PI Planetary Asteroid Defense

Philip Lubin has been funded by NASA NIAC to study a practical and effective method of planetary asteroid defense that allows for extremely short mitigation time scales if required. 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. The proposed system allows a practical, low-cost terminal defense solution to planetary defense using existing technologies.

The hypervelocity penetrators use the kinetic energy of the asteroid to help break it apart. Fragmentation into sizes of less than about 10 meter diameter is sufficient for most threatening objects. The Earth’s atmosphere to convert the parent object kinetic energy primarily into acoustical waves and secondarily into an optical signature, both of which have acceptable fluxes. Given the extreme impact speeds,
passive penetrators carry much more energy per mass than chemical explosives.

Above – Graphic depiction of PI – Terminal Defense for Humanity
Credits: Philip Lubin

Arxiv – Terminal Planetary Defense

In 2013, the 45 meter diameter asteroid 2012 DA14 approached to within 27,743km of Earth’s surface— inside the orbit of geosynchronous satellites. If DA14 were to strike Earth, it would deliver approximately 7.2 Million tons TNT. Although the Chelyabinsk meteorite and DA14 arrived at or near Earth on the same day, the two objects were not linked to each other, coming from completely unrelated orbits. That two such seemingly improbable events could occur within hours of each other serves as a stark reminder that humanity is continually at risk of asteroid impact.

Asteroids at least the size of DA14 (~50m diam.) are expected to strike Earth approximately every 650 years, while objects at least the size of the Chelyabinsk asteroid (~20m diam.) are expected to strike Earth approximately every 50-100 years.

The new plan is to place an array of penetrator rods in the path of an asteroid to use the kinetic energy of the asteroid to tear itself apart.

The coupling between the penetrator KE and the fragment KE is a key part of the implementation of this program. We compute the mass of the penetrator required to deliver the needed disassembly KE for a 1m/s fragment speed for different intercept speeds, as well as the conversion efficiency of penetrator KE to disassembly KE for a possible future 100 ton delivered penetrator array capability. A 100-ton delivery capability of an interceptor array is possible with upcoming heavy lift options such as the Space X Starship.

The required efficiency is very low (less than 1%) even up to Apophis-class asteroids at 20km/s. This is very encouraging. The detailed nature and workings of the interceptor to achieve this efficiency requires detailed design, simulations, and ground testing of both passive and explosive-filled penetrators. While the addition of explosives to the penetrator does not add significantly to their overall energy, it may add significant to their efficiency of scattering the fragments.

The speeds of the penetrators for asteroid mitigation (typically over Mach 60) are far in excess of those used in “Earth-penetrating munitions” (typ. ~ Mach 1-3), and thus the basic physics will need to be refined to give accurate results.

A 1km bolide can be mitigated using the penetrator interceptor approach. They explored 100,000, 500,000 and 1 million fragment disruption for the 1km diameter case with mean fragment size of about 21.6, 12.7, and 10m diameter respectively. The case of 100,000 fragments at 60 days and 1m/s disruption allows for a successful mitigation with minimal damage, though some broken windows IF there are residential buildings very close to ground zero for some of the larger fragments, while for the 1 million case the damage is even less, with pressures less than 1kPa virtually everywhere on the ground. Allowing for a 60-day intercept allows the fragment cloud to expand to nearly the size of the Earth. Even longer term intercepts would allow virtually all of the fragments to miss the Earth.

Enhancing kinetic penetrators with nuclear fission devices could work for 10 km bolide asteroids. Asteroids over 40 km are beyond the entire current nuclear arsenal of the earth to gravitionally debind.

Asteroid Mitigation Methods

Several concepts for asteroid deflection have been described, which can be broadly generalized into seven distinct strategies:

(1) Kinetic penetrators, with or without explosive charges: an expendable spacecraft is sent to intercept the threatening object. Direct impact would modify the object’s orbit through momentum transfer. Enhanced momentum transfer can be accomplished using an explosive charge, such as a nuclear weapon.

(2) Gradual orbit deflection by surface albedo alteration: the albedo of an object could be changed using paint, mirrors, sails, etc. As the albedo is altered, a change in the object’s Yarkovsky thermal drag would gradually shift the object’s orbit.

(3) Direct motive force, such as by mounting a thruster directly or indirectly to the object: thrusters could include chemical propellants, solar or nuclear powered electric drives, or ion engines.

(4) Indirect orbit alteration, such as gravity tractors: a spacecraft with sufficient mass would be positioned near the object, and maintain a fixed station with respect to the object using onboard propulsion.

Gravitational attraction would tug the object toward the spacecraft, and modify the object’s orbit.

(5) Expulsion of surface material, e.g. by robotic mining: a robot on the surface of an asteroid would repeatedly eject material from the asteroid. The reaction force from ejected material affects the object’s trajectory.

(6) Vaporization of surface material: similar to robotic mining, vaporization on the surface of an object continually ejects the vaporized material, creating a reactionary force that pushes the object into a new path. Vaporization can be accomplished by solar concentrators or by lasers deployed on spacecraft stationed near the asteroid, the latter of which is proposed for the DE-STARLITE mission or done using long range stand-off laser ablation (DE-STAR).

During laser ablation, the asteroid itself becomes the “propellant”; thus a very modest spacecraft can deflect an asteroid much larger than would be possible with a system of similar mission mass using alternative techniques.

PI Planetary Asteroid Defense

The Lubin 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.

Gravitation Escape Speed and Disassembly Energy – The key to the effective use of the proposed program is the fracturing and disassembly of the parent asteroid. For this to be effective, we must overcome the self-gravity reassembly energy. [200m diameter) and ρ=2000 kg/m3 the escape velocity is get νesc ~ 0.11 m/s.]

PI – Interceptor Penetrators – The proposed system uses an array of penetrating disassembly rods (PDR) to gravitational de-bind and pulverize the asteroid. PI stands for “Pulverize It” or “Penetrator Interceptor”. Since the asteroid is moving faster than chemical propulsion, there is only modest gain to be added from the rocket speed relative to Earth compared to the asteroid speed relative to Earth. In a sense, we “just get in front of the asteroid” and wait for it to hit the penetrators rather than trying to “hit the asteroid” at high speed. The spatial timing sequence of the PDR’s can be handled either in waves with the outer ones hitting first or possibly by timed chemical detonation. The latter is complicated by the fact that the PDR’s will largely be vaporized in the impact, though it is conceivable that a clever PDR design could survive.

This is somewhat analogous to “Earth-penetrating bunker busters” weapons, BUT with a critical difference that the closing speed in our case is vastly higher than for a penetrating “bunker buster” and the impact speed is much higher than the penetrator material phonon speed. No known material can withstand the extreme frontal pressure and remain intact without a “staging” approach. High aspect ratio PDR units with various vaporization stages may be possible to allow impact survival and thus subsequent detonation. This is part of a detailed design phase that we will not discuss in this paper, but will form the basis for future research.

To visualize this, imagine pealing an onion layer by layer from the outside working inward. Optimizing the penetrator “disassembly procedure” will depend on the asteroid and the vehicle capability to deliver the PDR. Asteroids are not homogeneous objects and will generally have unknown or poorly known interior structures. Given the consequences of a failed intercept, any realistic system design will likely involve multiple interceptors and such systems will not be minimalist, but rather “over designed.”

It is important to understand the largest solid fragment from pulverization that we are willing to live with. Ideally, we would pulverize to a size scale of a meter or so, allowing the small fragments to burn up in the atmosphere with minimal shock waves. While desirable, we show that this is generally not necessary.

To determine the maximum fragment size scale requires that we set a damage threshold we are willing to accept and to have some minimal understanding of the asteroid in the sense of its “material yield strength.” The latter is effectively a “bulk” binding energy issue. In practical terms this will mean trying to understand if we are dealing with a loose rubble pile or a solid nickel-iron asteroid. Fortunately, there are vastly more, lower density (stony/loosely bound) asteroids than there are nickel-iron ones.

One example of this system would be a 10×10 array of large aspect ratio (long, multi-meter) hardened penetrators, each of which is 100kg for a total of 10 metric tons. Delivering 10 metric tons is easily within the capability of modern launchers and with the upcoming SpaceX Starship with refueling capability.

Another example would be a 50×50 array of penetrators, each of which is approximately 40kg for a 100 ton total penetrator delivered.

It may be more effective to have several waves of penetrators arrays on the same spacecraft that deploy sequentially to more effective pulverize the asteroid. The array size could be dynamically set in the spacecraft prior to impact to match the geometry of the target. Explosive filled penetrators are another option with detonation started just prior to or upon impact, or the use of a “cluster” explosive system or possible “solid balls” to macroscopically “erode” the asteroid. For large threats, waves of interceptor vehicles is yet another option. We show below that the actual total mass of the penetrators required to gravitationally disassemble and spread the fragment cloud is extremely small IF the coupling efficiency of the KE of the penetrators to the KE of the fragments is high.

The primary issue will be whether molecular binding will be dominant or not (i.e. solid or loose rubble pile) and the scale of disassembly (size distribution of fragments). If hit early enough, the broken asteroid fragments will miss the Earth altogether. If it is a terminal intercept, then the Earth’s atmosphere becomes a “beam dump” or “bullet proof vest.” In essentially all cases for this system, the requirement is to prevent a “ground impact” and to spread the fragments out spatially and temporally as much as possible to minimize the ground damage from the multiple fragment air bursts.

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. Even intercepts as close as the Moon with intercept times of a few hours prior to impact are viable.

Using the Moon as a planetary defense outpost with both detection as well as mitigation (launch) capability is one option to be considered for the future to protect the Earth. The Moon is nearly ideal given the lack of atmosphere allowing for long range optical/NIR LIDAR detection, and the low escape speed allows for rapid launch and interception capability. For an Earth launch-based system, we show that a single heavy lift launcher such as a Falcon Heavy, Starship, SLS etc. can mitigate a multi-hundred-meter diameter asteroid at intercept ranges within a lunar distance if needed.

As an example, they 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 diam., ~ 4 Gt yield) would largely 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 less than a 15-minute prior to impact intercept with a 10m/s disruption. Even a 2-minute prior to impact intercept of 20m class threats is viable. Pro-active mitigation of these threats is also an option. Having such a capability would allow humanity for the first time to take control over its destiny relative to asteroid and comet impacts.