The Case for Mars (41 page)

ELECTRIC PROPULSION

The key metric of a rocket’s performance is its specific impulse, the number of seconds it can use a pound of propellant to make a pound of thrust. The best chemical rockets available today have a specific impulse of about 450 seconds, while nuclear thermal rockets can get about 900 seconds.

But there is another way to make a high specific impulse. That is to ionize a gas by stripping some of the electrons off of its atoms and then accelerate it by means of the attractive and repulsive forces of an electrostatic grid. This technique is known as electric propulsion, or “ion drive.” Using electric propulsion you can generate specific impulses of many thousands of seconds, without ever heating the exhaust gas to very high temperatures. This is not just a theory, but a fact—ion drives are used for station-keeping propulsion on many satellites today. But you need a lot of electric power if you are going to create much thrust. For example, a 120-tonne spacecraft would need a power of 5 megawatts (about 70 times that planned for the international space station) to generate a thrust of 280 newtons (about 60 lbs) with a specific impulse of 5,000 seconds. Assu
ming it had this much power, however, it could generate the 30 km/s ΔV it would need to travel from low Earth orbit to Mars and back in about one year of continuous thrusting. Such a nuclear electric propulsion (NEP) spaceship could accomplish this incredible ΔV with a mass ratio of only about 1.82. The trajectories that electric propulsion vehicles must follow generally require rather more ΔV (typically a factor of two) to get from one point in the solar system to another than chemical propulsion systems do, but since the Isp is about 10 times higher, you still come out way ahead.

Electric ion thrusters already exist in kilowatt-sized units, and bulking them up to the megawatt sizes needed for NEP transportation systems offers no fundamental challenges. The real problem in enabling NEP propulsion systems to date has been to obtain the funds and sustained commitment required to develop a multi-megawatt space nuclear reactor.

SOLAR SAILS

Ships and sails proper for heavenly breezes should be fashioned ...

—Johannes Kepler, 1609

Nearly four hundred years ago our old friend Kepler observed that regardless of whether a comet is moving toward or away from the Sun, its tail always points away from the Sun. This caused him to guess that light emanating from the Sun exerts a force that pushes the comet’s tail away. He was right, although the fact that light exerts force had to wait till 1901 to be proven.

Well, if sunlight can push comet tails around, why can’t we use it to move spaceships around? Why can’t we just deploy big mirrors on our spacecraft, solar sails if you will, and have sunlight push on them to create propulsive force? The answer is that we can, but it takes an awful lot of sunlight to exert any significant amount of push. For example, at 1 AU, the Earth’s distance from the Sun, a solar sail the size of a square kilometer would receive a total force of 10 newtons, about 2.2 pounds, pushing on it from the Sun. So in order to make a solar sail into a practical propulsion system, you ne
ed to make it out of very thin material and cover a huge area. Let’s say we made a 1-square-kilometer sail 0.01 mm or 10 microns thick, about one fourth the thickness of a kitchen trash bag. In that case, the sail would weigh 10 tonnes, and it could accelerate itself 32 km/s in just about a year. Of course, if the sail were hauling a payload equal to its own weight, that would slow it down by a factor of two. Still, a 10-micron-thick solar sail would be in the ballpark for an effective propulsion device supporting Earth-Mars transportation. And if a 1-micron sail could be made, then we’d really be flying . . .

No one has ever flown a solar sail-propelled mission, but there was a very serious study done at NASA’s Jet Propulsion Laboratory in the 1970s to use one to drive a probe to Halley’s comet during its 1986 appearance. Unfortunately, the proposal went down the drain when Congress failed to fund the mission. Amateur groups, such as Robert Staehle’s World Space Foundation and the French Union pour la Promotion de la Propulsion Photonique, have built solar sails. They had hoped to fly a solar sail regatta race to the Moon during the Columbus anniversary year 1992, but have thus far failed to hitchhike rides on launch vehicles that would allow them to get their craft into space.

Solar sails do have some real technical problems that bear on packing, unpacking, deploying without damage, and controlling huge space structures made of very thin materials. Still, it has to be said that the main impediment to the demonstration of solar sails has not been the technical obstacles, but the refusal of the world’s space agencies to allocate any significant funds to the their development and testing. Let’s hope that the Martians will do better.

MAGNETIC SAILS

Sunlight is not the only forceful breeze that emanates from the Sun. There is another, known as the solar wind.

The solar wind is a flood of plasma, protons and electrons, that streams out constantly from the Sun in all directions at a velocity of about 500 km/s. We never encounter it here on Earth, because we are protected from it by the Earth’s magnetosphere.

If the E
arth’s magnetosphere blocks the solar wind, it must be creating drag, and therefore feel a force as a result. Why not create an artificial magnetosphere on a spacecraft and use the same effect for propulsion? This was an idea that Boeing engineer Dana Andrews and I hit on in 1988. The idea was timely. In 1987, high-temperature superconductors had been discovered. These are essential to making a magnetic propulsion device practical, as low-temperature superconductors require too much heavy cooling equipment and ordinary conductors require far too much power. The amount of force per square kilometer of solar wind is much less even than that created by snlight, but the area blocked off by a magnetic field could be made much larger than any practical solid solar sail. Working in collaboration, Dana and I derived equations and ran computer simulations of the solar wind impacting a spacecraft generating a large magnetic field. Our results: If practical high-temperature superconducting cable can be made that can conduct electrical current with the same density as the state-of-the art low-temperature superconductors such as niobium titanium (NbTi)—about 1 million amps per square centimeter—then
magnetic sails
or “magsails” can be made that will have thrust-to-weight ratios
a hundred times
better than that of a 10-micron-thick solar sail.
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Furthermore, unlike an ultra-thin solar sail, the magsail would not be difficult to deploy. Instead of being made of thin plastic film, it would
be made of rugged cable, which due to magnetic forces would automatically “inflate” itself into a stiff hoop shape as soon as electrical current was put in it. It would take power to get current flowing through the cable, but because superconducting wire has no electrical resistance, once the current was in the cable, no further power would be needed to keep it going. In addition, the magsail would shield the ship completely against solar flares.

A magsail can exert enough force in the direction away from the Sun to completely negate the Sun’s gravitational attraction, or it can have its current turned down so as to negate whatever portion of the Sun’s gravity is desired. Without going into details here, this capability would allow a ship co-orbiting the Sun with the Earth to shift itself into orbits that take it to any planet in the solar system, just by turning the magsail power up and down. And it all can be done without an ounce of propellant.

Magsails are not practical now, because the high-temperature superconducting cable they need does not exist. However, research in this field is proceeding fast. I think it’s a pretty good bet that ten or twenty years from now the type of cable required to make an excellent magsail will be widely available.

FUSION

Thermonuclear fusion reactors work by using magnetic fields to confine a plasma consisting of certain species of ultra-hot charged particles within a vacuum chamber where they can collide and react. Since high-energy particles have the ability gradually to fight their way out of the magnetic trap, the reactor chamber must be of a certain minimum size to stall the particles’ escape long enough for a reaction to occur. This minimum size requirement tends to make fusion power plants unattractive for low-power applications, but in the world of the future where human energy needs will be on a scale tens or hundreds of times greater than today, fusion will be far and away the cheapest game in town.

In addition to providing the power base needed for continued societal growth, fusion reactors can provide a very advanced spacecraft propulsion system, especially since in space the vacuum the reaction requires can be had for free in any size desired. The deuterium/helium-3 (D/He3) reaction provides the best performance, because the fuel has the highest energy-to-mass ratio of any substance found in nature, but the much cheaper pure deuterium fueled reaction (D-D) is about 60 percent as good. A rocket engine based upon controlled fusion could work simply by allowing the plasma to leak out of one end of a magnetic trap, adding ordinary hydrogen to the leaked plasma, and then directing the exhaust mixture away from the ship with a magnetic nozzle. The more hydrogen added, the higher the thrust, but the lower the exhaust velocity. For travel to Mars or the outer solar system, the exhaust would be about 99 percent ordinary hydrogen, and the exhaust velocity would be over 100 km/s (10,000 seconds Isp). If no hydrogen is added, a fusion configuration could theoretically yield exhaust velocities as high as 18,
000 km/s (1.8 million seconds Isp), or 6 percent the speed of light using deuterium/helium-3, or 4 percent the speed of light using pure deuterium! Although the thrust level of such pure D/He3 or D-D rockets would be too low for in-solar-system travel, the terrific exhaust velocity would make possible voyages to
nearby stars
with trip times of less than a century. Such a fusion-powered starship would only need to burn fuel to accelerate, since stopping could be accomplished by deploying a magsail to create drag with the interstellar plasma.

Fusion propulsion will ultimately make travel to Mars possible on a time-scale of weeks instead of months, travel to Jupiter and Saturn possible in months instead of years, and travel to other solar systems on time scales of decades instead of millennia. It may be that fusion spacecraft propulsion will evolve as an outgrowth of terrestrial power plants, but the reverse is at least equally likely. Recall that the first really reliable steam engines were built to power steamships, and the first practical nuclear power plants were on nuclear submarines. There is a reason why this happened. Mobile systems constantly demand higher technology, whereas static systems do not. To a consumer, a kilowatt is a kilowatt, whether produced by thermonuclear fusion or burning coal. But a fusion-powered spacecraft offers totally new and dramatically superior possibilities over any lower technology. Thus, the most forceful initial driver for the introduction of fusion may well be space propulsion, servicing the demands of business people engaged in Earth-Mars trade for increasingly fast means of transportation.

Currently the world’s fusion research programs are proceeding at a snail’s pace, devastated by budget cuts from shortsighted politicians who have neither the capacity nor the inclination to address future necessities.

By forcing us to tackle the problems of fusion technology development, the growth of Martian civilization may well provide the basis for the survival of technological society.

9: TERRAFORMING MARS

God made the world, but the Dutch made Holland.

—Traditional saying in the Netherlands

Thus far in this book we have discussed the prospects for relatively near-term exploration and settlement of Mars. Now we’ll address the ultimate challenge that the Red Planet presents to humanity—terraforming.
³⁹
,
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Can we transform Mars to make it fully habitable?

On the surface the idea appears to be utterl
y fantastical, simply science fiction. Yet, not so long ago, the subject of human voyages to the Moon was the domain of science fiction. Today lunar expeditions are a subject for
historians
, and manned Mars exploration the province of working engineers. Most people may
believe
that the prospect of drastically changing the Red Planet’s temperature and atmosphere to create more Earthlike conditions—“terraforming” Mars—is either sheer fantasy or at best a technological challenge for the far distant future. However, unlike some other extreme engineering concepts—faster-than-light travel or nanotechnology, say—terraforming has some history to it, about four billion years’ worth.

The history of life on Earth is one of terraforming—that’s why our beautiful blue planet exists as it does. When the Earth was born, it had no oxygen in its atmosphere, only carbon dioxide and nitrogen, and the land was composed of barren rock. It was fortunate that the Sun was only about 70 percent as bright then as it i
s now, because if the present-day Sun had shined down on that Earth, the thick layer of carbon dioxide in the atmosphere would have provided enough of a greenhouse effect to turn the planet into a boiling Venus-like hell. Fortunately, however, photosynthetic organisms evolved that transformed the carbon dioxide in Earth’s atmosphere into oxygen, in the process completely changing the surface chemistry of the planet. As a result of this activity not only was a runaway greenhouse effect on Earth avoided, but the evolution of aerobic organisms, those using oxygen-based respiration, was begun. These animals and plants then proceeded to modify the Earth still more, colonizing the land, creating soil, and drastically modifying global climate. Life is selfish, so it’s not surprising that all of the modifications that life has made to the Earth have contributed to enhancing its prospects, expanding the biosphere, and accelerating its rate of development for new capabilities to improve the Earth as a home for life still more.