William J. Cooke
Computer Sciences Corporation
Huntsville, AL 35812
(256) 544-9136
It may seem to many that the procedures used by NASA and the international partners to protect the International Space Station (ISS) and the still-to-be developed scheme for protecting the Earth from an asteroid or comet impact have very little similarity; however, the truth is that they have a great deal in common. Both must deal with protecting lives and property from a trackable subset of the threat population, and both have to develop ways of dealing with the untracked remainder that are capable of doing damage. In the case of ISS, this untracked group consists of particles between 2 cm and 10 cm in diameter - the region between the protection offered by the station's bumper shields and the observable lower limit (95% completeness) of the Space Surveillance Network (SSN). The corresponding group for the Near-Earth Objects (NEOs) consists of asteroids (meteoroids) ranging in size from 1 km (the presently targeted goal of existing surveys) to roughly 50 m (below which the atmosphere will fragment the object) and long period comets, which will give little advance warning in the event of a collision course with Earth.
As the ISS program has evolved from design to an existing vehicle, NASA has codified methods for dealing with an orbital debris threat to the station. The same cannot be said for the NEO program, where the emphasis has been placed on detection and subsequent tracking of the objects, rather than mitigation of the risk (Chapman, Durda, and Gold, 2001). It was the dearth of activity of the mitigation side, particularly in the quasi-political area of determining when a defense should be implemented, that sparked the idea behind this paper. Given the similarities outlined above, and others to be mentioned later, the author feels that it is worthwhile to consider the possibility of applying some of the techniques used to protect ISS from debris to planetary defense.
The current procedure for determining whether or not the International Space Station (or the Space Shuttle, when it is on-orbit) must maneuver out of the way of an incoming piece of debris is based on the intersection of two position error ellipsoids. The first of these describes the uncertainty in the ISS position and is typically small (not much bigger than the ISS itself). The second measures the uncertainty in the position of the debris object at the projected time of ISS encounter; it can be rather large, depending on how well the object's orbit is determined. The degree to which these ellipsoids intersect determines the probability of a collision; if it is above the “red” threshold, ISS or Shuttle must maneuver. If it is between the red threshold and a lower “yellow” threshold, then the maneuver occurs at the discretion of management. Critical to the success of this scheme is the ability of SPACECOM's Space Surveillance Network (SSN) to track the debris object with sufficient accuracy far enough in advance to construct a meaningful error ellipsoid.
While such a scheme based on collision probabilities could easily be done for NEO's, it would prove inadequate as an implementation criterion, as it neglects the size/speed of the asteroid, which determines the amount of damage done should it strike Earth. Any trackable debris striking ISS would result in its destruction; such is not the case for planet Earth. Therefore, it would seem necessary to replace the collision probability with another quantity, one which accounts for both the probability of collision and the damage done by the strike. A natural choice for this would appear to be the Torino Scale devised by Binzel (2000); it does indeed include both the collision probability and potential for damage. However, the scale is too coarse to easily lend itself to establishing a threshold. Clearly, one would want to defend against Torino 10 events, but how far down into the Torino 9 events or how far to the left into Torino 7, 6, or 5 events does this need extend? One could establish a criterion stating that all events of 5 and above on the Torino Scale (i.e., all threatening objects) will necessitate the implementation of an asteroid defense, but would this be adequate?
Figure 1. The Torino Scale.
The author suggests using the Palermo Scale, developed by Chelsey et al. (2002), in which the relative degree of risk is measured by the logarithm of the collision probability divided by the product of the background annual frequency of impact for the kinetic energy of the asteroid and the time to close approach or impact:
|
|
(1) |
where PS is the scale number, Pcollision is the collision probability, E is the kinetic energy (yield) of the asteroid in megatons, and
T is the time to closest approach in years.
Figure 2. Palermo scale number as a function of collision probability and asteroid size, assuming a
T of 10 years.
Using PS or a quantity like it overcomes the coarseness of the Torino Scale, and, most importantly, it takes into account the available time before the potential impact, reflecting in a quantitative manner the urgency of the situation. A high-probability impact centuries in the future would be given a far lower risk number than a less-certain approach circumstance decades away, which makes sense in light of a host of obvious reasons. The choice of years as the units of T can also be easily defended, as populations tend to care significantly about events occurring in a span of time that affects them or their immediate descendants. It is the neglect of this human element, the “attention span” of the average person, that renders the Torino Scale unsuitable for defense thresholding. Though many may deem this factor irrelevant from a scientific point of view, the political realities dictate that it must play a significant role, as the populace would not even pause at the announcement of an impact projected to occur 500 years from the present, but would demand action from governments should the impact be expected in 10 years.
Figure 2 is a sample plot of PS as a function of asteroid size and collision probability, with the time to close approach held constant at 10 years. This plot should look familiar to those involved with safety of the Space Shuttle and ISS, as it is nothing more than the graphical depiction of a risk matrix.
Having defined a quantity suitable for the thresholding, it then becomes necessary to make the techno-political decision as to where to set the lower limit on the activation of an asteroid defense program. The logical starting place is to take the asteroid size at the lower limit of the current surveys (1 km) and decide the maximum acceptable collision probability for which inaction is suitable for this size object over a century. Does the community choose to ignore 1 km asteroids for which the probability of collision is 0.1? 0.25? 0.5?
Perhaps the choice has already been made, or at least constrained, by the decisions of various agencies who have defined levels of acceptable risk. Taking NASA as an example, figure 2 defines (red lines) two risk thresholds. The first of these (labeled `survivable debris') defines the Palermo scale value for a 1 km asteroid with an impact probability of 10-4 - the maximum acceptable level of risk for a human to be struck by a piece of re-entering orbital debris, according to current NASA safety standards. The second gives the PS value for a 1 km asteroid with an impact probability roughly equal to the risk of a catastrophic space shuttle launch failure (~0.01). The author suggests that the appropriate lower limit lies between these bounds, i.e.,
0.83 < PSdefense < 2.83
The establishment of PSdefense is the first - and perhaps most important - step that society needs to take in protecting the planet from extraterrestrial impactors. Without this value, an adequate strategy cannot be constructed, as the range of asteroid/comet sizes to be defended against remains undefined. Lacking this information, an adequate mitigation means cannot be designed and developed.
Another procedure used in ISS collision avoidance also pertains here. When a debris object has been initially determined to pass close enough to the station to force an avoidance maneuver, the SSN is tasked to provide better observational data so that the miss distance can be further refined. The collision probability is recomputed and passed on to management, where the decision to maneuver is made. It is important to note that this decision is now never made more than 40 hours in advance of the expected encounter, as sufficient time must be given to allow for convergence of the collision probability. The application of this scheme to planetary defense argues against the 72 hour (or any other fixed value) “waiting time” currently in use - PS or the like quantity should be allowed to converge before any announcement (or decision) is made. Debates over the “waiting time” should be refocused into debates over the acceptable amount of variation in the risk parameter - once this degree of convergence has been achieved, then action may be taken.
A penetration of a pressurized module of the ISS would result in all but one member of the crew proceeding to the escape vehicles (Soyuz capsules), with the remaining crew person trying to isolate the source of the leak by sealing off the damaged module, provided the depressurization is not too rapid. Once the leak is contained, a repair with a specially designed tool kit may be attempted via EVA. In the event of a rapid depressurization, the entire crew evacuates the station using the escape vehicles.
The first important point here is implicit, being that the tracking network (SSN) is not able to detect all objects capable of penetrating a station pressure wall. The same holds true of the current NEO surveys - “city killers” of the order of a few tens of meters in size are often not detected, or are picked up after close approach to the Earth. In both instances, given a long enough period of time, objects will get through and cause damage. The real difference lies in the second important point, namely, that the inhabitants of ISS are fully cognizant of this risk and that a scheme is in place to help them deal with such an event if it happens. The same is not true of Earth's population - the scientific community downplays the risks associated with small asteroids (Torino 8 objects) by dismissing them as causing only “local damage”, whereas the publicity generated by the search campaigns and media hype (movies like “Armageddon” and “Deep Impact”) have combined to create a complacency on the part of society, which has the mistaken notion that some sort of protection scheme is in place. Experience in space programs has shown that an impact need not destroy the ISS in order to create outrage among the general public and accusations in the halls of Congress - all that is needed is a penetration large enough to put astronaut lives at risk. This is applicable to small asteroid impactors as well, as the loss of a city or small town would be more than sufficient to cause public and governmental damnation of astronomers and others associated with NEO programs.
Those involved with the safety of ISS have repeatedly looked at various means of detecting all objects capable of penetrating the station's armor; none have proved practical or “cost-effective.” Such is not the case with regard to the “city killer” asteroids, as our current technological capabilities are better suited to deal with these objects. A weapon with less than few kilotons of striking power could totally disrupt such a threat, rendering the “deflect or shatter” arguments moot. Of course, the search programs would have to be augmented so that their completeness limit is lowered to lees than 100 meters, the cost of which would be minute compared to the loss of New York City or Phoenix. Even if no defense is established, the search programs need to be able to detect Torino 8 objects far enough in advance to allow for evacuation of the predicted impact zone. It is very important that the NEO community comes to the realization that, in a socio-political sense, any major loss of life and property is not acceptable (especially in the United States, which in times of war seeks to minimize casualties not only to its forces, but those of the enemy as well) and that schemes must be put into place to mitigate damage from all impactors that can cause damage.
Summarizing, in order of precedence:
If areas of commonality between ISS debris protection and asteroid defense are considered, it is clear that the procedures used with the International Space Station to protect life and property for orbital debris can be reformulated, in some cases with little effort, into schemes for defending the planetary population. There is no reason why such schemes cannot be developed now; the author is very much in agreement with Clark Chapman and his colleagues when they state that “procrastination is a serious issue” (Chapman, Durda, and Gold, 2001). It is time to not only look, but also to do.
Binzel, R.P., 2000. The Torino Impact Hazard Scale, Planetary and Space Science, v. 48, p. 297-303.
Chapman, C.R., Durda, D.D., and Gold, R.E., “The Comet/Asteroid Impact Hazard: A Systems Approach”, web document, 2001.
Chesley et al., “Quantifying the risk posed by potential Earth impacts”, Icarus, 2002 (in press), web document
Yeomans, D.K., 2002. Personal communication.
