Was the use of plutonium as an energy source for the Cassini spacecraft both safe and justifiable

views updated

Was the use of plutonium as an energy source for the Cassini spacecraft both safe and justifiable?

Viewpoint: Yes, using plutonium as an energy source for Cassini carried only minimal safety risks, and it was the only currently feasible way to reach the outer planets in the solar system.

Viewpoint: No, using plutonium as an energy source for Cassini was not safe, given the known dangers of plutonium and the legacy of rocket launch failures.

Spacecraft must do a variety of things during a mission. They must be able to send information to mission scientists on Earth, and they must be able to receive and respond to instructions sent from Earth. The instruments they carry must function, and the computers that record images and data must be kept operational. Mechanical parts must be kept operational, and backup systems kept ready for use if needed. All of these tasks require a significant amount of power, and a perennial challenge for space mission planners has been providing power to the spacecraft in question.

Familiar methods of providing power are not feasible. One cannot just plug a satellite into an electric wall socket, and batteries have a limited lifetime, have a limited capacity to provide power, and are extremely heavy. For these reasons, many Earth-orbiting missions have used solar panels. From the Skylab space stations of the 1970s to the Hubble Space Telescope, satellites have sported large panels that collect sunlight and convert it to power, just as solar-powered homes and vehicles use similar panels on Earth.

Not long after the Apollo landings on the Moon, our vision for space exploration turned to the outer solar system. The Voyager spacecraft generated immense public excitement with their grand tours of the giant planets in the 1970s and 1980s. More recently a variety of advanced, robotic missions have returned to these distant worlds—Galileo, designed to return a wealth of images and data from its orbits of Jupiter and its moons, and, beginning in 1997, a large, complex spacecraft called Cassini. Launched in October 1997, Cassini will arrive at Saturn in 2004.

Cassini faces a problem that the Hubble Space Telescope does not: Saturn is a long way from the Sun. Saturn is nearly 10 times farther from the Sun than Earth, and receives only 1% as much sunlight as Earth. At this distance, solar panels are either ineffective or must be prohibitively large. Another source of power is required, and the solution used for Cassini—and other missions—has been radioisotope thermoelectric generators (RTGs). These components use the decay of radioactive elements such as plutonium to provide an ongoing power source for a spacecraft. They do not care how far they are from the Sun and, even better, they are small and light—a critical advantage for missions where every gram of the payload is carefully considered to maximize the spacecraft's capabilities and benefits.

The use of radioactive materials in space missions, however, has sparked a heated debate that has been partly scientific but largely political. The development, stockpiling, and use of nuclear weapons has been one of the defining debates of the last half-century. By extension, any project in which the word "radioactive" appears has become the object of close scrutiny, and RTGs have been no exception.

Opponents of RTGs point out that substances such as plutonium are lethal to humans in sufficient quantity. Since present-day launch vehicles have an undeniable track record of occasionally exploding during or shortly after launch, the possible dispersion of plutonium into the atmosphere has been held forth as a principal argument against the use of RTGs. Others counter this argument by pointing out that RTGs are specifically designed to withstand a launch failure, that inhalation of plutonium is impossible since the particles will be much larger than the dust-sized grains necessary for inhalation, and that the total amount of radioactivity produced in a given area by dispersed RTG plutonium would be less than that from naturally occurring sources.

Cassini faced additional opposition because after it was launched, it flew around the Sun and returned for a close pass by Earth before heading out to Saturn. This is standard practice for missions to the outer solar system—spacecraft use a gravitational slingshot maneuver to pick up speed and get them to their destination. This flight path provides the velocity necessary to climb away from the Sun's gravitational field and journey to the outer solar system. Spacecraft coast to their destination, with the initial impetus provided by the launch vehicle. Heavy spacecraft—and Cassini is very heavy—need an additional boost. The Earth flyby caused concern, of course: what if Cassini's trajectory were a bit wrong, and it crashed into Earth? The same counterarguments about the small quantity of plutonium in the RTGs applied in this case as well, but few critics' fears were assuaged.

A final criticism of Cassini touches a broad policy debate at the National Aeronautics and Space Administration (NASA). Cassini is a huge spacecraft, bristling with instruments. When it enters the Saturn system, the returns will be monumental and will revolutionize our understanding of this distant planet and its moons. Cassini's hardware requires lots of power, however, and precludes the use of solar panels. Smaller spacecraft, some argue, could use alternatives to RTGs and would be less expensive to build. A larger number of smaller, more focused missions could return as much information and understanding as one monolithic one, without excessive power requirements. The former NASA administrator Daniel Goldin's slogan of "faster, better, cheaper" signaled NASA's interest in this type of mission, although after some well-publicized failures in the 1990s, cynics in the industry suggested "pick any two" as an appropriate postscript.

In the end, the Cassini launch and Earth flyby went without a hitch, but future missions employing RTGs will no doubt raise similar debates.

—JEFFREY HALL

Viewpoint: Yes, using plutonium as an energy source for Cassini carried only minimal safety risks, and it was the only currently feasible way to reach the outer planets in the solar system.

Past the Orbit of Mars

Most spacecraft use solar energy as a power source to provide electricity for operations and to heat the spacecraft's instruments, systems, and structures. However, in some cases solar and other traditional power-source technologies are not practical, and an alternate power source is required for the spacecraft. One of these instances occurs when spacecraft travel to the far reaches of the solar system, beyond the effective use of the Sun as an energy source. According to the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA), the Cassini mission requires the use of nuclear material because other power sources are inadequate for (1) its extensive science objectives; (2) the current launch systems available to lift Cassini's mass into orbit; (3) the travel time required to reach Cassini's destination, the planet Saturn; and (4) the far distance of Saturn from the Sun. Specifically, NASA has determined that the normally used energy of the Sun is inadequate as a practical power source for any spacecraft that operates beyond the orbit of Mars. This is primarily because solar panels are impractical due to the weak sunlight at those distances. NASA has further determined that the only practical source of power at distances beyond Mars is nuclear material. In the case of the Cassini mission, the nuclear material chosen was plutonium 238.

Radioisotope Thermoelectric Generators (RTGs)

The electrical power supply for the Cassini spacecraft and its instruments is provided by three plutonium batteries called general purpose heat source radioisotope thermoelectric generators (GPHS RTGs). Each device (usually abbreviated RTG) is designed to use the slow decay of plutonium 238 (denoted Pu-238) in order to generate heat. The heat generated by this process is then changed into electricity by a solid-state thermoelectric converter, converting heat into about 850 watts of electrical power for all three RTGs. Leftover RTG heat is passed through the spacecraft to warm operational components and systems.

RTGs are compact and lightweight spacecraft power systems that are very reliable, possess an outstanding safety record, and contain no moving parts. This proven technology has been used since the early 1960s within about two dozen U.S. space projects, including the Lincoln Experimental Satellites, Apollo lunar landings, Pioneer missions to Jupiter and Saturn, Viking landers sent to Mars, Voyager missions to Jupiter, Saturn, Uranus, and Neptune, Galileo mission to Jupiter, and Ulysses mission to the Sun's polar regions.

The RTGs have never caused a spacecraft failure; however, three accidents with spacecraft that contained RTGs have occurred. In each case the RTGs performed as designed, while the malfunctions involved other, unrelated systems. In 1964 a SNAP-9A RTG burned up in the atmosphere when a U.S. Navy Transit 5-BN-3 satellite reentered Earth's atmosphere. The plutonium was scattered in the atmosphere, but after six years only slightly higher levels of radioactivity were detected in soil samples. In 1968 a Nimbus B-1 weather satellite was targeted into the Pacific Ocean when its launch rocket failed. After being retrieved from the ocean floor, the RTG was still intact and was later reused on another satellite. In 1970 the Apollo 13 lunar module reentered Earth's atmosphere after successfully returning three astronauts from their life-or-death voyage around the Moon. No evidence of increased radiation was found from the affected impact area. These three accidents—where the RTGs performed as expected—affirm the safety of the RTGs when used in spacecraft such as Cassini.

Plutonium 238

The nuclear fuel source within Cassini is 72 lb (33 kg) of plutonium 238, with a half-life of 88 years. Plutonium 238 is a naturally radioactive, silver-metallic element used both as a reactor fuel in nuclear weapons and as a fuel source in spacecraft. Pu-238 emits alpha particles as its nuclei spontaneously fission at a very slow rate.

A major problem with alpha particles occurs when radioactive plutonium is ingested or inhaled. Alpha particles can then be emitted within the body. When this happens, alpha particles can inflict cell damage in any living organism, which can lead to cancer. To be inhaled or ingested by humans, plutonium must be ground into dust particles. However, the plutonium onboard Cassini is in the form of plutonium dioxide, a ceramic material that only breaks down into large chunks that are impossible to inhale and very difficult to crush into dust. This fuel is also highly insoluble in water, which makes it difficult for it to enter the food chain; is heat resistant, which reduces its chance of vaporizing in fire or reentry environments; and has a low chemical reactivity, which makes it much less likely to cause damage. All of these safety features aboard Cassini help to reduce the potential health effects from accidents involving a Pu-238 release.

If, by chance, dust-sized particles were released, very little exposure to humans would result. Extensive studies by NASA have shown that over a 50-year period, a person exposed to plutonium dust released in a launch accident would be subjected to about 1 millirem. This upper limit is 500 millirem if a person should inhale all of the plutonium dust in an 11 sq ft (1 sq m) area of the contaminated area. In contrast, according to the Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry, natural sources—such as house radon, rocks, cosmic rays, and the decay of naturally occurring radioactive elements in the human body—would release radiation in the amount of 15,000 to 18,000 millirem in that same 50-year time period.

Plutonium inside RTGs

The RTGs used aboard Cassini are the most advanced version of thermoelectric generators that were safely used on past successful missions. Each of the 216 Pu-238 pellets onboard Cassini is encased in capsules of heat-resistant iridium—a dense, corrosive-resistant, highly heat-resistant metal. Two pellets are then wrapped in a thermal graphite shell. Two of these shells are encased in a graphite block—a lightweight, highly corrosive-resistant, high-strength carbon-based material. The graphite block is about the size of two cell phones, and 18 of these blocks go into each of the RTGs. Previous tests conducted by NASA have shown that these modules will remain structurally sound even in an explosion like that of the Challenger in 1986 or other such devastating accident. These characteristics help to diminish the potential health effects from accidents involving the release of plutonium.

RTGs Are Not Nuclear Reactors

RTGs are often compared to nuclear reactors because both use nuclear substances. However, RTGs are not nuclear reactors. While nuclear reactors use a "man-made" fission chain reaction to produce their power, RTGs use the "natural" radioactive decay of an isotope called Pu-238 to produce heat and ultimately to produce electricity. Since RTGs do not use an artificial fission chain reaction process, they could not cause an accident like those that have occurred within nuclear reactors (such as at Chernobyl in 1986 and Three Mile Island in 1979), and could not explode like nuclear bombs. Therefore, it is invalid to associate an RTG accident, or a potential accident, with any past radiation accidents involving nuclear reactors or with nuclear bombs. RTGs do not use any nuclear reactor type of process and could never explode like a nuclear bomb. Neither could an accident involving an RTG create the acute radiation sickness similar to that associated with nuclear explosions.

Safety Design of RTGs

Extensive engineering and safety analysis and testing have shown that RTGs can withstand severe accidents of the sort that can happen during space missions. Safety features incorporated into the design of RTGs have demonstrated that they can withstand physical conditions more severe than those expected from most accidents. Even before Cassini was launched, NASA performed detailed safety analysis under the guidance of internal safety requirements and reviews in order to reduce the risks of using RTGs. The U.S. Department of Energy has conducted extensive impact tests to ensure that the plutonium canisters remain intact in the event of a catastrophic accident. In addition, an Interagency Nuclear Safety Review Panel, composed of experts from academia, government, and industry, performed external safety evaluations as part of the nuclear launch safety approval process.

Justifiable Use of RTGs

According to comprehensive research performed by JPL, the requirements placed upon Cassini by its primary scientific objectives, the available launch systems, the extensive distance necessary to reach Saturn from Earth, and the great distance of Saturn from the Sun necessitated the use of RTGs. These mission requirements determined the RTG power source to be far superior over other power sources with respect to power output, reliability, and durability when used in the outer solar system.

For Cassini to complete its scientific objectives it must carry about 6,000 lb (2,720 kg) of fuel to Saturn and its four-year Saturnian orbit. Because so much fuel is required to be carried onboard, the spacecraft must be as light as possible in order to accommodate about 795 lb (360 kg) of scientific instruments to conduct experiments including photographic and radar imaging, atmospheric sampling, and various studies of Saturn's planetary satellites and rings. As a result its power system must supply electricity to multiple scientific instruments at specific times, plus continuously power the spacecraft itself. To do all of these things, a lightweight and highly efficient power supply is required. The RTGs meet all of these requirements.

Alternatives to RTGs

JPL has concluded that neither fuel cells nor chemical batteries demonstrate the necessary operational life for Cassini, whose duration is expected to be 10.75 years, but could be extended to 16 years. In addition, the large amount of batteries that would be needed to power a Cassini mission greatly exceeds current launch vehicle lift capabilities.

Most NASA missions that fly through the inner solar system use solar panels to generate power because they are able to use the plentiful supply of sunlight to generate electricity. Even missions that have operated as far out as Mars—such as the Viking 1 and Viking 2 spacecraft—were all solar-powered missions. Solar panels are cheaper and lighter than RTGs and do not carry the same safety concerns as RTGs. But missions through the outer solar system (basically beyond Mars) exceed the functional use of solar technology, primarily because strong sunlight does not exist in those regions of space. JPL scientists have researched the recent advances in solar-power technologies, such as the high-efficiency solar arrays developed at the European Space Agency, and believe that solar technology is not capable of providing the necessary electrical power for space voyages beyond Mars. For the most part, the amount of solar arrays required for such an extended mission would make the spacecraft too heavy for any existing rockets to launch Cassini. Even if a rocket could lift such a heavy solar-powered spacecraft, significant risk to the success of a solar-powered mission would be introduced due to the lack of knowledge of such vehicles in space.

In addition, according to JPL, Cassini would require solar arrays with an area of just over 5,380 sq ft (500 sq m), or about the size of two tennis courts. With the need for two solar arrays plus supporting structures, including deployment equipment, increased complexity of the spacecraft's design would result. The operation of Cassini would also be more complex, limiting its ability to maneuver and communicate. This complexity would severely hurt Cassini's ability to achieve its scientific purpose. The enormous size of the solar arrays also would interfere with the fields of view of the science experiments. Equally bad, the solar arrays would limit the scope of the navigation sensors, further hindering Cassini in achieving its objectives. Lastly, the solar arrays could generate serious electromagnetic and electrostatic interference, adversely affecting the communications equipment and computers. Because of these reasons, NASA has concluded that RTGs are the only power source capable of reliably accomplishing the mission objectives of Cassini.

Conclusion

A two-foot-thick document details the government's six-year comprehensive safety analysis of Cassini, and independent experts have substantiated every assertion. Conclusions showed that the fuel modules were unlikely to be damaged in an accident. Even if all of the coatings and containers were to fail, there was little chance that any person would consume enough material to experience adverse health problems. NASA estimated that there was a 1 in 1,400 chance of a plutonium release accident early in the launch; a 1 in 476 chance of such a mishap later in the launch; and a less than 1 in a million chance of Cassini reentering the atmosphere and releasing plutonium during its Earth flyby.

Despite the opposition to its launch, Cassini promises to provide a wealth of scientific information. In-depth NASA studies show that RTGs were the only feasible power system for the Cassini mission, and are a safe and justifiable power system to use when other systems cannot be used.

—WILLIAM ARTHUR ATKINS

Viewpoint: No, using plutonium as an energy source for Cassini was not safe, given the known dangers of plutonium and the legacy of rocket launch failures.

The Cassini spacecraft was successfully launched in 1997. Leading up to Cassini's launch, serious concerns were raised about its onboard plutonium, which is used to supply the spacecraft's electrical power. Years after Cassini's launch, many people continue to have concerns regarding the use of radioactive substances as power sources on such spacecraft.

Divergent Views about Nuclear-Powered Satellites

The particular power units installed on Cassini are called radioisotope thermoelectric generators (RTGs). Cassini's RTGs use radioactive plutonium to supply the electrical power it needs to operate its various devices. Additionally, leftover heat from the RTGs is used to warm Cassini's electronic circuitry. RTGs have been utilized by the United States and the Soviet Union (and now Russia) on a variety of spacecraft starting in the 1960s, and continuing up to the present. At the beginning of the twenty-first century, there is a range of opinion regarding the use of radioactive power units aboard satellites. At one extreme is the assertion that RTGs possess such a negligible safety risk that the probability of an RTG accident causing ill effects in people or the environment is essentially zero. At the other extreme are claims that the chances of an accident (such as a launch failure) involving an RTG are considerable, and that under the right circumstances hundreds of thousands of people could be injured or killed. Obviously, strong disagreements exist regarding the safety of RTGs in certain situations. Even if one takes the dire warnings regarding RTGs with a grain of salt, one must still address the question of why they are used at all, given that most satellites do not possess them and that many people oppose their use.

In addressing why RTGs are used as spacecraft power supplies, it is instructive to first examine the structure of the Cassini spacecraft as well as its mission of exploration. This examination of Cassini and its mission is important because the use of plutonium, as a power source, is motivated by particular types of exploratory missions and spacecraft, as typified by Cassini.

Cassini and Its Mission

At a mass of 6.3 tons (5.7 metric tons), Cassini is one of the largest nonmilitary, unmanned spacecraft constructed to date. Built by an international team, it is a complex spacecraft with a total project cost of about $3.4 billion. Cassini, along with its attached probe, Huygens, was designed to explore the planet Saturn and its many moons. The great distance of Saturn from the Sun is a principal reason why RTGs were chosen as Cassini's power source instead of a solar array. A quantitative comparison can be made between the light received from the Sun at Earth's distance and at Saturn's distance. Light intensity from a source decreases as the distance from the light source increases; specifically, light intensity varies as the inverse of the distance squared. For example, a doubling of the distance from a light source results in one-fourth the light intensity per unit area, while a tripling of the distance results in one-ninth the light intensity. Earth orbits the Sun at a distance of approximately 93 million mi (150 million km), whereas Saturn orbits the Sun at around 886 million mi (1.4 billion km). A given solar array near Saturn only derives about one-hundredth the power from sunlight that it would near Earth.

So the Cassini project combines an exploratory mission far from the Sun with a large, complex spacecraft requiring a relatively large power supply. This combination of factors meant that the only reasonable option available to power Cassini consisted of RTGs, the advantages of which are further described below.

Advantages of RTGs

From an engineering standpoint, RTGs as satellite power sources (at least for certain types of missions) are a robust and attractive solution for several reasons, including: (1) There are no moving parts in the basic mechanism, which means that there are no bearings, valves, etc., to fail; in contrast, many other types of existing or potential satellite power sources require some sort of moving mechanism to generate electricity. Even solar cells—although in principle they require no moving parts to generate electricity—must still be aimed at the Sun by some mechanism in order to generate power. (2) The source of energy in RTGs, i.e., the radioactive plutonium, is long-lived. According to the National Aeronautics and Space Administration (NASA), the Pioneer 10 spacecraft launched in 1972 to study the outer planets still possesses an operating RTG power unit (although with a reduced power output). Pioneer 10 was still intermittently transmitting data to Earth in 2001. (3) The power output by an RTG is constant regardless of the satellite's location. In contrast, solar cells do not operate if solar illumination ceases, as when the satellite passes into a planet's shadow. And, as noted previously for solar cells, solar illumination decreases rapidly with distance, which in turn translates into a drastic reduction in power output. (4) Finally, RTGs are attractive because of their compact size, as compared to other power sources like solar arrays.

Judging strictly by the technical attributes of various types of power supplies, RTGs should be the premier choice for use in a variety of space missions, including deep-space missions of the kind Cassini was designed to complete. However, there are reasons that strongly argue against the continued use of RTGs. The stance taken in this essay is that satellites should not utilize radioactive materials to provide their power because of two overriding considerations: First, there are other, nonnuclear devices that could reasonably provide power to satellites such as Cassini. Second, an accident involving RTGs could have dire consequences for the cause of space exploration, and in extreme situations could cause injury to people and/or the environment on Earth. Both of these assertions will be examined more fully below, beginning with the latter point.

Possible Ramifications of an RTG Accident

NASA funded several studies to gauge the probability of the Cassini spacecraft crashing to Earth in various accident scenarios, as well as the deleterious effects that could be caused by the radioactive material in RTGs if such an event should occur. One of these reports, the Final Environmental Impact Statement (FEIS), asserted that there was a low probability of a serious accident involving Cassini, and furthermore, if an accident did occur, its health effects upon people would be relatively minor.

However, Dr. Michio Kaku, professor of theoretical physics at the City University of New York, performed a detailed review of NASA's FEIS and concluded that by making convenient assumptions, the report downplayed the very real health risks that could result from an accident involving RTGs, such as those used on Cassini. Kaku stated, "True casualty figures for a maximum accident might number over 200,000."

Many scientists and engineers disagree with assessments like Kaku's. For the sake of argument, assume that such analyses are terribly unrealistic, and that dire predictions of RTG-induced health disasters are completely overblown. Even in such a scenario, there could still be serious and negative side effects from an accident involving an RTG. Even if only a small quantity of radiation were released, an accident involving an RTG over a populated area could lead to public panic, extremely negative publicity for NASA, and a slew of lawsuits (if not against the government directly, then against the companies that made the rocket and spacecraft).

A Reasonable Alternative to Radioactive Power Supplies

If the use of RTGs aboard Earth-launched spacecraft is a bad idea, what can feasibly be developed in the near future to replace them? Although several alternatives to RTGs are in the realm of possibility, the most promising technology seems to be some form of solar power. Solar arrays power the vast majority of satellites in orbit around Earth. Communications satellites are typical examples of satellites whose instruments are powered solely (via arrays of solar cells) by the Sun's energy. Some solar-powered satellites store energy in batteries to power the satellite during those times when the solar arrays are temporarily not producing electricity—for instance, when a satellite is in a planet's shadow. Nevertheless, the ultimate power source is the solar array.

As noted previously, the solar energy per unit of area is dramatically less for a deep-space vehicle such as Cassini than it is for satellites considerably closer to the Sun. While a much lower solar-power density is available in locales far from the Sun (such as Jupiter, Saturn, and beyond), sunlight can nonetheless be a viable energy source for spacecraft. Dr. Ross McCluney, a research scientist at the Florida Solar Energy Center of the University of Central Florida, favors the use of solar devices to replace RTGs for deep-space missions. McCluney has investigated the use of structures to concentrate light (somewhat analogous to the way in which a lens focuses light). These solar concentrators would concentrate the weak sunlight found in regions far from the Sun onto solar cells. The solar cells would then produce electricity. By effectively boosting the power density of sunlight using low-mass solar concentrators, far fewer solar cells would be needed to produce a given amount of power. A solar concentrator/solar cell array would be far less massive than an all-solar cell array, thereby reducing the total spacecraft mass. Further efficiencies and hence mass reductions in such a power unit could be achieved by using new, higher-efficiency solar cells.

Besides developing a solar-based power supply, spacecraft engineers should reduce the power demand of satellites. This can be done in two ways. First, smaller satellites should be used. According to NASA's Jet Propulsion Laboratory (JPL), the Pioneer 10 and Pioneer 11 spacecraft that explored the outer planets during the 1970s each possessed a mass of approximately 575 lb (260 kg). In contrast, Cassini has a mass of over 12,600 lb (5,700 kg). Cassini carries some 18 scientific instruments, along with computers and data storage devices, as well as one large and two small antennas. All of this onboard instrumentation required a considerable power supply. For future missions, using several smaller spacecraft carrying fewer instruments and equipment would require much less power per spacecraft. A second technique for reducing power consumption depends upon developing electronic circuitry that requires much less power to operate than similar circuits in use today.

To recap, the four technologies described above were (1) the development of solar concentrators; (2) higher-efficiency solar cells, to be used in tandem with solar concentrators; (3) smaller and lighter spacecraft, but more of them; and (4) low-power electronics. McCluney envisions combining all four of these advances in spacecraft technology in order to eliminate the need for RTGs on future space missions.

Some might argue that developing these technologies, such as solar concentrators for spacecraft, may not be desirable or even practical. However, NASA claims to be in the business of developing innovative technologies of this type. Indeed, in the early 1990s, under the leadership of the administrator Daniel Goldin, a new philosophy regarding space exploration began at NASA. This new attitude was most forcefully directed toward unmanned (robotic) space missions and was embodied by the catchphrase "faster, better, cheaper," meaning a move toward less complex and considerably less expensive space missions. To compensate for less sophisticated and cheaper satellites, many more missions would be flown.

A "faster, better, cheaper" philosophy contains many attractive features. The amount of time from mission conception to launch is greatly reduced, meaning that the latest technology and techniques (such as new solar-concentrator arrays) can be incorporated into a particular spacecraft. Also, highly expensive and complex vehicles, such as Cassini, are replaced by multiple, smaller spacecraft, the loss of any one of which can be tolerated. In contrast, the loss of a Cassini-type spacecraft means the loss of every instrument and mission goal. Thus, it would seem that development of new technologies and approaches to space exploration, such as the development of solar-concentrator arrays to replace RTGs, would be especially suited to NASA's well-publicized "faster, better, cheaper" philosophy.

The Future of RTGs in Space Exploration

Although RTGs have been used successfully in a variety of space missions, they also have been aboard spacecraft that have crashed back to Earth. Several U.S. spacecraft with RTGs aboard have been destroyed in launch accidents. In at least one such launch failure, a small amount of radioactive material was released into the environment. Furthermore, spacecraft already traveling in outer space can sometimes reenter Earth's atmosphere, in which case extreme temperatures and pressures can be encountered due to the spacecraft's high speed. According to the environmental organization Greenpeace, a Russian spacecraft carrying an RTG power unit reentered Earth's atmosphere in 1996 and is thought to have crash-landed in South America. The fate of the plutonium that was onboard the spacecraft is unknown, at least to the general public.

If radioactive power supplies continue to be used on spacecraft, it is reasonable to assume that sooner or later another launch failure or reentry involving an RTG will occur. The specter of accidents involving radioactive materials aboard spacecraft poses unacceptable financial, environmental, health, and negative publicity risks. Some scientists and engineers contend that nonnuclear power supplies can, and should, be developed and deployed on future space missions that currently have RTGs as their only viable power-supply option. Using alternative power units for missions such as Cassini's will involve developing new technologies. These new devices, such as solar arrays employing concentrators and high-efficiency solar cells, may well find applications in areas not yet contemplated; high-tech spin-offs may ensue from the research and development efforts to replace RTGs. At the same time, the potentially serious problems associated with RTGs will be avoided.

—PHILIP KOTH

KEY TERMS

ALPHA PARTICLES:

Positively charged particles consisting of two protons and two neutrons (the nuclei of helium atoms) that are emitted by several radioactive substances.

DEEP SPACE:

There is no single, generally accepted definition; for the purposes of this article it is defined as the region of space beyond the orbit of Mars.

HALF-LIFE:

The time it takes for a radionuclide to lose half of its own radioactivity.

NUCLEAR REACTORS:

Any of several devices in which a fission chain reaction is initiated and controlled with the consequent production of heat.

PLUTONIUM:

A radioactive metallic element with the chemical symbol Pu, atomic number 94, and atomic weight (for its most stable isotope) of 244.

RADIOACTIVE:

Exhibiting the property possessed by some elements (such as uranium) or isotopes (such as carbon 14) of spontaneously emitting energetic particles (such as electrons or alpha particles) by the disintegration of their atomic nuclei.

REACTOR FUEL:

The particular power source used within a device in which self-sustained, controlled nuclear fission takes place.

REM:

A unit of ionizing radiation, equal to the amount that causes damage to humans in the form of one roentgen of high-voltage x rays. A millirem is one-thousandth of a rem.

ROBOTIC:

Having characteristics of a machine that looks or works like a human being and performs various complex acts, such as walking or talking, similar to a human being.

SOLAR ARRAY:

A mechanism composed of solar cells that convert sunlight into power.

SOLAR PANELS:

Devices that convert light into electricity.They are called "solar" after the Sun, because the Sun is the most powerful source of light commonly used.

AN UPDATE ON CASSINI'S JOURNEY

At the close of 2001, the Cassini spacecraft—headed for a rendezvous in mid-2004 with the planet Saturn—was operating normally. This clean bill of health for Cassini was derived from the telemetry it sends to Earth. (Telemetry is the transmission of data, usually from remote sources, to a receiving station for recording and analysis.)

Cassini flew by Jupiter on December 30, 2000. During the six months that Cassini was closest to Jupiter, its Ion and Neutral Camera (INCA) monitored the fluctuations in the solar wind and how it affected the planet's magnetosphere. (Jupiter's magnetosphere is a gigantic region of charged particles trapped inside the planet's magnetic field.) Cassini also examined Jupiter's moons, rings, and storm clouds.

On November 26, 2001, Cassini began a 40-day search for gravitational waves. By analyzing tiny fluctuations in the speed of the Cassini spacecraft (with the use of NASA's Deep Space Network), scientists are attempting for the first time to directly detect gravitational waves. If scientists are able to detect these "gravity ripples," valuable information may be learned on how the universe behaves.

The Cassini spacecraft is expected to arrive at Saturn on July 4, 2004. At that time, the Huygens probe will separate from Cassini, and then enter and aerodynamically brake into the atmosphere of Saturn's moon Titan. (The probe was named after Christiaan Huygens, who discovered Titan in 1655.) The probe will parachute a robotic laboratory down to Titan's surface while the probe support equipment (PSE), which remains with the orbiting spacecraft, will recover the data gathered during descent and landing. The PSE will then transmit the data to Earth. By exploring Saturn and its moons, scientists will gain valuable clues for understanding the early history and evolution of the solar system.

—William Arthur Atkins

Science in Dispute