Galaxy's Edge Magazine: Issue 2, May 2013 (25 page)

Far bigger accidents have already happened. Four large nuclear reactors have fallen from orbit, none has caused any distribution of radioactive debris. In fact, a Soviet reactor plunged into the Canadian woods and emitted so little radioactivity we could never find it. Embedded in tough ceramic nuggets, the plutonium cannot be powdered and inhaled.

Beyond this return to our past capabilities, NASA is considering building a nuclear-driven ion rocket. This will yield exhaust velocities (jetting pure hydrogen) of 250 km/s—a great improvement. But the total thrust of these is small, suitable only for long missions and light payloads.

Using hydrogen as fuel maximizes exhaust velocity (for a given temperature, lighter molecules move faster). And we can get hydrogen from water, wherever it can be found. We’ve discovered from our Mars orbiters that Mars has plenty of ice within meters of the surface. Comets, the Jovian ice moons—all are potential refueling stations.

But holding hydrogen at liquid temperatures demands heavy technology and careful handling. Water is easier to pump, but provides only a third the exhaust velocity. Many believe that ease of handling will drive our expansion into space to use not more exotic fuels, but plain old… water.

Living Off the Land

What could our space program be like right now, if we hadn’t shut down the nuclear program? The road not taken could already have led us to the planets.

The key to the solar system may well be nuclear rockets—
nukes
to friend and foe alike. The very idea of them had of course suffered decades of oblivion, from the early 1970s until the early days of the 21st century. Uranium and plutonium carry over ten thousand times as much energy per gram as do chemical rockets, such as liquid hydrogen burning liquid oxygen.

So in the end, advanced rockets may well be steam rockets, all the way from the launch pad to Pluto. Chemical boosters can get a nuke rocket into orbit, where it turns on. Whether with liquid hydrogen married to liquid oxygen, or with water passing by slabs of hot plutonium, they all flash into plumes of steam.

Real space commerce demands high energy efficiency. Realization of this returned to NASA in 2002, with the hesitant first steps of its nuclear Project Prometheus (bureaucracy loves resplendent names).

The first rush of heavy Mars exploration will probably prove the essential principle: refuel at the destination. Live off the land. Don’t haul reaction mass with you. Nuclear rockets are far easier to refuel because they need only water—easy to pump, and easy to find, if you pick the right destination. Nearly all the inner solar system is dry as a bone, or drier. If ordinary sidewalk concrete were on the moon, it would be mined for its water, because everything around it would be far drier.

Mars is another story. It bears out the general rule that the lighter elements were blown outward by the radiation pressure of the early, hot sun, soon after its birth. This dried the worlds forming nearby, and wettened those further out—principally the gas giants, whose thick atmospheres churn with ices and gases. Mars has recently proved to be wet beneath its ultraviolet-blasted surface. Without much atmosphere, its crust has been sucked dry by the near-vacuum. Beneath the crust are thick slabs of ice, and at the poles lie snow and even glaciers. So explorers there could readily refuel by melting the buried ice and pumping it into their tanks.

The moons of Jupiter and the other gas giants are similar gas stations, though they orbit far down into the gravitational well of those massive worlds, requiring big delta-V to reach. Pluto, though, is a surprisingly easy mission destination. Small, deeply cold, with a large ice moon like a younger twin, it is far away but reachable with a smaller delta-V.

Of course, there are more sophisticated ways to use water. One could run electricity through it and break off the oxygen, saving it to breathe, and then chill the hydrogen into liquid fuel. That would be the most efficient fuel of all for a nuclear rocket.

But the equipment to keep hydrogen liquefied is bulky and prone to error—imagine the problems of pumps that have to operate in deep space at 200 degrees below zero, over periods of years. An easier method would be to use that hydrogen to combine with the Martian atmosphere, which is mostly CO
₂
, carbon dioxide. Together they make oxygen and methane, CH
₄
, both easy to store. Burning them together in a nozzle gives a fairly high-efficiency chemical rocket. A utility reactor on Mars could provide the substantial power needed for this.

Still, that would demand an infrastructure at both ends of the route. Genuine exploration—say, a mission to explore the deep oceans of Jupiter’s moon Europa—would need to carry a large nuclear reactor for propulsion and power, gathering its reaction mass from the icy worlds.

NASA is studying an expedition to Europa using a nuclear-driven ion rocket, which would carry its own fuel. It will have to fire steadily for
seven
years to get to Europa, land and begin sending out rovers. Testing the reliability of such a long-lived propulsion scheme demands decades of work, effectively putting off the mission until the 2020s.

Far better would be a true nuclear fission rocket throwing hot gas out the back. If it could melt surface ice on Europa and tank up with water, it could then fly samples back to us.

The true use of a big nuclear reactor opens far more ambitious missions. The real job of studying that deep ocean is boring through the ice layer, which is quite possibly miles thick, and maybe even hundreds of miles. No conceivable drill could do it. But simple hot water could, if piped down and kept running, slowly opening a bore hole. Hot water has been tried in Antarctica and it works.

To test for life on Europa would demand that we send a deep-sea-style submarine into those dark, chilly waters. To power it we could play out a thick, tough power cord, just as do the undersea robot explorers that now nose about in the hulks of the
Titanic
and the
Bismarck
—power cord tens of kilometers long. Only nuclear can provide such vast powers in space.

Dreadnoughts of Space

Space is big. Moving asteroids and other large masses demands scale. This leads to a future using
big
nukes.

The payload would be a pod sitting atop a big fuel tank, loaded probably with ordinary water, which in turn would feed into the reactor. Of course, for manned flights the parts have to line up that way, because the water in the tank shields the crew from the reactor and from the plasma plume in the magnetic nozzle. To even see the plume, and diagnose it, they will need a rearview mirror floating out to the side. The whole stack will run most of its trajectory in zero-G, when the rocket is off and the reactor provides onboard power.

A top thick disk would spin to create centrifugal gravity, so the crew could choose what fractional G they would wish to live in. Perhaps forty meters in diameter, looking like an angel food cake, it would spin lazily around. The outer walls would be meter-thick and filled with water for radiation shielding. Nobody could eyeball the outside except through electronic feeds.

Plausible early designs envision a ship a hundred meters long, riding a blue-white flare that stretches back ten kilometers before fraying into steamy streamers. Plasma fumes and blares along the exhaust length, ions and electrons finding each other at last and reuniting into atoms, spitting out a harsh glare. This blue pencil points dead astern, so bright that, leaving Earth orbit, it could be seen from the ground by naked eye.

Ordinary fission nuclear power plants are quite good at generating electrical power but they are starved for the neutrons that slam into nuclei and break them down. That is why power reactors are regulated by pulling carbon rods in and out of the “pile” of fissionable elements—the carbon can absorb neutrons, cooling the whole ensemble and preventing overheating.

The next big revolution in nukes would then come with the invention of practical thermonuclear
fusion
machines.

Fusion slams light nuclei like hydrogen or helium together, also yielding energy, as in the hydrogen bomb. Unlike fission, fusion is rich in hot particles but has trouble making much energy.

Most spaceflight engineers have paid little attention to fusion, believing—as the skeptics have said for half a century—that controlled fusion power plants lie twenty years ahead, and always will. Fusion has to hold hot plasma in magnetic bottles, because ordinary materials cannot take the punishment. The most successful bottle is a magnetic doughnut, most prominently the Russian-inspired Tokamak.

To make it into a rocket, let the doughnut collapse. Fusion rockets are the opposite of fusion electric power plants—they work by letting confinement fail. Ions fly out. Repeat, by building the doughnut and starting the reaction again.

The rocket engine core is this come-and-go doughnut, holding the plasma, then letting it escape down a magnetic gullet that shapes the plasma into a jet out the back. Rather than straining to confine the fusing, burning plasma, as our so-far-unsuccessful power plant designs do, a rocket could just relax the magnetic bottle.

So these fusion nukes are a wholly different sort of vehicle. They can promise far higher exhaust velocities than the fission nukes.

Leaving high Earth orbit, such ships will not ignite their fusion drives until they are well outside the Van Allen belts, the magnetic zones where particles are trapped—or otherwise the spray of plasma would short out innumerable communications and scientific satellites ringing the Earth. (This actually happened in 1962, when the USA project Starfish Prime set off a hydrogen bomb in the Van Allen belts. People have trouble believing anybody ever did this, but those were different days, indeed. The ions and electrons built up charge on our communications satellites, most of which belonged to the Department of Defense, and electrically shorted them out. Presto, billions of dollars lost in surveillance satellites gone dead within the first hour. A colossal embarrassment, never repeated.)

The Long Prospect

So will we have a space operatic future? If that means huge spacecraft driven by spectacular engines, maybe so. Interstellar flight lies beyond the technologically foreseeable, alas.

But the rest of the space opera agenda depends on your political prognostications. Will Iain Banks’s anarchist/socialist empire arise from remorseless economic forces? Or perhaps Robert Heinlein’s libertarian frontier?

Currently we’re “developing” space mostly with tax dollars that go into hugely inefficient projects like the International Space Station, which does very little research. We now pay the Russians to deliver our crews and Elon Musk’s SpaceX to deliver freight. What we need is
Ad Astra, Contra Bureaucratica
. The private opening of space will drive forward now, as low-orbit tourism and the first efforts to carry out repair and resource gathering like asteroid mining, at much greater distances. Still, this is a mere toe in the ocean.

Humanity’s current dilemma is exploding populations amid, and
versus
, environmental decay and dwindling resources. Of course we’ve dodged most of the bullets, thanks to the engineers and scientists. But we cannot count on them forever to solve our social problems.

Rick Tumlinson, a leading space advocate, put it this way:

Ultimately, nearly anything you want to do in a “sustainable” world will be something someone else cannot—and that will mean limits. Limits to when and where and how you travel, how much you consume, the size of your home, the foods you eat, the job where you work, even how long you are allowed to live… Yet Earth’s population continues to grow.

Quite Heinleinesque. Robert Zubrin, an eloquent exponent of space as the last and greatest frontier, puts it eloquently:

We see around us now an ever more apparent loss of vigor of American society: increasing fixity of the power structure and bureaucratization of all levels of society; impotence of political institutions to carry off great projects; the cancerous proliferation of regulations affecting all aspects of public, private and commercial life; the spread of irrationalism; the balkanization of popular culture; the loss of willingness by individuals to take risks, to fend or think for themselves; economic stagnation and decline; the deceleration of the rate of technological innovation and a loss of belief in the idea of progress itself. Everywhere you look, the writing is on the wall.

This is a neat way to summarize the agenda of an entire culture: the space frontier revolutionaries. They tend to be Heinleiner-style libertarians. It galls them that the future of space still lies in government hands.

I’ve been talking about the nuts and bolts of moving large masses around the solar system, for exploration or economics. But the ultimate agenda is one that has lain at the core of our society for centuries: the promise that expanding spatial horizons in turn opens those enlightening horizons of the mind that have made the modern age.

Many concepts will fail, and staying the course will require leadership.

Consider how John F. Kennedy voiced the goals of the Apollo program:

We choose to go to the Moon in this decade, and to do the other things, not because they are easy, but because they are hard. Because that goal will serve to organize and measure the best of our energies and skills.