Is it appropriate to allow the use of nuclear technology in space

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ASTRONOMY AND SPACEEXPLORATION

Is it appropriate to allow the use of nuclear technology in space?

Viewpoint: Yes, nuclear technology offers the only feasible method for many future space missions.

Viewpoint: No, nuclear technology in space is a dangerous proposition that should be avoided.

Few topics generate as much impassioned debate as the use of nuclear energy—the extraction of energy from radioactive nuclei by fission (splitting of the atomic nuclei) or fusion (slamming two lighter nuclei together to produce a heavier one and an attendant release of energy). Nuclear energy is efficient and clean and produces markedly greater energy return per unit amount of fuel than do other methods of energy generation.

Nuclear energy was first harnessed as a means of developing and delivering weapons of mass destruction: the A-bombs that were dropped on Hiroshima and Nagasaki on August 6 and 9, 1945, to end World War II. Although even this action has generated fierce debate (whether it was better to end the war catastrophically but quickly or to allow it to continue as a conventional conflict), mass killing remains the introduction of nuclear technology to human history. Decades of nuclear saber rattling by various world powers have served as a constant reminder of this fact. Since Hiroshima, nuclear technology has been put to more benign means, and today nuclear power plants deliver efficient energy in many parts of the world. Widely publicized power plant accidents, however, such as those at Three Mile Island, near Harrisburg, Pennsylvania, and the far worse accident at Chernobyl, Ukraine, are an ongoing source of concern about the use and abuse of nuclear power. It is therefore not surprising that debate has surrounded the use of nuclear propellant and power systems in spacecraft.

The so-called space age began in 1957, when the Soviet Union launched the tiny 10-inch satellite Sputnik ("Explorer") into Earth orbit. Since then, the question of providing power to spacecraft that need it has been a constant challenge for engineers.

The bulk motion of a spacecraft is not the problem. Once launched into orbit or onto an interplanetary trajectory by a rocket, a spacecraft needs little fuel for propulsion. With no atmospheric friction to slow it, the spacecraft coasts solely under the influence of the gravity of the Sun and that of any planets it happens to be near. Planetary gravitational fields frequently are used to accelerate spacecraft on their journeys—the ultimate example of clean energy use. Onboard propellant is used only for small course corrections. Some form of control is essential, however, as is power for the space-craft's control systems and any scientific instrumentation it carries.

Critical to the choice of power generation, and to almost every other design decision for a spacecraft, is weight. Even in the age of the space shuttle, most satellites and spacecraft are launched from Earth with various types of rockets. The Delta rocket is commonly used, and the Titan can be used for larger payloads. The rocket has to launch out of Earth's gravitational well not only the weight of its payload but also its own weight. Although the idea of "staging" rockets—discarding portions of the rocket in which fuel is spent to reduce the remaining weight—works well, most rockets are limited in their payloads. Every ounce not spent on propulsion and power equipment can be used for instrumentation and communication, a vital design goal for interplanetary spacecraft intended to perform ongoing scientific observations of other planets.

To send spacecraft to explore the outer parts of the solar system, using the Sun as a power source becomes more difficult. Many spacecraft in Earth's orbit, such as the 1970s Skylab, the Hubble space telescope, and the extraordinarily long-lived International Ultraviolet Explorer, have used solar panels to harness the Sun's energy as a power source. In the cold, dark outer reaches of the solar system, the Sun's feeble energy is a less satisfactory option, and radioisotopic thermal generators (RTGs) are preferred.

Although recent debate has centered on the RTGs aboard the Cassini spacecraft due to reach Saturn in 2004, the general issue of use of nuclear energy sources in spacecraft has been a political hot button since these fuels were first used.

Concerns about the use of nuclear technology in spacecraft center on the possibility of dispersal of radioactive material into the atmosphere after a failed launch or after reentry and breakup of a satellite bearing the material. Although the subject matter of many disputes in science leads them to be of interest to the scientific community only, the issue of spacecraft power sources has boiled over to the general public and the political arena. The dispute stems from understandable long-term unease about the widespread destruction that would result from a full nuclear exchange and the history of nuclear accidents since 1945. The concerns have served to cloud with rhetoric the rational statements of both sides. The proponents of nuclear power in space occasionally dismiss their counterparts as naive fools while sidestepping the opponents' questions and supplying no reasoned arguments of their own. There also have been loud and at times almost violent denouncements from protesters of nuclear power. The following articles examine the evidence of successful and failed missions in which nuclear power was used, the payload and efficiency reasons that lead many scientists to support the use of nuclear energy aboard spacecraft, and the likely consequences of failed missions involving the amounts of radioactive material commonly involved. Debates such as this one, in which firmly held personal beliefs are involved, are the most difficult ones to evaluate methodically. The essays provide a foundation for readers to do so.

—JEFFREY C. HALL

Viewpoint: Yes, nuclear technology offers the only feasible method for many future space missions.

In the early years of exploration of outer space, small fuel cells, batteries, and solar modules provided energy for space missions. Even though only 100 to 500 watts of power were needed for these first-generation spacecraft, increasing energy requirements in space travel soon became obvious as increasingly sophisticated missions were flown. Increased power requirements caused engineers to investigate a variety of options, such as chemical, solar, beamed space energy, and nuclear sources. Today, future manned Moon and Mars missions will likely require one to two million watts (commonly called megawatts) of power.

Nuclear Space Technology

The assertion of this essay is that the use of nuclear technology in space is appropriate; that is, nuclear propellant and power systems (PPSs) meet the essential requirements needed for space missions: minimum weight and size and maximum efficiency, durability, and reliability all attained with appropriate levels of safety. In this article, power system refers to the on-board energy requirements of a spacecraft (components such as heaters and communications), and propulsion system refers to the energy storage, transfer, and conversion requirements to propel or maneuver a spacecraft (components such as the space shuttle's main engines). Nuclear technology can be applied to both onboard power systems and to the propulsion of spacecraft (for example, nuclear rockets).

With the projected growth in energy required by increasingly complex and extended missions, nuclear PPS technology is the only feasible performance option when it is compared with chemical PPS technology (the only other realistic possibility and the one most often used in the past).

NASA Nuclear Agenda

For its fiscal 2003 budget, the National Aeronautics and Space Administration (NASA) is providing additional funds for nuclear systems because agency officials realize that for the near future only nuclear systems will adequately propel and power spacecraft for the exploration of the solar system and beyond. Administrator Sean O'Keefe has declared that NASA has been restricted in the past in its ability to explore long distances in relatively short times because of its reliance on current chemical propulsion technology. O'Keefe contends that nuclear propulsion and power capability is needed to overcome this distance and time problem.

The strength of nuclear propulsion is that it is more efficient than traditional chemically propulsion. Stanley Borowski, a nuclear and aerospace engineer at NASA's Glenn Research Center in Cleveland, Ohio, states that rockets launched with nuclear propulsion have twice the propellant mileage of chemical rockets currently in use.

After NASA's announcement regarding expanded use of nuclear PPSs, experts from agencies such as the Department of Energy (DOE), Los Alamos National Laboratory, and Sandia National Laboratories (SNL) expressed satisfaction that nuclear systems would once-again be used on a regular basis. Scientist Roger Lenard of SNL asserts that space travel into unexplored parts of the solar system requires development of a better PPS.

Past Nuclear Research and Development

Research and development of nuclear PPSs have progressed markedly in the two major space powers: the United States and Russia. During the last three decades of the twentieth century, both countries used nuclear power sources to meet the electrical and thermal energy requirements of selected spacecraft. These power requirements include operation of onboard scientific experiments, spacecraft maintenance and monitoring, temperature control, and communication. Nuclear fuel has proved an ideal energy source in space because of its high power, acceptable weight and volume, and excellent reliability and safety in systems such as RTGs.

RTGs were first developed in the 1960s for the U.S. Space Nuclear Auxiliary Power (SNAP) program. The United States launched RTGs for test satellites in the 1960s, the Apollo lunar missions of the early 1970s, and missions to the planets past Mars (such as the Galileo, Voyager, and Cassini missions). RTGs have safely and effectively been used in robotic missions beyond the orbit of Pluto, where use of other systems would have proved extremely cumbersome and exorbitantly expensive.

RTGs have proved to be highly reliable and almost maintenance-free power supplies capable of producing as much as several kilowatts of electrical power. These generators have operated efficiently for decades under adverse conditions, such as deep space exploration. Even at lower-than-normal power levels, communication with Pioneer 10 (launched in 1972) has been maintained for 30 years. The Voyager 1 and Voyager 2 spacecraft (both launched in 1977) have operated for 25 years and are expected to function for at least 15 years more. The reliability of RTGs stems from the ability to convert heat to electricity without moving parts. That is, RTGs convert thermal energy generated from radioisotopic decay into electricity. The radioisotope consists of plutonium-238 oxide fuel used with static electrical converter systems. As of 2002, the DOE has provided 44 RTGs for use on 26 space missions to provide some or all of the onboard electric power. RTG reliability is extremely important in space applications in which there is a large investment and equipment repair or replacement is not feasible. Nuclear power systems have been largely ignored in the United States as effective power generation options because of political pressure and antinuclear sentiment.

Both the United States and Russia have developed prototypes of rockets that have their exhaust gases heated not by a chemical reaction ("burning") but with a reaction in which a nuclear fuel such as uranium is heated and turned from a liquid into a high-temperature gas. These nuclear rockets have been shown to be much more efficient than conventional chemical rockets.

Research and development of U.S. nuclear-propelled rockets was a major endeavor from the mid 1950s to the early 1970s. In 1956 a project named Rover began at the Los Alamos Scientific Laboratory. The aim of Rover was to develop and test nuclear reactors that would form the basis of a future nuclear rocket. Between 1959 and 1972, Los Alamos scientists built and tested 13 research reactors. These scientists found that one of the main advantages of a nuclear rocket is its high exhaust velocity, which is more than two times greater than that of chemical rockets. The higher velocity translates into lower initial fuel mass and shorter travel times in space. A spin-off of Rover was NERVA (nuclear engine for rocket vehicle application). Initiated by NASA in 1963, NERVA was designed to take the graphite-based reactor built in the Rover program and develop a working rocket engine for space flight. The system designed from NERVA was a graphite-core nuclear reactor located between a liquid hydrogen propellant tank and a rocket nozzle. The nuclear reactor would heat hydrogen to a high temperature, and the gas would be expelled out the rocket nozzle. The Rover/NERVA nuclear rocket program, in which six reactor-nuclear engines were tested, successfully demonstrated that a nuclear reactor could be used to heat liquid hydrogen for spacecraft propulsion.

Chemical-Based Rocket Propulsion

To put the performance of nuclear propulsion in perspective, it is instructive to examine the basic design and performance of chemical rocket systems, which currently dominate all types of rocketry. Chemical rocket technology typically entails use of a fuel and an oxidizer. For example, the space shuttle's main engines operate through a chemical reaction between liquid oxygen (oxidizer) and liquid hydrogen (fuel). The combustion of the oxygen-hydrogen mixture releases heat in the form of steam and excess hydrogen. These hot gases are then expelled through a thermodynamic nozzle and provide thrust to lift the rocket.

The chemical propulsion system has limited effectiveness because of its specific impulse (ISP). ISP is the pounds of thrust produced per pound of propellant consumed per second, measured in seconds. High ISP, like a high "miles per gallon" for a car, is desirable to minimize propellant consumption, maximize payload, and increase spacecraft velocity, which translate to shorter travel time. The problem with chemical rockets is that most of the vehicle's weight is propellant; the result is a low ISP. To increase ISP, chemical rockets are built in stages—each stage is ejected once its propellant is consumed. Even with this improvement in reducing weight (that is, use of rocket staging), a maximum ISP for chemical engines usually is 400 to 500 seconds, which is relatively small compared with the ISP of nuclear-based propulsion systems.

ISP effectiveness is limited in chemical rockets because the same materials are used for heat source and propellant. The most efficient chemical rockets burn hydrogen and oxygen (an element of relatively high molecular weight) as the heat source to form superheated steam as the exhaust gas. The molecular formula of steam is H2O (water). Because the oxygen atom (O) is 16 times heavier than the hydrogen (H) atom, a low-molecular-weight propellant, the water molecule has 18 times the weight of the hydrogen atom and nine times the weight of the H2 molecule, the usual form of hydrogen. Such chemical systems must rely on heavier materials, which directly contribute to a lower ISP, and limit the amount of energy released. Finally, chemical rockets can generate high power levels only for short periods. It is impractical to dramatically increase "burn time" because the resulting rocket would become too massive and too expensive to launch.

Nuclear Propulsion: Overcoming Chemical Specific Impulse Limitations

Nuclear propulsion systems overcome the ISP limitations of chemical rockets because the sources of energy and propellant are independent of each other. In current designs, the energy source comes from a nuclear reactor in which neutrons split isotopes, such as uranium-235. The heat produced in this process is used to heat a low-molecular-weight propellant, such as hydrogen. The propellant is then accelerated through a thermodynamic nozzle, as it is in chemical rockets. At the same temperature attainable in chemical rockets, the propellant molecules of nuclear rockets (because of their lower molecular weights) move many times faster than those used in chemical rockets. Nuclear propulsion systems, therefore, allow higher ISPs, thousands of seconds as opposed to the 400 to 500 seconds in chemical systems.

The heat released in this nuclear process can be used for propelling a spaceship over long periods to high speeds and can be converted directly to electricity, either through static processes (nonmoving devices, such as a thermoelectric device) or dynamic processes (moving devices, such as a turbine). Such processes can provide approximately one megawatt of continuous power each day with only one gram of uranium.

Promising Nuclear Propulsion

Nuclear propulsion holds much promise for many future applications in space exploration. Nuclear propulsion will enable entire classes of missions that are currently unattainable with chemical systems. Nuclear propulsion systems are classified as being either thermal or electric.

The graphite-core nuclear reactors built in the NERVA program are nuclear thermal reactors. The nuclear reactor heats a propellant to a high temperature, and the propellant is exhausted out of an engine's nozzle, attaining an ISP of approximately 1,000 seconds. Although the NERVA program was terminated in 1973, nuclear consultant Michael Stancati of Science Applications International Corporation in Schaumburg, Illinois, recently declared that the nuclear thermal reactor is a very credible option for long-duration space missions.

Nuclear electric propulsion (NEP) is another possible system. In this method, the propellant is heated to a very high temperature and becomes plasma (an ionized gas). It is then accelerated by electrostatic or electromagnetic fields to increase the exhaust velocity. Because ISP is a function of exhaust velocity, a higher ISP results. Such a nuclear electric plasma rocket (similar to an ion rocket) could attain an ISP ranging from 800 to 30,000 seconds. Specialists claim NEP can reduce the travel time to Mars from nine months to three to five months.

NASA's Future Nuclear Program

NASA's future nuclear plans consist of two parts: one developed and one not. Initially, existing RTG technology will be further developed. Without RTGs, NASA and DOE leaders believe that the ability to explore neighboring planets and deep space will not occur in accordance with current project requirements.

Further into the future, several advanced technologies are envisioned for nuclear space systems. One new technology, called Stirling radioisotope generator (SRG), will be used as the dynamic part of nuclear reactors. SRG has the potential to be a high-efficiency power source for missions of long duration, eventually replacing the low-efficiency RTGs. Edward Weiler, NASA's head of space sciences, has said that NASA's nuclear initiative will conduct research in nuclear power and propulsion in areas such as nuclear fission reactors joined with ion drive engines (similar to NEP).

Safety Concerns

Nuclear PPSs are not without risk. Safety concerns have always reduced their usability and in many cases has stopped them from being deployed. Although the risk from nuclear PPSs is greater than that from chemical systems, former space shuttle astronaut and senior NASA scientist Roger Crouch says that people's fears about nuclear materials are not grounded on a realistic risk assessment. More than 30 years have been invested in the engineering, safety analysis, and testing of RTGs. This proven technology has been used in 26 U.S. space projects, including the Apollo lunar landings, the Ulysses mission to the Sun's poles, the Viking landings on Mars, the Pioneer missions to Jupiter and Saturn, and the Cassini mission to Saturn and Titan. The RTGs have never caused a spacecraft failure. Three accidents with spacecraft that contained RTGs have occurred, but in each case the RTGs performed as designed, and the malfunctions involved unrelated systems.

Summary

Because of its many advantages, nuclear energy will continue to provide propulsion and power on space missions well through the twentieth-first century, whether as RTGs, other advanced generators, or nuclear reactors. For spacecraft missions that require up to a few kilowatts of power, an RTG could be the most cost-effective solution. However, for spacecraft missions with large power requirements (in the range of megawatts, or thousands of kilowatts, of power), a nuclear reactor is a better alternative.

Wesley Huntress, president of The Planetary Society, the world's largest space advocacy group, said recently that the development of nuclear propulsion and power technology will make exploration of the solar system more accessible with much shorter flight times and more powerful investigations of the planets. Huntress compared the hundreds of nuclear submarines that ply the world's oceans with the nuclear-powered spaceships that will one day explore outer space.

Even though nuclear space technology was essentially scrapped in the 1980s owing to public fears about its safety, the space community was, and still is, clear that the capabilities of this technology will dramatically reduce flight time to the planets and provide almost unlimited power for operation in space and on the planets. With a return to nuclear technology, the two limiting problems with exploring space—travel time and available power—will be solved.

Compared with the best chemical rockets, nuclear PPSs are more reliable, more efficient, and more flexible for long-distance, complex missions that require high amounts of power for long amounts of time. The American Nuclear Society supports and advocates the development and use of radioisotopic and reactor-based nuclear systems for current and future use to explore and develop space with both manned and robotic spacecraft.

—WILLIAM ARTHUR ATKINS

Viewpoint: No, nuclear technology in space is a dangerous proposition that should be avoided.

"Remember the old Hollywood movies when a mad scientist would risk the world to carry out his particular project? Well, those mad scientists have moved to NASA." This quote would be amusing if not for its source. Dr. Horst Albin Poehler worked as a scientist for NASA contractors for 22 years, 15 of those as senior scientist. Poehler was talking about the Cassini mission NASA launched in 1997. The nuclear-powered probe, headed for Saturn, carries 72 pounds (32.7 kg) of plutonium-238 to power its instruments.

Plutonium-238 is a reliable source of energy, and the amount required to produce this energy is not prohibitively large. Because plutonium-238 emits alpha radiation, no heavy shielding of electronics is required on board a spacecraft. (Alpha radiation, actually decay, is the process by which a particle consisting of two neutrons and two protons is ejected from the nucleus of an atom. The particles lose energy rapidly and thus are readily absorbed by air.) For all these reasons, and because it is easy to obtain, plutonium-238 is a favorite of engineers looking for inexpensive, sustainable power sources.

Plutonium-238 is also 280 times more radioactive than is plutonium-239, the plutonium isotope used in nuclear weapons. Its oxide, the compound produced when plutonium-238 and oxygen combine, is extremely dangerous to humans. A fraction of a gram of plutonium-238 is considered a carcinogenic dose if inhaled. Plutonium-238 also can be transported in the blood to the bone marrow and other parts of the body, causing further destruction. The RTGs aboard a spacecraft carry plutonium dioxide.

The Record to Date

If we are to accept the use of nuclear technology in space, we must recognize that there are inherent risks in the use of such technology. Atomic energy, although attractive on many levels, can cause untold suffering in successive generations when things go wrong. We accept this risk with nuclear power plants situated too close to populated areas. We believe scientists when they tell us that sending 72 pounds of plutonium-238 to space is a risk-free proposition. This amount is the equivalent of 17,000 pounds (7.7 metric tons) of plutonium-239 (the plutonium used to build nuclear bombs), according to nuclear physicist Richard E. Webb, author of The Accident Hazards of Nuclear Power Plants.

Should We Really Be So Trusting?

The United States launched 26 missions to space with nuclear material on board. Three missions resulted in accidents. The most famous, though not the worst, mishap was the Apollo 13 accident. The lunar excursion module that returned to Earth and was deposited by NASA in the Pacific Ocean carried an RTG containing more than 8 pounds (3.6 kg) of plutonium-238. Water sampling in the area revealed no radioactivity.

The worst accident involving a U.S. RTG in space occurred in 1964. A satellite powered by an RTG designated SNAP 9-A failed to achieve orbit. The burn-up on reentry caused the release and dispersal of the plutonium core, in a fine dust, the way NASA had planned. At the time the belief was that dispersing the contamination was preferable to a concentrated rain of plutonium. The SNAP 9-A accident, according to Dr. John Gofman of the University of California at Berkeley, contributed to a high global rate of lung cancer. Gofman is a physicist who worked on the Manhattan Project. He earned a medical degree and became a world authority on the dangers of low-level radiation. NASA admits that radioactivity from SNAP 9-A was found in the soil and water samples taken by the Atomic Energy Commission. After the SNAP accident, NASA changed its RTG design to achieve "full fuel containment." This change came about, however, only after SNAP 9-A contamination had been detected on almost every continent and people had died because of the contamination.

Russia, and the Soviet Union, has had more than 40 launches involving nuclear power. Six accidents have occurred, most recently in 1996, when a Mars probe fell to earth and burned over Chile and Bolivia. The amount of radiation released is labeled "unknown," and no attempt has been made to retrieve the radioactive core of the probe or its remains.

In 1978 the Soviet satellite Cosmos 954 disintegrated over the Northwest Territories in Canada. Scattered across a vast area were thousands of radioactive particles, pieces of the satellite's nuclear power core that survived reentry. Dr. Michio Kaku, professor of theoretical physics at the City University of New York declared, "If Cosmos 954 had sprayed debris over populated land, it would have created a catastrophe of nightmarish proportions." Later estimates were that close to 75% of the radioactive material was vaporized on reentry and dispersed over the planet.

This safety record on two continents shows that we keep paying the price for trusting nuclear technology. It is a lesson that we failed to learn after Three Mile Island and Chernobyl.

Claims for Safety

In the early 1980s NASA estimated the chance of a catastrophic space shuttle accident was one in 100,000. After theChallenger accident on the twenty-fifth shuttle mission, January 28, 1986, NASA lowered its estimate to one in 76. Nobel laureate physicist Richard Feynman, a member of the Rogers Commission that investigated the Challenger accident, declared in an appendix to the Rogers report that NASA management "exaggerates the reliability of its product, to the point of fantasy." NASA ignored incidents on previous shuttle missions that clearly pointed to a design flaw or design problem. Feynman points out that NASA took the fact that no accidents had occurred as "evidence of safety." In fact, NASA had known since 1977 there was a design flaw in the O-ring. Independent risk assessment put the probability of a solid rocket booster (SRB) failure at one in 100 at best (it was the O-ring in the SRB that failed). But before the Challenger accident, NASA stuck by its numbers, claiming the shuttle was safe simply because it had not crashed yet.

One would have hoped that the seven lives on the Challenger had not been lost in vain. But the NASA public relations machine overrode common sense when it came to Cassini. First came the claims that the Titan 4 rocket, which lifted Cassini into space, was safe. This rocket in fact had a spotted record over the years, memorably exploding in 1993, destroying a one-billion-dollar satellite system it was launching.

NASA reassured the public that even if the rocket were to explode in the atmosphere, the RTG would remain intact. NASA chose to ignore a test conducted by GE, the manufacturer of the RTG, that resulted in destruction of the generator. NASA claimed the test conditions were not realistic. Michio Kaku, however, showed quite clearly that under the extreme conditions of a launch-pad explosion, the RTG was very likely to rupture or be destroyed.

NASA then estimated that any plutonium released during a launch accident would remain in the launch area. As Kaku put it, "NASA engineers have discovered a new law of physics: the winds stop blowing during a rocket launch."

If we cannot trust NASA to honestly inform us of risks, should we trust them to launch plutonium dioxide over our heads?

The Human Error Factor

Even if the hardware is safe and the plutonium is well shielded, one factor in technology that cannot be resolved is that the likelihood of human error has no statistical value. As Kaku points out, "[O]ne can design a car such that the chances of an accident approach a million to one, with air bags … etc. However, this does not foresee the fact that someone might drive this car over a cliff."

Three Mile Island. Chernobyl. The Hubble space telescope. The Exxon Valdez. All these events were the results of human error, sometimes a chain of errors compounding one another. We can design the safest RTG, launch it with the safest rocket in the world, and still end up with plutonium raining down on us because of a simple human error. The Mars climate orbiter was lost because of a failure to convert pounds of thrust (an English unit of measure) to newtons (a metric unit). Two nuclear power plants in California were installed backward. Human error is inevitable.

The Alternative

Proponents of nuclear energy in space say that deep space missions require nuclear power because solar energy in deep space is insufficient. The fact is that solar energy cells and concentrators have improved vastly over the years. In 2003 the European Space Agency will be launching Rosetta, a deep space mission that will study the nucleus of comet 46P/Wirtanen. Rosetta will reach its target in 2012, with solar-powered instruments. The probe is designed to conserve energy and achieve its longevity in part by putting the electrical instruments on board into hibernation mode for a lengthy period. Rosetta will use solar energy and fuel; no nuclear material will be on board.

Dr. Ross McCluney, an optical physicist, pointed out in 1997 that NASA has several alternatives to nuclear power in space. Newer solar cells are highly efficient, so fewer are needed. New electronic components require less power to do their work. Instead of launching a massive probe, as in Cassini, the mission will be split into smaller probes, which will be less expensive and faster to operate than previous missions.

McCluney argues that hybrid power systems can be used for propulsion of probes going as far as nine astronomical units from the sun. Beyond that, McCluney reasons, we can wait for technology that does not require nuclear power. There is no reason to rush into very deep space if it entails risk to people on Earth.

Weapons in Space

The U.S. Space Command considers space the future battlefield. The George W. Bush administration has denounced the 1972 Antiballistic Missile Treaty, which among other things forbids antimissile tests. Although the 1967 Outer Space Treaty forbids weapons of mass destruction in space, the United States is researching combat technology in space and is testing space antiballistic systems. The effect of a U.S. military presence in space will most likely be an arms race with many other countries. In addition to the inherent dangers of sending plutonium into space in vehicles and containers of doubtful safety, the specter of a space nuclear arms race has entered the picture.

The twenty-sixth Challenger mission would have carried the Ulysses space probe, with 24.2 pounds (11 kg) of plutonium. Such a mission would have been far more dangerous than the one in which seven people died. Richard Feynman wrote in his appendix to the Rogers report: "When playing Russian roulette the fact that the first shot got off safely is little comfort for the next." Nuclear technology in space is a game of Russian roulette that must be stopped before the bullet hits us.

—ADI R. FERRARA

KEY TERMS

ENERGY:

The capacity for doing work.

FISSION:

The action of dividing or splitting something into two or more parts.

ISOTOPE:

Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.

ISP:

See specific impulse.

PLUTONIUM:

A synthetic element that emits alpha radiation. Plutonium is used in nuclear weapons and as an energy source. Chemical symbol, Pu.

POWER:

The rate of doing work.

RADIOACTIVE:

Emitting or relating to the emission of ionizing radiation or particles.

RADIOISOTOPIC THERMAL GENERATOR (RTG):

A small nuclear reactor onboard a space probe. The RTG generates energy for the probe's instruments.

RADIOISOTOPIC:

Related to a radioactive isotope.

SOLAR CELL:

The device used to convert the sun's energy to electricity.

SPECIFIC IMPULSE (ISP):

The pounds of thrust produced per pound of propellant consumed per second; measured in seconds.

ANTIMATTER FOR FUTURISTIC SPACECRAFT

Antimatter is one of the most recognized energy sources in science fiction literature. Every particle in the universe is said by physicists to possesses a mirror image of itself, an antiparticle, except that its charge is reversed. When matter meets antimatter, the two substances obliterate themselves and are converted into pure energy. The famous fictional spaceship the Enterprise from the television show Star Trek uses engines with an antimatter-powered warp drive to journey through interstellar space. Although warp drive is quite impossible with current and near-future technologies, power from antimatter is possible.

NASA is considering antimatter as a rocket propellant to travel throughout the solar system. One gram of antimatter would carry as much potential energy as that now carried on board approximately 1,000 space shuttle external tanks. Although antimatter-powered human space flight could work from a physics point of view, it currently (and in the near future) is unfeasible from the engineering and economic standpoints.

Gamma rays, which are given off by anti-matter, are deadly to humans. The necessary shielding for human protection from gamma rays would be so heavy that it would offset the enormous energy gains of using antimatter. Also, at $100 billion per milligram, less than one grain of sand, antimatter is the most expensive material in the universe.

Pennsylvania State University physicist Gerald Smith says antimatter can be used in alternative forms. Instead of using antimatter as the sole source of energy, Smith's group is investigating the use of tiny amounts of antimatter as "triggers" to begin a nuclear fusion reaction with pellets of hydrogen. Smith comments that using antimatter in this way is like using "a lot of little hydrogen bombs." For example, the Pennsylvania State research team is designing a high-speed ion compressed antimatter nuclear (ICAN-II) engine that would use antiprotons to implode pellets with nuclear fusion particles at their cores. Located between the ICAN-II engine and the crew compartment would be massive shock absorbers that would cushion the ship as a series of small blasts propelled it through space.

Harold P. Gerrish, Jr., of the Marshall Space Flight Center propulsion laboratory says that such future spacecraft will use only a few billionths of a gram of antimatter to propel, for example, a 400-ton spacecraft to Mars and back in as little as three months. The same trip with traditional chemical engines would take eight to ten months.

—William ArthurAtkins

Science in Dispute