Jueves 1 de Enero de 2009

The efficient future of deep-space travel--electric rockets
Por Edgar Y. Choueiri

Alone amid the cosmic blackness, NASA's Dawn space probe speeds beyond the orbit of Mars toward the asteroid belt. Launched to search for insights into the birth of the solar system, the robotic spacecraft is on its way to study the asteroids Vesta and Ceres, two of the largest remnants of the planetary embryos that collided and combined some 4.57 billion years ago to form today's planets.

But the goals of the mission are not all that make this flight notable. Dawn, which took off in September 2007, is powered by a kind of space propulsion technology that is starting to take center stage for long-distance missions a plasma rocket engine. Such engines, now being developed in several advanced forms, generate thrust by electrically producing and manipulating ionized gas propellants rather than by burning liquid or solid chemical fuels, as conventional rockets do.

Dawn's mission designers at the NASA Jet Propulsion Laboratory selected a plasma engine as the probe's rocket system because it is highly efficient, requiring only one tenth of the fuel that a chemical rocket motor would have needed to reach the asteroid belt. If project planners had chosen to install a traditional engine, the vehicle would have been able to reach either Vesta or Ceres, but not both.

Indeed, electric rockets, as the engines are also known, are quickly becoming the best option for sending probes to far-off targets. Recent successes made possible by electric propulsion include a visit by NASA's Deep Space 1 vehicle to a comet, a bonus journey that was made feasible by propellant that was left over after the spacecraft had accomplished its primary goal. Plasma engines have also provided propulsion for an attempted landing on an asteroid by the Japanese Hayabusa probe, as well as a trip to the moon by the European Space Agency's SMART-1 spacecraft. In light of the technology's demonstrated advantages, deep-space mission planners in the U.S., Europe and Japan are opting to employ plasma drives for future missions that will explore the outer planets, search for extrasolar, Earth-like planets and use the void of space as a laboratory in which to study fundamental physics.

Although plasma thrusters are only now making their way into long-range spacecraft, the technology has been under development for that purpose for some time and is already used for other tasks in space.

As early as the first decade of the 20th century, rocket pioneers speculated about using electricity to power spacecraft. But the late Ernst Stuhl inger a member of Wernher von Braun's legendary team of German rocket scientists that spearheaded the U.S. space program finally turned the concept into a practical technology in the mid-1950s. A few years later engineers at the NASA Glenn Research Center (then known as Lewis) built the first operating electric rocket. That engine made a suborbital flight in 1964 onboard Space Electric Rocket Test 1, operating for half an hour before the craft fell back to Earth.

In the meantime, researchers in the former Soviet Union worked independently on concepts for electric rockets. Since the 1970s mission planners have selected the technology because it can save propellant while performing such tasks as maintaining the attitude and orbital position of telecommunications satellites in geosynchronous orbit.

The benefits afforded by plasma engines become most striking in light of the drawbacks of conventional rockets. When people imagine a ship streaking through the dark void toward a distant planet, they usually envision it trailing a long, fiery plume from its nozzles. Yet the truth is altogether different: expeditions to the outer solar system have been mostly rocketless affairs, because most of the fuel is typically expended in the first few minutes of operation, leaving the spacecraft to coast the rest of the way to its goal. True, chemical rockets do launch all spacecraft from Earth's surface and can make midcourse corrections. But they are impractical for powering deep-space explorations because they would require huge quantities of fuel too much to be lifted into orbit practically and affordably. Placing a pound (0.45 kilogram) of anything into Earth orbit costs as much as $10,000.

To achieve the necessary trajectories and high speeds for lengthy, high-precision journeys without additional fuel, many deep-space probes of the past have had to spend time often years detouring out of their way to planets or moons that provided gravitational kicks able to accel erate them in the desired direction (slingshot moves called gravity-assist maneuvers). Such circuitous flight paths limit missions to relatively small launch windows; only blasting off within a certain short time frame will ensure a precision swing past a cosmic body serving as a gravitational booster.

Even worse, after years of travel toward its destination, a vehicle with a chemical rocket motor would typically have no fuel left for braking. Such a probe would need the ability to fire its rocket so that it could slow enough to achieve orbit around its target and thus conduct extended scientific observations. Unable to brake, it would be limited to just a fleeting encounter with the object it aimed to study. Indeed, after a trip of more than nine years, New Horizons, a NASA deep-space probe launched in 2006, will get only a brief encounter of not more than a single Earth day with its ultimate object of study, the recently demoted "dwarf planet" Pluto.

For those who wonder why engineers have been unable to come up with ways to send enough chemical fuel into space to avoid such difficulties for long missions, let me clarify the immense hurdles they face. The explanation derives from what is called the rocket equation, a formula used by mission planners to calculate the mass of propellant required for a given mission. Russian scientist Konstantin E. Tsiolkovsky, one of the fathers of rocketry and spaceflight, first introduced this basic formula in 1903.

In plain English, the rocket equation states 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 baseball pitcher (a rocket motor) with a bucket of baseballs (propellant) standing on a skateboard (a spacecraft). The faster the pitcher flings the balls rearward (that is, the higher the exhaust speed), the faster the vehicle will be traveling in the opposite direction when the last ball is thrown or, equivalently, the fewer baseballs (less propellant) the pitcher would have to hurl to raise the skateboard's speed by a desired amount at any given time. Scientists call this incremental increase of the skateboard's velocity "delta-v."

In more specific terms, the equation relates the mass of propellant required by a rocket to carry out a particular mission in outer space to two key velocities: the velocity at which the rocket's exhaust will be ejected from the vehicle and the mission's delta-v how much the vehicle's velocity will increase as a result of the exhaust's ejection. Delta-v corresponds to the energy a craft must expend to alter its inertial motion and execute a desired space maneuver. For a given rocket technology (that is, one that produces a given rocket exhaust speed), the rocket equation translates the delta-v for a desired mission into the mass of propellant required to complete it. The delta-v metric can therefore be thought of as a kind of "price tag" of a mission, because the cost of conducting one is typically dominated by the cost of launching the needed propellant.

Conventional chemical rockets achieve only low exhaust velocities (three to four kilometers per second, or km/s). This feature alone makes them problematic to use. Also, the exponential nature of the rocket equation dictates that the fraction of the vehicle's initial mass that is composed of fuel the "propellant mass fraction" grows exponentially with delta-v. Hence, the fuel needed for the high delta-v required for a deep-space mission could take up almost all the starting mass of the spacecraft, leaving little room for anything else.

Consider a couple of examples: To travel to Mars from low-Earth orbit requires a delta-v of about 4.5 km/s. The rocket equation says that a conventional chemical rocket would require that more than two thirds of the spacecraft's mass be propellant to carry out such an interplanetary transfer. For more ambitious trips such as expeditions to the outer planets, which have delta-v requirements that range from 35 to 70 km/s chemical rockets would need to be more than 99.98 percent fuel. That configuration would leave no space for other hardware or useful payloads. As probes journey farther out into the solar system, chemical rockets become increasingly useless unless engineers can find a way to significantly raise their exhaust speeds.

So far that goal has proved very difficult to accomplish because generating ultrahigh exhaust speeds demands extremely high fuel combustion temperatures. The ability to reach the needed temperatures is limited both by the amount of energy that can be released by known chemical reactions and by the melting point of the rocket's walls.

Plasma propulsion systems, in contrast, offer much greater exhaust speeds. Instead of burning chemical fuel to generate thrust, the plasma engine accelerates plasmas clouds of electrically charged atoms or molecules to very high velocities. A plasma is produced by adding energy to a gas, for instance, by radiating it with lasers, microwaves or radio-frequency waves or by subjecting it to strong electric fields. The extra energy liberates electrons from the atoms or molecules of the gas, leaving the latter with a positive charge and the former free to move freely in the gas, which makes the ionized gas a far better electrical conductor than copper metal. Because plasmas contain charged particles, whose motion is strongly affected by electric and magnetic fields, application of electric or electromagnetic fields to a plasma can accelerate its constituents and send them out the back of a vehicle as thrust-producing exhaust. The necessary fields can be generated by electrodes and magnets, using induction by external antennas or wire coils, or by driving electric currents through the plasma.

The electric power for creating and accelerating the plasmas typically comes from solar panels that collect energy from the sun. But deep-space vehicles going past Mars must rely on nuclear power sources, because solar energy gets too weak at long distances from the sun. Today's small robotic probes use thermoelectric devices heated by the decay of a nuclear isotope, but the more ambitious missions of the future would need nuclear fission (or even fusion) reactors. Any nuclear reactor would be activated only after the vessel reached a stable orbit at a safe distance from Earth. Its fuel would be secured in an inert state during liftoff.

Three kinds of plasma propulsion systems have matured enough to be employed on long-distance missions. The one in most use and the kind powering Dawn is the ion drive.

The ion engine, one of the more successful electric propulsion concepts, traces its roots to the ideas of American rocketry pioneer Robert H. Goddard, formed when he was still a graduate student at Worcester Polytechnic Institute a century ago. Ion engines are able to achieve exhaust velocities ranging from 20 to 50 km/s [see box on next page].

In its most common incarnation, the ion engine gets its electric power from photovoltaic panels. It is a squat cylinder, not much larger than a bucket, that is set astern. Inside the bucket, xenon gas from the propellant tank flows into an ionization chamber where an electromagnetic field tears electrons off the xenon gas atoms to create a plasma. The plasma's positive ions are then extracted and accelerated to high speeds through the action of an electric field that is applied between two electrode grids. Each positive ion in the field feels the strong tug of the aft-mounted, negatively charged electrode and therefore accelerates rearward.

The positive ions in the exhaust leave a spacecraft with a net negative charge, which, if left to build up, would attract the ions back to the spacecraft, thus canceling out the thrust. To avoid this problem, an external electron source (a negative cathode or an electron gun) injects electrons into the positive flow to electrically neutralize it, which leaves the spacecraft neutral.

Dozens of ion drives are currently operating on commercial spacecraft mostly communications satellites in geosynchronous orbit for orbital "station-keeping" and attitude control. They were selected because they save millions of dollars per spacecraft by greatly shrinking the mass of propellant that would be required for chemical propulsion.

At the end of the 20th century, Deep Space 1 became the world's first spacecraft using an electric propulsion system to escape Earth's gravitation from orbit. The probe then accelerated by about 4.3 km/s, while consuming less than 74 kilograms of xenon propellant (about the mass of an untapped beer keg), to fly through the dusty tail of the comet Borrelly. This is the highest velocity increment gained via propulsion (as opposed to gravity assists) by any spacecraft to date. Dawn should soon break that record by adding 10 km/s to its velocity. Engineers at the Jet Propulsion Laboratory have recently demonstrated ion drives able to function flawlessly for more than three years of continuous operation.

A plasma rocket's performance is determined not only by the speed of the exhaust particles but also by its thrust density, which is the amount of thrust force an engine produces per unit area of its exhaust aperture. Ion engines and similar electrostatic thrusters suffer from a major shortcoming, called space-charge limitation, that severely reduces their thrust density: as the positive ions pass between the electrostatic grids in an ion engine, a positive charge inevitably builds up in this region. This buildup limits the attainable electric field to drive the acceleration.

Because of this phenomenon, Deep Space 1's ion engine produces a thrust force that is roughly equivalent to the weight of a single sheet of paper hardly the thundering rocket engine of sci-fi movies and more akin to a car that takes two days to accelerate from zero to 60 miles per hour. As long as one is willing to wait long enough (typically, many months), though, these engines can eventually attain the high delta-vs needed for distant journeys. That feat is possible because in the vacuum of space, which offers no resistance, even a tiny push, if constantly applied, will lead to high propulsion speeds.

A plasma propulsion system called the Hall thruster [see box at right] avoids the space-charge limitation and can therefore accelerate a vessel to high speeds more quickly (by way of its greater thrust density) than a comparably sized ion engine can. This technology has been gaining acceptance in the West since the early 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 discovered in 1879 by Edwin H. Hall, then a physics graduate student at Johns Hopkins University. Hall showed that when electric and magnetic fields are set perpendicular to each other inside a conductor, an electric current (called the Hall current) flows in a direction that is perpendicular to both fields.

In a Hall thruster a plasma is produced when an electric discharge between an internal positive anode and a negative cathode situated outside the device tears through a neutral gas inside the device. The resulting plasma fluid is then accelerated out of the cylindrical engine by the Lorentz force, which results from the interaction of an applied radial magnetic field and an electric current (in this case, the Hall current) that flows in an azimuthal direction that is, in a circular "orbit" around the central anode. The Hall current is caused by the electron's motion in the magnetic and electric fields. Depending on the available power, exhaust velocities can range from 10 to more than 50 km/s.

This form of electric rocket avoids a space-charge buildup by accelerating the entire plasma (of both positive ions and negative electrons), with the result that its thrust density and thus its thrust force (and so its potential delta-v) is many times that of an ion engine of the same size. More than 200 Hall thrusters have been flown on satellites in Earth orbit. And it was a Hall thruster that the European Space Agency used to efficiently propel its SMART-1 spacecraft frugally to the moon.

Engineers are now trying to scale up today's rather small Hall thrusters so that they can handle higher amounts of power to generate greater exhaust speeds and thrust levels. The work also aims to extend their operating lifetimes to the multiyear durations needed for deep-space exploration.

Scientists at the Princeton Plasma Physics Laboratory have taken a step toward these goals by implanting segmented electrodes in the walls of a Hall thruster. The electrodes shape the internal electric field in a way that helps to focus the plasma into a thin exhaust beam. This design reduces the useless nonaxial component of thrust and improves the system's operating lifetime by keeping the plasma beam away from the thruster walls. German engineers have achieved similar results using specially shaped magnetic fields. Researchers at Stanford University have meanwhile shown that lining the walls with tough, synthetic-polycrystalline diamond substantially boosts the device's resistance to plasma erosion. Such improvements will eventually make Hall thrusters suitable for deep-space missions.

One way to further raise the thrust density of plasma propulsion is to increase the total amount of plasma that is accelerated in the engine. But as the plasma density in a Hall thruster is raised, electrons collide more frequently with atoms and ions, which makes it more difficult for the electrons to carry the Hall current needed for acceleration. An alternative known as the magnetoplasmadynamic thruster (MPDT) allows for a denser plasma by forgoing the Hall current in favor of a current component that is mostly aligned with the electric field [see box at left] and far less prone than the Hall current to disruption by atomic collisions.

In general, an MPDT consists of a central cathode sitting within a larger cylindrical anode. A gas, typically lithium, is pumped into the annular space between the cathode and the anode. There it is ionized by an electric current flowing radially from the cathode to the anode. This current induces an azimuthal magnetic field (one that encircles the central cathode), which interacts with the same current that induced it to generate the thrust-producing Lorentz force.

A single MPD engine about the size of an average household pail can process about a million watts of electric power from a solar or nuclear source into thrust (enough to energize more than 10,000 standard lightbulbs), which is substantially larger than the maximum power limits of ion or Hall thrusters of the same size. An MPDT can produce exhaust velocities from 15 to 60 km/s. It truly is the little engine that could.

This design also offers the advantage of throttling; its exhaust speed and thrust can be easily adjusted by varying the electric current level or the flow rate of the propellant. Throttling allows a mission planner to alter a spacecraft's engine thrust and exhaust velocity as needed to optimize its trajectory.

Intensive research on mechanisms that hamper the performance and lifetimes of MPD devices, such as electrode erosion, plasma instabilities and power dissipation in the plasma, has led to new, high-performance engines that rely on lithium and barium vapors for propellants. These elements ionize easily, yield lower internal energy losses in the plasma and help to keep the cathode cooler. The adoption of these liquid-metal propellants and an unusual cathode design that contains channels that alter how the electric current interacts with its surface has resulted in substantially less erosion of the cathode. These innovations are leading to more reliable MPDTs.

A team of academic and NASA researchers has recently completed the design of a state-of-the-art lithium-fed MPDT called 2, which could potentially drive a nuclear-powered vessel hauling heavy cargo and people to the moon and Mars as well as robotic missions to the outer planets.

Ion, Hall and MPD thrusters are but three variants of electric plasma rocket technology, albeit the most mature. During the past few decades researchers have developed many other promising related concepts to various degrees of readiness. Some are pulsed engines that operate intermittently; others run continuously. Some generate plasmas through electrode-based electric discharge; others use coil-based magnetic induction or antenna-generated radiation. The mechanisms they apply to accelerate plasmas vary as well: some use Lorentz forces; others accelerate the plasmas by entraining them in magnetically produced current sheets or in traveling electromagnetic waves. One type even aims to exhaust the plasma through invisible "rocket nozzles" composed of magnetic fields.

In all cases, plasma rockets will get up to speed more slowly than conventional rockets. And yet, in what has been called the "slower but faster paradox," they can often make their way to distant destinations more quickly by ultimately reaching higher spacecraft velocities than standard propulsion systems can using the same mass of propellant. They thus avoid time-consuming detours for gravity boosts. Much as the fabled slow and steady tortoise beats out the intermittently sprinting hare, in the marathon flights that will become increasingly common in the coming era of deep-space exploration, the tortoise wins.

So far the most advanced designs could impart a delta-v of 100 km/s much too slow to take a spacecraft to the far-off stars but plenty enough to visit the outer planets in a reasonable amount of time. One particularly exciting deep-space mission that has been proposed would return samples from Saturn's largest moon, Titan, which space scientists believe has an atmosphere that is very similar to Earth's eons ago.

A sample from Titan's surface would offer researchers a rare chance to search for signs of chemical precursors to life. The mission would be impossible with chemical propulsion. And, with no in-course propulsion, the journey would require multiple planetary gravity assists, adding more than three years to the total trip time. A probe fitted with "the little plasma engine that could" would be able to do the job in a sig nificantly shorter period.


  Enero de 2009. Scientific American Magazine.