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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

Did two sub-moons collide to form our Moon? From Jutzi and Asphaug, Nature 476, 4 August 2011.

A recent paper suggests that early in the history of the Solar System, two sub-moons collided to create Earth’s present-day Moon.  Several people have asked for my opinion on this new concept, so I will examine how this result was obtained, along with some general remarks on the nature of modern scientific research.

Over 25 years ago, a popular model for the origin of the Moon emerged at a special conference on the Moon held in Kona, Hawaii.  Whenever I mention that we had a conference in Hawaii, snickering about exotic travel boondoggles invariably follows, but you should note that at this particular conference, it was hard to get attendees out of the meeting room – the tension and excitement of a new and revolutionary discovery was that great.  The collective understanding of the then-current models of lunar origin was that they were all inadequate in one way or another.  But at Kona, a “new idea” was advocated – that a giant impact sprayed material into orbit around the Earth and that debris coalesced into the Moon.   This concept was supported by nearly all attendees and affectionately became known as the “Big Whack” model.  It seemed to satisfy most of the important physical and chemical constraints on lunar origin.  Subsequent work elaborated on the details concerning this model, but its salient features were pretty well defined at Kona in 1984.

The Big Whack has subsequently entered the realm of “settled science” in regard to lunar origin, although some dissenters remain.  But a “consensus” of working lunar scientists seemed satisfied that the origin of the Moon had become a “solved problem.”  Much of the detailed information on such a planetary scale collision comes from computer modeling, in which the basic physical parameters such as size of the two bodies, impact speed, angle of encounter, and composition in broad terms are specified as input variables.  The output of the computer model tells us how much material was vaporized, melted and ejected, and how fast the ejecta was squirted out and where it was deposited.  As you might expect, these calculations are extremely involved, requiring advanced supercomputers working day and night for weeks to churn out the results.

Some scientists tend to be skeptical of purely computational results.  In computer modeling, results are only as good as the input values and assumptions, the realism of the model, the inevitable simplification necessary to make the model fit into the computer and how carefully and thoughtfully the results are interpreted.  After the first few Big Whack computer models were run and presented at scientific conferences, various lunar workers would advance questions or problems that weren’t well explained by the existing models.  The models were tweaked to accommodate the difficulties.  In fact, it seemed that the models were amenable to endless tweaking.  If a tweak couldn’t be found, the observation was questioned or deemed irrelevant.  Models should be flexible enough to explain data outliers and the odd inconvenient fact, but they should also make predictions that can be tested by experiment or observation.  A model that is infinitely flexible ultimately is scientifically worthless.

So in regard to the origin of the Moon, we find ourselves with a solved problem for which a strong consensus of the experts exists.  Big Whack skeptics either have poor or irrelevant observations or are right-brained, qualitative geoscientists incapable of understanding complex planetary “physics.”

Which brings us back to Two Moon Junction.  The recent study suggesting that the Moon is the product of the collision of two sub-moons is an outgrowth of the same type of computer modeling done on problems in planetary accretion, including the Big Whack.  What’s unusual in the new scenario is that the two objects are relatively small to begin with (not Earth-sized, but a few hundreds of kilometers across) and collide at relatively low velocities, less than 2 km/sec.  The result of these unusual conditions, it is claimed, is that the impactor “plastered” itself onto the larger object, without forming a crater.  This “spackling” of matter adds an anomalously thick crust to the far side of the Moon and shoves semi-molten, late-stage liquids around to the near side, simultaneously accounting for two major lunar conundrums – the thicker far side crust and the concentration of KREEP (potassium, rare earths, and phosphorus) on the western near side of the Moon.

Sounds pretty good, eh?  Well, there are some issues with it.  The idea that a low velocity impact does not make a crater is counter-indicated by the existence of secondary impact craters on the Moon.  Secondary craters are made when blocks and clouds of debris ejected from an impact crater land on the Moon and dig up new craters, either as isolated single holes or as chains and clusters of multiple craters.  Since these features are formed by material thrown from the Moon’s surface, they cannot have been created at speeds greater than lunar orbital velocity (about 1600 m/sec).  Since the ballistic range for most secondaries is typically less than a few tens of kilometers from the primary, most were formed by impacts at much lower speeds, typically less than 1 km/sec.  Moreover, the addition of the far side crust as a sedimentary layer does not jibe with the observation that the lunar crust is a laterally contiguous global layer, composed everywhere of similar rocks (but varying in proportion).  The authors of the study acknowledge this is an issue, but suggest that the two sub-moons would have already formed their own crusts, probably of the same composition since they come from the same region of the Solar System.  This explanation appears rather ad hoc and elastic to me, an example of the “flexibility” for which computer models are renowned.

The Big Splat has not yet been embraced by most of the lunar science community, but will doubtless be examined and considered by many.  At this stage, it remains a model and not a description of reality, but rather, the description of a possible reality.  The distinction is important.  Neither the “votes” of the lunar science community nor the “elegance” of the model are relevant in terms of its validity.  The authors describe some possible tests of their model in the paper, but these seem to me neither particularly conclusive nor easy to accomplish.

So were there originally two moons over Miami (or rather, where Miami would one day exist)?  Maybe.  But the fact that someone can make a computer model of a complex process is not proof of its reality.  In this and similar cases, the burden is on its proponents to offer experimental tests or observations to prove their case.  In the mean time, nothing is settled and consensus is irrelevant.

Top: Map of thorium concentrations near Compton crater on the lunar far side. Bottom: LRO view of the felsic highland volcano. After Jolliff et al. (2011), Nature Geoscience 4, 566.

The flood of new data from the Moon continues to enlighten and puzzle lunar scientists.  Members of the Lunar Reconnaissance Orbiter Camera team have noticed an unusual landform on the far side of the Moon that was as unexpected as it might be significant.

We’ve known for many years that early in its history, the Moon was volcanically active.  The dark, smooth maria of the Moon is made up of lava flows, individually erupted over at least a billion year time span and possibly for much longer.  In total volume, the mare lavas make up only a percent or so of the crust, so the Moon is not the volcanic cauldron that Io, the large moon of Jupiter, appears to be.  But these lavas indicate that the early Moon was hot and that melted material spilled onto its surface in the past.

The volcanic rocks of the Moon are what geologists call mafic, meaning that they are enriched in iron and magnesium.  Mafic lavas (basalt) are commonly found as plains and low relief shield volcanoes, such as the Hawaiian islands.  On Earth, volcanic rocks can be mafic (and in fact, are the most abundant rocks on Earth, comprising the ocean floor bedrock) or they can be felsic, meaning enriched in silica (SiO₂) and depleted in iron.  On Earth, felsic rocks occur in mid-continental volcanoes and the stratovolcanoes that are found along the margins of the giant tectonic plates that make up Earth’s outer, rigid layer (lithosphere).  Felsic lavas are often associated with explosive, violent eruptions, such as the Mt. St. Helens eruption of 1980.

All of the volcanic rocks returned from the Moon by the Apollo astronauts are mafic.  Most of them are basalts – lava erupted as quiet, fissure-fed sheets.  A few of the samples are tiny glass beads of mafic composition, erupted when low viscosity (runny) fluid lava squirted into space as a spray of lava called a fire-fountain.  Sprayed droplets of lava cool in ballistic flight and land on the Moon as a uniform deposit of tiny (40 micron diameter) glass beads, forming a lunar ash bed.  Although we did find tiny fragments of felsic material in some of the complex breccias from the highlands of the Moon, no felsic lavas or ash were collected on Apollo.

In April 1972, the Apollo 16 mission was sent to the Descartes highlands on the near side of the Moon.  Pre-mission mapping and studies indicated to the geology team that the plains and mountains of Descartes are felsic volcanoes, having the morphology of lava domes and ash flows on Earth.  The Apollo 16 crew was given intensive instruction in the recognition and mapping of volcanic units on the Earth, so they would recognize the abundant felsic volcanics thought to make up the Descartes highlands.

John Young and Charlie Duke put their geological training to good use when they landed on the Moon.  Not only were the rocks at Descartes not silica-rich volcanics, they weren’t even volcanic!  The crew immediately recognized that all the rocks they found were breccias – aggregates of many rocks assembled by impact.  As Command Module Pilot Ken Mattingly wryly noted, “Well, it’s back to the drawing boards – or wherever geologists go!”  (They usually go for a beer.)

After that sobering experience, lunar geologists were hesitant to map felsic volcanoes on the Moon again.  In fact, the pendulum swung away from such a process ever having occurred on the Moon at all.  Nevertheless, we continued to note small geological anomalies around the Moon, hills and domes that are difficult to explain as impact features.  Additionally, some of these small landforms apparently have unusual composition as they have unique spectral properties, being anomalously “redder” (i.e., higher reflectance at longer visible wavelengths) than surrounding terrain.  These features were imaginatively named “red spots.”  Although we could determine that lunar red spots were compositionally distinct, we did not know exactly what those compositions were.  Now, with new data from the orbiting lunar missions, the mystery of the red spots is finally solved.

The red spots are small volcanoes made up of felsic rocks.  We know from data returned by the DIVINER thermal imaging spectrometer on Lunar Reconnaissance Orbiter that these landforms are rich in silica.  From the Lunar Prospector gamma-ray data, we have determined that they are also enriched in the element thorium, a key indicator of chemically evolved rock types.  Finally, data from both Earth-based telescopes and from the Moon Mineralogy Mapper on the Chandrayaan-1 mission, show that some of the red spots are made of almost pure glass.  On Earth, silica-rich volcanic glass forms a deposit called obsidian; its crystallized form is rhyolite.  New, remotely sensed compositional data show that the lunar red spots are felsic domes of obsidian and rhyolite.  Red spots occur mostly on the western near side of the Moon, the area in and around Oceanus Procellarum.  The new finding of an isolated felsic volcano on the far side of the Moon indicates that such eruptions were a global phenomenon.

These features are not volumetrically major and occur as small geological oddities set within the predominantly mafic, basaltic volcanic terrain of the lunar surface.  Their presence was not predicted by the prevailing model of lunar volcanism.  After the fiasco of the mistaken Apollo 16 prediction, geologists were hesitant to pronounce any dome on the Moon to be a felsic volcano.  Suitably chastened, they re-interpreted those dome-like landforms of the highlands to be by-products of basin-forming impacts.  We see by the existence of these features that in some cases, the volcanic interpretation is viable.  This information adds to our understanding of the incredibly rich and complex geological story of the Moon.