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.