Ion Propulsion

Summary

Ion propulsion is the ejection of charged particles from a spacecraft, causing the craft to move in the opposite direction from the ejected particles. Chemical rocket engines move spacecraft and other vehicles in much the same way, but the speed of their exhaust is limited by the energy released in the chemical reaction. Ions, however, can be accelerated by electric and magnetic fields to far higher speeds, so ion engines require a far smaller mass of fuel than chemical engines.

Definition and Basic Principles

The fundamental principle of rocket propulsion is Sir Isaac Newton's third law of motionor every action, there is an equal and opposite reaction. The mass of fuel ejected rearward from a rocket results in the rocket being propelled forward. More precisely, the mass of fuel multiplied by the exhaust velocity equals the force pushing the rocket forward multiplied by the burn time. It follows then that the greatest amount of mass possible must be ejected each second at the highest possible speed to provide maximum acceleration to a rocket. The maximum exhaust speed for a chemical rocket is about 3 to 5 kilometers per second, but the exhaust speed for an ion engine can be 30 to 50 kilometers per second or greater.

The best thrust of an ion engine is only a small fraction of its weight, so ion engines cannot be used to lift a spaceship into orbit. Chemical engines must perform that task. However, once the spaceship is in orbit, ion engines can be very useful. Short bursts from an ion engine are ideal for station keeping (maintaining position and orientation concerning another craft or object). Because ion engines use so little mass, they may be fired for days or months instead of the minutes a chemical engine can burn. The low thrust of an ion engine fired for a long time can make major changes in the orbit of a spaceship. For example, an ion engine took the National Aeronautics and Space Administration's (NASA) Deep Space 1 from low Earth orbit to fly by asteroid 9969 Braille and comet Borrelly.

Background and History

Anyone working with electric fields and charged particles soon discovers that it is easy to accelerate charged particles to high speeds. Rocket pioneers Robert Goddard (in 1906) and Konstantin Tsiolkovsky (in 1911) discussed the idea of an ion engine. In 1916, Goddard built an ion engine and demonstrated that it produced thrust. In 1964, NASA scientist Harold R. Kaufman built and successfully tested an ion engine that used mercury as reaction mass in the suborbital flight of the Space Electric Rocket Test 1 (SERT 1).

During the 1950s and 1960s, the United States and the Soviet Union (modern-day Russia) worked on a Hall-effect ion engine that used magnetic fields to accelerate ions. The United States continued to work on the electric field ion engine but dropped out of the competition for the Hall-effect engine. Russia eventually developed Hall-effect engines and began using them in space in 1972.

In 1992, the West adopted some Russian technology and started using Hall-effect engines as well as the electric field ion engine. In 1998, NASA launched Deep Space 1, which used an ion engine and flew by asteroid 9969 Braille and Comet Borrelly. The Deep Space 1 used an ion engine as its main source of propulsion, and its success paved the way for future missions powered by ion engines. In the 2000s, NASA, the European Space Agency (ESA), and the Japan Aerospace Exploration Agency launched spacecraft that used versions of ion engines.

How It Works

Heavy atoms are usually selected as the reaction mass for an ion engine because heavy ions increase the mass ejected per second. It is also desirable to use a substance that requires relatively little energy to ionize it. Mercury, cesium, xenon, bismuth, and argon have all been used for reaction mass. However, mercury and cesium are poisonous and require special handling, so the substance most commonly used for reaction mass has been xenon. Xenon can be stored under pressure as a liquid for long periods. As a gas, it is easy to transfer from storage to the rocket motor, the molecules are relatively heavy and easily ionized, and it is chemically inert, so it will not corrode the engine. Ion engine bodies are often made of boron nitride, a good insulator that can withstand the engine’s operating conditions.

A small amount of the reaction mass is leaked at a controlled rate into a plasma chamber, where it is bombarded with electrons to ionize it. Fields from permanent magnets are used to confine and control the resulting plasma.

The walls of the plasma chamber are maintained at a high positive voltage (such as +1,100 volts) so that free electrons migrate to the walls and are absorbed. Two grids or wire screens are at the rear of the plasma chamber. The first grid is at a lower voltage, such as +1,065 volts, so positive ions from the chamber are attracted to it. A second grid is placed 2 or 3 millimeters beyond the first grid and electrified at −180 volts. Positive ions are accelerated by the electric field in the gap between the two grids and gain 1,245 electron volts of energy. This means xenon ions will exit the engine at an impressive 43 kilometers per second.

Three areas have to be addressed for ion propulsion to work. First, because the craft is ejecting a positive charge into space, it is becoming more negative. If nothing were done about this, the craft would eventually become so negative that the positive ion exhaust would be attracted back to the ship, and there would be no propulsion. To deal with this, a special electrode sprays a negative charge onto the ion plume behind the craft to keep the craft's charge neutral. Second, a positive charge being accelerated between the grids constitutes a space charge density. This space charge repels the approaching positive charge. This limits the ion current to fairly small values, which limits the thrust of an ion engine to small values. These limits on thrust must be carefully calculated and considered. Third, high-energy ions strike electrodes and erode them. Careful attention to the electrode design and materials used can extend electrode lives to 10,000 hours.

Hall-Effect Thrusters. The Hall effect (named for Edwin Hall, who discovered it in 1879) states that if a conductor carrying a current is placed in a magnetic field perpendicular to the current, the magnetic field will cause a new current to flow perpendicular to both the magnetic field and the original current.

For example, consider a horizontal tube along the bottom of a page. Let the tube contain a vertical electric field pointing from the bottom to the top of the page. Suppose further that there is a horizontal magnetic field pointing out of the paper. If electrons and ionized xenon atoms are introduced at the left end of the tube, electrons will be propelled by the electric field to sink to the bottom side of the tube along the lower edge of the page. Because of the greater mass of the xenon ions, their progress toward the upper side of the tube will be far slower. A charged particle moving in a magnetic field experiences a force perpendicular to its velocity and the magnetic field. This causes the electrons sinking to the bottom of the tube to move from left to right down the axis of the tube. These electrons will strike other electrons and ions, forcing them to move down the tube. This plasma now moving along the tube's axis will become the exhaust jet that propels the spacecraft. The mass exhausted is not limited by space charge and can be as large as the device can handle.

VASIMR. The VASIMR (Variable Specific Impulse Magnetoplasma Rocket) project of the Ad Astra Rocket Company in Texas, is one of the leading proptypes for more powerful ion engines. The VASIMR engine uses microwaves to ionize and heat the propulsive gas. The gas’s temperature can be controlled by increasing or decreasing microwave intensity. The absence of electrodes means they cannot be eroded by hot plasma and suffer degraded performance as in a normal ion engine. Magnetic fields confine, direct, and accelerate the plasma to exhaust speeds up to 50 kilometers per second. Magnetic fields form a rocket nozzle inside the metal nozzle. Gas introduced between the metal nozzle and the plasma is heated by the plasma and adds to the thrust. This thrust can be modified by changing the microwaves heating the plasma, by changing the rate at which reaction mass is delivered to the engine, or by changing the size of the magnetic field nozzle. Engines using 100 and 200 kilowatts have been built and ground-tested. One day, it may be possible to build a 200-megawatt engine powered by a small nuclear reactor. A craft powered by such an engine could make the trip from Earth to Mars in only 39 days, a considerable improvement over the 255 days required by conventional rockets.

Applications and Products

Numerous ion engines have been used for station keeping. They are the most common primary engines used in satellites, and missions increasingly use them as main engines.

SERT 1. Launched by NASA in 1964, SERT 1 showed that an ion engine functioned as expected in space. In 1970, SERT 2 was sent into space to test two ion engines using mercury as reaction mass. The engines operated for 2,011 hours and 3,781 hours respectively and were restarted about three hundred times.

Deep Space 1. Launched by NASA in 1998, Deep Space 1 tested twelve high-risk technologies, including the NSTAR ion engine. The engine drew up to 2.5 kilowatts from solar cells and produced 0.092 newtons of thrust. It carried 81.5 kilograms of xenon reaction mass, enough to last 678 days. Each day, the engine added 24 to 32 kilometers per hour to the craft's speed. It flew by asteroid 9969 Braille and Comet Borrelly. Because of a software crash, the pictures of Braille were not as good as had been hoped, but the pictures of Comet Borrelly were outstanding. A camera had to be reprogrammed to take over tracking duties when the star tracker failed, but most of the other instruments worked well. The mission ended with an engine shutdown on December 18, 2001.

Artemis. The European Space Agency launched the Artemis in 2001, but the telecommunications satellite failed to reach its intended orbit. It used its remaining chemical fuel to transfer to a higher orbit, then used its xenon ion engine for eighteen months to raise it to the intended geostationary Earth orbit.

Hayabusa. The Japan Aerospace Exploration Agency launched Hayabusa in 2003. It reached the asteroid Itokawa in 2005, but the lander module MINERVA flew by the asteroid instead of landing as planned. Astronomers have long suspected that some asteroids are rubble piles held together by only self-gravitation. Images sent from Hayabusa seem to show exactly that. Hayabusa successfully landed and activated a collection capsule. The craft was propelled by four xenon ion engines that amassed a combined total of 31,400 operating hours. These were the first engines in space to use microwaves to form and heat the xenon plasma. Unfortunately, the engines failed one by one. None was operational to bring Hayabusa home, but operators could use the ion generator from one engine with the electron gun neutralizer from another engine to make one working engine. The sample capsule returned to Earth on June 13, 2010. It had several grains of dust inside, the first such sample ever obtained.

SMART-1. The European Space Agency launched SMART-1 (Small Missions for Advanced Research in Technology 1) in 2003. A Hall-effect ion engine, using xenon as the reaction mass, allowed SMART-1 to travel from low Earth orbit to lunar orbit. SMART-1 surveyed the chemical elements on the lunar surface and was driven into the Moon in a controlled crash in 2006. Although the Russians had used Hall-effect engines for station keeping for many years, this was the first use of a Hall-effect engine as the main engine.

Dawn and GOCE. In 2007, NASA launched Dawn to explore the asteroid Vesta in 2011 and the dwarf planetCeres in 2015. The spacecraft was propelled by three xenon ion engines but typically used only one at a time until the mission’s 2018 completion. The European Space Agency launched GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) in 2009. Until 2013, GOCE orbited only 260 kilometers above the Earth's surface to map small changes in the Earth's field. It used a xenon ion engine to make up for losses caused by air drag.

GSAT-4. The Indian Space Research Organization launched the GSAT-4, a geostationary satellite for navigation and communications, in April 2010. It had an ion engine for station keeping, but the third stage failed to ignite, and the satellite was lost. The ion engine would have extended GSAT-4's normal ten-year life to fifteen years.

LISA. The LISA Pathfinder (originally SMART-2) was launched by the European Space Agency (ESA) in 2015 to test technologies for the LISA (Laser Interferometer Space Antenna) mission. LISA is a very sensitive device for detecting gravity waves, which would consist of three satellites containing special masses, mirrors, and lasers. The satellites will be 5 × 10⁶ kilometers apart, each at a different corner of an equilateral triangle. The lasers should be able to measure small changes in the positions of the satellites caused by a gravity wave. Among the technologies LISA Pathfinder was meant to test were colloid thrusters, ion engines that use charged liquid drops for reaction mass. Their thrust is very small, perhaps only 20 × 10⁻⁶ newtons. Such small forces are useful in making tiny adjustments to the speed or position of a satellite.

Psyche. NASA launched the tennis-court-sized spacecraft Psyche in October 2023 to observe asteroid Psyche, a metal-rich asteroid between Mars and Jupiter. Four Hall-effect thrusters propel the spacecraft at 240 millinewtons of thrust using solar electric propulsion. Psyche was also equipped with 22 tanks of xenon propellant containing 82 liters each.

Careers and Course Work

Jobs in design and development will be exciting and interesting to those who enjoy solving problems and seeing ideas become reality. A strong background in physical sciences—at least a bachelor's degree in physics, engineering, electrical engineering, mechanical engineering, materials science, or chemistry—is required to design and develop ion engines or other space hardware. High school students should take all available physics, chemistry, and mathematics courses. The same is true for college students, who need at least one year of basic chemistry, calculus-based physics with laboratory practice, and mathematics through differential equations and matrices. Classes in statistics and computer programming may be helpful.

Other useful classes include writing and speech (for reports and presentations) and a simple business course. Those wishing to work in research and development need a feel for how things work and some creativity. Those working in quality control may need to design tests that demonstrate that a device works and will continue to work reliably.

Social Context and Future Prospects

As Hall-effect ion engines and other ion propulsion systems such as VASIMR engines become more powerful, rocket fuel will account for less of the spacecraft's total weight. This will allow larger payloads and enable robot missions to conduct more scientific experiments. Research and development programs such as NASA's Next Space Technologies for Exploration Partnerships (NextSTEP) have continued to push the performance of Hall thrusters, as in the example of the X3 thruster that broke power generation records for ion engines during tests in 2017. While xenon gas has been the main propellent used in ion engines, the substance is relatively rare on Earth and therefore difficult to obtain. In 2021, scientists successfully tested an ion thruster that uses iodine as a propellant. Iodine is far more prevalent on Earth and has been demonstrated to be more efficient than xenon gas.

If even higher-powered engines can be developed, the likely source capable of providing enough power would be a small nuclear reactor. Russia has used over thirty fission reactors in space to power satellites. The United States flew a test reactor, the SNAP-10A, in 1965. With a nuclear reactor for power and a large ion engine, a spacecraft could travel from Earth to Mars in a few dozen days instead of the hundreds of days it would take a conventionally powered vessel. This may make a crewed mission possible because of the reduction in days of exposure to cosmic rays. Such an engine might make it possible to fly to an asteroid and exploit its minerals. (A single metallic asteroid could contain more nickel than has ever been mined on Earth.) An additional benefit is that people's apprehensions about using nuclear reactors for power on Earth might lessen if nuclear reactors in space built up a good safety record.

Bibliography

Cho, Charles Q. "Promising New Electric Iodine Thruster Passes Key Test in Orbit." Space.com, 26 Nov. 2021, www.space.com/spacecraft-propulsion-iodine-ion-engine-tested-orbit. Accessed 28 Feb. 2022.

"Dawn: Ion Propulsion." NASA, 7 Aug. 2023, science.nasa.gov/mission/dawn/technology/ion-propulsion. Accessed 25 May 2024.

Doody, Dave. Deep Space Craft: An Overview of Interplanetary Flight. Springer, 2009.

Gilster, Paul. Centauri Dreams: Imagining and Planning Interstellar Exploration. Springer, 2011.

Goebel, Dan M., et al. Fundamentals of Electric Propulsion. 2nd ed. Wiley, 2024.

National Academies of Sciences, Engineering, and Medicine. Space Nuclear Propulsion for Human Mars Exploration. National Academies Press, 2021.

Orf, Darren. "A Remarkable New Thruster Could Achieve Escape Velocity—and Interplanetary Travel." Popular Mechanics, 3 May 2024, www.popularmechanics.com/space/rockets/a60654632/next-generation-ion-thruster-nasa. Accessed 20 May 2024.

Pultarova, Tereza. "Ion Thruster Prototype Breaks Record in Tests, Could Send Humans to Mars." Space.com, 13 Oct. 2017, www.space.com/38444-mars-thruster-design-breaks-records.html. Accessed 24 Aug. 2018.

Turner, Martin J. L. Rocket and Spacecraft Propulsion: Principles, Practice and New Developments. 3rd ed. Praxis, 2009.