|Fig. 1: Likelihood and danger of different disasters, including natural and human- made incidents. Source: H. Shabb, after Chapman and Morrison. |
Millions of asteroids orbit the sun, mostly in the asteroid belt between Jupiter and Mars, according to Nahum Melamed, a project leader in the Aerospace Corporations Vehicle Systems Division. When these asteroids collide with each other, fragments are ejected and can move towards Earth. Although rare, occasionally these fragments can orbit near the sun, and potentially cross Earth's orbit. Any asteroid or comet that is within 45 million kilometers of Earth is called a Near Earth Object, or NEO. 
Currently, the US Space Command estimates there are around 8,500 objects in the earth's orbit larger than twenty centimeters, with many more slightly smaller in size. The collisions of this debris creates a multiplying effect, which makes it very difficult for spacecraft to reside in those regions.  There is a constant barrage of interplanetary particles reaching the Earth's surface, but the vast majority of this material is in the form of small dust particles. Although it is very rare, it is possible that a larger mass will collide with the Earth, which is an event that must be prepared for due to its potential devastation of our planet. 
Since the beginning of the Earths history, meteor impacts have had a large effect on the patterns of life on Earth. The Alvarez hypothesis states that the Cretaceous extinction took place due to a large 10 kilometer object impacting Earth, which has evidence based on a large crater in the Yucatan.  Impacts this large completely altered evolution and life on the planet, and proof of impacts this large have instilled fear in people starting as early as 1705. 
According to Chapman and Morrison, only certain sized NEOs should be worried about.  NEOs less than 50 meters in diameter will generate less than 10 megatons of energy, and will diffuse in the upper atmosphere and have no negative affects on the Earth. However, objects with diameters larger than 50 meters can create a lot of damage locally if they impact a populated area.  Although these impacts can cause local damage, they are a lesser threat than other natural disasters such as fires, earthquakes, and floods. The real fear should be from objects large enough to cause cause both local and global disasters, creating problems such as acid rain, firestorms, and the blocking of sunlight from all of the debris. The size required to create these damages is largely debated, but is likely between .5 and 5 kilometers in diameter. 
On February 15, 2013, an eighteen meter mass exploded twenty three kilometers above Chelyabinsk, Russia, and created major damage. This asteroid exploded destroyed many buildings, and injured more than one thousand people. According to Nahum Melamed, this size of asteroid is projected to impact Earth around once every thirty or forty years, but usually landing in the ocean rather than populated areas.  Although objects larger than these are very unlikely to impact Earth, if it did happen, the effects would be detrimental to the survival on our planet.
There are two different ways that have been suggested to use nuclear weapons to decrease the detriments of an asteroid impact on Earth. One strategy is to use nuclear fusion weapons to create an explosion that alters the course of the asteroid, and the other is to completely explode the asteroid itself using a bomb. Both of these strategies have been discussed, with the latter being the most recent strategy recommended. However, neither strategy has been found particularly productive, and it is likely that nuclear weapons are not strong enough to affect large NEOs enough to protect Earth. 
The initial strategy requires nuclear fusion weapons to be set off over the course of several years, which would vary the velocity of the asteroid, and cause it to completely miss Earth. Although this strategy doesnt create the issue of fragments after a direct explosion, it requires several years to do, and therefore is a longer process that needs early recognition of the NEO, which requires technology that we dont yet have. 
The more recent strategy, as described by Bong Wie, the founding director of the Asteroid Deflection Research Center explains that a more direct strategy has benefits. He states that nuclear bombs are the answer, a strategy that can be implemented much quicker, even in less than a year. He is creating a spacecraft called the Hypervelocity Asteroid Impact Vehicle (HAIV), which will burrow into the asteroid to create a crater, and will be followed immediately by a nuclear bomb, which will detonate inside. Due to the creation of the crater, the destructive power of the bomb is increased by a factor of twenty. Although this explosion would shatter the asteroid into millions of tiny fragments and some have the potential to impact Earth, the effects would be minimal.  However what Wie fails to realize, is in the case of large asteroids, even if the nuclear explosion is very large, the asteroid is too big, and therefore will be barely affected by the explosion.
In a report written by Colin McInnes, a Scottish aerospace engineer, he notes the importance of recognizing the energy loss that will occur in an explosion from a nuclear weapon on an NEO: While the specific yield of nuclear explosives is great (~ 1 × 1012 J kg-1) for a complete fusion devidce, the coupling of liberated energy to the asteroid may be low. He states that the conversion efficiency from the initial energy to the heat on the asteroids surface could be as low as .03, resulting in a potential yield of 9 × 109 J kg-1. 
The immediate threat of an NEO impacting Earth is very small, but if it were to happen, damage to our planet could be colossal. In the meantime, it is extremely important for scientists to find and keep an eye on NEOs, so that we can become aware if one poses danger to Earth.  Additionally, the challenge is how the world can work together in a peaceful manner to search for solutions to deter the impact of asteroids on Earth, which may not be a solution of nuclear weapons.  Many challenges come with international collaboration, especially those that involve weapons of such power.
© Hannah Shabb. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 N. Melamed and A. Melamed, "Planetary Defense Against Asteroid Strikes," The Aerospace Corporation, OTR2017-00050, January 2018.
 C. Chapman and D. Morrison, "Impact on the Earth by Asteroids and Comets," Nature 367, 33 (1994).
 J. Aron, "Deflector Selector Says Nuke Asteroids," New Scientist 237, No. 3165, 6 (February 2018).
 P. Farinella, L. Anselmo and B. Bertotti, "Nuclear Power in Space: A Dual-Use Conflict," in Technology Transfer, ed. by D. Schroeer and M. Elena (Routledge, 2017).
 C. McInnes, "Deflection of Near-Earth Asteroids by Kinetic Energy Impacts From Retrograde Orbits," Planet. Space Sci. 52, 587 (2004).