Publicado: 1ero de abril de 2009
Air & Space Power Journal - Español  Primer  Trimestre 2009


Planetary Defense

Near-Earth Object Deflection Strategies

Brent W. Barbee

Background

Meteor Impact
Source: Don Davis

Throughout history our planet has been bombarded by Near-Earth Objects (NEOs), which are asteroids and comets whose orbits around the Sun cause them to pass near Earth. The orbits of these celestial objects gradually change over time, causing some of their orbits to eventually intersect Earth's orbit. An object whose orbit intersects Earth's orbit will collide with Earth, if the timing is right, at the point where the orbital paths intersect. We see evidence of this throughout each year as we witness the wide variety of annual meteor showers caused by Earth passing through debris left in the wake of comets orbiting the Sun.

As our ability to detect NEOs has improved we have discovered more and more of them in our celestial neighborhood. Earth's orbital region around the Sun is densely populated with NEOs, as shown in fig. 1, and it is only the vastness of space and the comparatively tiny sizes of celestial bodies that makes collisions infrequent.

While collisions are infrequent, they are also inevitable. Our Moon's surface is covered in impact craters and many craters have been discovered and are still being discovered on Earth. The Moon's surface does not experience weathering due to meteorological and geological processes and so the craters are preserved and easy to see. By contrast, Earth is a very meteorologically and geologically active world so the signs of impact craters are often masked over time. Nevertheless, some terrestrial craters are quite obvious, such as the Barringer Crater, shown in fig. 2, located near Winslow Arizona. The crater is 1200 m wide and 170 m deep. It was created approximately 50,000 years ago by a nickel-iron NEO only about 50 m in size whose impact energy was between 20 and 40 Mt, devastating an area with a radius of 10 to 24 km and creating hurricane-force winds out to a radius of 40 km .2

Known NEOs as of August, 2007

Figure 1. Known NEOs as of August, 20071

NEO impact events range in consequence from local devastation to extinction-level events. In 1908 a relatively small NEO (perhaps 20 meters in size) exploded over the Tunguska river in Siberia, raining destruction over a 2000 square kilometer area4 (about the size of Washington, DC). Approximately 65 million years ago a relatively large NEO, about 10 km in size, slammed with terrible destructive force into the Yucatan peninsula and is believed to have caused the extinction of more than 70% of the species living at the time, including the dinosaurs .5

More recently, in October of 2008 we were able to just barely predict the collision with Earth of a very small NEO named 2008 TC3 a mere six hours before it entered our atmosphere and disintegrated at high altitude over Sudan. 6 The asteroid disintegrated rather than striking the ground because it was only about 5 meters in size.

El Cráter del Meteorito Barringer

Figura 2. El Cráter del Meteorito Barringer3

Perhaps the most unique aspect of these natural disasters is that for the first time in known history humanity may have the technology to anticipate and prevent them by discovering and deflecting incoming NEOs before they collide with Earth. However, to date no NEO deflection systems have been built or tested and no agency has been given the responsibility of defending Earth from hazardous NEOs.

The Role of Uncertainty in NEO Defense Strategies

If it were possible to accurately know the orbits of all the NEOs in the solar system, we would know exactly when the next impacting NEO will arrive and could plan a scientific reconnaissance mission and follow-up deflection mission accordingly. Moreover, if we were able to find and track every single NEO in the solar system, we could say definitively which ones will strike us and when.

Unfortunately, we are faced with a rather difficult partial information problem. We don't know how many NEOs are in the total population and for those that we have discovered we only know their orbits approximately. The reasons for this are that there are no exact solutions to the equations of orbital motion (we approximate the solutions using computers), we do not have exact models for all the forces (e.g., solar radiation pressure) acting on NEOs, our observations of the NEOs from ground-based observatories are imperfect (containing measurement noise and bias), and we cannot track NEOs continuously from the ground due to the relative geometry between the NEOs, the Earth, and the Sun. The result of all this is that we estimate the current and future shape, size, and orientation of an ellipsoid around where we think the NEO is, and if that ellipsoid ever intersects Earth we can calculate the probability of that NEO colliding with our planet.

The best way to collapse that ellipsoid is to send a spacecraft out to rendezvous with the NEO and closely follow it along its orbit, taking relative position measurements while carrying a transponder itself. The reconnaissance spacecraft beams its measurements back to Earth and allows us to know the NEO's orbit much more accurately. Generally the increase in accuracy shrinks the ellipsoid enough to either rule out a collision with Earth or raise the probability to a level that is alarming enough to motivate us to take action.

However, spacecraft missions to NEOs are quite costly, even if the sole goal of the mission is to refine our knowledge of the NEO's orbit. To date full science missions to asteroids and comets have generally fallen within the Discovery-Class mission cost cap of NASA. However, the cost of a scaled-down NEO mission can be small relative to routine scientific spacecraft missions. For instance, the design team that won the Planetary Society competition for designing a reconnaissance mission to the asteroid Apophis estimated the cost of their mission in 2007 dollars to be $81.59 M , compared to previous full science missions to asteroid/comets that ranged between $100 M and $440 M.7

It is interesting that we have already spent more than $81.59 M on movies about fictitious responses to NEOs that threaten Earth. The movie Deep Impact (May 1998) had a production budget of $75 M and had a worldwide box office total of $348 M (in 2005 dollars).8 The movie Armageddon (July 1998) had a production budget of $140 M and had a worldwide box office total of $554 M (in 2005 dollars).9

We have known for several years now that the asteroid Apophis will closely approach our planet on Friday, April 13th, 2029 and if it passes through a gravitational keyhole during that close approach it will return to strike us in 2036. The currently calculated probability for Apophis colliding with us in 2036 is 1/45000, however the explanation for the impact probability value includes a caveat:10

The probability computation is complex and depends on a number of assumptions that are difficult to verify. For these reasons the stated probability can easily be inaccurate by a factor of a few, and occasionally by a factor of ten or more.

The close approach of Apophis on April 13th, 2029 will be historic due to how close it will come to us and its size. It will pass closer to Earth than our geosynchronous satellites, at an altitude of approximately 32,000 km, and its size is currently estimated at about 270 m. It will be visible to the naked eye in some parts of the world during the close approach.

We currently cannot track Apophis from the ground and will not be able to again until the 2012-2013 time frame, which also happens to be the next available launch window to rendezvous with Apophis; the next launch window after that will not arrive until the 2021 time frame. Rather than taking the opportunity to rendezvous with Apophis (in which case the mission planning would need to start very soon), so far we are waiting until we can see Apophis again and hoping that additional ground observations will rule out the 2036 collision. What will happen if a probability of impact remains after the additional observations is unclear. Apophis could deliver approximately 500 Mt of energy10 if it hits us and there is no question that would want to deflect it, but every rendezvous opportunity we miss makes it more difficult to deflect. It is interesting to note that there is another NEO out there that we cannot currently observe named VK184 that currently has an Earth collision probability of 1/3030 in the year 2048 and would impact with 150 Mt of energy.11

It is generally accepted that statistics and probability theory is the best way to handle partial information problems. Gamblers and insurance companies employ it extensively. However, one of the underlying premises is that it is acceptable to be wrong sometimes. If a gambler makes a bad play, the hope is that the gambler has made more good plays than bad ones and still comes out ahead. This however is not applicable to planetary defense against NEOs. Being wrong just once may prove fatal to millions of people or to our entire species. If we trust our statistical estimates of the NEO population and our perceived collision probabilities too much, we risk horrific damage or even extinction. This is how we must define the limit for how useful probability theory is in the decision-making process for defense against NEOs.

Spacecraft Mission Heritage

Some proposed NEO deflection strategies require significant enabling technologies while others are feasible with current or near-term technologies. However, all NEO deflection strategies rely on proven spacecraft mission hardware such as launch vehicles and the wide variety of fundamental spacecraft subsystems including propulsion (rendezvous thrusters), communications, command and data handling, navigation sensors, guidance and control systems, and scientific sensors.

Although no deflection systems have ever been tested, the foundations are being laid through the successful science missions sent to asteroids and comets over the years. The NEAR mission (1996-2001) performed a flyby of the asteroid Mathilde, rendezvoused with and orbited the asteroid Eros, and culminated with a soft landing on Eros.12 Deep Space 1 was launched in 1998 and performed flybys of the asteroid Braille and the comet Borrelly.13 The Stardust mission was launched in 1999, investigated comet Wild 2 and its coma, and returned samples of the coma material in 2006.14 The Hayabusa/MUSES-C mission was launched in 2003, rendezvoused with the asteroid Itokawa in 2005,15 possibly collected samples, and will attempt to return the possibly collected samples in 2010.16 The Deep Impact mission was launched in 2005 and delivered a small impactor to comet Tempel 1 in the same year; the impactor created a crater on the comet, producing ejecta, shown in fig. 3,17 for the main spacecraft to study during a flyby.18

Collision of impactor with Comet Tempel 1

Figure 3. Collision of impactor with Comet Tempel 1 during the Deep Impact mission.17

However, the kinetic energy of the impactor was not intended to be sufficient to measurably alter the velocity (or orbit) of the comet,19 which would have constituted a deflection and the first demonstration of the kinetic impactor concept for NEO deflection. The Dawn mission was launched in 2007 and is currently on its way to study Vesta (the second largest main belt asteroid) and will then proceed to rendezvous with and study the dwarf planet Ceres (also located in the main asteroid belt), completing its mission in 2015.20

These missions have demonstrated that we have the capability to launch spacecraft from Earth, guide them to rendezvous with NEOs, operate them and their subsystems while in the vicinity of a NEO for an extended period of time, and collect important data about the physical properties of a NEO. This constitutes virtually everything that goes into a deflection mission except for the deployment of the deflection system itself. Although these capabilities have been successfully demonstrated at a basic level and in a general sense, they will have to be tailored and scaled depending on particular deflection system requirements. There are also a variety of enabling technologies required for some deflection strategies that have not yet been demonstrated, such as attaching equipment to a NEO.

Near-Earth Object Deflection Strategies

There are several possible approaches to preventing an incoming NEO from colliding with Earth. If sufficiently energetic systems were available, the NEO could be annihilated (perhaps vaporized or pulverized). Alternatively, the NEO could be fragmented into small pieces that are then dispersed. However, it would be difficult to ensure that all of the pieces would be small enough to burn up in Earth's atmosphere should any still accidentally collide with Earth, and fragmentation schemes designed to be sufficiently controllable have been found to require many launches due to the high overall mass of the fragmentation system.21 Another option is to deflect the NEO, which is feasible in a variety of scenarios using current technology.

The goal of a NEO deflection is to employ some mechanism to change the NEO's velocity, placing the NEO onto a different orbit that will not collide with Earth. Fig. 4 illustrates this concept for an impulsive velocity change. The displacement between the NEO's undeflected and deflected trajectories is exaggerated in fig. 4 for illustration purposes. The velocity change is virtually instantaneous in an impulsive deflection while in a low-thrust deflection the velocity change is very gradually applied over a long period of time.

NEO's velocity

 

Figure 4. A small change in the NEO's velocity places it onto a new trajectory.

While fig. 4 shows the velocity change in an arbitrary direction for illustration purposes, studies have shown that the best direction for the velocity change is nearly aligned with the NEO's velocity direction, as shown in fig. 5. This causes a maximum cumulative difference between the NEO's post-deflection position and where the NEO would have been if no deflection were performed.

NEO's velocity change
 

Figure 5. The optimal direction for the velocity change is generally near the NEO's velocity direction.

Another important consideration is when to apply the deflection to the NEO. Generally it is best to apply the deflection as far in advance of the predicted Earth impact date as possible, with the constraint that deflections are most efficient if applied when the NEO is at the point in its orbit that is closest to the Sun. This point is referred to as perihelion and a given velocity change magnitude has maximum effect if applied at the NEO's perihelion due to orbit dynamics. Fig. 6 illustrates this concept by showing computer analysis results22 for a hypothetical deflection of 1 cm/s applied to the asteroid Apophis at various points in time. The blue data markers correspond to deflections being applied at perihelion while the red markers correspond to deflections applied at a different point in the NEO's orbit. Perihelion deflections clearly outperform non-perihelion deflections by a substantial margin, though the margin shrinks as the time between deflection and Earth impact shrinks.

Deflections applied in simulation to the asteroid Apophis

Figure 6. Deflections applied in simulation to the asteroid Apophis. Each deflection is in the optimal direction (mostly along the asteroid's velocity direction) but the blue markers correspond to the deflection being applied at the asteroid's perihelion while the red markers correspond to the deflection being applied elsewhere along the asteroid's orbit.

In summary, the best way to deflect an incoming NEO is to apply the velocity change as far in advance of the predicted Earth impact time as possible, with the constraint that deflecting at the NEO's perihelion maximizes performance, and to align the applied velocity change with the NEO's velocity direction. The magnitude of the velocity change depends on the particular deflection mechanism chosen; the 1 cm/s velocity change presented in fig. 6 as an example is a representative value.

Apart from the basic details of how the deflection is performed, the general strategy is largely dictated by logistics. The sequence of events comprising a hazardous NEO scenario is shown in fig. 7.

Hazardous NEO scenario timeline

Figure 7. Hazardous NEO scenario timeline.

The NEO is initially discovered and then additional time is required to determine that the probability of Earth collision is large enough to require action. Once that determination is made a scientific survey mission to the NEO should be sent, if time permits, to more accurately determine the NEO's orbit and to gain a better understanding of the NEO's physical properties such as size, shape, mass distribution, spin state, surface composition, and internal structure. These data allow the best deflection methodology to be selected and the specific deflection system to be appropriately designed and sized for the particular NEO at hand. The deflection spacecraft is then built and launched to rendezvous with the NEO and the deflection is carried out subsequent to rendezvous. Note that some deflection techniques, such as the kinetic impactor, require interception of the NEO rather than rendezvous, meaning that they simply target and collide with the NEO rather than perform a maneuver to match the NEO's orbit near the NEO. If the deflection spacecraft performs a rendezvous with the NEO, some time may be allowed to elapse before the deflection system is activated, e.g., waiting until the NEO reaches perihelion if the deflection spacecraft's best opportunity to rendezvous with the NEO (dictated by orbit dynamics) has it arriving at the NEO before the NEO reaches perihelion. An observer spacecraft should be incorporated into the mission so that the result of the deflection action can be verified.

Impulsive Deflection Techniques

Impulsive deflection techniques impart all of the required velocity change to the NEO virtually instantaneously. As intuition suggests, these techniques are necessarily explosive in nature. One possibility is to use a nuclear explosive. Nuclear explosives posses tremendous energy density, which is an important consideration when seeking to minimize the required spacecraft launch mass. There are several possible modes in which a nuclear explosive might be employed to deflect a NEO. In a standoff nuclear detonation, the nuclear device is positioned near the surface of the NEO and then detonated, as seen in the artistic conception presented in fig. 8.23

Artistic conception of a standoff nuclear detonation for NEO deflection
Figure 8. Artistic conception of a standoff nuclear detonation for NEO deflection.23

The radiation generated by the nuclear explosion immediately vaporizes a thin layer of NEO surface material, which then blows off and imparts thrust to the NEO. As mentioned previously, the direction of the imparted velocity change due to the thrust is important, so the nuclear device must be appropriately positioned relative to the NEO before detonation. The associated geometry is shown in fig. 9.

Various modeling efforts have indicated that the magnitude of the imparted velocity change is sensitive to the height of the explosion above the NEO's surface so care must be taken to place the nuclear explosive at the proper height above the NEO. It is possible that detonating the nuclear explosive on the NEO's surface or burying it beneath the NEO's surface before detonation could yield favorable results in terms of deflection magnitude, but these modes of application may increase the risk of accidentally fragmenting the NEO in an uncontrolled manner. Conventional explosives might also be used on the NEO's surface or beneath it, but their energy density is orders of magnitude lower than that of nuclear explosives and this might cause the required spacecraft mass to be too high.

The main advantages of a standoff nuclear detonation are that we have experience positioning spacecraft near a NEO, the spin state of the NEO is not a factor, no contact with the NEO's surface is required, the ultra-high energy density of nuclear devices allows them to handle large NEOs or handle small to medium size NEOs with less lead time than other deflection methods, and the concept of operations for the mission is very simple: rendezvous with the NEO, position the nuclear device, and detonate. Fewer “moving parts” in the mission design greatly improves the probability of mission success. The disadvantages are that this deflection method has never been tested and it carries obvious political and social stigmas since it involves the use of nuclear explosives.

Imparted velocity change
 

Figure 9. The location of the nuclear explosive relative to the NEO determines the direction of the imparted
velocity change.

It is possible to reduce the required spacecraft mass for the mission by eliminating the fuel required for rendezvous with the NEO if a flyby of the NEO is performed instead and the nuclear device is detonated at the correct instant using a proximity fuse. However, the extremely high closing velocities dictated by orbit dynamics and the sensitivity of the outcome to the height of the nuclear explosive above the NEO's surface at the time of detonation might make a flyby detonation too risky.

The NASA study report to Congress in 2007 that provided an analysis of alternatives for deflecting NEOs suggested that standoff nuclear detonations are 10–100 times more effective than non-nuclear alternatives,24 though this issue is still being analyzed.

Another option for impulsive deflection is the kinetic impactor. Similar to the Deep Impact mission, a spacecraft is guided to a high velocity collision with the NEO. The kinetic energy of the impactor changes the momentum and hence velocity of the NEO, achieving the deflection. Kinetic impactors have the advantage of not requiring any sort of explosive warhead, but their performance is limited by the amount of mass that our launch vehicles can handle, making kinetic impactors best suited to small or medium size NEOs with generally at least a decade of lead time.

Low-Thrust Deflection Techniques

Low-thrust deflection techniques change the velocity of the NEO by a small amount over a long period of time. The low-thrust nature of these techniques makes them best suited to small NEOs with long lead times, preferably on the order of decades. The main advantage of these techniques is that they are more controllable than impulsive techniques since the action is more gentle and can generally be modified as sensor feedback is processed.

An excellent example of this technique is the Gravity Tractor (GT). The GT is essentially a spacecraft propelled by a low-thrust engine that rendezvouses with the NEO and maintains position near the NEO for a long period of time. The gravity of the GT spacecraft, due to its mass, exerts a tiny but persistent pull on the NEO, gradually changing the NEO's velocity and hence its orbit. Of course, the NEO's much, much larger mass is pulling much, much harder on the GT and so the GT must use its low-thrust engines continuously to maintain its distance from the NEO. Studies have also suggested using the GT as a follow-on to a kinetic impactor when there is not enough time available for the GT to achieve the deflection on its own.25 The chief enabling technology required is an efficient, reliable, and powerful spacecraft thruster that can create as much thrust as possible while using comparatively little fuel, allowing very long operation times. Progress has been made in this direction in the form of low-thrust ion engines already demonstrated in previous spacecraft missions such as Deep Space 113 and current missions such as Dawn.20

Other proposed low-thrust NEO deflection techniques include attaching thrusters to the NEO, sending a large solar concentrator mirror to rendezvous with the NEO and focus a beam of sunlight on the NEO's surface to create a small jet of vaporized material, attaching a mass driver to the NEO that collects NEO material and propels it away to change the NEO's momentum, sending a laser beam emitter to rendezvous with the NEO and fire laser pulses at the NEO to vaporize material and thereby impart thrust to the NEO, and even painting the surface of the NEO a different color in order to change the amount of solar radiation the NEO absorbs and thereby alter the natural forces acting upon it in order to alter its orbit. All of these techniques have some merit but they all also currently require substantial enabling technologies and all but the GT and the painting technique have to deal with the fact that NEOs are generally rotating, which causes the imparted thrust vector to not always be aligned with the desired thrust direction (along the NEO's velocity direction). The attached thrusters and mass driver also require attachment to the NEO's surface, which is extremely challenging and has not yet been attempted. The pulsed laser would require a tremendous power supply, likely a nuclear reactor. The amount of paint required to cover a sufficient portion of the NEO's surface would likely constitute a considerable amount of mass to be launched from Earth, and the paint deployment method is unclear. One study suggests that the vaporized material jet created by a solar concentrator mirror would foul the mirror optics within a few minutes.26 Nevertheless, all of these proposed techniques bear further study.

The Near-Earth Object Threat and Our National Security

A variety of observatories around the world scan the skies to provide observations (when available) of known NEOs and to discover NEOs we have not seen yet. The Jet Propulsion Laboratories (JPL) here in the US and The University of Pisa in Italy both use observation data to determine the orbits (and associated uncertainties) of all known NEOs and assess Earth collision probabilities. However, no agency anywhere in the world is currently responsible for carrying out the deflection of an incoming NEO should it be required.

The development of a global political and organizational framework for responding to a threatening NEO through the United Nations (UN) will be discussed at a UN meeting in February 2009.27 The Association of Space Explorers (ASE) has written a report28 outlining their conclusions regarding the scope of the NEO threat and issues relevant to threat response in terms of organization, politics, and legalities. Their fundamental concern is that an effective response framework be established prior to needing it during an emergency. The wisdom of being properly prepared to act before being required to act is certainly indisputable, as is the fact that NEO impact is generally a global problem that warrants an international response.

But not all NEOs are created equal. In fact, the majority of the known and predicted NEO population consists of NEOs several hundred meters in mean diameter or smaller. This size class of NEO is capable of causing devastation on local or regional levels, whereas impacts by NEOs one kilometer or larger in size would have global effects and impacts by NEOs several kilometers in size or larger constitute extinction-level events. This raises of the possibility of one nation being targeted by an incoming NEO that is small enough to localize the devastation to that particular nation or small region of that nation. If the US ever finds itself in such a situation, it does not want to be dependent on other nations to defend US citizens from the incoming NEO. If a large NEO is discovered to be on a course to impact any part of the world and the NEO is large enough to cause adverse effects on a global scale, the US does not want to be at the mercy of the decisions and capabilities of other nations when it comes to deflecting such a NEO.

The NASA Authorization Act of 200529 included that NASA has responsibility for detecting, tracking, cataloging, and characterizing NEOs in order to provide warning and mitigation of the threat they pose to Earth. NASA was also directed to discover 90 percent of the NEO population consisting of NEOs 140 meters in size and larger by the end of 2020. However, no specific funding has been provided for these tasks and NASA is not officially responsible for defending the US from NEO impacts. Nevertheless, the key to our response strategy is to find incoming NEOs as early as possible. This requires that discovery and accurate threat assessment of NEOs happens far enough in advance of the predicted times of Earth impact for us to take effective action.

A small community of scientists and engineers interested in the NEO deflection problem has steadily grown in recent years. In April of 2008 the Asteroid Deflection Research Center (ADRC) was founded at Iowa State University with the goal of creating an interdisciplinary research program aimed at developing both impulsive and low-thrust NEO deflection techniques.30

While interest is increasing and there is a growing body of technical work created largely by small independent groups of researchers, most of the work is either unfunded or has only a small amount of funding behind it. Thus a possible threat to our national security is that other spacefaring nations will develop NEO deflection technologies before the US or that their deflection technologies will be superior to ours due to either a head start in development or their allocation of more resources.

The collisions of NEOs with Earth ought to be viewed and treated as a natural disaster that poses a legitimate threat to US national security, and one that can be prevented with appropriate technology development and preparation. However, it is important to note that the required technology development and deployment itself has national security implications, particularly if nuclear explosives are involved.

It is difficult to speculate as to the costs of a NEO defense program, but the costs to date of NASA missions to NEOs can serve as a starting point. The main cost driver is the need to design, launch, and test deflection systems on harmless NEOs in order to verify and calibrate our associated physics models, ensuring that we can reliably alter the orbits of NEOs in a predictable, controlled, and timely manner. It is important to note that the technologies and expertise developed along the way would be widely applicable to the spectrum of space technology areas and the data gathered would be intrinsically scientifically valuable in its own right. This is highly encouraging since we already spend hundreds of millions of dollars on purely scientific missions to NEOs. The logical next step is to incorporate deflection system testing into NEO science missions.

Conclusion

It is important to note that NEO deflection system test missions will also be NEO science missions. Conducting NEO science operations is inherent to the process of deploying and monitoring any NEO deflection system. Thus there is tremendous natural synergy between NEO deflection system test objectives and the usual NEO science mission objectives. Since NEO science missions are funded and deployed on a regular basis, it would be quite sensible to begin conducting combination NEO science and NEO deflection system test missions. This will make the best use of available spacecraft mission funds by accomplishing two important objectives with each spacecraft mission.

While NEOs are scientifically interesting in terms of understanding the origins of our solar system, it is their tremendous, even cataclysmic, destructive potential that merits our close attention and that ought to motivate us to develop and evolve the ability to stop them from colliding with Earth while we grow as a spacefaring species. That catastrophic collisions of NEOs with Earth are very infrequent is quite fortunate for us, but we do ourselves a disservice if we allow that to lull us into a false sense of security. For all we know, the next large asteroid to strike Earth could be discovered tomorrow and we would have relatively little time to mount an effective defense.

We find ourselves at a unique point in our evolution as a species. For the first time in known history we have the knowledge and technology to conceivably prevent a particular type of natural disaster that could cause us grievous harm or even make us extinct. While that is a tremendous milestone for a species, it is up to us to capitalize on the opportunity in a manner that enhances our national security now and for the generations to come.

Disclaimer

The conclusions and opinions presented in this document are solely those of the author and do necessarily reflect the official position of the U.S. Government, the Department of Defense, the National Aeronautics and Space Administration (NASA), Emergent Space Technologies, Inc., or any other institution or organization.

 Notes

1. “Inner Solar System Maps,” Armagh Observatory, http://www.arm.ac.uk/neos/ (http://www.arm.ac.uk/neos/media/2007.gif) (accessed December 7th, 2008).

2. “Barringer Meteor Crater * Meteorites Craters and Impacts,” http://barringercrater.com/news/ (accessed December 7th, 2008).

3. http://www.solarviews.com/raw/earth/meteor.jpg (accessed December 7th, 2008).

4. “BBC NEWS | Science/Nature | Team makes Tunguska crater claim,” http://news.bbc.co.uk/2/hi/science/nature/6239334.stm (accessed December 7th, 2008).

5. “A 'Smoking Gun' for Dinosaur Extinction,” NASA Jet Propulsion Laboratory – News, http://www.jpl.nasa.gov/news/features.cfm?feature=8 (accessed December 7th, 2008).

6. “Asteroid hits Earth ... Good news: Scientists predicted it. Bad News: Only six hours before it burst into our atmosphere,” http://www.dailymail.co.uk/sciencetech/article-1071393/Asteroid-hits-Earth--Good-news-Scientists-predicted-Bad-News-Only-hours-burst-atmosphere.html (accessed December 8th, 2008).

7. Schaffer, Mark G., Koening, Jesse, Charania, A.C., Olds, John R., et al, “Foresight: A Radio Tagging Mission to Near Earth Asteroid Apophis,” Planetary Society 2007 Apophis Mission Design Competition, August 31st, 2007.

8. “Lee's Movie Info – Deep Impact Box Office,” http://www.leesmovieinfo.net/title2.php?t=324 (accessed December 8th, 2008).

9. “Lee's Movie Info – Armageddon Box Office,” http://www.leesmovieinfo.net/title2.php?t=120 (accessed December 8th, 2008).

10. “99942 Apophis (2004 MN4) Impact Risk,” Near-Earth Object Program, http://neo.jpl.nasa.gov/risk/a99942.html (accessed December 7th, 2008).

11. “2007 VK184 Impact Risk,” Near-Earth Object Program, http://neo.jpl.nasa.gov/risk/2007vk184.html (accessed December 8th, 2008).

12. “NEAR Information,” Lunar and Planetary Science at the NSSDC (National Space Science Data Center), http://nssdc.gsfc.nasa.gov/planetary/near.html (accessed December 7th, 2008).

13. “Solar System Exploration: Missions: By Target: Comets: Past: DS1,” http://solarsystem.nasa.gov/missions/profile.cfm?MCode=DS1 (accessed December 7th, 2008).

14. “Solar System Exploration: Missions: By Target: Comets: Past: Stardust,” http://solarsystem.nasa.gov/missions/profile.cfm?MCode=STARDUST (accessed December 7th, 2008).

15. “ISAS | Asteroid Exploration HAYABUSA (MUSES-C) / Missions,” http://www.isas.jaxa.jp/e/enterp/missions/hayabusa/index.shtml (accessed December 7th, 2008).

16. “ISAS | Asteroid Explorer Information about HAYABUSA / Asteroid Explorer HAYABUSA,” http://www.isas.jaxa.jp/e/enterp/missions/hayabusa/today.shtml (accessed December 7th, 2008).

17. http://discovery.nasa.gov/images/67_secs_after_impact.jpg (accessed December 7th, 2008).

18. “Solar System Exploration: Missions: By Target: Comets: Present: Deep Impact,” http://solarsystem.nasa.gov/missions/profile.cfm?MCode=DeepImpact (accessed December 7th, 2008).

19. “Court Rejects Russian Astrologer's Lawsuit Against NASA – NEWS – MOSNEWS.COM,” http://web.archive.org/web/20070521082005/http://www.mosnews.com/news/2005/11/08/marinabai.shtml (accessed December 7th, 2008).

20. “Solar System Exploration: Missions: By Target: Asteroids: Present: Dawn,” http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Dawn (accessed December 7th, 2008).

21. Barbee, B., Gertsch, L., and Fowler, W., “Spacecraft Mission Design for the Destruction of Hazardous Near-Earth Objects (NEOs) via Distributed-Energy Explosives,” Planetary Defense Conference, Washington DC, 2007.

22. Barbee, B., Fowler, W., “Spacecraft Mission Design for the Optimal Impulsive Deflection of Hazardous Near-Earth Objects (NEOs) using Nuclear Explosive Technology,” Planetary Defense Conference, Washington DC, 2007.

23. Wilkins, Peter A. Computer rendering of asteroid model (November 2005).

24. “Near-Earth Objects Survey and Deflection Analysis of Alternatives,” National Aeronautics and Space Administration, NASA Authorization Act of 2005, 2007.

25. “'Gravity tractor' could deflect asteroids – space – 28 July 2008 – New Scientist,” http://www.newscientist.com/article/dn14414 (accessed December 8th, 2008).

26. Kahle, Ralph, et al, “Physical limits of solar collectors in deflecting Earth-threatening asteroids,” Aerospace Science and Technology, 256-263, Elsevier, February 2nd, 2006.

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Contributor

Mr. Brent William Barbee Mr. Brent William Barbee (BS, Aerospace Engineering degree from UT Austin; MS in Engineering from the Department of Aerospace Engineering and Engineering Mechanics at the University of Texas, Austin specializing in Astrodynamics and Spacecraft Mission Design), is currently working as an Aerospace Engineer and Planetary Defense Scientist with the Emergent Space Technologies company in Greenbelt, Maryland. He also teaches graduate Astrodynamics in the Department of Aerospace Engineering at The University of Maryland, College Park. Mr. Barbee's research interests include deflecting hazardous Near-Earth Objects, spacecraft mission design, spacecraft rendezvous and proximity operations, spacecraft trajectory design, and spacecraft simulation and modeling.

Disclaimer : The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.


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