February 2016 Astronautics

Nuclear Flashlamps in Space

Launching nuclear explosives into space seems like a bad idea and is not something to be treated lightly but there might be times when we need to think seriously about why it might be useful to use nuclear explosives in deep space - and ponder what can be learned just by thinking about it.

Tim Livengood Tim Livengood NASA Goddard Space Flight Center

We can be thankful that nuclear weapons continue to be unused in the fashion for which they were designed - and we can imagine a future in which nations may no longer feel the need to keep nuclear devices at all. When that day comes it would be helpful to dispose of the ones that already exist. There is no method that is more completely effective than detonating them somewhere safe. Strapping on a few solid rocket boosters to each missile could generate enough thrust to achieve escape velocity and send them as far away as possible. The challenge is to get some use out of them in the process.

Let’s think seriously about why it might be useful to launch nuclear explosives into space

A nuclear explosion releases a pulse of energy that vaporises the device and anything nearby. The nuclear reactions at the heart of the explosion release a burst of radiation of every kind: gamma rays and X-rays, which are very high energy particles (photons) of light, as well as lots of less energetic light in ultraviolet, visible wavelengths, infrared, microwaves, and radio waves, each carried by successively lesser-energy photons. Also neutrons and other subatomic particles. The blast wave of radiation vaporises and ionizes the nuclear device and its surroundings, creating a rapidly expanding cloud of superhot radioactive plasma. In short: bright light, big bang, radioactive afterglow.

Nuclear explosives have been considered for space propulsion of large objects. Spacecraft designs envision pitching nuclear bombs out the back door, one by one, to detonate and propel the spacecraft. Several thousand such events in succession could really get something moving. Since nuclear pulse propulsion requires launching large numbers of nuclear explosives and leaves a trail of radioactive debris behind the spacecraft, it is not hard to see why no one has ever actually built one of these vehicles.

Nuclear explosives could also deflect the orbit of comets or asteroids. The energy from the detonation would vaporise the surface of a nearby comet or asteroid and generate propulsive force.

Plans to deflect comets and asteroids assume that we could spot dangerous objects with enough time to prepare a response. So far, so good: within human history, we have observed no major objects on collision course with Earth. Alas, the failure to notice any such objects has not stopped them from hitting us, anyway. We have been lucky and damage has been minor - within human history. Only the meteoroid that exploded over Chelyabinsk caused any significant human harm, many orders of magnitude less than the Chicxulub impact that terminated the dominance of the dinosaurs, 65 million years ago.

The problem with finding small rocks and chunks of dirty ice in space - ‘small’ meaning ‘the size of mountains or cities’, like the Chicxulub impactor - is that they are small, while space is vast; they are usually very dark in colour; and typically, they are far from the Sun during the period when we would like to detect them, long before they get onto an orbit that poses any kind of impact hazard.

There are good scientific reasons to learn more about small objects in the space around our Sun, apart from our fear of murderous asteroids. These tiny objects remain from the gravitational self-assembly of the Solar System. Their material composition is evidence of conditions in the pre-solar nebula that formed our Sun and planets. The modern orbits of these objects result from orbital dynamics processes in the ancient past. The story of our origins is written in the bodies of comets and asteroids and in the impact craters they have left on rocky bodies of the Solar System. If we could just deploy a really bright light to someplace where we think small bodies can be found, we could increase our chances of discovering them and determine how many there are and of what size.

Spacecraft designs envision pitching nuclear bombs out the back door, one by one, to detonate and propel the spacecraft

We have good evidence that there are two orbital regions, distant from the Sun, that are occupied by innumerable small bodies: the Kuiper Belt and the Oort Cloud. The Edgeworth-Kuiper Belt (EKB) extends from around the orbit of the planet Neptune out to perhaps a couple times the orbital distance of Pluto.

Pluto itself is the first known resident of the EKB but there are over a thousand Edgeworth- Kuiper Belt Objects (EKBOs) known by now. The Oort Cloud is a notionally spherical cloud of small bodies that orbit the Sun within interstellar space, at distances of a large fraction of a light year out to perhaps two light years, roughly halfway to the next nearest star. Cometary bodies on orbits that eventually reach the inner Solar System come from the outer reaches of the EKB, on orbits deflected over time by the gravitational influence of Neptune, by passing stars, and possibly by other bodies of which we know little or nothing.

High altitude (48 km) nuclear explosion Bluegill Triple Prime from Operation Fishbowl nuclear test series,. Yield believed to be 0.2-0.4 megaton TNT-equivalent. Duration of flash about 0.15 second. This is more or less what a space nuclear explosion would look like, an expanding bubble of hot glowing plasma High altitude (48 km) nuclear explosion Bluegill Triple Prime from Operation Fishbowl nuclear test series,. Yield believed to be 0.2-0.4 megaton TNT-equivalent. Duration of flash about 0.15 second. This is more or less what a space nuclear explosion would look like, an expanding bubble of hot glowing plasma

We can make some rough estimates for crowding in the EKB. The belt extends from about 20 AU (Astronomical Unit - the average distance from the Sun to Earth) out to about 100 AU. The belt forms a donut with a volume of around three quarters of a million cubic AU. There are estimated to be around one hundred thousand KBOs of greater than 100 km diameter, so the average volume of empty space around each one is about 7.5 cubic AU and the distance between objects is thus around 2.4 AU.

Objects of 100 km diameter are about ten times as big as the Chicxulub impactor. If we assume that the total mass of objects that are about ten times smaller than 100 km is about ten times as much as the 100 km objects, then the number of such objects is about ten thousand times greater, or about a billion bodies of around 10 km diameter. That brings the spacing between such objects down to around 0.1 AU.

Radar image of near-Earth asteroid Radar image of near-Earth asteroid

A nuclear flash at some random point in space within the Kuiper Belt would be, on average, about 0.05 AU away from the nearest 10 km object but around 100 AU from us. A one megaton blast releases about 4.1 million billion joules of energy. In the absence of better information, assume it takes about a second to stop radiating light energy - it takes milliseconds for the initial nuclear explosion but the plasma from the vaporised device takes a little while to radiate its energy. By the time it reaches the surface of an EKBO at a distance of 0.05 AU, the energy of the blast is highly diluted by distance to a one-second pulse of about five microwatts per square metre. It turns out that is much less than the total power received from the Sun, which comes out to about 0.14 watts per square metre. Even at our most destructive, the energies controlled by humanity are puny compared to a star.

Even at our most destructive, the energies controlled by humanity are puny compared to a star

Both the Sun and the nuclear explosion emit radiation over a broad wavelength band, as well as particle radiation. Unlike the Sun, it is conceivable to engineer the radiation output from a nuclear explosion to make it detectable despite being relatively weak. By comparison, the radio transmitter in the Voyager 2 spacecraft emits only 20 watts of power in a narrow frequency range at its current distance of about 103 AU. If we assume that focusing of the Voyager radio signal increases its efficiency by a factor of around 100, it is equivalent to a 2000W broadcast transmitter; still far, far less than the nuclear explosion. Limiting the energy output from the detonation to radio wavelengths, if it can be done, could dramatically increase the likelihood of a detectable signal reflected from KBOs.

When a nuclear explosion occurs in Earth’s atmosphere, it produces an electromagnetic pulse (EMP), a powerful burst of radio emission from the ionized air that surrounds the device, compressed by the shock wave of the explosion. That would not happen in space, where the nuclear explosion expands into vacuum with only the small amount of plasma (ionized gas) that comes from vaporising the nuclear device itself.

Near-Earth asteroid 2006 DP14 Near-Earth asteroid 2006 DP14

It may be possible to engineer a wrapper or a shell for the nuclear explosive that would fill the same role as Earth’s atmosphere but with designed properties. Down-converting the energetic photons and particle radiation from the nuclear pulse to radiate almost entirely in the radio spectrum would enable the nuclear pulse to be used for radar. Radio scatters much more efficiently from icy bodies than visible light, which is mostly absorbed. Megawatt-level radar transmitters are used regularly at the Goldstone and Arecibo observatories to investigate Solar System objects at multiple-AU distances, still far less power than a one megaton nuclear pulse.

We have had nuclear devices for 70 years but there was never a reason to consider turning them into flashlamps

The engineering problem of making a nuclear explosion into a radar source is the same whether the problem is solved by us, or solved by an alien civilization orbiting another star. This means that a nuclear radar source operated by an alien civilization would have whatever distinctive properties might result from solving the intellectual problem ourselves, whether or not we ever choose to build and operate such a device.

Comparing to the Voyager 2 radio transmitter, the 2 kilowatt equivalent broadcast power from Voyager is about 21,012 times less powerful than a 1 megaton blast. That means that the blast could perhaps be detected at a million times the range at which we currently detect Voyager 2. This translates to about 2300 light years, a range that includes millions of stars.

Artist’s conception of the view from the surface of Centaur object 10199 Chariklo that orbits between Saturn and Uranus Artist’s conception of the view from the surface of Centaur object 10199 Chariklo that orbits between Saturn and Uranus

The Search for Extraterrestrial Intelligence (SETI) has always tacitly assumed that any civilization that it might detect would be far beyond our own technology, simply because we lack the ability to manipulate radio or optical transmissions of power great enough to detect over interstellar distances. In fact, that may not be so. We have had nuclear devices for 70 years but there was never a reason to consider turning them into flashlamps.

The capability to transmit a signal to another star may not be so far outside our grasp, and thus any civilization we detect might not be so far beyond us. The information content that could be transmitted with an isolated nuclear pulse is small, but contains the most important piece of information imaginable: we exist.

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