Terminal Planetary Defense with Hypervelocity Impactors

A recent post here discussed “Diverting Asteroids by Painting Them”, a particularly inexpensive and gentle way of diverting an asteroid on an Earth impact trajectory which is detected sufficiently early. But what if you don’t have the luxury of decades or centuries to mitigate a threatened impact, as will always be the case with comets inbound from the outer solar system, which are rarely detected more than a year before they cross the Earth’s orbit? A long (137 page!) and detailed paper posted on arXiv, “PI — Terminal Planetary Defense”, discusses the feasibility of disrupting an inbound object with hypervelocity kinetic impactors so the resulting pieces will either miss the Earth entirely (in the case of sufficiently early warning) or airburst in the atmosphere causing little or no damage on the surface. Here is the abstract.

We present a practical and effective method of planetary defense that allows for extremely short mitigation time scales. The method involves an array of small hypervelocity non-nuclear kinetic penetrators that pulverize and disassemble an asteroid or small comet. This mitigates the threat using the Earth’s atmosphere to dissipate the energy in the fragment cloud. The system allows a planetary defense solution using existing technologies. This approach will work in extended time scale modes where there is a large warning time, as well as in short interdiction time scenarios with intercepts of minutes 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 and/or air burst, with the primary channel of energy going into spatially and temporally de-correlated shock waves. It is the de-correlated blast waves that are the key to why PI works so well. The effectiveness of the approach depends on the intercept time and size of the asteroid, but allows for effective defense against asteroids in the 20–1000m diameter class and could virtually eliminate the threat of mass destruction posed by these threats with very short warning times even in a terminal defense mode. A 20m diameter asteroid (∼0.5Mt, similar to Chelyabinsk) can be mitigated with a 100 seconds prior to impact intercept with a 10m/s disruption. With 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.

The key insight is that in terminal defence the incoming object’s own velocity is used against it, providing most of the energy released by the swarm of kinetic impactors. Adding chemical explosives to the impactors has a negligible effect on their operation, since their energy density and ejecta velocity is much smaller than the impact speed, and while nuclear explosives might help, particularly with larger objects, we have no experience designing warheads that could survive an impact at tens of kilometres per second and the perceived risks of wide-scale deployment of such weapons are probably unacceptable.

A variety of intercept scenarios are discussed, involving both present-day technological capabilities and future options such as lunar-based surveillance and interceptors.

This is a fascinating study which crunches the numbers and goes into detail in a fashion I’ve rarely seen since the work of Herman Kahn.


I assert that there is little political motivation to fund such projects, however worthwhile they are. As with hardening the grid against CME’s or intentional EMP, the political class understands there will be no surviving constituency to complain about their failure to act to protect against mass casualties/human extinction. It is a simple political calculation for our omniscient betters: no possibility of complaint after-the-fact - no impetus to act preemptively.

It’s entirely possible such a system could be put in place by SpaceX or Blue Origin once they get their reusable heavy lifters in operation. Elon Musk has spoken repeatedly about the need for humans to become multiplanetary in order to survive. The investment required to spin up a terminal defence system as suggested in the article is modest compared to even starting a small permanent settlement on Mars. The search for and surveillance of potentially hazardous near-Earth objects is something for which NASA has received adequate funding over a long period, and is constantly pushing down the size threshold above which we are confident all objects are known.

Imagine if government simply declared that spending on a planetary defence system was a charitable tax deduction.


Thank you. I sit corrected (too old to stand corrected). Speaking of standing, this stands for the proposition that, had Scanalyzer been around for 10 years or so, I would have known this - such is its revelatory power.


Another perspective might be that our omniscient betters are using a very high discount rate on the future. They are not looking to the long term future of humanity; not even looking to their own life spans; they are interested in the next election; but really focused only on the current news cycle.

Consider the necessity of supplying the human race with power – the only way to avoid an unpleasant regression to the Stone Age. Those Best & Brightest know that fossil fuels are finite; they know that unpredictably unreliable bird-whackers and solar panels will never be able to power the mines and smelters required to replace themselves; they know that nuclear fission is technically feasible, and economically attractive if relieved from the burden of excessive unnecessary regulation – yet they have chosen a path which may well lead to disaster within their own lifespans. What are the prospects for getting that kind of political creature to be concerned about the inevitable but unpredictable eventual meteor strike on Earth?