Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: PI -- Terminal Planetary Defense   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 $\sim$10-meter diameter allow the Earth's atmosphere to
act as a "beam dump" where the fragments either burn up in the atmosphere
and/or air burst, with the primary channel of energy going into spatially and
temporally de-correlated shock waves. It is the de-correlated blast waves that
are the key to why PI works so well. The effectiveness of the approach depends
on the intercept time and size of the asteroid, but allows for effective
defense against asteroids in the 20-1000m diameter class and could virtually
eliminate the threat of mass destruction posed by these threats with very short
warning times even in a terminal defense mode. A 20m diameter asteroid
($\sim$0.5Mt, similar to Chelyabinsk) can be mitigated with a 100s prior to
impact intercept with a 10m/s disruption. With a 1m/s internal disruption, a 5
hours prior to impact intercept of a 50m diameter asteroid ($\sim$10Mt yield,
similar to Tunguska), a 1 day prior to impact intercept of 100m diameter
asteroid ($\sim$100Mt yield), or a 10 day prior to impact intercept of Apophis
($\sim$370m diameter, $\sim$4Gt yield) would mitigate these threats.

35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an