INT 86TN Planetary Defense – Spring 2024

Planetary Defense

Professor: Philip Lubin

Office 2015C Broida


Classes: Th 5-6 PM in Broida 3302


Synopsis: This is an interdisciplinary course that is designed to explore the science and policy of planetary defense.  This course has a goal of bringing together students from various background in the humanities and the sciences to look at the issues of defending the planet from asteroid and comet strikes. We will explore the reality of the dangerous neighborhood we live in and the consequences from small daily strikes of sub meter diameter meteoritic material (roughly 100 tons/day) the recent significant events like the 2013 Chelyabinsk and 1908 Tunguska impacts to the existential threats from large asteroids and comets. Our previous tenants, the dinosaurs, did not have a course in planetary defense and sadly for them (good for us) they all perished about 66 million years ago when a 10 km diameter asteroid (some think it was a comet) struck and extiguished most of the species of life on Earth. Such cataclysmic events are rare but there is strogng evidence from geologic and biologic records that existential threats and the mass extinction of kife has  happened multiple time in the history of ther Earth. A related issue in the early history of our solar system is the possible collision of the Earth by a large body which subsequently led to the formation of moon from the ejected material which coalesced into our moon. If this event happened, it would have occurred billions of years ago in the early formation phase of our solar system (~ 4.5 Gyr ago). This idea is known as the “Giant Impact Hypothesis” . There is some support for this idea in lunar sample returns from the Apollo landings.  The idea is still controversial but is indicative of the profound effects that impacts have on solar system planetary formation. If you look up at the moon you will see the many craters formed by asteroid and comet impacts. The reason you do not see as many on the Earth is that the Earth atmophere, rain, weathering and geolgic activity makes it much harder to see these craters.,accreted%20to%20form%20the%20Moon.

At present there is NO operational planetary defense system in place but there are promising paths forward that can allow humanity to protect our planet from extinction. In this course we will explore the many aspects of the problems we face in both situational awareness (what and when a significant strike will occur) as well as realistic paths forward to protect our planet. We will discuss a variety of technical issues related to implementing an operational planetary defense program from better detection to mitigation strategies to launch vehicle interceptors to basing option (Earth, orbital, lunar) to planetary defense on the moon and Mars as humanity moves out into the solar system. We will discuss mitigation options from “doing nothing” for smaller threats to passive interception techniques (no explosives) to the use of nuclear weapons mitigation (NED – Nuclear Explosive Device). The use of nuclear weapons for large threats is briefly discussed in our paper “Don’t Forget to Look Up” which is a response to the movie “Don’t Look Up”.  We will also look at a variety of techniques from painting the threat (to increase solar radiation pressure, wrapping in reflective films to ion engine drives and stand-off laser ablation among others.


Topic Study Areas:

Students will form groups centering around specific topics. Some suggestions include:

  1. Ethics of PD both for proactive (preventative destruction of possible future threats) as well as “known or imminent threat” mitigation
  2. Policy – both at the nation state and international
  3. Situational Awareness – both existing as well as future detection strategies
  4. Mitigation Strategies – deflection (eg DART mission), albedo modification, laser ablation, ion engine, fragmentation (PI), conventional vs nuclear
  5. Launch Vehicles for interception – current and planned launcher capability (mass, C3 vs mass)
  6. Ground effects –  in cases where the mitigation fails or in terminal defense situations shock waves, optical pulse, ground motion and ejecta (if ground hit)
Some recent threats:
Asteroid 2021 CD6

2023 NT1


UCSB NASA Planetary Defense Program – PI (π)

Study this area for a summary of the problems we face as well as a new path forward. Lots of material and references.

Some of our class will focus on this new approach and its implications.

NASA Planetary Defense Office (PDCO)

NASA JPL Threat Assessment

Asteroid Material Retrieval Mission from Asteroid Bennu (~ 500m diameter) – Osiris-Rex

DART Mission – NASA deflection mission – impact Sept 26, 2022 of binary asteroid – Dimorphos/ Didymos

Chelyabinsk Event 2013

Laser Ablation Defense

US Interagency Report on NEO Impact Threat Emergency Protocols – Jan 2021


Planetary Society and Planetary Defense

USSF – US Space Force interest

Meteorites and Meteors



Apophis (Apep or god of Chaos in ancient Egyptian ) Asteroid (~ 350m diameter) – coming soon to a planet near you (Friday the 13th, April 2029 – mark your calendar – please do not invite Jason or Freddy

Extremely close approach (inside geosync belt) – it will miss but just barely. While not an existential threat, IF it were to hit the Earth it would have an impact energy comparable to ALL the nuclear weapons on Earth.

Heavy Impact Bombardment in the Early Solar System

The death of the dinosaurs – Chicxulub Impactor,and%2015%20kilometres%20in%20diameter

In the popular realm – movies
Don’t Look Up

based on a real comet (NEOWISE) that was detected in March 2020, four months before closest approach to Earth

Had this comet hit, most human life would have been extinguished

See our technical response paper to such a threat:

Don’t Forget to Look Up
A Physicist’s View of Hollywood and Existential Threats


see if you can find the scientific flaws

Deep Impact

see if you can find the scientific flaws