These ideas are, in themselves, realistic proposals for work to improve planetary defense, but their primary purpose is to illustrate the kinds of efforts that are encouraged by the prize.
Keyholes and other complexities
Predicting a NEO impact and subsequently deflecting that NEO from a pending impact are primary components of planetary defense. Both, however, involve subtle complexities not immediately evident. Many issues involve uncertainties in our knowledge; some involve timing challenges and others the complexity of geopolitical decision-making. One of the most technically challenging is the subtle, but profound effect of multi-body gravitational influences.
Virtually all NEO Earth impacts have been, and will be preceded by prior close approaches of that NEO to the earth. Of particular interest are those prior approaches close enough to pass within the Hill sphere of the earth. The NEO's orbit when in close proximity to Earth (or any massive body) is populated by a series of small regions referred to as keyholes, or gravitational keyholes. If, as it passes close by the earth, the NEO happens to pass through one of these keyholes it's orbit will be slightly altered by the earth's influence such that, on it's next approach to Earth it will impact the planet.
If a NEO is determined to be headed for a keyhole as it approaches the planet, the potential future impact can be prevented by altering its orbit such that it misses, rather than passing through the keyhole. Since keyholes are considerably (generally orders of magnitude) smaller than Earth itself, the deflection deltaV required to avoid a keyhole, and therefore the subsequent impact with Earth, is dramatically smaller than that required several years later when a direct impact deflection would be required.
Keyhole passages, if anticipated with sufficient early warning, are useful in the sense of requiring only a modest deflection effort to avoid them compared with a subsequent direct impact deflection. Conversely, for any direct deflection by self-destructive means (e.g. kinetic impact, nuclear explosion, etc.) the NEO's post deflection path, while now avoiding Earth impact, may now pass through a keyhole as it bypasses Earth, thereby simply postponing an impact until the next Earth approach.
Given that keyholes are relatively small regions of space, assuring that a primary deflection (I.e. deflection from an immediate pending impact) does not result in a post-deflection keyhole passage requires precise knowledge of the NEO's post-deflection orbit. Given that self-destructive deflection techniques generally eliminate the possibility of an associated post-deflection (I.e. in-situ) NEO orbit determination, a precise, alternative means of orbit determination is required.
This challenge, and other considerations of consequence, were recognized and addressed in the Association of Space Explorer's 2008 report to the United Nations. (Asteroid Threats: A Call for Global Response - https://www.space-explorers.org/resources/Pictures/photos/ATACGR.pdf) A key finding of this report was the desirability, if not the necessity, for thinking of a deflection challenge as a "deflection campaign" vs. a "deflection mission". Appendix II of this report (Key concepts in Asteroid Threat Mitigation) is recommended reading.
Risk Corridor Issues — Economic, legal, social, political
[Note: Generally speaking, when thinking about Near Earth Asteroids (NEOs) and potential impacts, one pictures diagrams with circles and ellipses with points representing the sun, planets, and the asteroids themselves. The reality of potential NEO impacts is far more meaningfully communicated by the risk corridor associated with each potential impact. Seeing a risk corridor crossing a map, especially including one's hometown, immediately makes it personal, and in short order, raises a host of important questions, only some of which can be answered with today’s knowledge. Hence we are emphasizing the risk corridor as a primary communication tool to highlight the many issues that require attention for planetary defense to be truly effective when an impact is imminent.]
Each NEO with a non-zero probability of impacting the earth has associated with it a narrow corridor extending across the entire planet. It is within these boundaries that the NEO will land if it is confirmed to be on an impact trajectory. This "risk corridor" is primarily determined by the intersection of the plane of the asteroid with the surface of the earth at the time of nominal impact. In simple terms, if an NEO is actually on an impact trajectory, it will impact somewhere within its risk corridor. Each NEO with a non-zero impact probability has one or more of such risk corridors. Thanks to the technology of today, there are thousands of risk corridors across the planetary face, each of which is associated with a specific potential NEO impact.
The image on the upper left shows the plane of an asteroid intersecting the surface of the earth. To its right is a Mercator projection of the earth's surface showing the sinusoidally shaped risk corridor of this potential impact. Similarly, the third image below shows a Mercator projection of the earth's surface with 101 random (but representative) risk corridors. Note that typically the +/- 3 sigma width of these lines is less than 100 km wide and therefore remains almost invisible at this scale.
Each risk corridor has associated with it all of the characteristics of the associated NEO; size, impact energy, date and time of impact, probability of impact, etc. The characteristic of special interest is that of the probability of impact, which while generally very low, will on rare occasions increase to levels that justify public concern. In the very improbable instance of an actual impact, the impact probability will, with more and more tracking, rise to 100%.
The uncertainty implicit in this situation is due, almost entirely to the imperfect knowledge of the precise position of the asteroid along its orbital path. Most NEOs, and particularly the substantial number of smaller ones, are only rarely seen (and therefore tracked). While soon after discovery many NEOs have an extremely small probability of impact with Earth, subsequent improvement in tracking accuracy over the years almost always shows that the NEO impact will not happen. In other words, the impact probability almost always drops to zero thanks to the improved knowledge of the NEO's precise orbit. Almost always... but not always.
It is, of course, in the occasional instances where the impact probability increases over time that many of the most challenging planetary defense issues arise.
Most fundamental, is whether to deflect the asteroid or instead evacuate the impact zone. How does one decide? Who decides? At what size or impact energy level is an evacuation impractical? How much would a deflection cost compared with an evacuation? Who pays for a deflection? For an evacuation? How much warning time will we have? What would happen to local real estate values as the local impact probability increases? Should a multi-million $ sewage treatment plant be built within a known risk corridor - or should it be placed elsewhere? Is it legal for insurance companies to raise rates for property in a known risk corridor with rising impact probability? Can private insurers legally hire private space operators (think SpaceX, e.g.) to deflect an asteroid if government authorities have decided to evacuate instead? Etc, etc, etc.
These and many other issues are currently poorly defined, studied, and understood. While some are technical in nature (e.g. deflection, impact probability determination), many of the most intractable problems are economic, legal, social, or political.
Low Delta-V Pro-active Options
While most planetary defense technology has been developed to address the prevention of asteroid impacts with Earth, there are some interesting potential uses of low thrust propulsion systems (electric propulsion) that validate the technology but provide other benefits. The next examples illustrate two of these possibilities which would both validate the technology, and provide interesting space exploration results. Both are pro-active opportunities available for exploitation independent of the emergence of an actual impact threat.
NEO capture for ISRU
Asteroids are great potential sources of valuable resources already in space. No need to dig into the earth and pay the high price of lifting them out of the gravity well. However they move about at very high speeds in orbits independent of the earth and are therefore inconvenient and expensive to exploit.
There are, however, unique gravitational environments which, if cleverly capitalized on, may dramatically reduce the cost of exploring these resource bodies. These are the 5 Lagrangian points characteristic of all 3-body gravitational systems. In very specific and precise orbital situations the delta-V required to move between these unique gravitational points is very low.
In extremely unusual natural circumstances (e.g. the instance of Shoemaker-Levy-9) a comet may pass between Lagrangian points to become a planetary satellite and in the instance of SL-9 actually impact the planet.
Low thrust planetary defense systems (e.g. the gravity tractor, or equivalent) may well be able to capitalize on these unusual gravitational circumstances to bring a small NEO into a high Earth orbit. The known inventory of small asteroids is, with the advent of the Vera Rubin telescope, about to expand dramatically, thereby potentially enabling exploitation of this unique gravitational situation to facilitate such a capture. Even a small asteroid in high Earth orbit would be very useful in exploring, at modest cost, in-situ resource utilization (ISRU).
NEO Removal
While planetary defense addresses specific means to mitigate the potential damage to life on Earth by future asteroid impact threats, it generally does not eliminate the impact threat, even from a single asteroid, forever. If a threatening asteroid is large enough to warrant a deflection campaign, it will usually be deflected from a specific predicted impact only. The NEO at issue will remain within the NEO population and, may again present an impact risk over an extended period of time. In some instances, such a return threat may manifest within only a few years from the initial deflection.
In the long run (think 1000+ years) it would be desirable to reduce the NEO population, per se, by permanently removing asteroids from the class, one at a time. For most members of the NEO population, this could be accomplished by raising each NEO's perihelion to a value > 1AU.
Such a concept, when applied directly, would require extremely large and specifically directed momentum changes, a goal that is well beyond our limits even with an optimistic forecast for space technology.
However, by carefully executing a series of small, accurate maneuvers over time, NEOs passing close by Earth could be pushed to accurately approach (but remain outside) gravitational keyholes, such that over time their orbit would be adjusted by Earth's gravity to pass close by Mars, which could in turn, if properly planned and executed, gravitationally raise their perihelion to >1AU.
In other words, small "trim" maneuvers (perhaps via gravity tractor) in combination with resonance opportunities made available by approaching available gravitational keyholes, would enable the use of Earth's gravity to cause a precise nearby approach to Mars, resulting in the maneuvered NEO losing its status of being near Earth.
Given that NEOs with non-zero impact probability will periodically (and frequently) pass by Earth, there will likely be a variety of opportunities for such a campaign on an annual basis. Passing through gravitational keyholes is generally understood to be a common trigger for subsequent NEO impacts. However, passing near a gravitational keyhole, if done precisely, can cause the subsequent gravitational influence of the earth to precisely maneuver the NEO into a desired new trajectory.
Ultimately such a "gravitational trim tab" application should enable reducing, or even eliminating, the NEO population over extended periods of time.
Government vs. Commercial Deflection
In the foreseeable future, it is highly likely that commercial space operators may be capable of providing asteroid deflection services. The logical assumption in this regard is that governments, whether individually or collectively, might contract with private entities for such services when a NEO deflection campaign is determined to be required. Current efforts within the COPUOS (Committee on the Peaceful Uses of Outer Space) work on planetary defense (in particular IAWN, International Asteroid Warning Network, and SMPAG, Space Mission Planning Advisory Group) essentially operate on this assumption.
As the probability of an impact within a specific risk corridor and the public’s understanding of the threat of NEO impacts increase over time, the likely decreasing property values within that risk corridor may well form a collective "private" economic incentive of considerable significance.
It is entirely possible that the overall economic interest of private entities within such a risk corridor may be sufficient to justify the contracting of deflection services independent from any determination by COPUOS or other international governmental structures. This possibility raises many legal questions regarding private vs. public interest.
If a risk corridor does not cross the territory of any major space power but does create sufficient economic losses to contract for a NEO deflection mission by a private space service provider, the tension between "official inaction" and "private initiative" becomes evident. Many issues within this scenario deserve analysis.
Furthermore, given improvements in space technology, the cost of a deflection campaign will be reduced. As a result, the decision of whether to deflect or evacuate when faced with an impact threat becomes a dynamic economic tradeoff. This reality begs further analysis and understanding.
Insurance Implications
Within a few years, due largely to the dramatically increased NEO discovery rate associated with the Vera Rubin space telescope, it is very likely that there will be 5-7,000 risk corridors crisscrossing the surface of the planet. Each of these risk corridors, typically less than 100 km wide (+/- 3 sigma) and with a length equal to the width of the entire planet, is associated with a specific known asteroid holding a non-zero probability of impact occurring within the next 100 years.
While the occurrence of these impact probabilities can range widely from 1 in a few thousand to 1 in a few million years, the vast majority of them are below the size that would warrant deflection (currently those exceeding about 45 meters in diameter). Those with diameters between 20-45 meters, if they were to impact, would release energies of ~ 0.35-5.0 megatons of TNT equivalent.
As each of these asteroids is tracked over time, our knowledge of their orbits increases and the impact probabilities generally decrease. However, for those with a confirmed impact trajectory, the opposite will be the case, and the likelihood of a nuclear weapon-level explosion increases until collision. For asteroids that will eventually come close, but not impact Earth, the impact probability is often seen to increase for some time and then eventually drop to zero when their orbits have been confirmed to not impact Earth.
In either of these two scenarios, the perceived risk to both existing or planned assets within the asteroid’s relevant risk corridor will increase until the risk corridor moves off Earth or shrinks to less than Earth's width with the assets placed outside the reduced impact zone.
However, for future planned assets with expected lifetimes of decades, investors and stakeholders may well benefit from the knowledge of such risk corridors when making decisions regarding the specific geographic options for locating those assets.
Local Impact Evacuation Simulation
For NEOs in the size range of 20-45 meters in diameter, with an impact trajectory, it is possible that they would be detected on their "final approach" by the ATLAS telescope system (or similar). Current technology would provide approximately 2 days warning for a NEO of ~20 meters in diameter, or a week or so of warning for a 45-meter NEO. Since such NEOs are very close and about to impact, their impact zone will be quite accurately determined.
With this level of warning, an evacuation of the projected impact zone is quite possible, provided that such a warning is understood by the public to be legitimate and that appropriate information and instructions are provided. Such information would include the date and time of the projected impact, the anticipated destructive power of the impending impact, and the appropriate evacuation direction, routes, and destinations that are well-specified and selected.
Planning and simulation of a local evacuation are clearly appropriate given that the general public, while having a basic understanding of natural disasters such as a hurricane or tornado, has little to no intuitive knowledge of an asteroid impact. However, unlike the uncertainties of the path of a hurricane or the last-minute nature of a tornado, the information available at the time of an asteroid impact warning would be quite extensive.
Such a low probability natural disaster would likely not justify the need for wide public preparation or involvement. Nevertheless, a local disaster agency may well benefit from a desk-top type simulation in order to become familiar with the many characteristics and considerations associated with an impact scenario. Of particular importance would be the public's confidence in the expertise involved in such an unusual public safety action.
Public Credibility Enhancement of PD Expertise
It is very clear, given the recent experience with the COVID-19 pandemic, that public trust in the relevant experts powerfully influences the public response to the threat at issue. The information provided by experts regarding the relevant threat along with the advice or recommendations offered by them can make a very large impact on public safety. When considering asteroid impacts and the lack of public familiarity with the threat, there are substantial challenges of credibility which can make a huge difference in the lives affected by an asteroid impact.
On the downside, very few people will have any pre-existing knowledge of who is or is not a legitimate expert in regard to a pending asteroid impact. The actual existence of this type of threat could easily be called into question. Furthermore, it is highly likely that there will be pseudo-experts who will seek the public’s attention for themselves or simply be uninformed on the matter.
On the upside, there is little behavioral change that would be asked of people, other than potentially evacuating in the instance of a local impact warning.
Serious thinking and planning regarding public communication will clearly be critical for the success of public safety measures and yet thinking in this regard is currently minimal.