SDE: Strategic Defense of Earth

Source: NASA/JPL-Caltech

Source: NASA/JPL-Caltech

Energy densities for wood, coal, and petroleum, do not include the mass of oxygen required for combustion, since in their typical applications, it is simply drawn from the atmosphere.  Values for hydrogen combustion are given with and without considering the mass of oxygen.

Source: 21^st Century Science & Technology

The mass-ratio values given here correspond to a particular trip made on an inertial (rather than continually accelerating) path.  Changing the distance of destination and the desired acceleration rate would alter the values.  For example, a three-day trip to Mars, undertaken with a constant acceleration and deceleration of 1-g, would give mass ratios of 1.007 for fusion, and an astronomical 10²⁶ for chemical propulsion.  Even 20,000K fission would have a mass-ratio of 50 for such an ambitious trip.  Since constant acceleration also requires accelerating, at each moment, all the fuel for the remainder of the trip, the fuel requirements increase exponentially with trip distance.

Source: IAEA, LANL, 21^st Century Science & Technology

Source: “Nuclear Explosive Propelled Interceptor for Deflecting Comets and Asteroids on a Collision Course with Earth,” J. C. Solem, Proceedings of the Near-Earth Object Interception Workshop, Los Alamos National Laboratory, New Mexico. 1992, January 14-16, page 121-130.

It is difficult to gain a visceral sense of the immensity of energy involved in an asteroid or comet impact on Earth. Although asteroids and comets can range anywhere from meters to many kilometers in diameter ((All sizes of comets or asteroids will be given in the length of the diameter of the object, unless otherwise noted. E.g., a “1 km asteroid” refers to an asteroid with a diameter of 1 km across.)) (imagine Mt. Everest falling from the sky!), the actual effect of an impact is greatly enhanced by the enormous speeds involved. The total kinetic energy released in such a collision is the product of the mass of the impactor multiplied by the square of the velocity, and the impact speeds range from 10 to 70 km/second, or 20,000 to 150,000 miles per hour! ((For comparison, a typical passenger jet travels at around 500-600 mph (~250 m/s); the speed of sound (at sea-level) is about 770 mph (343 m/s); and the fastest jet ever flown (unmanned) was NASA’s X-43A scramjet, which reached mach 9.8, which is 7,500 mph or 3.1 km/s.))

For example, take two notable cases: 1) the impact of an extremely large object, ~10 km, creating the 180 km diameter Chicxulub crater in the Yucatán Peninsula in Mexico, formed around 65 million years ago, which may have helped put an end to the dinosaurs; and 2) the Tunguska event in Siberia, Russia, in 1908, which, though believed to have been caused by a much smaller object, only about 30-50 meters across, resulted in local devastation. These two significant cases will help provide a sense of a range of possible scenarios.

Based on studies of Mexico’s Chicxulub crater, it has been estimated that a roughly 10 km object, hurtling at around 20 km/s (~45,000 mph), slammed into the Earth ~65 million years ago. Though the exact details of the effects are left to models and simulations, we can certainly get an idea of the scale of destruction: mega-tsunamis ((Megatsunami is a term used to describe a tsunami that has wave heights which are much larger than normal tsunamis. They originate from a large scale landslide or collision event, rather than from tectonic activity. A recent example is the 1958 Lituya Bay megatsunami, near Alaska, which resulted in a wave hundreds of meters high, the largest known in modern times.)) thousands of meters high; an expanding cloud of boiling dust, vapor, and ash; rock and other surface material ejected out of the atmosphere, raining back down over a huge area, redhot from its atmospheric re-entry; and shock waves that trigger volcanic eruptions and earthquakes around the entire globe.

To give a rough sense of scale, the energy released by such an impact is estimated to be in the range of 100 million megatons of TNT, 20,000 times larger than public estimations of the entire global thermonuclear weapons stockpile (see table I). In addition, besides the immediate effects of collision, an impact this large would launch so much dust and debris into the atmosphere that a dust cloud would cover the entire planet, blocking out the Sun for years: the impact winter, only one of many possible long-term, global effects.

Fortunately, the Chicxulub case represents an extreme, and relatively rare threat. Such large impacts, though more destructive, are much less frequent than smaller impacts. As will be expanded shortly, our neighborhood in the Solar System is populated with many asteroids and comets; however, the frequency of impact by these objects, generally, is inversely proportional to their size. Nevertheless, while a big object, in the range of 1 km or larger, can create massive global damage, even a relatively small object, can cause significant damage.

One often-cited example of an impact thought to be caused by a smaller object is the Tunguska event, in which a sudden explosion leveled roughly 80 million trees over an area of 2,150 square kilometers in Siberia, Russia. Though some mystery and debate still surrounds this 1908 case, the most well-supported theory is that the blast was due to a comet or asteroid exploding as it impacted the atmosphere, disintegrating before it could hit the Earth’s surface, and generating a massive blast wave. ((Though the Tunguska event drew and has continued to draw intense interest and study, no unambiguous, single impact crater has been found. For example, there is some evidence that it could have been generated by a massive release and explosion of natural gas from underneath the Siberian crust. In any case, we investigate the asteroid-impact theory in this report.))

Table 1: Impacts, Energy Release, and Effects (Source: NASA/JPL-Caltech)

Setting aside any lingering debates on the subject, studies have been conducted to determine what size asteroid or comet could have flattened 80 million trees over a region the size of a major metropolitan area. The results of these studies have shown that an object only 30-50 meters across could have generated such a blast wave. ((See, Comet/Asteroid Impacts and Human Society: An Interdisciplinary Approach, Peter T. Bobrowsky, Hans Rickman, Springer, Feb 21, 2007 – 546 pages.))

In order to put the range of threats further into perspective, this table presents a comparison of the levels of energy released from various types of events, both manmade and natural.

Structure and Composition

It is also highly important that we determine the physical composition of the interplanetary bodies. Some of the deeper implications of this will be discussed in the sections on defense options and exploratory missions, but here we must note that not all of these objects are structurally similar. Some are almost solid iron-nickel, some solid rock, while many others are loose piles of smaller objects held together by their gravity (sometimes referred to as flying rubble piles).

The Objects

The next question is, where do these objects come from? Our solar neighborhood is much more populated than you may realize. Here, we concentrate only on two specific classes of objects: near-Earth objects and long-period comets. The classical image of our Solar System, four inner planets, then the asteroid belt, followed by four outer planets, while true, does not present the full picture. As Johannes Kepler indicated, and as Karl Gauss proved, there is a major discontinuity between Mars and Jupiter separating the inner from outer planets, which is the home of the majority of the asteroids in our Solar System. However, in addition to this “main belt” of asteroids, there are other populations of asteroids and comets. Some share Jupiter’s orbit. Some dwell in between Saturn and Uranus. Many populate the area of the inner planets, including around Earth.

The most successful way to further investigations of the ordering of the entire Solar System will be an elaboration of the methodological approach of Kepler and Gauss, the great minds who discovered the ordering of the Solar System. Instead of starting from pairwise interactions, we must investigate the Solar System as a single, harmonic system, taking a top-down view of the orbital systems and subsystems. Ultimately, applying those methodological considerations will be the key to improving our understanding of the orbits, and determining well into the future what bodies may threaten our planet.

Consider, first, a class of objects known as near-Earth objects (NEOs). This class of potentially threatening objects are mostly asteroids, but include some short-period comets. ((The comets included in the near-Earth objects grouping (sometimes referred to as short-period comets) have dramatically different orbits than the long-period comets mentioned above. Some of these short-period comets can have orbits that are similar to that of asteroids, and constitute a small part of the total near-Earth object population.))

The defining character of NEOs is that they orbit the Sun in paths that are either in the same general region as the Earth’s orbit, or can even cross the Earth’s orbit on a regular basis, raising the possibility of a collision with the Earth at some point in the future.

Though not all NEOs pose a threat to the Earth, a large number could. Of these, a number have orbits which come within 0.05 AU of the Earth’s orbit, and are large enough to cause damage to the Earth. These are referred to as potentially hazardous objects (PHOs). ((AU stands for astronomical unit, the average distance from the Earth to the Sun. It is used as a standard measure of distance in the Solar System. Also don’t be fooled by the image above, as bodies in the Solar System orbit within a thin volume, not a flat plane. Two orbits that may look like they intersect, when drawn on paper, may not, because one could be above the other.)) This particular class of objects are of great concern for government agencies and scientific organizations all over the world, who have set out to find and track them, in order to identify potential threats, and to give advanced warning time to prepare defensive actions if needed.

Before going into the current estimations of the NEO population, how to observe and track them, as well as defense options, we must first identify a second class of potentially threatening objects, long period comets (LPCs). The orbits of these comets are completely different from those of NEOs. Whereas NEOs spend their entire life in the inner Solar System, long period comets spend the vast majority of their lifetime out in the farthest depths of the Solar System (often well beyond the orbit of Pluto.) The extreme ellipticity of some of these distant creatures can take them on rapid trips through the interior of the Solar System, and possibly across Earth’s orbit.

These create a number of significant problems for defending the Earth. First, the key to planetary defense is early detection. While we have had success in detecting NEOs which populate the inner region of the Solar System, it is basically impossible, with present technology, to see the vast majority of these long period comets when they are farther away. Not only does this dramatically shorten warning times, but, since the majority of these comets take hundreds of thousands of years to complete a single orbit around the Sun (some even take millions of years), we know little to nothing about the nature of the long period comet population. In addition, from what we do know, they are often very large, and can have impact speeds of up to about 70 kilometers per second (over 150,000 mph), significantly greater than asteroids. ((Remember that the energy released on impact goes up with the square of the speed. To give one example, the 70 km/second impact speed of a comet, going three and a half times faster than the 20 km/second impact speed of an asteroid of the same size, would deliver over 12 times more energy.))

Currently, compared to NEOs, we see far fewer long period comets passing our region of the Solar System, so it is expected that their impacts with the Earth are much less frequent. However, they have hit the Earth in the past, and if one were on a future impact trajectory, its great speed, large mass, and undetectability until close to the Earth would make it a particularly dangerous global threat. These are the type of bodies that could eliminate all human civilization with one impact.

There is also reason to believe that the population of long period comets which pass into the interior of the Solar System is not completely random. The current hypothesis is that these long period comets may originate from an extremely distant spherical structure surrounding the Sun, at the farthest reaches of the Solar System, known as the Oort cloud. Presently we do not have the observational capability to see comets that far away (a 10 km object at 10,000 times the distance of the Earth from the Sun is hard to spot), but it is thought that the number of large comets (larger than 1 km) in the Oort cloud is in the range of trillions.

Since they extend so far beyond the Solar System, these comets become sensitive to galactic factors. Other stars coming close to our Solar System can perturb the Oort cloud, changing the orbits of potentially millions of comets. Beside individual influences, at these distances, the gravitational effect of the Sun is less dominant and the general gravitational field of the galaxy begins to have an influence—an effect which varies as the galaxy evolves, and as our Solar System travels through it.

Even though our current scope of understanding regards these galactic processes as having slow, long-term effects, they are the type of considerations that mankind must begin to take into account at this stage. First and foremost, there is still little in the way of solid knowledge about these outer regions of the Solar System, and much less known about our Solar System’s relationship with our galaxy and how those galactic changes affect us here on Earth. There are many theories and models, but as we are reminded by the fact that recent readings from the two 35-year old Voyager spacecraft continue to surprise the scientific community, we cannot assume that we understand these neighboring regions, or the solar-galactic interactions, until we go out and investigate.

If there is some doubt as to why mankind has an imperative to understand our solar and galactic environment, let long period comets draw for us a larger neighborhood.

While our current capability to defend against the threat of long-period comets is limited, the state of our knowledge of near-Earth objects is less uncertain.

Population and Impact Frequency Estimations

Due to their close orbits, near-Earth objects can be observed and tracked with Earth-based and space-based telescopes. Following on a few decades of observation programs, astronomers have developed a significant catalogue of known near-Earth objects. Depending on how well and for how long each individual NEO is observed, computer models can be used to extrapolate each NEO’s orbit and trajectory, years or decades into the future. ((Obviously, the more observations of an object we have, and the better those observations are, the better the forecast will be. Still there are certain subtle effects which require greater investigation, such as composition, spin rates, and non-gravitational effects, such as an uneven heating/emission action referred to as the Yarkovsky effect. Moreover, there are questions about the methodology of the computer models themselves: they generally rely on only a few dozen large bodies to model the field through which the others pass.)) These multi-decade extrapolations are crucial, since the key to defense against a potentially threatening asteroid is having as much advanced warning time as possible.

Presently, we are far from having discovered and tracked every NEO, and that must be done. The limited population that has been characterized by current surveys has been used to extrapolate statistical estimations of the expected total NEO populations. For example, in September 2011, a NASA-led team published updated estimations of NEO populations based on the data obtained from the Wide-field Infrared Survey Explorer (WISE) space telescope.

Since then, various estimates continue to be refined as increasing amounts of data from Earth-based telescopic surveys are received. One of the more recent available estimates was released in April of 2012, and presented by the head of NASA’s NEO program, Lindley Johnson, at a May 2012 Workshop on Potentially Hazardous Asteroids. ((http://neo.jpl.nasa.gov/neo/2011_AG5_LN_intro_wksp.pdf ; April 17, 2012, Alan B. Chamberin (JPL).))

As is clear in table 2, we have been rather successful in identifying most of the larger NEOs. Of the discovered populations, some fit the specific category of potentially hazardous objects, meaning that their orbits come close to or even directly cross the Earth’s orbit. Currently, 152 of the discovered 850 near-Earth asteroids larger than 1 km are classified as PHOs, although none are expected to collide with Earth over the coming century. This is important, since 1 km is a rough division line between objects which would create truly global effects if they struck the Earth, and objects whose impact would produce a local or regional effect.

Table 2: Percentage found of the estimated total population of near-Earth asteroids of various size categories (Source: NASA/JPL-Caltech)

Still, this leaves the vast majority of medium and small-sized objects undiscovered: ~80% (over 21,000) of the middle-sized NEOs, ranging from 100 to 1000 meters; and ~99.5% of smaller NEOs, 30-100 meters (recall that the Tunguska-sized event is associated with objects in the range of 30-50 meters).

Any of these undiscovered objects could already be on a short-term collision course with Earth, unbeknownst to us. Some are guaranteed to be, at some point in the future. We are still essentially flying blind through our populated region of the inner Solar System.

Further analysis has provided estimations of the frequency with which various sized NEOs and comets impact the Earth. ((For example see, Catastrophic Events Caused by Cosmic Objects; 2008, Springer; Chapter 2, “Size-frequency distribution of asteroids and impact craters: estimates of impact rate.”)) As implied by the NEO population estimates referenced above, and as indicated in the graph on the preceding page, there is a direct relationship between the size of the NEO, the population level, and the impact rate.

These estimations of NEO populations and impact frequencies are still approximations, and should only be taken as temporary reference points, paving the way for more rigorous investigations. We cannot entrust human lives, or potentially human civilization, to betting on statistics which purely extrapolate from past events. They can be utilized in limited applications where useful, but only on the path to obtaining a principled—causal—understanding of the system. This requires both a dramatic expansion of our observational systems and our space-faring capability generally, as well as renewed methodological approaches to understanding the organization of the Solar System, and its relationship with the galaxy. Reliance on statistical extrapolations from the past leaves mankind completely blind to unexpected shifts away from present trends, driven by the development and evolution of the Solar System and galaxy—a process driven by future changes.

Fortunately, mankind has not been completely negligent on the issue of tracking potentially threatening near-Earth objects (NEOs) and comets. In the United States, serious recognition of the threat of potential impacts with the Earth started to grow in the 1980s, and by the 1990s the U.S. Congress issued a mandate to NASA, tasking them to find and track 90% of all NEOs larger than 1 kilometer, in order to determine if any pose a threat to the Earth in the future. This mandate led to the development of the “Spaceguard” program, which is a loose alliance of survey programs which receive money from NASA to find and track NEOs.

The past decades of observation under these programs have provided a start to addressing this planetary challenge, but, as is seen from the asteroid population estimates discussed above, we are still far from identifying all the potentially threatening NEOs which populate our immediate neighborhood. Looking to greater challenges, these existing NEO survey programs are not designed to deal with the challenge posed by the second class of rarer, but potentially more threatening objects, long-period comets, which come from the farthest depths of the Solar System, and are, for all practical purposes, impossible to detect at their great distances.

To successfully tackle both of these threats, mankind must rapidly expand its sensory systems based on existing designs, while at the same time developing new technologies to handle threats outside of our current technological capability.

The best possible option is for the United States, Russia, and China to cooperate in a joint science driver program, the beginnings of which have already been put forward by the Russian government in the form of the Strategic Defense of Earth proposal to the United States.

Existing Programs

A series of ground-based observation programs have been developed to search for near-Earth objects. The following chart indicates how many near-Earth asteroids have been discovered each year and by which observation programs.

Observations from these and other telescopes are then centralized in the computer systems of the International Astronomical Union’s Minor Planet Center (Smithsonian Astrophysical Observatory, Massachusetts). These systems, along with NASA’s Jet Propulsion Laboratory (the Horizons Ephemeris Computation Facility) and the Near Earth Object Dynamics Site (NEODyS) at the University of Pisa, Italy, can then approximate the orbits of NEOs, and extrapolate their trajectories decades into the future.

These surveys and orbital extrapolations provide the first line of defense. Detecting a threat decades before it may impact would provide the necessary time to launch a mission to change the threatening object’s trajectory.

For these reasons, early warning is of the utmost importance. However, current computer simulations can not provide absolutely precise determinations of asteroid or comet orbits that far into the future, and instead provide a range of possible trajectories based on various uncertainties. While it is true that more observations lead to more accurate orbits, there are still limiting factors which keep scientists from achieving the accuracy needed. One crucial aspect of this problem is taken up in the interview with Claudio Maccone, on page 46 of this magazine.

In addition to ground-based observatories, the first steps have been made to utilize space-based telescopes to improve our view of the Solar System. This option was demonstrated with NASA’s Wide-field Infrared Survey Explorer (WISE), an infrared space telescope which only operated for a short period of time, but opened a completely new window to view our neighboring asteroids and comets. Seeing these objects in the infrared end of the spectrum (which can only be done from space) provides an improved capability to determine their size, and to see small dark objects.

These first steps have been useful, but even with a decent discovery rate by present systems, it will take us decades to begin to come close to identifying the total population of near-Earth objects alone. It is time for nations to take up this challenge in a serious way. Up to the present, these efforts have been limited to a small number of concerned scientists who have demonstrated the existential importance of asteroid and comet defense. Their initial accomplishments could be rapidly expanded by an international mission.

Existing Proposals

In April of 2010, a NASA ad hoc task force was commissioned to advise the relevant agency officials on how best to further efforts to defend our planet from threatened NEOs. A short report was released in October of that year which provided a series of recommendations. ((“Report of the NASA Advisory Council Ad Hoc Task Force on Planetary Defense;” October 2010.))

Included in the recommendations is the construction of one or more space-based infrared telescopes dedicated to accelerating the detection and characterization of the NEO population. As discussed above, utilizing the infrared region of the spectrum provides an improved ability to see and identify these bodies. It was recommended that one or more of these infrared space telescopes be placed in orbit around the Sun, but at a distance similar to that of Venus. Because we can only see these objects when looking away from the Sun, this position, being inside the Earth’s orbit, provides a better viewing angle to see a larger section of the NEO population.

This would be a significant step towards identifying and tracking the NEO population, but unfortunately NASA has not been able to take up this recommendation. ((Because of the inability of NASA to pursue this, a non-profit organization dedicated to the issue of planetary defense, the B612 Foundation (whose board of directors includes key participants of the 2010 NASA ad hoc report), has recently announced plans to build and launch an infrared telescope of this type supported by purely private funding. The “Sentinel Mission” is planned for launch in either 2017 or 2018.))

This is a step in the right direction, but we must also consider what it will take to tackle the second class of problems, long-period comets. The 2010 NASA report on planetary defense chose to focus solely on the NEO threat, leaving out the issue of long-period comets, for the following reasons:

“The population of long-period comets, with orbits originating in the outer Solar System, represents a small part of the total comet threat, and thus an even smaller part of the total impact hazard. Because the tasks of effectively detecting and deflecting objects of this size and velocity are beyond our present technology, the Task Force report does not address long-period comets.”

While long-period comets do have a lower frequency of impact, this does not mean that we should ignore the problem.

There have been preliminary investigations into what would be required for adequate detection times for long-period comets, most of which focus on developing space telescopes with much larger apertures to be able to see deeper into space. For example, one analysis discussed using light-weight structures to construct 25, 50, or even 75 meter telescope diameters. ((“Obtaining long warning times on long-period comets and small asteroids – Extremely large yet extremely lightweight space telescope systems,” Ivan Bekey, 2004 Planetary Defense Conference: Protecting Earth from Asteroids; Orange County, CA; Feb. 23-26, 2004.)) This would be a dramatic improvement, ((The Hubble space telescope is 2.4 meters across, and the James Webb space telescope will have a diameter of 6.5 meters.)) and is thought to allow us to look deep enough into space to provide warning times for long-period comets on the order of 6, 11, and 16 years respectively. Given the sizes and speed of many of these objects, even these warning times would be minimal, if not still too short.

These telescope designs are just two key examples of proposals to expand the observational capability in order to meet the demands posed by both NEOs and comets. Much more detailed analyses have been made for these and other systems, and more analyses can be done, but we must begin to move to the construction phase of such systems immediately.

Mankind is battling an array of natural disasters which continually pose a threat to life on this planet.  Thanks to advancements in satellites and weather monitoring systems, our ability to forecast major storms and other extreme weather events is improving.  Progress is being made in developing earthquake forecasting systems, designed to detect precursors signals which can provide early warnings before seismic events. ((See, IGMASS: Towards International Collaboration in the Defense of Mankind, “Progress in Seismic Forecasting,” 21st Century Science and Technology Magazine, Fall/Winter 2012.  See also, Science Can Predict Earthquakes, in the Winter 2011 issue. ))  Even our Sun is being watched and analyzed more closely than ever, in attempt to forecast “space weather” events and their effects here on the Earth.  However there is another class of events that can not only be foreseen, but can the stopped from ever occurring.  Asteroid and comet impacts represent a unique challenge, as we can take the necessary actions to see them coming, but also to ensure the Earth is never again struck in a catastrophic event.  While it is likely that we will be able to control storms and certain extreme weather events in the not-too-distant future (if appropriate scientific/economic programs are pursued), for now asteroid defense can hold the title of the only currently preventable natural disaster.

But what are the factors determining our ability to defend the planet, and how can these limits be expanded?  In defending the Earth from impacts, there are many possible scenarios we could face: a relatively small near-Earth asteroid on a short-term collision course, giving us little time to act; a large asteroid threatening a possible impact in a few decades, proving more time to act, but proving a larger foe; a worst case scenario of a large long-period comet only months away; and any number of possible variations in between.

The first line of defense is clear: early detection.  No matter how large the threat is, the more warning time we have, the better off we will be.  While asteroid and comet detection systems have been discussed in other locations, ((see, Strategic Defense of the Earth, “Part 2: Observation Systems,”)) the subject here is our ability to act on this knowledge.  This takes us beyond just asteroid or comets per se, to a general consideration of our power for action within the universe.

Initial Considerations

To state the question in simple terms: 100 years ago we would have had no chance to defend the Earth from an asteroid or comet impact, while presently we have a limited ability to defend the Earth under certain circumstances, and in the future we could foreseeably develop the means to defend against threats currently outside of our defense capability – what determines these qualitative changes?

While there are countless important discoveries and technological innovations which have contributed to this process (and shouldn’t have their importance dismissed), the subsuming role of energy-flux density will be considered here. ((“Energy-flux density” as specifically defined by Lyndon LaRouche, in his science of physical economics.  For example, see, So, You Wish to Learn All About Economics?: A Text on Elementary Mathematical Economics, New York: New Benjamin Franklin Pub. House, 1984.)) This can be illustrated in first approximation by comparing the energy densities of successive power sources.

Table 1: The Energy Density of Fuels (Source: 21^st Century Science & Technology)

Energy densities for wood, coal, and petroleum, do not include the mass of oxygen required for combustion, since in their typical applications, it is simply drawn from the atmosphere.  Values for hydrogen combustion are given with and without considering the mass of oxygen.

The significance is not simply found in the increase in energy, but in the physical economic implications: fundamental changes in the human species’ space-time relationship with the universe, where leaps from one level to the next define new (previously impossible) modes of action.  As in transportation, for example, development of systems associated with successive fuel sources create fundamentally new possibilities.  On the Earth’s surface, the locomotive revolution was associated with coal-fired engines, whereas the internal combustion engine required the advancement to petroleum.  Airplane flight depends upon the higher energy to weight ratios of petroleum, but rocket travel from the Earth’s surface to Earth orbit (and beyond) has demanded the most efficient chemical combustion reactions possible.

Although transportation is only one expression of a broader qualitative change, it is enough to introduce the concept of transformations in the physical boundaries of mankind’s action within the universe.  Taking this investigation further, only the energy densities of nuclear fission, to a limited degree, but ultimately thermonuclear fusion and matter-antimatter reactions, can truly provide mankind with efficient and timely access to the solar system, as this reality is expressed in basic fuel and mass limitations.  For example, we can measure the ratio of the total starting mass of a spacecraft (including all of its fuel) to its final mass upon arrival at its destination (in other words, measuring how much of the initial mass is the fuel required for the trip), and then compare how this ratio changes for different fuel sources (mass ratio).  Or, the specific impulse can be determined by comparing how long one pound of fuel can provide one pound of thrust.  For example, see the following theoretical calculations of the mass ratio and the specific impulse for different fuel sources from Los Alamos National Laboratory. ((“The Role of Nuclear Power and Nuclear Propulsion in the Peaceful Exploration of Space,” IAEA, 2005; page 34, and Appendix VI (page 116).  ((

Beyond the consideration of the energy density of a fuel source for transportation, higher levels of energy-flux density have systemic effects for the entire economy.  Consideration of the the transitions from the hydrocarbon-based economy to the nuclear economy, and the yet-to-be realized, but desperately needed, transition to the fusion economy, are premier examples. ((For example, regarding mankind’s entry into the nuclear age, see, “The Isotope Economy,” J. Tennenbaum, 21st Century Science and Technology magazine, Fall-Winter 2006.  Pertaining to fusion-related directed energy research see, “The Economic Impact of Relativistic Beam Technology,” June 15, 1983; EIR Research Inc. )) Although a fuller treatment is beyond the scope of this writing, the specific applications to the defense of Earth will be discussed here.

Planetary Defense

For the asteroid and comet threats specifically, and ultimately the defense of all life on our planet, the ability to wield higher energy densities becomes crucial.  We know for certain that there will be significant asteroid or comet impacts in the future.  The question, then, becomes, will we take the necessary actions to deflect or destroy prospective threats before they hit?

This brings two interrelated aspects into focus: the energy required to influence the asteroids or comets themselves, and, even prior to that, the ability to reach the body in the first place. ((Again, this is not to dismiss the crucial role of finding and tracking asteroids and comets long before they may become a threat.  While that absolutely must be done, here we focus on the ability to act on that knowledge.))

Moving spacecraft around the solar system is not as simple as moving from location A to location B, because we are dealing with orbits within a gravitational field.  For example, current missions to Mars can only be launched at specific times (about every 2.17 years).  This is not to wait for the planets to be close in terms of distance across Euclidean space, but it is when the orbital relationships of Earth and Mars provide a least-energy orbital pathway between them.  Because changing your orbit requires a change in speed, space travel is often discussed in terms of the change in velocity required (or delta-V requirements).

Table 2: Mass Ratio of Various Rocket Fuels (Source: IAEA, LANL, 21^st Century Science & Technology)

The mass-ratio values given here correspond to a particular trip made on an inertial (rather than continually accelerating) path.  Changing the distance of destination and the desired acceleration rate would alter the values.  For example, a three-day trip to Mars, undertaken with a constant acceleration and deceleration of 1-g, would give mass ratios of 1.007 for fusion, and an astronomical 10²⁶ for chemical propulsion.  Even 20,000K fission would have a mass-ratio of 50 for such an ambitious trip.  Since constant acceleration also requires accelerating, at each moment, all the fuel for the remainder of the trip, the fuel requirements increase exponentially with trip distance.

In the case of a potentially threatening near-Earth asteroid, for example, when decades of warning time are available, a minimal energy trajectory can be determined to intercept the asteroid, and the launch date can wait until the trajectories of the Earth and the target reach the positions which provide that relatively low energy path.

However, when there is not sufficient warning time to wait for this optimal timing, then the energy requirements can quickly jump many fold. (( See, “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies,” page 80-84. National Research Council, 2010.)) This would then require more fuel, meaning either a heavier spacecraft to start with, or a greater proportion of an unchanged total mass going towards fuel, leaving less mass free for your spacecraft upon arrival.  For chemical propulsion, with its inherently low energy density, this is problematic, and can easily become untenable.  But, relative to any specific scenario, higher levels of energy-flux density inherently have the potential to provide a greater delta-V.  This underscores the need for more advanced propulsion systems, with fission playing a useful part, but a greater focus on the propulsion potential of fusion (while looking towards harnessing matter-antimatter reactions), in order to truly open up mankind’s efficient access to the solar system. ((The difficulties of scenarios offering only a short warning time before impact can hopefully be avoided with better observation systems for early detection, but only higher levels of energy-flux density can fundamentally transform mankind’s space-time access to the solar system.))

Defense

When it comes to altering the path of an asteroid or comet to ensure it misses the Earth, various methods have been considered, and are often categorized into different types.  For example, there are “slow-push-pull” methods, in which a small amount of force is exerted over a long period of time to slowly alter the path of the asteroid or comet, and there are “quick” methods, in which a large amount of force is applied over a short period of time. ((For a more detailed description of each of the following methods, and the particular benefits or limitations of each, see Chapter 4, “Preventing or mitigating an impact,” of Dealing with the Threat to Earth from Asteroids and Comets, IAA, 2009 (pages 50 to 67); and Chapter 5, “Mitigation,” of Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies, National Research Council, 2010 (pages 66-88).))

Relative to many of the asteroids or comets in question, even applying an intense burst of energy quickly may not amount to much of an effect.  To use the example provided in the 2010 National Research Council report cited above, if we want to change the position of an asteroid by 2.5 Earth radii (enough to ensure if misses the Earth), this could be done by hitting the target with a kinetic impactor, to either speed it up or slow it down by a tiny amount (only 1 centimeter per second), if that speed change is induced 10 years prior to the feared impact.  The 10 year period is required for the small speed change to culminate in a large enough displacement of the target’s future position.  For certain medium-sized asteroids this is possible with current technologies (assuming we have a few decades of warning time).

If, instead of a kinetic impact, a gravity tractor was used instead, it would also have to begin exerting a small gravitational pull on the asteroid in question years to decades before the impact date (depending on the target’s size), but in this case continuously applying its gravitational potential for the entire time, in order to ensure the asteroid misses the Earth.

As a function of the size of the asteroid in question and the amount of time available to act, different deflection options can be compared together on one chart, showing their effectiveness for different time and size scenarios.  Such comparisons have been done as part of comprehensive reports on planetary defense, such as the following two examples.

These comparisons of mitigation options consistently show that nuclear explosive devices are the most powerful currently available, and, hence, the only option in the cases of short warning times or large objects.  However, returning to the opening thesis of this presentation, this is an expression of an underlying principle.  As will be seen in the following examples, indicating what can be done with new technological developments, we must look to the role of energy-flux density as the determining factor of various mitigation methods.

Hypervelocity Kinetic Impact:

The 1992 Near-Earth Object Interception Workshop, held at Los Alamos National Laboratory, brought together an interesting array of specialists contributing to various aspects of the planetary defense challenge.  Included in the proceedings was a study demonstrating that in certain scenarios, a kinetic impactor can actually match the deflection potential previously only thought achievable with a thermonuclear warhead, but only when utilizing speeds achievable only by a variation of nuclear propulsion.  This hypervelocity kinetic impact was based on the famous Project Orion, a 1960s program to develop a spacecraft that would be propelled by a series of small nuclear bombs, released out the back of the ship and then detonated behind its “pusher-plate,” propelling the spacecraft.  Although a fair amount of design and preparatory testing was done, Orion never got off the ground. ((Despite this, the general concept is still sound, and could even be advanced farther with current technologies.))

This 1992 study ends with a specific scenario in which we would only have a short warning time, and our intercepting spacecraft could only be launched when the asteroid was only 17 hours from impact (at a distance of 1.5 million km).  Comparing an Orion-like propulsion system and a standard chemical propulsion system, the author showed that the nuclear explosive propulsion design would be able to reach the target in less than 1/25^th the time, and at a speed 85 times greater!  As the author concluded, “the exceedingly high relative velocities provide sufficient kinetic energy to deflect these malignant astral bodies without resorting to an explosive warhead, nuclear or otherwise.”  ((“Nuclear Explosive Propelled Interceptor for Deflecting Comets and Asteroids on a Collision Course with Earth,” J. C. Solem, Proceedings of the Near-Earth Object Interception Workshop, Los Alamos National Laboratory, New Mexico, 1992, January 14-16, page 121-130.))

Nuclear-Electric Propulsion:

Currently Russia’s Keldysh Center, Energia, and Rosatom are developing the first ever megawatt-class electric spacecraft, using a small nuclear reactor to generate electricity to power an ion propulsion system.  Despite the low thrust of electric propulsion, ((Nuclear-electric will not be adequate for manned missions.)) the very high specific impulse of this system and the ability for continuous propulsion throughout the mission expands our capability for rendezvous missions, for either mitigation (e.g. gravity tractor) or for science and characterization (determining what the asteroid or comet is made of).  This will be a vast improvement over existing solar-electric propulsion systems, and entering megawatt levels of electricity generation in space will expand the number and power of scientific instruments available to spacecraft and satellites (current systems are measured in the tens of kilowatts). ((“The role of space power in solving prospective problems in the interests of global safety, science and social economic sphere,” 2010, presentation by ?. S. Koroteev, Director of SSC Keldysh Research Centre, Academician of Russian Academy of Sciences.))

Nuclear-Thermal Propulsion:

Part of the 1992 Los Alamos Workshop was a technology assessment, indicating future technologies which could be developed with applications to planetary defense.  Included was a brief analysis of the general benefits of nuclear-thermal propulsion systems, in which a nuclear reactor is used to heat and expel hydrogen as a propellant.  Compared with existing chemical systems, nuclear-thermal propulsion promises either substantially lower launch mass for comparable missions, or quicker intercept speeds. ((Workshop Summary, “Assessment of Current and Future Technologies,” Proceedings of the Near-Earth Object Interception Workshop, Los Alamos National Laboratory, New Mexico, 1992, January 14-16, pages 225-234.))

“Nuclear rockets with hydrogen propellant offer significant performance benefits over chemical rockets. They have much higher specific impulse, on the order of ~1,000 seconds compared to 450 seconds for H₂/O₂ rockets. This higher specific impulse allows nuclear rockets to achieve substantially higher final velocities than chemical rockets, at least twice as great for comparable launch weight.  Alternatively, for comparable final velocities and payload, nuclear rockets can be a factor of three to four lower in launch mass. These performance advantages are of potential benefit for NEO-intercept missions.  For close-in intercepts, high velocity translates into quicker intercepts, reducing the level of risk and amount of delta-V deflection required. For distant intercepts, lower launch mass translates into lower cost. Extensive testing of nuclear engines has been carried out by the U.S. in the NERVA program, and by the former USSR. The basic feasibility of nuclear rockets has been well established. Recently, the SNTP particle bed nuclear rocket program has been disclosed by the U.S. Department of Defense. This program [was] developing a compact nuclear rocket with very high thrust/weight ratio.”

In 2011 a more detailed study examined how nuclear thermal systems can increase our capability to handle worst-case scenarios.  Long-period comets, which can come at us with little warning, and often at higher speeds than asteroids, represent a particularly dangerous threat.  While thermonuclear explosives provide the greatest deflection capability, the propulsion systems available to deploy them still remains a limiting factor.  The 2011 study, “Near-Earth object interception using nuclear-thermal rocket propulsion,” showed that by reducing fuel weight requirements, nuclear-thermal propulsion increases the maximum size that could possibly be dealt with. ((X. L. Zhang, E. Ball, L. Kochmanski, S. D. Howe, “Near-Earth object interception using nuclear thermal rocket propulsion,” Proceedings of the Institution of Mechanical Engineers Part G – Journal of Aerospace Engineering, 2011; 225 (G2 Sp. Iss): 181-193.))

“Comparison of propulsion technologies for this mission shows that NTR [nuclear thermal rocket] outperforms other options substantially. The discussion concludes with an estimate of the comet size (5 km) that could be deflected using NTR propulsion, given current launch capabilities.”

Table 3: Propulsion Comparisons

Source: “Nuclear Explosive Propelled Interceptor for Deflecting Comets and Asteroids on a Collision Course with Earth,” J. C. Solem, Proceedings of the Near-Earth Object Interception Workshop, Los Alamos National Laboratory, New Mexico. 1992, January 14-16, page 121-130.

In Defense of Progress

A variety of different mitigation options have been considered, each with particular benefits and short falls relative to specific scenarios.  Given our current technological capabilities, only a few of these options are currently available, although studies, such as those cited above, do provide an indication of what can be possible with future technological developments.  However, the point here is not to advocate one specific option, but to examine the considerations which cut across various options, and can provide mankind with a broad-based capability to act in the solar system.

As discussed above, kinetic impacts can reach the capabilities of thermonuclear explosives, but only when accelerated with nuclear-explosive propulsion.  The capabilities of electric propulsion for rendezvous missions to characterize and study asteroids or comets, or to utilize a gravitational tractor method to alter their trajectories, can be greatly improved when nuclear-electric is utilized instead of solar-electric.  With nuclear-thermal propulsion for planetary defense, launch mass and intercept times can be reduced, and we can handle larger threats than we could with chemical propulsion systems.  Even the fundamental geometry of our access to the solar system can be revolutionized with the capabilities of nuclear fission and fusion propulsion systems.

Although a more thorough analysis can be done, these considerations show that nuclear power is an invariant in improving our capabilities, and the concept of energy-flux density must be taken as a determining factor in planetary defense.  Our nuclear fission and thermonuclear fusion capabilities in space, as a broad set of technologies, must be pursued to qualitatively transform our time-space access to, and action within, our solar system.  The best path to do this is to adopt a science-driver mission to force the challenge of making these breakthroughs.  For example, developing fusion propulsion systems capable of transporting human beings to and from Mars at a constant acceleration/deceleration of 1-gravity (1-G) could be that challenge.  Achieving this capability for 1-G space travel over the course of a generation or two will provide the technologies to deal with the threats posed to the Earth.  This applies to defense, but also situates defense as a subsumed factor of general scientific and economic advance, in space and on Earth.