PI-Terminal Planetary Defense

Links to papers:

Detailed technical paper:

PI-Terminal Planetary Defense

arXiv archive link:

http://arxiv.org/abs/2110.07559

Scientific American Article:

https://www.scientificamerican.com/article/planetary-defense-is-good-but-is-planetary-offense-better

UCSB Press Release 10-11-21:

https://www.news.ucsb.edu/2021/020432/planetary-defense-pi-sky

 

π

Terminal Planetary Defense

 

PI is a practical and effective method of planetary defense that allows for extremely short mitigation time scales as well as general use for longer warning time cases . The method involves an array of small hypervelocity kinetic penetrators that pulverize and disassemble an asteroid or small comet. This effectively mitigates the threat using the Earth’s atmosphere to dissipate the fragment cloud where the atmosphere acts as a “bullet proof vest” or “body armor”.  The effects of doing so are remarkable in their effectiveness. The key is to de-correlate the resulting acoustic blast waves on the ground and to take into account the optical signature due to fragment “burnup”.  Understanding both effects and and spreading the fragment cloud both spatially and temporally allows for virtually any threat to be mitigated. PI stands for “Pulverize It”.

The system allows a terminal defense solution to planetary defense using existing technologies.  This approach will work in extended time scale interdiction modes where there is a large warning time, as well as in short interdiction time scenarios with intercepts of hours to days before impact. In longer time intercept scenarios, the disassembled asteroid fragments largely miss the Earth. In short intercept scenarios, the asteroid fragments of maximum ~10-meter diameter allow the Earth’s atmosphere to act as a “beam dump” where the fragments either burn up in the atmosphere or air burst, with the primary channel of energy going into spatially and temporally de-correlated shock waves. Compared to other threat reduction scenarios, this approach represents an extremely cost effective, testable, and deployable approach with a logical roadmap of development and testing. Pre-deployment of the system into orbit or a lunar base allows for rapid response on the order of less than a day if needed. The effectiveness of the approach depends on the time to intercept and size of the asteroid, but allows for effective defense against asteroids in the multi-hundred-meter diameter class and could virtually eliminate the threat of mass destruction caused by these threats. The great advantage of this approach is that it allows for terminal defense in the event of short warning times and target distance mitigation where orbital deflection is not feasible.

      As an example, we show that with only a 1m/s internal disruption, a 5 hours prior to impact intercept of a 50m diameter asteroid (~10Mt yield, similar to Tunguska), a 1 day prior to impact intercept of 100m diameter asteroid (~100Mt yield), or a 10 day prior to impact intercept of Apophis (~370m diameter, ~ 4 Gt yield) would mitigate these threats. Mitigation of a 1km diameter threat with a 60-day intercept is also viable. We also show that a 20m diameter asteroid (~0.5Mt, similar to Chelyabinsk) can be mitigated with a 100 second prior to impact intercept with a 10m/s disruption and 1000 second prior to impact with a 1m/s disruption. Zero-time intercept of 20m class objects are possible due to atmospheric dispersion effects. The theoretical details and the analytic and numerical simulations showing the program works is discussed in our technical papers. 

     Such a capability would allow humanity for the first time to take control over its destiny relative to asteroid and comet impacts.

 

 

How does PI work?

PI works by intercepting the parent threat, whether asteroid or comet, by intercepting the parent bolide with an array of penetrating rods, some filled with conventional explosives, early enough to both fracture the parent bolide into small fragments that are typically below 15m diameter. These fragments are spread out into a fragment cloud that then eventually either hit the Earth’s atmosphere or if intercepted early enough the fragments miss the Earth completely. The fragment cloud enters the Earth’s atmosphere at hypersonic speeds (typically around Mach 60 or 60 times the speed of sound)  and like a re-entering spacecraft it experience extreme levels of “ram pressure” and heating due to high speed impacts from the atmosphere, causing breakup due to structural failure of each fragment at altitudes depending on the fragment size (typically around 30-50 km altitude for our system). The resulting cascade effect causes a “detonation” with a large acoustical blast wave and optical pulse generated. The acoustical blast is similar to the “sonic boom” from a re-entering spacecraft or supersonic spacecraft. While the energy of each fragment can be similar to a modern strategic thermonuclear weapon there is no radiation hazard, only a large “light and sound” show. What could have killed millions had no intercept taken place is now reduced to a large number of displays of thunder and lighting like effects. The key is that the acoustic shock waves are de-correlated in time so an observer “hears” a series of small blast wave that rise and fall rapidly on a time scale of order a second and hence what would have been a deadly high pressure blast wave is reduced to a series of much lower pressure blast waves that do not overlap in time in general. There is also a small amount of dust generated from the fragment detonation in the atmosphere. We show that the dust is not significant in terms of causing a “nuclear winter” scenario.

 

Energy Scale Comparison

Comparing energy scale is helpful in understand the basic problem and solution. Consider that the 2013 Chelyabinsk asteroid impacts with a diameter of approximately 20m had the energy of about 40 times that of the Hiroshima nuclear weapon.  We know the blast wave and optical pulse measured in Chelyabinsk is fairly consistent with our calculations as discussed in the PI technical paper.

In PI, by spreading the fragments out spatially and temporally, the acoustic (blast wave) is de-correlated from each fragment due to the slow speed of sound compared to both the bolide speed (~ Mach 60 @ 20 km/s) and compared to the speed of light of the optical pulse (~ one million times faster than the acoustic blast).

Consider the energy of the extreme case of a 1 km impactor compared to the energy of sunlight in one hour on the Earth.

A 1 km bolide with density 2.6 g/cc @ 20 km/s has a KE ~ 3×1020 J

Energy of one hour of sunlight on the Earth ~ 5×1020 J

Comparable but we worry about the 1 km impact, as we should, but not about one hour of sunlight. Yet they have about the same energy on the Earth. Why? Because of the time scale of energy deposition (the power).  What PI does is spread the damage from the bolide in both space and time similar to sunlight as an analogy. Of course they are different but the analogy is useful.

The figures below show the overall acoustic, optical and dust production as well as the de-correlated blast waves seen at two ground observers.

 

 

 

Impact threats

The threat of impacts is real and well recorded. Looking at the Moon you can see the craters formed by impacts on the Moon largely frozen in time due to the lack of a lunar atmosphere and hence the lack of weathering and no significant large scale geological upwelling. The Earth has been similarly bombarded but with a few exceptions the evidence is not as immediately visible due to weathering and geological activity as well as most impacts occur in the oceans as the surface of the Earth is largely covered in water.

The time between impacts is reasonably modeled by a power law of time between impacts and the diameter and impact energy of the threat, From geological records we know there have been a number of mass extinction of life on Earth that appear to be associated with impacts from asteroids and comets. The last mass extinction event was about 65 million years ago at the K-T boundary when the dinosaurs were wiped out. In the last century we have witnessed two major events and hundreds of smaller events. Every day approximately 100 tons of small  debris rains down on the Earth, burning up in the atmosphere with little effect on us but sometimes the effects can be catastrophic.

 

Airburst vs Ground Impacts – For common stony asteroids with diameters less than about 80m the asteroid will break up in the atmosphere and not hit the ground. This is known as an “air burst”. For diameters great than 80m there will be a ground impact which is generally far more devastating locally. Our program fractures the bolides (asteroid or comet) so it never hits the ground intact. Very small fragments may survive at hit the ground but they are generally not a danger as they slow down dramatically due to atmospheric drag.

 

Tunguska – A large event occurred in 1908 in Russia (Siberia – Tunguska event) where a modest sized asteroid or comet fragment estimated to be between 50 and 100m in diameter with and impact energy of between 3 and 30 Mt (megatons TNT) or like that of a large thermonuclear weapon. This event happened in a very remote area that was extremely sparsely populated with the acoustical shock wave knocking down millions of trees over about a 1000 km2 area and was an “air burst event” that did not hit the ground. Had this event occurred over a major modern city it could have killed millions of people. See: Tunguska event – Wikipedia

Chelyabinsk – A recent event that was extremely well recorded was in 2013 on Feb 15 in Chelyabinsk Russia. It had a diameter of approximately 20m and a yield of about 0.5 Mt or like that of a modern ICBM deployed strategic thermonuclear weapon. This was an “air burst event” that did not hit the ground.

This can seen on numerous “dash cams” from the early morning event. Approximately 1600 people were injured from the blast wave and the resulting window and building damage. Had the impact angle been steeper (closer to vertical) and had this occurred over a large city it could have threatened 10’s to 100’s of thousands of people. See: Chelyabinsk meteor – Wikipedia

Luck vs Threat – So far humanity has been spared large scale catastrophe as was visited upon our previous tenants but counting upon being “lucky” is a poor strategy in the longer term.

 

Approximate power law relationship between impact recurrence times and exo-atmospheric impact KE. Power law fits based on fitting data known and extrapolated impacts from [23],[24].

 

Map of recent 873 events greater than 0.073 Kt from April 15, 1988 to Sept 29, 2021 air burst impacts from atmospheric infrasonic sensors. The Feb 15, 2013 Chelyabinsk 500 Kt event is clearly seen over Russia. It is important to note that the energy ranges of many of these events of asteroid strikes are in the ranges of tactical to strategic nuclear weapons. White colored points lack altitude data. Data from Alan B. Chamberlin (JPL/Caltech) https://cneos.jpl.nasa.gov/fireballs/.

 

Exo-atmospheric kinetic energy (KE) vs diameter and atmospheric entry speed.  Nuclear weapon yields and total human nuclear arsenal shown for comparison.

Intercept Time Sales for various Threats

The PI program is remarkable in the extremely short response times needed for mitigation. These times are vastly less than deflection based ideas for planetary defense. We have done thousands of simulations for various threat scenarios and show some results below for mitigations from 15 to 1000m diameter threats. Below 15m diameter for the most common stony asteroids, no action is needed as the Earth’s atmosphere takes care of the “problem” for us. However, as humanity stretches out into the solar system with lunar and Martian bases with no or lesser atmospheres it is important to implement planetary defense on these outposts as well. In the plot below we show mitigation intercept time scale required for 15, 20, 30, 40, 50, 100, 370 (Apophis), 490 (Bennu) and 1000m diameter threats. We assume a nominal stony asteroid density (2.6 g/cc) and internal binding strength (~1-2 MPa). Note that the impact energy of a threat like Apophis which will visit us next on Friday the 13th of April , 2029 and come closer to the Earth’s surface than geosynchronous satellites, has equivalent energy comparable to the entire nuclear arsenal of the Earth.

Intercept time prior to impact, blast wave pressure, and optical pulse at observer and maximum pressure directly underneath all fragments vs bolide diameter. The simulations were done for a speed of 20km/s, density of 2.6g/cc and 45-degree angle of attack for 15, 20, 30, 40, 50 , 100, 370 (Apophis), 500 (Bennu) and 1000m diameters. The disruption speed dispersion is 30% of the mean speed and the density dispersion is 30% of the mean density. The disruption speed has a mean of 1m/s for all bolides except the 15 and 20m diameters, which have a mean disruption speed of 10m/s. The intercept time for the smaller diameters (<40m) is dominated by the dispersion in the burst altitude (zb), while the very large diameters (>300m) are dominated by the finite size of the Earth. In both extremes (small and large), the intercept time is lower than expected analytically. Note that zero time intercepts are possible for bolides less than 30m due to the shock wave arrival time dispersion from the burst altitude dispersion dominating over the lateral dispersion. As mentioned, the intercept time  can always be shortened by making the mean disruption speed (vave) larger, but this increases the disruption energy required. In general, even larger bolides have acceptable intercept times when the cloud diameter exceeds the Earth diameter. For 1m/s disruption, intercept times greater than 75 days are sufficient for bolides even larger than 10km .

Intercept Decision Making

Detection and threat analysis are key to any planetary defense system.  Improvements in this area are crucial. Our decision process for a given threat is summarized in the logic flow diagram below with the metric of threat on the surface of the Earth being set by the threshold to keep the blast wave peak pressure low enough to prevent significant damage and the optical pulse kept low enough to prevent fires. While the peak pressure allowed and the peak optical energy flux are free parameters in our simulation we set a limit that is typically <2 KPa for the peak blast pressure and 0.2 MJ/mfor the sum of all optical pulses. These are shown in the figure above at the bottom for each intercept scenario. These are very conservative assumptions.  Every fragment is tracked in our simulations and the atmospheric absorption for both the acoustic and the optical signature is accounted for. A robust planetary “civil defense” notification system is highly desirable to alert people to take shelter and close their eyes and cover their ears.

 

 

 

Robust Defense Capability

A planetary defense system will need both robust detection and a multi layered mitigation capability analogous to national defense systems. In national defense systems there is almost always a terminal defense mode in case the long range mitigation fails or a surprise short time threat occurs. PI offers both a terminal intercept mode for rapid response or as a last line of defense as well as a long range disruption capability if threat detection is early enough, all within the same general “family of mitigation”.  Having such a general purpose layered threat mitigation strategy is important to assure compete defense capability as well as reducing the number of mitigation methods for cost effective defense.

Proactive Threat Mitigation

Just as we do in many other areas of life, such as public health in dealing with pandemics proactively by taking protection against many diseases, so too can we treat planetary defense in a proactive way IF we wanted to. While not all threats are “repeat offenders” in that not all threats visit us multiple times before striking, we do know that some threats such as Apophis and Bennu do. One option is to eliminate these threats by preempting them. We could eliminate threats such as Apophis and Bennu with PI by intercepting them on a close approach and eliminating them as a future threat. This is both a policy and a technical area to be discussed.

 

International Cooperation in Planetary Defense

Ideally the area of planetary defense would transcend national border as the threat respects no borders. An ideal scenario would be to bring together an international effort to defend our planet for future generations.

 

Conclusion

PI represents a radical new approach to planetary defense. It is practical, testable both in computation, in ground testing of synthetic threats and in orbit. It is a logical and scalable approach that will for the first time allow humanity to take control over its fate.

 

Blast Wave Simulation Movies

Using the physics discussed in the main paper (http://arxiv.org/abs/2110.07559) we have developed blast wave movies that how the time evolution of the acoustic wave as measured by observers on the Earth’s surface. The pressure is shown vs time and spatial position of the observer We assume the observer are on a flat plane for simplicity. A flat plane us a good approximation over several hundred kilometers. We will continue to add additional simulations as we develop them.

1 day prior to impact intercept of 100m diameter parent asteroid  fragmented into 1000 pieces with mean diameters of 10m.

Parent density 2.6 g/cc, 20 km/s speed, 1m/s disruption.

Student Research Team

We thank the many Physics and other department students who have worked on this project. Upcoming papers will feature their work as authors.

Jeeya Khetia (Physics)

Teagan Costa (Physics – graduated 2021)

Hannah Shabtian (Geophysics  with Astronomy and Planetary Sciences minor)

Alexander Korn (CCS Physics with Music minor)

Dharv Patel (Physics with Astronomy minor )

Alok Thakrar (DPHS)

Marlon Munoz (Physics)

Kellan Colburn (Physics – graduated 2021)

Albert Ho (Physics)

 

Related PI References

Lubin, P. “PI – Terminal Planetary Defense”, ASR, 2021

Lubin, P. and Cohen, A., “Asteroid Interception and Disassembly for Terminal Planetary Defense”, ASR, 2021

Lubin, P. and Cohen, A, “Planetary Defense is Good – but is Planetary Offense Better?”, Scientific American, October, 2021

Multiple student papers in progress

Funding

Funding for this program comes from the NASA California Space Grant NASA NNX10AT93H as well as a generous gift from the Emmett and Gladys W. fund.