The Case For A UN Earth Defense Force

Protecting Earth from extraterrestrial impacts is a "Space Force" mission that makes sense, and one that clearly justifies the use of nuclear weapons. But, it really ought to be a global effort, since it protects everyone on Earth. 

The preprint below discusses the threat from a quite practical perspective. It highlights the fact that early detection of a threat, preparation to respond to it as quickly as possible if one is detected, and decisive action to commit us to responding to the threat as early as possible are critical to the continued survival of the human species if a threat like this manifests itself. 

All of this is because, for the first time in the history of life on Earth, we are advanced enough technologically to overcome this threat is we act swiftly. Previously, it would have been futile to try to take action to prevent a devastating impact, even if we knew well in advance that a dangerous impactor was on a collision course with Earth.  

The title of the paper is a reference to the recently released film, "Don't Look Up" by a Denver based team, that while fictional, appeals to the documentary crowd and is heavy on social commentary. The introduction reminds readers why they should care:

Asteroid and comet impacts pose a continual threat to life on Earth. For example, on February 15, 2013, a 20 m diameter asteroid airburst over Chelyabinsk, Russia with a total energy of about 0.5 Mt, or roughly that of a modern ICBM thermonuclear warhead. About 50% of its energy went into the atmospheric blast wave that injured approximately 1,600 people. The same day, a 50 m diameter asteroid (2012 DA14) passed within the geosynchronous satellite belt. Had it impacted, it would have had a yield of about 10 Mt. Such an impact over a major city could have killed and injured millions. Asteroids the size of 2012 DA14 (∼ 50 m diameter) are expected to strike Earth approximately every 650 years, while objects at least the size of the Chelyabinsk asteroid (∼ 20 m diameter) are expected to strike Earth approximately every 50–100 years. Another recent example is the Tunguska 1908 event that could have caused large scale loss of life, but did not due to the remote area in Russia it airburst over. This event is estimated to have been a roughly 65 m diameter asteroid (or possibly an atmospheric grazing comet) and resulted in an air blast with total energy yield of between 3 and 30 Mt. The resulting blast wave flattened more than 2000 km^2 of forest, as seen from the 1927 Soviet Academy of Sciences expedition. Larger objects pose an even more severe threat. For example, the total kinetic energy associated with an impact of a 100 m asteroid is equivalent to approximately 100 Mt, and that of the well-known ∼ 350 m threat, Apophis, would have an impact yield of approximately 3–4 Gt, or about 1/2 of the Earth’s total nuclear arsenal, while Bennu at 490 m could have a yield greater than the entire Earth’s nuclear arsenal. This is summarized in Figure 1, wherein we show the bolide kinetic energy as a function of its diameter. For reference, Apophis will next visit Earth on Friday April 13, 2029 and come within the geosynchronous belt.

The paper and its abstract are as follows: 

We discuss a hypothetical existential threat from a 10 km diameter comet discovered 6 months prior to impact. 

We show that an extension of our work on bolide fragmentation using an array of penetrators, but modified with small nuclear explosive devices (NED) in the penetrators, combined with soon-to-be-realized heavy lift launch assets with positive C3 such as NASA SLS or SpaceX Starship (with in-orbit refueling) is sufficient to mitigate this existential threat. 

A threat of this magnitude hitting the Earth at a closing speed of 40 km/s would have an impact energy of roughly 300 Teratons TNT, or about 40 thousand times larger than the current combined nuclear arsenal of the entire world. This is similar in energy to the KT extinction event that killed the dinosaurs some 66 million years ago. Such an event, if not mitigated, would be an existential threat to humanity. 

We show that mitigation is conceivable using existing technology, even with the short time scale of 6 months warning, but that the efficient coupling of the NED energy is critical.

Philip Lubin, Alexander N. Cohen, "Don't Forget To Look Up, arXiv:2201.10663 (January 25, 2022).