by Dr. Greg Matloff
Introduction: The Cosmos Comes to Earth
Modern civilization has many advantages. People live longer and are generally healthier than in past eras. Infant and child mortality are greatly reduced. More information is available than in any past century. And this can be accessed at the click of a mouse. In the not-too-distant past, most people never traveled more than a few miles from their place of birth.
But the inertia of the modern world is quite frustrating. Getting complex and sophisticated societies to change their ways apparently requires a present danger or visible catastrophe. The promise of eventual catastrophe or the lure of eventual riches just does not seem to work.
Such was the case a few weeks ago. Many people were aware that Near Earth Objects (NEOs)—celestial objects of asteroidal or cometary origin—occasionally strike the Earth. In parts of the world, some people followed the progress of asteroid 2012 DA14, which was scheduled to pass within 27,000 kilometers of the Earth on February 15, 2013. Only a small segment of the populace actively followed plans to divert Earth-threatening NEOs or mine them for their vast resources.
Peril or Promise
All this complacency was about to change on February 15. We were due for such an event. At intervals of tens-of-millions of years, really large celestial objects—perhaps 5 -10 kilometers across strike the Earth. One of these struck in what is now the Yucatan about 65 million years ago. The horrendous environmental consequences resulting from the near-instantaneous release of the energy equivalent of a million thermonuclear bombs contributed to the extinction of many terrestrial life forms, including the non-feathered dinosaurs. The ascendancy of feathered dinosaurs (birds) and mammals (including humans) might never have occurred without this celestial visitation.
Smaller celestial objects strike the Earth at greater frequencies. City-killers, such as the one that struck Tunguska Siberia in 1908 with the force of a 5-megaton hydrogen bomb, reach Earth’s surface about once every century. In early 2013, we were overdue for another. By an amazing celestial coincidence, this one also targeted Siberia.
If the Feb. 15 meteorite that exploded above Chelyabinsk, Siberia had been much larger than 20 meters across, it would have reached the ground with devastating effects. As it was, the 500 kilotons of energy released in the sky during the air burst of this object (about 20 times the energy equivalent of the atomic bombs that destroyed Hiroshima and Nagasaki in 1945).
As the celestial visitor streaked across the sky, thousands rushed to windows to view the fiery spectacle. What it the end of the world? Or was it an American secret weapon? Or perhaps it was the first stage of an alien invasion.
When the small asteroid vaporized, a shock wave was generated in the upper air. Broken window glass resulting from this pressure front was the cause of most property damage and injuries. Remarkably, although 1,200 were injured, no fatalities were reported.
One result of this widely publicized incident was the slow realization in US governmental circles that the NEO threat is real and immediate. With an interplanetary capability under development, perhaps we can practice our fledgling NEO diversion techniques in the near future. And this could lead to commercial application. The NEO threat might be converted into an activity that could improve the life of all terrestrials and assist in the development of an off-planet civilization.
In early March 2013, I was contacted by Les Johnson, who is affiliated with the NASA Marshall Space Flight Center in Huntsville Alabama. In 2007, I had participated in a study performed by Les’ team on how we might use our nascent interplanetary capabilities to divert an Earth-threatening asteroid or comet of moderate size—perhaps a few football fields across. Suddenly, the study was sprouting wings. The often-derided US House of Representatives was requesting input from the relevant agencies on how we might meet the NEO threat. Perhaps our early human ventures to the NEOs in around 2020 might even test some of these concepts. In these deficit-ridden times, Congress is to be encouraged and congratulated for rising to the challenge of protecting the home world from these celestial intruders.
Preventing NEO Impacts
In considering suggested approaches to diverting NEOs on Earth-threatening trajectories, it is worthwhile to first consider the classes of objects that might threaten our planet. Most or all solar system objects that might threaten our planet are classed as comets or asteroids.
Comets: Icebergs of the Sky
One celestial beast that could take us out, or at least cause serious damage, is a comet. Comets far from the Sun consist of a porous rocky nucleus perhaps a few miles across that is coated with layers of water, ammonia, and methane ice and dust. Close to the Sun, some of this stuff melts and forms the tenuous coma that surrounds the nucleus and the even more diffuse tail, which always (due to the pressure of sunlight) points away from the Sun. The coma might be ten thousand kilometers in diameter and is often visible from Earth. The tail is frequently tens of millions of kilometers long. But the tail is so diffuse that if were very patient and obsessive, you could probably pack it in your suitcase and carry it along on your next trip!
Our solar system may contain as many as a trillion comets. But luckily for Earth, most of them spend their entire existence in the frigid wastes beyond Neptune. Only a comparatively few are disrupted from their stable orbits on this so-called “Oort Comet Cloud” by passing stars or close planetary alignments to venture on elliptical paths into the inner solar system.
Although comets (because of their mass and high velocity) create quite a wallop when they strike a planet, they are actually quite fragile. One class of comets that closely approaches the Sun, the so-called Sun grazers, sometimes break apart or entirely evaporate during close solar approaches. In 1994, comet Shoemaker/Levy 9 became a temporary satellite of Jupiter. Before it impacted that giant planet’s atmosphere, it was observed to split into many smaller fragments (Fig. 1). The scars in Jupiter’s atmosphere from the impacts were visible for years.
Figure 1. Shoemaker/ Levy 9 after its First Jupiter Encounter (courtesy NASA)
Although ancient people considered a comet in the sky to be a bad omen and a comet might well have been the dino killer, there is a good side to these sky denizens. The oceans and atmosphere of our Earth are thought to be largely due to comet impacts early in the history of the solar system.
Deflecting an Earth-threatening comet will be challenging. Unlike asteroids that are mainly concentrated in or near the ecliptic plane, comets can approach from any direction and at any inclination. Impact warning time might be limited since an Earth-threatening comet might not be detected until it is close to our planet.
Asteroids: Threat and Opportunity
Figure 2. Asteroid 253 Mathilde photographed by NEAR probe in 1997 (courtesy NASA)
As is true for comets, asteroids have been explored by spacecraft and observed through telescopes (Fig. 2). We have learned that there are at least four classes of asteroids: iron-rich objects, stony asteroids, extinct comet nuclei and rubble piles. The dividing line between comets and asteroids seems very diffuse. At least one object, usually classified as an asteroid, occasionally displays a cometlike tail. Most meteorites in museum and university collections tend to be iron-rich, largely because these objects tend to survive the rigors of entry into Earth’s atmosphere and erode slowly in Earth’s atmosphere.
Most asteroids safely orbit in the asteroid belt between Mars and Jupiter. Some have been captured as satellites of Jupiter and the other giant planets and others lead or follow Jupiter (and other planets) in their orbits at the gravitationally stable Lagrange-4 or Lagrange-5 points. But those of most interest to humans either to explore, defend against or for resource utilization, are the Near Earth Objects or NEOs.
Many government-funded and private searches search for and track NEOs. An up-to-date survey of this material is available on-line at a NASA Jet Propulsion Laboratory website: http://neo.jpl.nasa.gov/.
As of March 16, 2013, 10:30 AM Eastern Daylight Savings Time, a total of 9,688 NEOs were included in the Jet Propulsion Laboratory data bank. Of this sample, 434 are listed as NEOs that are potentially hazardous to the Earth, with impact risks over the next few centuries tabulated.
Because NASA and other space-related entities plan human missions to NEOs in the near future, 24 NEOs are listed as NHATS (Near Earth Object Human Space Flight Accessible Targets). These must have low inclinations to the ecliptic plane to reduce fuel requirements. All have, for a launch time in the interval 2015-2040, flight durations under 360 days with stay times greater than 8 days and velocity increments less than 6 kilometers per second.
The JPL NEO website also includes some discussion of the potential value of minerals in the asteroids. If we can economically tap the main-belt asteroids, the estimated cumulative mineral content of these objects could enrich every human by about $100 billion! More will be said about such forecasts later.
NEO searches are increasing in frequency and accuracy. Probably, we have reliable orbital information on most NEOs of ~1 kilometer dimension and many smaller ones. But small objects such as the Feb. 15, 2013 Siberian visitor, cannot be readily detected and tracked using today’s equipment. It should also not be assumed that published trajectory elements for small NEOs are accurate over the long run. Orbital perturbations including differential solar radiation pressure can result in small trajectory changes. So even though an Earth impact by a city-killer does not seem likely in the next few decades, the possibility cannot be ignored. For that reason, studies of methods of altering NEO orbits are receiving higher priority. Some of these may be tried in forthcoming robotic and human space missions to accessible NEOs.
Altering Asteroid & Comet Trajectories to Prevent Earth Impacts
Hollywood special effects experts like pyrotechnics, possibly because simulated explosions are exciting and they increase profits. For that reason, cinematic versions of human missions to divert Earth-threatening comets and asteroids usually feature the biggest bangs of all: thermonuclear weapons (often called “devices” in this era of political correctness).
The reader might initially suspect that this fictional approach is quite reasonable for application to a real-world situation. But nature is not always so agreeable. The following sub-sections consider advantages and disadvantages of nuclear device application to NEO solar-orbit modification as well as some of the suggested less dramatic alternative techniques. As the reader will see, some of the suggested approaches are also applicable to resource retrieval from NEOs
The Nuclear Option
The destructive potential of nuclear weapons is well understood. Even a relatively small thermonuclear weapon, detonated over a major city, will result in millions of deaths and immeasurable property damage. A larger device, such as the 20-30 megaton H-bombs tested by the US and USSR during the Cold War, could render much of a continent uninhabitable.
But a nuclear or thermonuclear detonation in space would be a totally different matter. There will be no mushroom cloud or shockwave in the airless vacuum. Instead, there will be a wave front containing gamma rays, followed by an intense flux of neutrons and electrically charged sub-atomic particles.
If the target object is a fragile comet, as was the case in the Hollywood film Deep Impact, the comet might fragment or “calve” into two or more smaller objects that still target the Earth. And these fragments would now be intensely radioactive! The same might be the result if the Earth-threatening object were a rubble pile.
Let’s say instead that the offending object is an asteroid composed of pure iron. Iron, like most metals, is a good conductor. The melting and boiling points of iron are respectively 1535 Celsius and 2750 Celsius. So much of the energy released in the nuclear blast might go to heat the NEO rather than altering its orbit or fragmenting it.
The nuclear option might be most effective in diverting a NEO with a dense nucleus that is coated with a dust layer. In such a case, the photon and particle flux might energize the dust layer and raise it in an escaping, high-velocity plume. By Newton’s Third Law, the reaction to that plume would alter the NEO’s solar trajectory.
Most of the alternative NEO deflection schemes discussed below require long impact warning times—years or decades. But if we discover a massive comet targeting the Earth, the warning time might be a lot shorter. Nukes might be the Earth defense of last resort.
Kinetic NEO Deflection
On January 12, 2005, NASA launched the Deep Impact probe towards Comet 9P/Tempel. After a successful injection into its interplanetary trajectory, the probe separated into two sections. One would crash at high velocity into the nucleus of the comet on July 4 of that year; the second would photograph the event.
The main purpose of this experiment was to gain additional understanding of the physical and chemical composition of comets. But a secondary benefit was a demonstration of a possible NEO deflection technique: kinetic deflection.
Imagine a solar sail spacecraft injected into a solar orbit near the Earth. Given a period of 1-2 years, it is possible to “crank” the sail’s orbit without the expenditure of fuel so that it is in a retrograde (reverse) solar orbit. If it encounters an Earth-threatening NEO in an orbit similar to the Earth’s, the relative velocity of the two objects will be about 60 kilometers per second!
It the sail is directed to smash into the NEO, both the linear momentum and energy transfer will be enormous. Provided that the Earth-impact warning time is sufficient, the alteration in the NEO’s orbit could convert and Earth impact into a near miss.
As with all suggested NEO deflection schemes, there are issues with kinetic deflection. Aim must be perfect, as must be our knowledge of the NEO’s solar orbit. And it won’t work with all classes of NEOs. A very loosely bound NEO, called a “rubble pile” would likely be fragmented, not diverted by the encounter. But kinetic deflection is conceptually simple and is certainly worthy of further study.
Foil Wrap and Paintballs
If the offending NEO is small or if Earth-impact warning time is measured in decades, Earth defenders might try a gentler solar sail approach. An astronaut crew or intelligent robot could place a reflective coating around the NEO in the form of wrapped foil or reflective paintballs. The increased reflectivity of the originally dark NEO would result in a higher linear momentum transfer from impinging solar photons. This solar reflection increase would gradually alter the NEO’s solar orbit and hopefully reduce the chances of an Earth impact.
This is not an ideal approach for larger NEOs or in the case of short warning times. But the technology is very simple.
The Gravity Tractor
Physicists have known for centuries that the Earth’s gravitational field produces an acceleration towards Earth’s center that is a constant and is independent of an object’s composition and mass. Anything with mass will produce a similar effect on other objects.
Consider a solar sail orbiting the Sun and using solar radiation pressure on the sail to maintain a constant distance from an Earth threatening NEO. According to Newtonian gravitational theory, the NEO will be attracted to the sail with an acceleration that is directly proportional to the Universal Gravitational Constant and the sail mass and inversely proportional to the square of the distance between the sail and the NEO’s center of mass.
The gravity tractor will work with any type of NEO. Another advantage is that low-mass and high-mass objects will “fall” towards the sail at identical accelerations. But years or decades will be required to divert an Earth-threatening NEO. Since we have thus far only flown two solar sails in space, we need to know how rapidly a sail’s thin reflective layer will degrade in the interplanetary environment.
Mass Drivers and Rotary Pellet Launchers
During the 1970s, engineers considered a class of solar-powered machines that could be placed on the surface of a NEO and used either to alter a NEO’s solar orbit or launch NEO material towards Earth for industrial processing and application. The mass driver is a kind of electromagnetic catapult in which pellets of NEO material are magnetically levitated and accelerated to velocities in excess of 1 kilometer per second. Rotary pellet launchers accomplish the same task in a spinning mechanical object reminiscent of a huge slingshot.
Although these ideas have merit for NEO resource retrieval, they both require that an industrial infrastructure is established on the surface of the NEO. Also, it would likely be necessary to cancel NEO rotation for optimum operation. The first human visits to NEOs will probably not be able to test these concepts in practice.
Solar and Laser Collectors
If the threatening object is resource rich, such as an extinct comet and the Earth-impact warning time is at least a year, the solar and laser collectors are candidate NEO-deflection schemes. Both use a modified solar sail to collect lots of solar energy. The solar collector concentrates this energy in a “hotspot” on the rotating NEO’s surface in a manner analogous to a child using magnified sunlight to burn a hole in a sheet of paper. The laser collector uses sunlight to energize a laser that is directed towards the NEO.
In the case of a volatile-rich NEO, both devices will energize a jet of material from the NEO’s surface. By Newton’s Third Law (for every action there is an equal and opposite reaction) the energized jet will result in an alteration of the NEO’s solar trajectory.
For a volatile-rich object, Earth-impact warning times are not as stringent as for some other approaches. It is also possible that solar or laser collectors could be used to steer certain types of NEOs into high-Earth orbit for resource retrieval. If it is installed on a NEO’s surface, a solar collector could also be used to boil off material for separation and collection in a resource-mining process
But there are concerns regarding degradation of collector objects caused by the energized jet. And this approach will be useless in diverting large, iron-rich NEOs.
Robotic and Remote Exploration of Comets and Asteroids
Regardless of the method(s) we ultimately apply to divert and mine offending asteroids and comets, astronomers and planetary scientists use a number of tools to detect and explore them.
Most detection schemes involve small or medium size telescopes. Essentially, a star field is photographed at two different times. Objects that have shifted their location are likely asteroids or comets and are studied further with higher resolution equipment.
More than a few NEOs have been investigated using planetary radar. A collimated radio signal is transmitted from the aperture of a large radio telescope (such as the 305-meter dish in Arecibo, Puerto Rico). The reflected radiation from the NEO is intercepted by the radio telescope and analyzed. A fair amount of information regarding rotation rate, large surface features, and basic composition can be obtained in this fashion.
Starting in 1986 when a flotilla of robotic spacecraft originating in Europe, Japan, Russia and the US flew near or through Halley’s Comet, the major space agencies have launched many probes to the vicinity of these small solar system bodies.
As of March 2013, probes from the aforementioned nations and China have flown by, orbited, landed upon, or impacted seven main belt asteroids, two NEOs, two Mars-crossing asteroids, and seven comets. In 2001, the US Near probe accomplished the first soft landing on a NEO, 433 Eros. In 2010, the Japanese Hayabusa returned a small sample of material from another NEO, 25143, to Earth. Samples were returned from a comet’s coma in 2006 by NASA’s Stardust probe.
In 2014, NASA’s Dawn probe is scheduled to orbit 1 Ceres, a large main-belt asteroid. Also in 2014, the European Rosetta probe is scheduled to land softly on the nucleus of a comet.
But a few years after these milestones are accomplished, the nature of deep-space exploration is scheduled to take a dramatic new turn. Starting around 2020, NASA and several private entities plan to conduct human visits to suitable NEOs. These flights and practice missions to lunar space will be the first time since 1972 that humans have ventured above near Earth orbit.
Humans to the NEOs: Two Scenarios
With the retirement of the NASA space shuttles some months ago, many believe that human interplanetary aspirations have become moribund. These beliefs are far from the truth.
Recently, a number of options have appeared for human exploration and exploitation missions beyond the confines of the Earth-Moon system. Some are government-sponsored programs; others are privately funded initiatives. Before we consider two of these approaches, it is worthwhile to consider why we seem to be bypassing the Moon.
In the 1960s, many US and USSR robots explored the surface of our planet’s natural satellite. Three Apollo crews circled or orbited the Moon and six of these missions touched down on the lunar surface. Hundreds of kilograms of lunar samples were returned to Earth and much scientific data were collected. It would seem that a natural follow-on to this Cold War effort would be the partial colonization of the Moon and exploitation of lunar resources.
But there are a number of problems with this. One is the lunar day-night cycle. Unlike the 24-hour Earth day, our satellite experiences 14 days of sunlight followed by 14 days of darkness. A solar-powered lunar power grid would be difficult to develop and maintain unless it were located in certain polar regions of perpetual light. So unless nuclear fission plants were incorporated into a human infrastructure on the Moon, Lunarians would have to adjust to long distance power transmission.
Another problem is water. Although we now realize that there is a great deal more water than originally suspected to exist on our planet’s natural satellite, most of this is concentrated in polar craters perpetually shaded from the Sun. So either we locate our lunar colony close to the poles or we arrange for transport of water as well as transmission of electricity.
Humans living on the Moon must also contend with the fact that lunar gravity is about 16 percent that experienced by terrestrial life forms. We simply don’t know how well people would readjust to terrestrial gravity after living in a lunar domed city for a period of years.
In addition, there is the economics of lunar settlement. As well as developing heavy lift launch vehicles and spacecraft qualified for interplanetary flight, agencies seeking to develop the Moon must also develop spacecraft capable of descending to and ascending from the lunar surface. Also, habitation modules and lunar rovers must be constructed and maintained on the lunar surface.
Finally, if lunar mining is attempted for the benefit of the Earth, lunar resources must be rocketed, catapulted or in some other way lofted into space from the lunar surface. Even though a lunar round trip takes less than two weeks, as opposed to the many months required to visit and return from a NEO, expeditions to appropriate near Earth asteroids and extinct comets require less propulsive energy in many cases than lunar ventures. When this is coupled with the fact that we must begin to alter the orbits of Earth-threatening NEOs if our global civilization is to survive, advocates of early NEO missions seem to have the upper hand in their competition with those who favor lunar development.
Human-occupied spacecraft visiting a NEO will dock with rather than land upon the low-gravity object. Unlike lunar explorers, they will live aboard their interplanetary spacecraft during the NEO visit rather than requiring a surface habitat. The solar panels of their spacecraft combined with on-board fuel cells should be adequate for electricity requirements on the NEO since the rotation period of many NEOs is measured in hours. But unlike lunar explorers, astronauts exploring a NEO will have to take care that a false step does not propel them to a velocity sufficient to orbit or escape the low-gravity object. In many ways, exploring a NEO will be a logical outgrowth of our experience constructing and living aboard the International Space Station.
The Government-Sponsored Option: NASA’s Planned NEO Visits
Figure 3. Two Configurations of the Space Launch System (courtesy NASA)
We first consider NASA’s current approach to NEO visits by astronauts. Even though other space agencies have interplanetary plans, it must be admitted that NASA should be considered first since after all, NASA astronauts of the Apollo program are the only humans to date who have actually visited a solar system object other than the Earth. But in this era of large deficits and political dysfunction, we must all realize that NASA’s interplanetary ambitions will not necessarily come to pass.
The NASA plans to venture beyond Low Earth Orbit (LEO) depend upon the Space Launch System (SLS), a two-stage, shuttle-derived heavy-lift launch vehicle currently under development (Fig. 3). According to Wikipedia, the SLS will have several configurations and be capable of placing 70,000-129,000 kilograms in LEO.
Human crews numbering 2-6 per space mission will ride aboard a Multi-Purpose Crew Exploration Vehicle (CEV) capable of a mission duration of 21 days, if extra supply modules are not attached to the core spacecraft. Unlike the space shuttle, the partially-reusable CEV is designed to perform an ocean landing using parachutes.
A number of CEV/SLS missions are under consideration including visits to Geosynchronous Earth Orbit (GEO), lunar fly-bys and orbits, visits to Earth-Moon gravitationally stable Lagrange points and trips to nearby NEOs. NEO trips under study include two that could be launched in 2026. One would entail a 155-day roundtrip to NEO 1999 AO10 with a 14-day stay time. The other would be a 304-day mission to NEO 2001 G82.
In collaboration with Boeing, NASA is also considering a 2024 mission to NEA2008 EV5. This mission would require 100 days for the outbound flight, 30 days for exploration and 235 days for the Earth return.
The CEV is designed to be at least ten times safer than the space shuttle. The capsule, which is manufactured by Lockheed-Martin, has a mass of about 8,900 kilograms. The service module is supplied by the European Space Agency (ESA) and is based upon the Automated Transfer Vehicle (ATV) that has been used to resupply the International Space Station. The mass of the service module is about 12,000 kilograms. The service module rocket motor is equipped with about 7,900 kilograms of fuel and is capable of a velocity increment of about 1.6 kilometers per second.
Falcon-9 Heavy, Dragon and Mr. Bigelow: A Non-Governmental Alternative
As NASA (and other US government agencies) have encountered funding issues in recent years, NASA has off-loaded its ferry service to resupply (and eventually recrew) the International Space Station to several private corporations. The most successful of these efforts to date is the Dragon capsule, which is orbited by the Falcon-9 booster. Both have been developed by the Space Exploration Technologies Corporation (more commonly referred to as Space-X).
Figure 4. A Space-X Dragon at the International Space Station (courtesy NASA)
The Dragon spacecraft is currently used for operational, unscrewed resupply missions to the International Space Station (Fig. 4). According to Wikipedia, it has a dry mass of about 4,200 kilograms and can carry a payload of 3,300 kilograms. Dragon is designed to endure the space environment for 1 week to 2 years. Currently equipped for parachute-aided water landings, future variants may use retro rockets to accomplish landings on the ground. Future crewed versions will be capable of accommodating as many as seven astronauts.
The booster for the Dragon is the very reliable, two-stage Falcon-9. Falcon-9 can place up to 13,000 kilograms in LEO. Partial reuse is possible since future versions of the first stage will be equipped with parachutes.
Space-X is developing a heavy lift launch vehicle based upon the Falcon-9. Called the Falcon-9 Heavy, this rocket will be capable of launching up to 53,000 kilograms to LEO. It will consist of a standard Falcon-9 first stage with two additional Falcon-9 strap on boosters.
If Falcon-9 Heavy is as reliable as Falcon-9, this Space-X booster could project a modified Dragon on an exploratory mission to a suitable NEO. Probably two Falcon-9 launches and rendezvous in orbit will be required for a mission to a NEO. The Dragon would require a more elaborate heat shield because of the higher reentry velocity experienced by an interplanetary-capable craft. A habitat/service-module might be based upon one of the Bigelow inflatable modules and an additional propulsive stage is necessary.
Other private space companies will almost certainly announce NEO-visit plans in the near future. Certainly, one of the government-sponsored or private interplanetary concepts will be ready for application before 2025.
Resources in the NEOs
Robotic probes of increasing sophistication will visit selected NEOs in the not very distant future. Within perhaps as little as a decade, these probes will be followed by the first human explorers. The first priority of these missions will be learning about aspects of NEO composition of interest to designers of impact mitigation and prevention schemes. But if we have to divert some of these celestial objects to protect the Earth, we can certainly steer them into high Earth orbit where they will be more readily accessible to astronaut teams. A survey of NEO resources is in order.
The first resource of interest is rock, which may serve to shield orbital and deep space outposts and human-occupied spacecraft from galactic cosmic radiation. There is certainly an abundance of this material in even a small NEO. Consider a spherical NEO with an average density of 2,000 kilograms per cubic centimeter and a radius of 50 meters. The mass of this object is about a billion kilograms!
Another in-space resource of great interest is water. This will be of use for life support (drinking, agriculture and hygiene) and can be dissociated using solar energy into oxygen and hydrogen. The resulting oxygen will find application in life support since humans and other animals consume this gas in respiration. The oxygen and hydrogen can be used to store energy in fuel cells and recombined as the most effective known chemical rocket fuel combination.
As discussed in the following section, there is at least one terrestrial application for raw asteroidal rock. Many studies have been performed of the material composition of meteorites in museum and private collections. Since it is likely that many of these objects originated among the NEOs, silicon will be another major expected constituent of NEO material. And NEO-derived silicon has application to electronics both in space and on the Earth.
The Earth and other major planets are differentiated, meaning that heavier metals tend to be rare near the surface and more common in lower layers. Since asteroids have lower interior pressures and probably coalesced by collisions of smaller bodies early in the history of the solar system, rare and valuable metals such as platinum may be more common in certain NEOs than in Earth’s crust. Diamonds are precious because of their rarity and formed deep beneath Earth’s surface when graphite and similar forms of carbon are compressed under high pressure. The same process may have occurred when carbon-rich NEOs collide and coalesce with other asteroids.
Some have suggested that early NEO miners may become rich by steering diamond- and platinum-rich bodies to impact the Earth in desolate, remote locations (perhaps Siberia?). There seems a bit optimistic regarding the operation of national and international commodities markets demonstrated in these suggestions. If markets were suddenly flooded by a million kilograms of platinum and/or diamond, these resources could no longer be considered rare and precious. Their value would crash almost immediately unless new applications were developed at about the same rate that resources become available.
It might make more sense to remove the precious commodity from the NEO in space and return it to Earth’s surface in a controlled and gradual fashion. Much thought has been devoted to the extension of terrestrial mining techniques to the NEOs.
The mineral wealth soon to become available in the NEOs is enormous. It will be interesting to see whether a “NEO-rush” analogous to the Gold Rush of 1849 develops as human capabilities to reach and exploit these objects improves.
What Might We Do With These Resources?
Almost certainly, the availability of NEO resources will begin to affect economies and lifestyles within a few decades. Can we predict likely applications?
Although new terrestrial applications for NEO material will certainly arise as this stuff becomes available in quantity, some possibilities have been explored. Most of these deal with energy production and climate control.
Space-Based Solar Power
The best explored terrestrial application for NEO material is space-based solar power. Let’s say that the Global Power Authority of the late 21st century desires to supply a substantial fraction of human energy requirements using solar power beamed from space, say 1.4 X 10^13 watts. One way to do this would be to construct a huge, thin-film array of silicon photovoltaic cells in geosynchronous Earth orbit (GEO) above the equator. Energy would be beamed down using microwaves or laser wavelengths that readily pass through the Earth’s atmosphere.
The solar constant, or solar power per unit area striking an object facing the Sun near Earth is about 1,400 watts per square meter. If the efficiency of the energy collection and transmission system is 10 percent, 1.4 X 10^14 watts of solar power are required, which implies an array area of 10^11 square meters. A square array in GEO would have a dimension of about 300 kilometers—large but not beyond the bounds of reason.
The specific gravity of silicon is about 2.3. If the array thickness is a conservative 10-4 meters, the volume of the array is 10 to the seventh cubic meters and its mass is about 2.3 X 10^10 kilograms.
Gerard K. O’Neill is his classic The High Frontier estimates that about 20 percent of typical Moon rock mass is silicon. If this is true for typical NEOs, we require a NEO mass of about 1011 kilograms to support this energy program. Assuming that the NEO’s specific gravity is 2, the NEO’s radius must be a bit more than 200 meters. There are very many NEOs in this size range and more than a few of them approach the Earth.
Economics and advances in launcher technology might put a damper on the prospects of large-scale application of NEO resources to construct solar power satellites in GEO. It is not impossible that advances in reusable heavy-lift launch vehicles and photovoltaic cell efficiency, lifetime and mass reduction might make Earth-launch of these huge satellites competitive with space manufacture.
Helium-3 for Thermonuclear Fusion Reactors
A second potential energy-related application of NEO resources relates to controlled thermonuclear fusion. Early fusion reactors that might come on line within the next few decades will probably burn a combination of two heavy hydrogen isotopes: deuterium and tritium. Although this fuel mixture is relatively easy to ignite, it has a major drawback. Lots of thermal (high-speed) neutrons are emitted in the process and these result a great deal of radioactivity.
The neutron flux from a fusion fuel mix composed of deuterium and a low-mass form of helium called helium-3 would be greatly reduced and second-generation fusion reactors may be capable of burning these reactants efficiently. The drawback is that helium-3 is exceedingly rare on Earth.
Some authors have suggested that we could obtain this isotope in sufficient quantities by strip mining the Moon, since helium-3 nuclei are present in the flux of electrically charged sub-atomic particles emanating from the Sun (the solar wind). As John Lewis suggests in Mining the Sky, another possible source is the atmospheres of the giant planets Jupiter, Saturn, Uranus and Neptune.
The NEOs are closer to home. Since we have to move some of them to protect the home planet, it is worth checking the dust and soil that may shroud some NEOs to determine the concentration of helium-3 deposited by the solar wind.
The L-1 Sunshade
Blocking the Sun
Almost all climatologists agree that Earth is warming and that human activity is a significant contributor to this climate change. Much of the public discourse seems to center upon what to do about this problem.
Although it is true that the United States has been a major consumer of fossil fuels and a producer of greenhouse gases such as carbon dioxide, the major climate problem for the future may not rest with the US and other developed nations. Instead, much of the future consumption of fossil fuels may be by citizens of developing economies such as China and India. Even if the US, Europe and Japan become totally green energy consumers, the problem of global climate change may be with us for decades.
For that reason, planetary engineering concepts have been proposed. These include seeding our planet’s upper atmosphere with aerosols (small particles) to block some of the incoming sunlight. Such geo-engineering proposals are quite controversial, largely because they would be difficult to terminate if things went wrong.
One possibility that could utilize NEO resources and be easier to control is the L-1 sunshade. Lagrange-1, or L-1, is a gravitationally stable location about 1.5 million kilometers closer to the Sun than the Earth. Objects at L-1 in the Earth-Sun system tend to maintain their position with a minimum of course adjustment.
A proposed method of alleviating climate change is to disassemble a NEO, convert it into a thin-film hundreds or thousands of kilometers across and place it at L-1 to partially reduce the amount of sunlight striking the Earth.
This sunshade could be moved if it were required to stop or reduce the climate-cooling process. Also, the sunshade could be coated with photovoltaic cells constructed using NEO material and beam copious quantities of solar energy to Earth.
Although the sunshade sounds like a win-win proposition, it does have its critics. If citizens of our planet become convinced that a technological fix such as the L-1 sunshade can solve the global climate-change problem, they may elect to continue consuming fossil fuels at a prodigious rate.
Conclusions: Risks of Developing a NEO-Based Civilization
To preserve and expand our global civilization, it is imperative that we alter the solar trajectories of Earth-threatening NEOs. We cannot do much to prevent Earthquakes, tsunamis, super volcanoes, cosmic gamma-ray bursts and other catastrophes that might threaten civilization. But preventing NEO impacts will soon be feasible.
If we can visit and divert these celestial bodies, it is inevitable that some public or private agency will steer one into Earth orbit and begin to tap its resources. Using NEO material for cosmic-ray shielding, habitat construction and rocket fuel, a NEO-based civilization may develop.
This may have a character not unlike Larry Niven’s “Belters” from his future-history stories. Raw materials may come in such a scenario from the NEOs. These might be traded for luxury items produced on Earth.
The downside to this might be conflicts between terrestrials and NEO-dwellers in which the principal armament would be small NEOs engineered as kinetic weapons. In such an unpleasant scenario, the February 15, 2013 Chelyabinsk event would pale into insignificance. Let us hope that human wisdom keeps up with human technology!
The results of the 2007 NASA Marshall Space Flight Center study on NEO deflection have been published in R. B. Adams, J. W. Campbell, R.Hopkins, S. Smith, W. Arnold, M. Baysinger, T. Crane, P. Capizzo, S. Sutherlin, J. Dankovich, G. Woodcock, G. Edlin, J. Rushing, L. Fabisiniki, D. Jones, S. McKamey, S. Thomas, C. Maccone, G. Matloff, and J. Remo, “Continuing efforts at MSFC to develop mitigation technologies for Near Earth Objects”. Presented at the AIAA/Aerospace Corporation 2007Planetary Defense Conference, George Washington University, Washington, D.C., March 5-8, 2007.
A classic source on the prevention of NEO and comet impacts is T. Gehrels, ed., Hazards Due to Comets & Asteroids, University of Arizona Press, Tucson, AZ (1994). For additional information on impact prevention and mining techniques, another source is J. Remo, ed., Near-Earth Objects: The United Nations International Conference, Annals of the New York Academy of Sciences, Vol. 822, (1997).
One excellent source on solar system resource availability is J. S. Lewis, Mining the Sky, Addison-Wesley, Reading, MA (1996). To check out early research on space habitats, mass drivers and space based solar power, see G. K. O’Neill, The High Frontier, Morrow, NY (1977).
The possibility of diverting Earth-threatening NEOs using foil wrap, from which the paint ball concept is derived, is discussed in G. L. Matloff, “Applying International Space Station and Solar Sail Technology to Explore/Divert Small, Dark NEOs,” Acta Astronautica, Vol. 44, 151-158 (1999). For more information on the gravity tractor, the reader is encouraged to consult E. T. Lu and S. G. Love, “Gravitational Tractor for Towing Asteroids,” Nature, Vol. 438, 177-178 (2005).
A recent paper on the solar collector that cites earlier work on this NEO-diversion technique is G. L. Matloff, “Deflecting Earth-Threatening Asteroids Using the Solar Collector: An Improved Model,” Acta Astronautica, Vol. 82, 209-214 (2013). A less technical; review of this approach and other NEO-deflection schemes by the same author is in the April 2012 issue of IEEE Spectrum.
Three of the scientists who have investigated the L-1 sunshade concept are K. Roy, T. Kennedy and D. Fields. They contributed a chapter to L. Johnson, G. L. Matloff and C Bangs, Paradise Regained: The Regreening of the Earth, Springer-Copernicus, NY (2010). A recent paper of theirs on this topic is in the February 2013 issue of Acta Astronautica.
Copyright © 2013 by Dr. Greg Matloff
Non-NASA art, Copyright © 2011 C Bangs and Dr. Greg Matloff
Dr. Gregory Matloff is Associate Emeritus and Adjunct Professor of Physics at New York City College of Technology, CUNY. He is a Fellow of the British Interplanetary Society, a Hayden Associate of the American Museum of Natural History, and a Full Member of the International Academy of Astronautics. He has authored or co-authored more than 100 research papers and nine books, and is known for his pioneering theoretical work on the concept of solar sails. His most recent popular science essays can be found in Baen Books’ Going Interstellar, edited by Jack McDevitt and Les Johnson. Featured art images are from artist/scientist collaborative book Biosphere Extension: Solar System Resources for the Earth. This book is included in the permanent collection of the Brooklyn Museum's artist books.