlikely.  Because the distance to a NEO varies widely, we cannot just go to one whenever we choose – launch windows open at certain times of the year and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days.  Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time-lags of tens of seconds to several minutes between transmission and reception.  Thus, the crew must be autonomous during operations.

Although there are several thousand NEOs, few of them are possible destinations for human missions.  This is a consequence of two factors.  First, space is very big and even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects.  Second, many of these objects are unreachable, requiring too much velocity change (“delta-v”) from an Earth departure stage; this can be a result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (all orbits are elliptical).  These factors result in reducing the field of possible destinations from thousands to a dozen or so at best.  Moreover, the few NEOs that can be reached are all very small, from a few meters to perhaps a km or two in size.  Not much exploratory area there, especially after a months-long trip in deep space.

That’s another consideration – transit time.  Not only are there few targets, it takes months to reach one of them.  Long transit time is sold as a benefit by asteroid advocates:  because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions.  But such systems do not yet exist.  On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy.  A human NEO mission will have to be self-sufficient to a degree that does not now exist.  Parts on the ISS fail all the time, but because it is only 400 km above the Earth, it is relatively straightforward to send replacement parts up on the next supply mission (unless your supply fleet is grounded, as currently it has been).  On a NEO mission, a broken system must be both fixable and fixed by the crew.  Even seemingly annoying malfunctions can become critical.  As ISS astronaut Don Pettit puts it, “If your toilet breaks, you’re dead.”

Crew exposure is another consequence of long flight times, in this case to the radiation environment of interplanetary space.  This hazard comes in two flavors – solar flares and galactic cosmic rays.  Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular intervals.  We must carry some type of high-mass shielding to protect the crew from this deadly radiation.  Because we cannot predict when a flare might occur, this massive solar “storm shelter” must be carried wherever we go in the Solar System (because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare).  Cosmic rays are much less intense, but constant.  The normal ones are relatively harmless, but high-energy versions (heavy nuclei from ancient supernovae) can cause serious tissue damage.  Although crew can be partly shielded from this hazard, they are never totally protected from it.  Astronauts in low Earth orbit are largely protected from radiation because they orbit beneath the van Allen radiation belts, which protect life on the Earth.  On the Moon, we can use regolith to shield crew but for now, such mass is not available to astronauts traveling in deep space.

When the crew finally arrives at their destination, more difficulties await.  Most NEOs spin very rapidly, with rotation periods on the order of a few hours at most.  This means that the object is approachable only near its polar area.  But because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but more like that of a wobbling toy top.  If material is disturbed on the surface, the rapid spin of the asteroid will launch the debris into space, creating a possible collision hazard to the human vehicle and crew.  The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object and just as occurs in Earth orbit, each touch of the object (action) will result in a propulsive maneuver away from the surface (reaction).

We need to learn how to work quickly at the asteroid because we don’t have much time there.  Loiter times near the asteroid for most opportunities are on the order of a few days.  Why so short?  Because the crew wants to be able to come home.  Both NEO and Earth continue to orbit the Sun and we need to make sure that the Earth is in the right place when we arrive back at its orbit.  So in effect, we will spend months traveling there, in a vehicle with the habitable volume of a large walk-in closet (OK, two walk-in closets maybe), a short time at the destination and then months for the trip home.  Is it worth it?  That will be the subject of my next post.

Destination:  Moon or Asteroid?

Part II:  Science Considerations

Part III: Resource Utilization Considerations