New Dawn for Electric Rockets

SPACE TECHNOLOGY

New Dawn for

KEY CONCEPTS Ef!cient electric plasma engines are ■ Conventional rockets propelling the next generation of space generate thrust by burning chemical fuel. Electric probes to the outer solar system By Edgar Y. Choueiri rockets propel space vehicles by applying electric or electromagnetic !elds to clouds of charged lone amid the cosmic blackness, NASA’s ing liquid or solid chemical fuels, as convention- particles, or plasmas, to Dawn space probe speeds beyond the al rockets do. accelerate them. A orbit of Mars toward the asteroid belt. Dawn’s mission designers at the NASA Jet Launched to search for insights into the birth of Propulsion Laboratory selected a plasma engine ■ Although electric rockets the solar system, the robotic spacecraft is on its as the probe’s rocket system because it is highly offer much lower thrust levels than their chemical way to study the asteroids Vesta and Ceres, two ef"cient, requiring only one tenth of the fuel cousins, they can even- of the largest remnants of the planetary embry- that a chemical rocket motor would have need- tually enable spacecraft os that collided and combined some 4.57 billion ed to reach the asteroid belt. If project planners to reach greater speeds years ago to form today’s planets. had chosen to install a traditional engine, the for the same amount But the goals of the mission are not all that vehicle would have been able to reach either of propellant. make this !ight notable. Dawn, which took off Vesta or Ceres, but not both.

■ Electric rockets’ high-speed in September 2007, is powered by a kind of space Indeed, electric rockets, as the engines are also capabilities and their propulsion technology that is starting to take known, are quickly becoming the best option for ef!cient use of propellant center stage for long-distance missions—a plas- sending probes to far-off targets. Recent success- make them valuable for

ma rocket engine. Such engines, now being de- es made possible by electric propulsion include a SAIC deep-space missions. veloped in several advanced forms, generate visit by NASA’s Deep Space 1 vehicle to a comet, a —The Editors thrust by electrically producing and manipulat- bonus journey that was made feasible by propel-

ing ionized gas propellants rather than by burn- lant that was left over after the spacecraft had ac- RAWLINGSPAT

complished its primary goal. Plasma engines turned the concept into a practical technology in NASA’S DAWN SPACE PROBE, which have also provided propulsion for an attempted the mid-1950s. A few years later engineers at the is propelled by an electric rocket landing on an asteroid by the Japanese Hayabusa NASA Glenn Research Center (then known as called an ion thruster, nears the probe, as well as a trip to the moon by the Euro- Lewis) built the "rst operating electric rocket. asteroid Vesta in this artist’s pean Space Agency’s SMART-1 spacecraft. In That engine made a suborbital !ight in 1964 on- conception. Vesta is its initial light of the technology’s demonstrated advantag- board Space Electric Rocket Test 1, operating for survey target; the asteroid Ceres, its second destination, es, deep-space mission planners in the U.S., Eu- half an hour before the craft fell back to Earth. "oats in the far distance in the rope and Japan are opting to employ plasma In the meantime, researchers in the former image (bright spot at upper drives for future missions that will explore the Soviet Union worked independently on con- right). A conventional chemical outer planets, search for extrasolar, Earth-like cepts for electric rockets. Since the 1970s mis- rocket engine would be able to planets and use the void of space as a laboratory sion planners have selected the technology be- carry enough fuel to reach only in which to study fundamental physics. cause it can save propellant while performing one of these asteroids. such tasks as maintaining the attitude and or- A Long Time Coming bital position of telecommunications satellites Although plasma thrusters are only now mak- in geosynchronous orbit. ing their way into long-range spacecraft, the technology has been under development for that Rocket Realities purpose for some time and is already used for The bene"ts afforded by plasma engines become other tasks in space. most striking in light of the drawbacks of con- As early as the "rst decade of the 20th centu- ventional rockets. When people imagine a ship ry, rocket pioneers speculated about using elec- streaking through the dark void toward a distant tricity to power spacecraft. But the late Ernst planet, they usually envision it trailing a long, Stuhl inger—a member of Wernher von Braun’s "ery plume from its nozzles. Yet the truth is alto- legendary team of German rocket scientists that gether different: expeditions to the outer solar spearheaded the U.S. space program—"nally system have been mostly rocketless affairs,

© 2009 SCIENTIFIC AMERICAN, INC. www.SciAm.com © 2009 SCIENTIFIC AMERICAN, INC. SCIENTIFIC AMERIC AN 59 [COMPARISON] motor would typically have no fuel left for brak- Chemical vs. ing. Such a probe would need the ability to "re its rocket so that it could slow enough to achieve Electric Rockets END orbit around its target and thus conduct extend- Thrust: low Chemical and electric propulsion ed scienti"c observations. Unable to brake, it systems are suited to different END Speed: very high Thrust: zero Tank: one-third full would be limited to just a !eeting encounter kinds of missions. Chemical Speed: high (enough for a with the object it aimed to study. Indeed, after rockets (left) produce large Tank: empty second mission) a trip of more than nine years, New Horizons, amounts of thrust quickly, so they can accelerate to high a NASA deep-space probe launched in 2006, will speeds rapidly, although they get only a brief encounter of not more than a sin- burn copious quantities of fuel gle Earth day with its ultimate object of study, to do so. These characteristics the recently demoted “dwarf planet” Pluto. make them appropriate for MIDDLE MIDDLE relatively short-range trips. Thrust: zero Thrust: low The Rocket Equation Electric rockets (right), which Speed: high Speed: high For those who wonder why engineers have been Tank: empty Tank: two-thirds full use a plasma (ionized gas) as unable to come up with ways to send enough propellant, generate much less chemical fuel into space to avoid such dif"cul- thrust, but their extremely frugal ties for long missions, let me clarify the immense consumption of propellant allows hurdles they face. The explanation derives from them to operate for much longer periods. And in the frictionless what is called the rocket equation, a formula environment of space, a small START START used by mission planners to calculate the mass Thrust: high Thrust: low of propellant required for a given mission. Rus- force applied over time can Speed: low Speed: low eventually achieve similarly Tank: full Tank: full sian scientist Konstantin E. Tsiolkovsky, one of high or greater speeds. These the fathers of rocketry and space!ight, "rst features make plasma rockets introduced this basic formula in 1903. well equipped for deep-space CHEMICAL ELECTRIC In plain English, the rocket equation states missions to multiple targets. ROCKET ROCKET the intuitive fact that the faster you throw propellant out from a spacecraft, the less you need to execute a rocket-borne maneuver. Think of a because most of the fuel is typically expended in baseball pitcher (a rocket motor) with a bucket the "rst few minutes of operation, leaving the of baseballs (propellant) standing on a skate- spacecraft to coast the rest of the way to its goal. board (a spacecraft). The faster the pitcher !ings True, chemical rockets do launch all spacecraft the balls rearward (that is, the higher the ex- from Earth’s surface and can make midcourse haust speed), the faster the vehicle will be travel- [THE AUTHOR] corrections. But they are impractical for powering in the opposite direction when the last ball is ing deep-space explorations because they would thrown—or, equivalently, the fewer baseballs Edgar Y. Choueiri teaches astro nautics and applied physics require huge quantities of fuel—too much to be (less propellant) the pitcher would have to hurl at Prince ton University, where he lifted into orbit practically and affordably. Plac- to raise the skateboard’s speed by a desired also directs the Electric Propulsion ing a pound (0.45 kilogram) of anything into amount at any given time. Scientists call this and Plasma Dynamics Laboratory Earth orbit costs as much as $10,000. incremental increase of the skateboard’s velocity (http://alfven.princeton.edu) To achieve the necessary trajectories and high “delta-v.” and the university’s Program in Engineering Physics. Aside from speeds for lengthy, high-precision journeys with- In more speci"c terms, the equation relates ) plasma propulsion research, he out additional fuel, many deep-space probes of the mass of propellant required by a rocket to is working on mathematical the past have had to spend time—often years— carry out a particular mission in outer space to techniques that could enable detouring out of their way to planets or moons two key velocities: the velocity at which the rock- illustration accurate recording and reproduc- that provided gravitational kicks able to accel- et’s exhaust will be ejected from the vehicle and tion of music in three dimensions. erate them in the desired direction (slingshot the mission’s delta-v—how much the vehicle’s ve- );KEVIN HAND ( moves called gravity-assist maneuvers). Such cir- locity will increase as a result of the exhaust’s cuitous !ight paths limit missions to relatively ejection. Delta-v corresponds to the energy a author small launch windows; only blasting off within craft must expend to alter its inertial motion and HOUEIRI( C .

a certain short time frame will ensure a precision execute a desired space maneuver. For a given Y swing past a cosmic body serving as a gravita- rocket technology (that is, one that produces a tional booster. given rocket exhaust speed), the rocket equation Even worse, after years of travel toward its translates the delta-v for a desired mission into

destination, a vehicle with a chemical rocket the mass of propellant required to complete it. COURTESYOF EDGAR

60 SCIENTIFIC AMERIC AN © 2009 SCIENTIFIC AMERICAN, INC. February 2009 The delta-v metric can therefore be thought of as better electrical conductor than copper metal. EARLY HISTORY a kind of “price tag” of a mission, because the Because plasmas contain charged particles, OF ELECTRIC cost of conducting one is typically dominated by whose motion is strongly affected by electric ROCKETS the cost of launching the needed propellant. and magnetic "elds, application of electric or Conventional chemical rockets achieve only electromagnetic "elds to a plasma can acceler- 1903: Konstantin E. Tsiolkovsky low exhaust velocities (three to four kilometers ate its constituents and send them out the back derives the “rocket equation,” per second, or km/s). This feature alone makes of a vehicle as thrust-producing exhaust. The which is widely used to calculate them problematic to use. Also, the exponential necessary "elds can be generated by electrodes fuel consumption for space missions. In 1911 he speculates nature of the rocket equation dictates that the and magnets, using induction by external anten- that electric !elds could accelerate fraction of the vehicle’s initial mass that is com- nas or wire coils, or by driving electric currents charged particles to produce posed of fuel—the “propellant mass fraction”— through the plasma. rocket thrust. grows exponentially with delta-v. Hence, the The electric power for creating and acceler- fuel needed for the high delta-v required for a ating the plasmas typically comes from solar 1906: Robert H. Goddard conceives deep-space mission could take up almost all the panels that collect energy from the sun. But of electrostatic acceleration of starting mass of the spacecraft, leaving little deep-space vehicles going past Mars must rely charged particles for rocket room for anything else. on nuclear power sources, because solar energy propulsion. He invents and patents a Consider a couple of examples: To travel to gets too weak at long distances from the sun. pre cursor to the ion engine in 1917. Mars from low-Earth orbit requires a delta-v of Today’s small robotic probes use thermoelectric 1954: Ernst Stuhlinger !gures out about 4.5 km/s. The rocket equation says that a devices heated by the decay of a nuclear isotope, how to optimize the performance of conventional chemical rocket would require that but the more ambitious missions of the future the electric ion rocket engine. more than two thirds of the spacecraft’s mass be would need nuclear "ssion (or even fusion) reac- propellant to carry out such an interplanetary tors. Any nuclear reactor would be activated 1962: Work by researchers in transfer. For more ambitious trips—such as ex- only after the vessel reached a stable orbit at a the Soviet Union, Europe and peditions to the outer planets, which have delta- safe distance from Earth. Its fuel would be se- the U.S. leads to the !rst published v requirements that range from 35 to 70 km/s— cured in an inert state during liftoff. description of the Hall thruster, chemical rockets would need to be more than Three kinds of plasma propulsion systems a more powerful class of 99.98 percent fuel. That con"guration would have matured enough to be employed on long- plasma rocket. leave no space for other hardware or useful pay- distance missions. The one in most use—and the 1962: Adriano Ducati discovers the loads. As probes journey farther out into the so- kind powering Dawn—is the ion drive. mechanism behind the magneto- lar system, chemical rockets become increasing- plasmadynamic thruster, the most ly useless—unless engineers can "nd a way to The Ion Drive powerful type of plasma rocket. signi"cantly raise their exhaust speeds. The ion engine, one of the more successful elec- So far that goal has proved very dif"cult to tric propulsion concepts, traces its roots to the 1964: NASA’s SERT I spacecraft accomplish because generating ultrahigh ex- ideas of American rocketry pioneer Robert H. conducts the !rst successful "ight haust speeds demands extremely high fuel com- Goddard, formed when he was still a graduate test of an ion engine in space. bustion temperatures. The ability to reach the student at Worcester Polytechnic Institute a cen- 1972: The Soviet Meteor satellite needed temperatures is limited both by the tury ago. Ion engines are able to achieve exhaust carries out the initial space"ight amount of energy that can be released by known velocities ranging from 20 to 50 km/s [see box on of a Hall thruster. chemical reactions and by the melting point of next page]. the rocket’s walls. In its most common incarnation, the ion en- 1999: NASA’s Jet Propulsion gine gets its electric power from photovoltaic Laboratory’s Deep Space 1 The Plasma Solution panels. It is a squat cylinder, not much larger demonstrates the !rst use of Plasma propulsion systems, in contrast, offer than a bucket, that is set astern. Inside the buck- an ion engine as the main much greater exhaust speeds. Instead of burn- et, xenon gas from the propellant tank !ows propulsion system on a ing chemical fuel to generate thrust, the plasma into an ionization chamber where an electro- spacecraft that escapes Earth’s gravitation engine accelerates plasmas—clouds of electri- magnetic "eld tears electrons off the xenon gas from orbit. cally charged atoms or molecules—to very high atoms to create a plasma. The plasma’s positive velocities. A plasma is produced by adding ener- ions are then extracted and accelerated to high gy to a gas, for instance, by radiating it with speeds through the action of an electric "eld lasers, microwaves or radio-frequency waves or that is applied between two electrode grids. MAGES I by subjecting it to strong electric "elds. The Each positive ion in the "eld feels the strong tug ETTY G extra energy liberates electrons from the atoms of the aft-mounted, negatively charged elec- HIVE/ C

R or molecules of the gas, leaving the latter with a trode and therefore accelerates rearward.

A ROBERT H. positive charge and the former free to move free- The positive ions in the exhaust leave a space- GODDARD, ULTON

H ly in the gas, which makes the ionized gas a far craft with a net negative charge, which, if left to circa 1935

© 2009 SCIENTIFIC AMERICAN, INC. w w w.SciAm.com © 2009 SCIENTIFIC AMERICAN, INC. [ION THRUSTER] The Proven Plasma Propulsion Workhorse This engine type creates a plasma propellant by bombarding a neutral the back of the craft by an electric !eld that is created by applying gas with electrons emitted from a hot electric !lament. The resulting a high voltage between two electrode grids. The ion exhaust generates positive ions are then extracted from the plasma and accelerated out thrust in the opposite direction.

4 The electric potential Status: Flight operational ● between oppositely charged 3 As the electrons bombard the xenon gas, Input power: 1 to 7 kilowatts ● electrode grids attracts the its atoms become positively charged ions. Exhaust velocity: 20 to 50 ions and accelerates them kilometers per second Magnet rings out the back of the craft to generate thrust. Thrust: 20 to 250 millinewtons ●2 Stored xenon Ef!ciency: 60 to 80 percent propellant is fed into Uses: Attitude control and orbital the ionization chamber. station-keeping for existing Negative grid Positive grid satellites; main propulsion for Xenon current small robotic spacecraft atom Plasma Thrust

From xenon Xenon ion propellant tank Ionization Hot chamber electrode Magnet ring

●1 A hot electrode (cathode) 5 Electrons emitted by a hot Electron ● emits electrons that are Hot electrode neutralize the energized by a magnetic !eld electrode positive ion beam as it in an ionization chamber. leaves the engine to keep the ions from being Electron injector/ neutralizer attracted back to the craft and reducing net thrust.

build up, would attract the ions back to the space- date. Dawn should soon break that record by craft, thus canceling out the thrust. To avoid this adding 10 km/s to its velocity. Engineers at the problem, an external electron source (a negative Jet Propulsion Laboratory have recently demon- cathode or an electron gun) injects electrons into strated ion drives able to function !awlessly for the positive flow to electrically neutralize it, more than three years of continuous operation. which leaves the spacecraft neutral. A plasma rocket’s performance is determined Dozens of ion drives are currently operating not only by the speed of the exhaust particles on commercial spacecraft—mostly communica- but also by its thrust density, which is the tions satellites in geosynchronous orbit for or- amount of thrust force an engine produces per

bital “station-keeping” and attitude control. unit area of its exhaust aperture. Ion engines ) They were selected because they save millions and similar electrostatic thrusters suffer from illustration of dollars per spacecraft by greatly shrinking a major shortcoming, called space-charge limi- (

the mass of propellant that would be required tation, that severely reduces their thrust density: SAIC for chemical propulsion. as the positive ions pass between the electrostat- At the end of the 20th century, Deep Space 1 ic grids in an ion engine, a positive charge inevi- ION THRUSTER, which is 40

became the world’s "rst spacecraft using an elec- tably builds up in this region. This buildup lim- );RAWLINGSPAT cent i meters in diameter, was test-!red inside a laboratory tric propulsion system to escape Earth’s gravita- its the attainable electric field to drive the vacuum chamber. Charged tion from orbit. The probe then accelerated by acceleration. ionengine xenon atoms account for the about 4.3 km/s, while consuming less than 74 ki- Because of this phenomenon, Deep Space 1’s /JPL(

blue color of the exhaust plume. lograms of xenon propellant (about the mass of ion engine produces a thrust force that is rough- NASA E: an untapped beer keg), to !y through the dusty ly equivalent to the weight of a single sheet of C tail of the comet Borrelly. This is the highest ve- paper—hardly the thundering rocket engine of locity increment gained via propulsion (as op- sci-" movies and more akin to a car that takes

posed to gravity assists) by any spacecraft to two days to accelerate from zero to 60 miles per DONFOLEY; SOUR

62 SCIENTIFIC AMERIC AN © 2009 SCIENTIFIC AMERICAN, INC. February 2009 [HALL THRUSTER] hour. As long as one is willing to wait long enough (typically, many months), though, these The Latest Plasma Engine Contender engines can eventually attain the high delta-vs This device generates propulsion by crossing a so-called Hall current and a radial needed for distant journeys. That feat is possi- magnetic !eld, which causes electrons to circle around the device’s axis. These ble because in the vacuum of space, which offers electrons tear electrons from xenon atoms, producing xenon ions, and an electric !eld no resistance, even a tiny push, if constantly ap- parallel to the axis accelerates the ions downstream. The density of propulsive force plied, will lead to high propulsion speeds. produced by a Hall thruster is greater than that of an ion engine because its exhaust contains both positive ions and electrons, which avoids the buildup of positive charge The Hall Thruster that can limit the strength of an accelerating electric !eld. A plasma propulsion system called the Hall thruster [see box at right] avoids the space- ●1 An electric potential established between an external negative cathode and internal positive anode creates a mostly axial charge limitation and can therefore accelerate a electric !eld inside the acceleration chamber. vessel to high speeds more quickly (by way of its greater thrust density) than a comparably ●2 As the cathode heats up, it emits electrons. Some of the electrons drift sized ion engine can. This technology has been upstream toward the anode. When the electrons enter the chamber, a gaining acceptance in the West since the early radial magnetic !eld and the axial electric !eld cause them to whirl around the axis of the thruster as a “Hall current.” 1990s, after three decades of steady development in the former Soviet Union. The Hall thruster will soon be ready to take on long- range missions. The system relies on a fundamental effect Magnetic Internal coils insulator wall discovered in 1879 by Edwin H. Hall, then a physics graduate student at Johns Hopkins Uni- External versity. Hall showed that when electric and cathode Anode/gas magnetic "elds are set perpendicular to each injector other inside a conductor, an electric current Radial magnetic (called the Hall current) !ows in a direction that !eld is perpendicular to both "elds. Hall Electrons In a Hall thruster a plasma is produced when current an electric discharge between an internal posi- Xenon tive anode and a negative cathode situated out- atoms side the device tears through a neutral gas inside the device. The resulting plasma !uid is then ac- Acceleration chamber Plasma

2000 celerated out of the cylindrical engine by the Xenon ions Lorentz force, which results from the interac- AUGUST tion of an applied radial magnetic "eld and an . 3;. NO

electric current (in this case, the Hall current)

,” ,” Axial electric !eld . 88,. that !ows in an azimuthal direction—that is, in VOL a circular “orbit” around the central anode. The THRUSTER Hall current is caused by the electron’s motion HALL in the magnetic and electric "elds. Depending ●3 Xenon gas propellant feeds through the

TRODE positive anode injector into the annular ●4 The plasma (containing both positive C on the available power, exhaust velocities can acceleration chamber, where the whirling ions and electrons) is accelerated ELE

range from 10 to more than 50 km/s. electrons collide with the xenon atoms, sternward by the electromagnetic This form of electric rocket avoids a space- turning them into positive ions. forces resulting from the interaction JOURNALOF APPLIED PHYSICS,

, between the predominantly radial SEGMENTED

charge buildup by accelerating the entire plasma H C IN magnetic !eld and the Hall current.

FIS (of both positive ions and negative electrons), . J TION . C N with the result that its thrust density and thus its AND REDU

K thrust force (and so its potential delta-v) is many Status: Flight operational Input power: 1.35 to 10 kilowatts

PLUME times that of an ion engine of the same size. LITVA . A E:“

. Exhaust velocity: 10 to 50 kilometers per second

C More than 200 Hall thrusters have been !own A , Thrust: 40 to 600 millinewtons

SOUR on satellites in Earth orbit. And it was a Hall DORF

. Ef!ciency: 45 to 60 percent A .

SAIC; thruster that the European Space Agency used L

, Uses: Satellite attitude control and station-keeping; used to ef"ciently propel its SMART-1 spacecraft fru- as main propulsion for medium-size robotic spacecraft

RAITSES gally to the moon. . Y

PAT RAWLINGSPAT B Engineers are now trying to scale up today’s

© 2009 SCIENTIFIC AMERICAN, INC. www.SciAm.com © 2009 SCIENTIFIC AMERICAN, INC. SCIENTIFIC AMERIC AN 63 rather small Hall thrusters so that they can ing the plasma beam away from the thruster handle higher amounts of power to generate walls. German engineers have achieved similar $10,000 greater exhaust speeds and thrust levels. The results using specially shaped magnetic "elds. is roughly what it costs to send a work also aims to extend their operating life- Researchers at Stanford University have mean- pound (0.45 kilogram) of payload times to the multiyear durations needed for deep- while shown that lining the walls with tough, into Earth orbit with conventional rocket boosters. This high price space exploration. synthetic-polycrystalline diamond substantially tag is one reason engineers go to Scientists at the Princeton Plasma Physics Lab- boosts the device’s resistance to plasma erosion. great lengths to shave as much oratory have taken a step toward these goals by Such improvements will eventually make Hall mass from spacecraft as is feasi- implanting segmented electrodes in the walls of thrusters suitable for deep-space missions. ble. The fuel and its storage tank a Hall thruster. The electrodes shape the internal are the heaviest parts of a vehicle powered by a chemical rocket. electric "eld in a way that helps to focus the plas- Next-Generation Thruster ma into a thin exhaust beam. This design reduc- One way to further raise the thrust density of es the useless nonaxial component of thrust and plasma propulsion is to increase the total improves the system’s operating lifetime by keep- amount of plasma that is accelerated in the engine. But as the plasma density in a Hall [MAGNETOPLASMADYNAMIC THRUSTER] thruster is raised, electrons collide more fre- The Future of Plasma Propulsion quently with atoms and ions, which makes it more dif"cult for the electrons to carry the Hall An MPDT relies on the Lorentz electromagnetic force to accelerate the plasma to current needed for acceleration. An alternative prod uce thrust. The Lorentz force (green arrows), which is mainly along the axis, is known as the magnetoplasmadynamic thruster created by the interaction of a mostly radial electric current pattern (red lines) with a (MPDT) allows for a denser plasma by forgoing concentric magnetic !eld (blue circle). the Hall current in favor of a current component that is mostly aligned with the electric "eld [see Status: Flight-tested but not yet operational box at left] and far less prone than the Hall cur- Input power: 100 to 500 kilowatts Exhaust velocity: 15 to 60 kilometers per second rent to disruption by atomic collisions. Thrust: 2.5 to 25 newtons In general, an MPDT consists of a central Ef!ciency: 40 to 60 percent cathode sitting within a larger cylindrical anode. Uses: Main propulsion for heavy cargo and A gas, typically lithium, is pumped into the an- piloted spacecraft; under development nular space between the cathode and the anode. There it is ionized by an electric current !owing ●4 The magnetic !eld radially from the cathode to the anode. This cur- ●2 As the lithium gas atoms emerge from the cath- interacts with the radial ode, they are positively ionized into a plasma current that induced it, rent induces an azimuthal magnetic "eld (one by an electric discharge that arcs between the producing a Lorentz force that encircles the central cathode), which inter- cathode and the surrounding cylindrical anode. that accelerates the acts with the same current that induced it to gen- Power ions rearward and out erate the thrust-producing Lorentz force. supply of the thruster. A single MPD engine about the size of an av- erage household pail can process about a mil- Lorentz force lion watts of electric power from a solar or nu- Magnetic !eld clear source into thrust (enough to energize more than 10,000 standard lightbulbs), which

Hollow is substantially larger than the maximum pow- cathode er limits of ion or Hall thrusters of the same Plasma size. An MPDT can produce exhaust velocities Lithium propellant from 15 to 60 km/s. It truly is the little engine Acceleration chamber that could. This design also offers the advantage of throt-

tling; its exhaust speed and thrust can be easily UNIVERSITY Radial electric Cylindrical anode adjusted by varying the electric current level or current ETON the !ow rate of the propellant. Throttling allows C PRIN : E

a mission planner to alter a spacecraft’s engine C ●1 Hot lithium propellant OUR

thrust and exhaust velocity as needed to opti- S is injected into a central ●3 The predominantly radial electric

hollow cathode. current discharge induces a circular mize its trajectory. SAIC; magnetic !eld in the annular space Intensive research on mechanisms that ham- between the cathode and anode. per the performance and lifetimes of MPD devic-

es, such as electrode erosion, plasma instabilities RAWLINGSPAT

64 SCIENTIFIC AMERIC AN © 2009 SCIENTIFIC AMERICAN, INC. February 2009 [POWER SOURCES] Solar and Nuclear Energy for Electric Rockets For trips to the inner solar system, where the sun’s rays are strong, generally require nuclear power sources. A large, heavy craft would need suf!cient electric power can be provided to plasma rocket engines by a nuclear reactor, but a smaller, lighter one might get by with a thermo- solar cells. But trips to the outer planets of the solar system would electric power-generation device heated by the decay of radioisotopes.

Mercury Kuiper belt Venus Earth Mars Neptune

Uranus

Asteroid belt Saturn Jupiter

INNER PLANETS AND ASTEROID BELT: OUTER PLANETS: Range of solar power capability Nuclear power required

and power dissipation in the plasma, has led to the plasma through invisible “rocket nozzles” new, high-performance engines that rely on lith- composed of magnetic "elds. ium and barium vapors for propellants. These el- In all cases, plasma rockets will get up to ements ionize easily, yield lower internal energy speed more slowly than conventional rockets. losses in the plasma and help to keep the cathode And yet, in what has been called the “slower but cooler. The adoption of these liquid-metal pro- faster paradox,” they can often make their way pellants and an unusual cathode design that con- to distant destinations more quickly by ultimate- tains channels that alter how the electric current ly reaching higher spacecraft velocities than stan- interacts with its surface has resulted in substan- dard propulsion systems can using the same mass tially less erosion of the cathode. These innova- of propellant. They thus avoid time-consuming tions are leading to more reliable MPDTs. detours for gravity boosts. Much as the fabled A team of academic and NASA researchers has slow and steady tortoise beats out the intermit- recently completed the design of a state-of-the- tently sprinting hare, in the marathon !ights that ➥ MORE TO art lithium-fed MPDT called B2, which could po- will become increasingly common in the coming EXPLORE tentially drive a nuclear-powered vessel hauling era of deep-space exploration, the tortoise wins. heavy cargo and people to the moon and Mars as So far the most advanced designs could im- Bene!ts of Nuclear Electric Propulsion for Outer Planet — well as robotic missions to the outer planets. part a delta-v of 100 km/s much too slow to Exploration. G. Woodcock et al. take a spacecraft to the far-off stars but plenty American Institute of Aeronautics The Tortoise Wins enough to visit the outer planets in a reasonable and Astronautics, 2002. Ion, Hall and MPD thrusters are but three vari- amount of time. One particularly exciting deep- ants of electric plasma rocket technology, albeit space mission that has been proposed would re- Electric Propulsion. Robert G. Jahn and Edgar Y. Choueiri in Encyclopedia the most mature. During the past few decades turn samples from Saturn’s largest moon, Titan, of Physical Science and Technology. researchers have developed many other promis- which space scientists believe has an atmosphere Third edition. Academic Press, 2002. ing related concepts to various degrees of readi- that is very similar to Earth’s eons ago. ness. Some are pulsed engines that operate inter- A sample from Titan’s surface would offer A Critical History of Electric mittently; others run continuously. Some gener- researchers a rare chance to search for signs of Propulsion: The First 50 Years (1906–1956). Edgar Y. Choueiri ate plasmas through electrode-based electric chemical precursors to life. The mission would in Journal of Propulsion and Power, discharge; others use coil-based magnetic induc- be impossible with chemical propulsion. And, Vol. 20, No. 2, pages 193–203; 2004. tion or antenna-generated radiation. The mech- with no in-course propulsion, the journey would anisms they apply to accelerate plasmas vary as require multiple planetary gravity assists, add- Physics of Electric Propulsion. Rob- well: some use Lorentz forces; others accelerate ing more than three years to the total trip time. ert G. Jahn. Dover Publications, 2006. the plasmas by entraining them in magnetically A probe "tted with “the little plasma engine

ON Fundamentals of Electric Propul- X

DI produced current sheets or in traveling electro- that could” would be able to do the job in a sig- sion: Ion and Hall Thrusters. Dan

DON magnetic waves. One type even aims to exhaust ni"cantly shorter period. ■ M. Goebel and Ira Katz. Wiley, 2008.