The dilemma of nuclear energy in space

Abstract

Nuclear energy, used in weapons as well as for electricity generation, has the potential to destroy life on Earth. But it also has the potential to save life as we know it. Currently, nuclear explosives are the only technology with the capability to deflect, on relatively short notice, a large asteroid fragment or comet headed for a collision with Earth. However, a number of international treaties prohibit the use of nuclear explosives in space. The peaceful and critically effective use of nuclear energy to prevent a civilization-threatening collision is at odds with its potential for inducing a catastrophic thermonuclear war on Earth. The author uses the Fermi paradox—if there are extraterrestrial civilizations in our galaxy, why haven’t they communicated with us?—to examine the dichotomy of whether nuclear explosives are a barrier or a path to long-term human survival.

We live in a dangerous time, navigating at the edges of potential natural and human-made catastrophes. The former, during past periods, have radically altered life on Earth. Mass extinction of the dinosaurs, at the time the dominant species, triggered by the impact of a massive asteroid about 67 million years ago, is a prime example. Fortunately, this event opened an evolutionary pathway for our mammalian ancestors to ascend.

The current lull between planetary disasters offers an opportunity for Earth’s now-dominant species, Homo sapiens, to implement a strategy and develop technologies that can help overcome some of the future limits to human evolution, thereby affording a greater chance of long-term survival in the face of inevitable natural threats. One technology in particular—nuclear energy—has the potential to mitigate threats from the Earth-crossing asteroids and comets known as near-Earth objects or NEOs. These objects may be unique among natural hazards because their impacts with Earth can be prevented through timely actions (Remo and Haubold, 2014). Standoff nuclear explosives could be used to deflect an Earth-threatening object from its orbit, and upper-stage nuclear thermal rockets could be used to rapidly deliver an explosive payload deep into space (Remo et al., 2013). Nuclear explosives are an extremely dangerous and controversial option in any context, because they are thought to present the ultimate human threat to world peace and human survival. The adaptation of nuclear weapons for use in space would require careful consideration from many perspectives.

Because of its high energy density, nuclear energy can play a unique and essential role in the protection against NEOs. While there are no guarantees of preventing an asteroid strike (or a much less probable comet impact), nuclear explosives—that is, specially modified thermonuclear weapons—may offer the best hope for an effective planetary defense. However, the use of weapons of mass destruction in space is prohibited by the UN Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, as well as by nuclear non-proliferation and test ban treaties. Given the growing awareness of the seriousness of the threats to Earth from NEOs, it is time to put these treaties and prohibitions into a new perspective that considers the possible use of nuclear energy for the defense of the planet.

Technology and destiny

The human race can use its intellect to enhance its own survival or to hasten its demise. Humans are especially capable of sowing the seeds of their own self-destruction through misuse of technology. Nuclear war, so far only narrowly avoided, and catastrophic anthropogenic climate change pose the greatest—and in some ways interrelated—short-term threats to human civilization. Other human-driven, long-term environmental disasters lurking in the background include unchecked biological warfare, disease epidemics, uncontrolled population growth, mismanagement of water resources, and irreversible environmental pollution.

These are solvable problems that are essentially educational, political, and economic in nature. Humankind must overcome its hubris in order to safely possess the means for planetary preservation. Even with our large brains, our species may be extinction-prone rather than extinction-immune, destroyed by the mismanagement of machines we have created to serve us. This is a common theme in science fiction. It is also one of many hypothetical explanations of the Fermi paradox: the mystery of why we have not heard from extraterrestrials, even though scientists estimate that there are billions of Earth-like planets in our galaxy alone. Perhaps these extraterrestrials destroyed themselves or allowed themselves to be destroyed.

There are many ways civilizations can destroy themselves, with or without nuclear explosives. For example, climate change and poisoning of the biosphere are currently prominent threats to human survival and biodiversity. One solution is to mitigate the climate change caused by rampant fossil fuel consumption. Societies must develop a variety of renewable energy technologies backed up by the carefully regulated use of nuclear reactors (Remo and Jacobsen, 2008). This fits well with the goal of mitigating danger from NEOs because the development of nuclear power sources for NEO interception, planetary exploration, and space colonization can proceed in parallel with the development of better nuclear reactors for energy production here on Earth. The extensive recycling of nuclear warheads to provide fuel for reactors can also be included in these activities, thereby lowering the world inventory of nuclear weapons, which are still a grave threat to humanity.

It may not seem so from the daily news chronicling the savage brutality among humans, but in terms of economic and cultural advancement and planetary catastrophe avoidance (so far) we are living in the best of times. Nevertheless, the solar system is a dangerous place in which threats to civilization can arise with little or no warning. Large NEOs (with a radius of 100 meters or more) with limited or no impact warning time would pose the most immediate natural catastrophic threat to Earth. Longer-term catastrophic natural threats against which there is currently no defense include large gamma-ray and supernova outbursts, solar hyperactivity, successive massive volcanic eruptions, externally driven fluctuations in Earth’s orbit, and natural catastrophic climate change.

Fortunately, humans already have the scientific and technical capabilities to defend ourselves against an NEO impact, given sufficient warning time, using chemical and/or nuclear rockets and a sequence of standoff explosive nuclear payloads to deflect (but not destroy) a threatening NEO. Little or nothing can be done to mitigate the other natural threats, but their near-term probability is thought to be much less than that of a catastrophic NEO impact with Earth.

The risk of a near-Earth object impact

NEOs with a mass millions of metric tons or greater, and an impact velocity of about 20 kilometers per second, pose a catastrophic danger to Earth’s biosphere. Less than one percent of the estimated 20 million near-Earth asteroids have been identified. Of the 12,250 near-Earth asteroids already found (as of March 11, 2015), more than 860 are at least one kilometer wide and 1,559 have been classified as potentially hazardous (NASA, 2014). Add to this the further complication that NEO orbits can be difficult to predict, which can cause false alarms or shrink warning times. The numbers, velocities, and sizes for Earth-crossing comets can be even more daunting.

Even a small NEO can do major damage. The asteroid fragment that exploded over Chelyabinsk, Russia in 2013 was only about 20 meters in diameter (Popova et al., 2013) and it approached Earth without warning (Isobe and Yoshikawa, 1997). Later that same day, coming from a different direction, the asteroid 2012 DA14, about 50 meters in diameter, came within about 27,000 kilometers of Earth. These sporadic events and the regular flux of smaller airbursts at high altitude (mostly above water) dramatically demonstrate the immediacy and regularity of the threat and damage that even a small NEO is capable of doing. Aside from the direct damage, such an NEO entering the atmosphere could be misinterpreted as a hostile act and trigger a massive ballistic missile response. In Chelyabinsk, the day (or minute) before the NEO impact was uneventful, with no observations foretelling what was to come. This underscores the need for strategic deployment of observational assets, early detection, exact orbit calculation and extrapolation, and sufficient warning time, especially for detection of NEOs approaching from blind directions and those that reflect primarily in the infrared region of the spectrum (Mainzer et al., 2011).

How nuclear explosives could prevent an impact

A near-Earth object with a diameter greater than 100 meters possesses more than a hundred times the energy and destructive power, assuming the same entry speed, of the object that exploded at Chelyabinsk. There are thought to be hundreds of thousands of objects this size with Earth-intersecting orbits (Irion, 2013). The high-energy-density radiation produced by nuclear explosives (Remo et al., 2013) would be the most effective and reliable way to deflect them, depending on their orbital characteristics and given at least several months of warning time. A NASA study of deflection alternatives assessed nuclear standoff explosions as 10 to 100 times more effective than the non-nuclear methods analyzed (NASA, 2007).

Detonating a nuclear explosive close to the surface of a threatening NEO would change the object’s orbit by a few centimeters per second; multiple explosions would deflect it even more. The device carrying the explosives would be guided, armed, and fired using an automated system. To prevent misuse, the nuclear explosive could be designed so that detonation could only occur in space, millions of kilometers from Earth. The deflected NEO would remain intact but now on a non-intersecting orbit, and the radiation fallout in space would be minimal and essentially zero on Earth. To reach the targeted NEO more rapidly, thereby improving the response envelope, ideally the interceptor would be propelled by an upper-stage nuclear thermal rocket, although the current inventory of chemical rockets can suffice until nuclear propulsion technology is further developed.

The Fermi paradox

The Fermi paradox offers a thought-provoking perspective on the question of whether to develop nuclear explosives for planetary defense. Briefly stated, the paradox is this: Considering the estimated age (about 13.8 billion years) and enormous size of the observable universe—hundreds of billions of galaxies, each with hundreds of billions of stars—and the high probability that many stars have planets orbiting in a habitable zone, it is reasonable to expect that advanced civilizations exist and have developed the means to communicate and travel at least within our galaxy. But although humans have discovered more than a thousand planets outside our solar system in the past decade alone, including some thought to be capable of supporting life, so far we have found no evidence of extraterrestrial life. But it’s too early in the discovery process to draw any meaningful conclusions. Perhaps the missing potential communicants were wiped out before they could defend their own planets against inevitable NEO impacts—which may pose a fundamental barrier to the survival of an otherwise dominant species. Alternatively, it is possible that extraterrestrials were wiped out by other natural disasters like the ones that threaten Earth from time to time. Compare these natural risks to those of being destroyed by your own technology, which could otherwise be a lifesaver, and you have the essence of the dilemma of planetary survival.

Is there a built-in doomsday mechanism that prevents intelligent life from clearly perceiving and mitigating the threats to its own existence? Surely there are other possible explanations for the puzzling failure to detect life in our region of the galaxy. The Fermi paradox, though, provides a context for warning humanity of the very real danger of actively or passively causing its own demise.

One interpretation of the Fermi paradox highlights the self-destructive tendencies of advanced civilization due to a lack, or misuse, of nuclear or other advanced technologies (such as bio- or cyber warfare). But the reverse may be true in some cases, such as planetary defense where nuclear technology could be the savior of advanced civilizations.

On the evolutionary path to a highly intelligent civilization capable of interplanetary communication, sooner or later the NEO hazard must be proactively dealt with. Eliminating the nuclear explosives option in space leaves humanity vulnerable to annihilation unless an equally effective and reliable alternative can be developed. But exercising the option may require the deployment of nuclear weapons in space, which may be destabilizing for nuclear proliferation. The resolution of the paradox is a test of human character in which humans must overcome their aggressive, self-destructive tendencies and use their creative minds for the betterment of humankind.

Other mitigation options

In principle, there are many other options for NEO deflection or destruction, including a kinetic impact to knock the NEO off course; lasers or concentrated sunlight to vaporize material on the NEO’s surface, creating a gas plume that would act like a rocket thruster; or a heavy “gravity tractor” unmanned spacecraft that would hover for years near the NEO and slowly but steadily tug it into a safer orbit by exerting a gravitational attraction (Remo, 2000). But each of these deflection agents is operationally ineffective in terms of at least two or more of the critical parameters, which include factors such as the overall energy density of the payload, how much impulsive power is delivered to the NEO, how much momentum change can be imparted to the NEO, and how reliable the deflection agent is. These issues are especially relevant when the deflection agent is delivered by a nuclear thermal rocket that can reach the NEO target in a fraction of the time required by a chemical rocket, because the earlier a change in velocity can be imparted to the NEO the greater the collision-avoiding deflection distance (Powell et al., 1997). Perhaps a future deflection method will have all of the advantages of nuclear explosives delivered via nuclear rocket but will also be totally benign and controllable. For the immediate time frame though, further development of known and tested technology is the most prudent approach.

Proliferation and international treaties

There has already been a significant proliferation of thousands of nuclear weapons on Earth, possessed by at least nine countries with various degrees of political stability (or instability) and with the possibility of even more following. If NEO-deflecting nuclear explosives were to be developed, their custody, maintenance, and security could be restricted to the leading nuclear powers, with United Nations operational jurisdiction in space. The critical political issue and apparent dichotomy is whether the possible testing and use of a few nuclear explosives (nuclear weapons with modified permissive action links for security purposes) in outer space, along with nuclear thermal propulsion technology, helps or hinders nuclear proliferation and the long-term survival of Earth’s inhabitants.

Parties to the 1967 UN treaty governing activities in outer space, including the United States and the former Soviet Union, are prohibited from positioning weapons of mass destruction in orbit around the Earth, moon, or any celestial body. Only peaceful activities are permitted on or near these bodies (Remo and Haubold, 2001). Clearly, the intent of the treaty is to prevent military conflict, disputes over sovereignty, and the degradation of space from commercial exploitation.

Other treaties relating to the potential use of nuclear explosives in space include: the Partial Test Ban Treaty of 1963, which prohibits all nuclear detonations except those conducted underground; the Nuclear Non-Proliferation Treaty, which entered into force in 1970; and the Comprehensive Nuclear Test Ban Treaty of 1996, which prohibits all nuclear explosions, including testing of nuclear weapons. Signature by UN member states is by no means unanimous, and efforts to reduce nuclear weapons have stalled during the past six years. Unfortunately, it appears that nuclear weapons will remain a major component of strategic planning for the foreseeable future (Kramer, 2014). However, the space and test ban treaties do not directly address issues of planetary defense. The threat from near-Earth objects was not recognized when the treaties were framed, which suggests that a reinterpretation of the treaties might be appropriate.

For objects on a collision course with Earth, with a year or less of warning time, nuclear explosives are currently the most effective—by a factor of 100 at the very least (Remo, 2000; Remo et al., 2013)—and reliable way to ensure deflection. The development of nuclear explosive devices for this purpose would increase the world’s nuclear weapons inventory by less than 0.1 percent. Placing these devices under the guardianship and collective control and security of those major nuclear powers that otherwise comply with the treaties mentioned above would limit misuse. Such an arrangement may even foster an environment for cooperative nuclear disarmament among the current and aspiring nuclear powers by expanding UN efforts and revising treaties to deal with the NEO threat to international security—while still curtailing proliferation and reducing the probability of nuclear conflict.

Possible endpoints

Assuming that NEOs are planetary debris common to the formation of solar systems, extraterrestrial worlds are also subject to bombardment. Dealing with this depends on the anticipated end point. Here on Earth, one of the possible end points of developing nuclear explosives for use in space, at least until another method is devised, is an effective planetary protection system with strong nuclear proliferation safeguards and disarmament incentives. A second possible outcome is that nuclear war and/or runaway climate change destroy Earth’s civilization as we know it before an NEO collision occurs. A third end point is to continue prohibiting nuclear explosives in space and ultimately be destroyed by a collision or other catastrophe. The status quo option—in which nuclear explosives for space are not developed but we get lucky and aren’t destroyed by an NEO collision before a more benign mitigation system is developed—is risky wishful thinking.

Clearly, the first end point is currently the most viable option. These end points could help explain why we have not heard from any extraterrestrial civilizations and provide some perspective about our own future options and chances for survival.

It appears that the United Nations may be heeding warnings about the NEO threat. In December 2013, the General Assembly embraced the recommendations for an international response (COPUOS, 2013). A senior officer at the United Nations Office for Outer Space Affairs in Vienna with whom I have worked is quoted as stating, “If a near-Earth object is approaching Earth and we are in great peril, then the U.N. Security Council can make its own decisions about what to do. The world powers could meet and determine what technology makes sense for dealing with a particular threat” (Nadis, 2015). This is a good approach provided there is enough time available to mount a defense. Warning time is proportional to early detection, which depends on observational assets being strategically deployed. This is an issue that must be promptly addressed.

To systematically deal with inevitable civilization-threatening NEOs, the emphasis in the coming decades should be on early discovery, reconnaissance (if possible), and deflection of large NEOs—rather than the relatively small ones that can be consumed by Earth’s atmosphere such as the asteroid fragment that exploded near Chelyabinsk. Focusing on large objects requires the development of very-high-energy-density deflection methods—in short, nuclear weapons specially designed for peaceful use in space. Nuclear thermal rockets can expand the operational envelope of the interception trajectory. Amendments or codicils to international treaties could make it permissible to develop nuclear explosives for planetary defense before an imminent threat is detected.

The genie cannot be put back in the bottle. Despite the risks, we must go forward with efforts to defend Earth from both homegrown and external threats. As an added benefit perhaps planetary defense, as well as the development of nuclear technologies (both fission and fusion) for energy production, can provide a model for collective security through international cooperation to stabilize the biosphere and support planetary exploration.