Friday, January 22, 2016

Applications of Ion Propulsion

This is the second part of a two-part post. The first post, describing the function of ion propulsion engines, may be found here.

Ion thrusters have many advantages over other forms of propulsion. In comparison with traditional chemical propellent, they are roughly 10 times more efficient, lightening the load of space-traveling craft and saving massive amounts of fuel for launch. This efficiency originates in part from the higher exhaust velocity of the xenon propellant particles, which are ejected from the spacecraft at speeds of 20-50 km/s! In addition, the electric power required to run ion engines is relatively small, on the order of a few kilowatts. In comparison, a typical microwave oven consumes 1.1 kW, and the power consumption is significantly less than that of a typical automobile. Solar panels can meet this power demand during flight, allowing ion thrusters to create smooth and continuous acceleration over the entire duration of a mission.

However, this efficiency comes at a price: ion propulsion produces very small thrusts. The first model of ion engine actually used in spaceflight, the NSTAR engine, produced a thrust of around 90 millinewtons. This is the same force that your hand would experience from a single piece of paper on Earth by gravity, a barely detectable force! However, this minute force can operate continuously, adding up to a significant acceleration over time in the frictionless environment of space. In comparison, space probes which operate on chemical propellant may exert thrusts in the hundreds or thousands of newtons (the equivalent of a couple hundred pounds at Earth's surface), but only for very short times.

The small thrust of ion engines pales even more in comparison to that of rockets that launch payloads from Earth's surface. To escape Earth's gravity, such rockets must exert a force greater than the force of gravity on the often huge rockets. For example, the Saturn V rocket (shown above) that launched humans to the Moon generated an astounding 34,500,000 newtons of thrust at launch. For this reason, ion engines cannot be used to launch spacecraft.

The idea of electric rock propulsion dates to the 1930's and the first test of ion engines in space came during the 1960's. However, no operational mission utilized ion thrusters until NASA's Deep Space 1 probe, launched in 1998. This spacecraft performed a flyby of the asteroid 9989 Braille and the comet 19P/Borrelly. To meet the acceleration requirements of the mission extension to comet Borrelly, Deep Space 1 changed its velocity by 4.3 km/s using less than 74 kilograms of xenon. The Hayabusa probe, launched by the Japan Aerospace Exploration Agency (JAXA) in 2003, was another important demonstration of ion thruster technology. This probe used ion thrusters on its mission to land on the near-Earth asteroid 25143 Itokawa, collect samples, and return them to Earth for analysis. In 2010, the probe successfully returned the sample to Earth, completing the first ever asteroid sample return mission. Hayabusa 2, another asteroid sample return mission, launched in 2014 with similar objectives.

An even greater demonstration of ion propulsion technology began in 2007 with the launch of the Dawn spacecraft. This spacecraft carried three ion engines, operating alternately throughout the mission. Thrusting frequently throughout its eight-year journey, Dawn followed a spiral outward from the Earth, past Mars, into orbit of the asteroid Vesta, and subsequently into orbit of the asteroid Ceres.

The above image shows Dawn's outward spiral as well as the intervals during which one of its ion engines was in operation. With the exception of the gravitational assist at Mars, Dawn thrusted almost continuously, moving outward under its own power. Its total velocity change exceeded 10 km/s, the greatest yet for a spacecraft under its own power. When it successfully entered orbit around Ceres in April 2015, Dawn became the first spacecraft to orbit two different extraterrestrial targets. This feat would not have been possible using traditional methods of chemical propulsion.

Meanwhile, advances continued in ion thruster technology. NASA's Evolutionary Xenon Thruster (NEXT) performed a 48000 hour test in a vacuum chamber lasting from 2004 to 2009 to demonstrate its successful operation. Using power at a higher rate but also providing somewhat greater thrust, the NEXT engine (shown above) is at least 30% more efficient than its predecessors and can operate for a longer time. Continued development of ion propulsion technology promises to provide the foundation necessary for more ambitious interplanetary space missions.


Wednesday, January 13, 2016

Hurricane Alex (2016)

Storm Active: January 13-15

On January 7, a low pressure system situated along a front northeast of the Bahamas began deepening, producing a large area of strong winds over the western Atlantic. Though strong upper-level winds and cool ocean temperatures precluded immediate development into a tropical or subtropical cyclone, the National Hurricane Center began to monitor the disturbance. The low moved eastward, remaining frontal in nature, but strengthened even more, producing maximum winds to hurricane force on January 10. Over the next few days, shower activity increased modestly near the system's center as it took a southeast heading into the far eastern Atlantic. By January 12, bands of shallow convection surrounded a well-defined center. The next day, despite marginal sea surface temperatures, thunderstorm activity increased near the center. Due to the relatively shallow convection and gale force winds associated with the system, it was classified Subtropical Storm Alex that afternoon. Alex was only the fourth known tropical or subtropical system to form in the north Atlantic basin in January.

By the time of its formation, Alex had turned toward the northeast and was headed in the direction of the Azores. Meanwhile, despite marginal sea surface temperatures, convection continued to deepen and Alex developed a well-defined eye feature. At the same time, the upper-level low situated over Alex moved away, allowing the cyclone to transition to a tropical cyclone. Since the eyewall now had hurricane force winds, Alex was upgraded to a hurricane during the morning of January 14. It became the first hurricane in January since 1955, and the first to form during the month since 1938. By the afternoon, the outer bands began to affect the Azores Islands. The same evening, it reached its peak intensity of 85 mph winds and a central pressure of 981 mb. The next morning, the center of circulation passed among the central Azores, bringing hurricane-force winds to the region as it sped northward. Meanwhile, the eyewall disintegrated and the convective structure became lopsided as Alex began extratropical transition and weakened slightly. The system became extratropical that afternoon.

The above image shows Alex at peak intensity less than a day before it passed over the Azores. The hurricane developed a remarkable eye feature highly unusual for an off-season storm.

The track of Alex includes several days during which the system was an extratropical system producing winds near hurricane force.

Friday, January 1, 2016

Introduction to Ion Propulsion

In physics, ion propulsion is a type of electric propulsion used by spacecraft. As with any traditional method of rocket propulsion, ion propulsion depends on Newton's Third Law: for every action, there is an equal and opposite reaction.

A typical rocket engine uses internal mechanisms to accelerate some type of exhaust away from the rocket. Since this constitutes a force on the exhaust, the engine experiences a force in the opposite direction. Crucially, propulsion requires that mass be lost from the rocket to exhaust. Other vehicles, such as cars, use friction between wheels and road to provide a force and therefore do not need to expel mass. Operating in space or the atmosphere in which friction is minimal (there is nothing to "push off" of), rockets instead carry extra mass to accelerate. As the name suggests, ion propulsion works by accelerating ions.

The above schematic illustrates the function of a gridded electrostatic ion thruster (which is usually what is meant by "ion propulsion"). On the left side, neutral atoms of the propellant move from storage tanks (not shown) into the ionization chamber. Simultaneously, an electrode fires electrons into the chamber with high velocity. These electrons knock other electrons out of the neutral propellant atoms to create ions. As a result, the ionization chamber becomes filled with electrons and positive ions.

At the other end of the chamber are two grids. They are connected to a voltage source that maintains a static positive charge on the inner grid and an equal and opposite negative charge on the outer one. Two effects combine to remove many of the free electrons from the ionization chamber. First, the positively charged plate attracts electrons, conducting them out of the chamber. Second, the contents of the chamber are very hot. Since electrons are much lighter than the positive ions, they move faster with the same amount of thermal energy and have a greater chance of collecting on the grid. Soon, positively ionized gas (plasma) builds up in the chamber.

This closeup on a gap in the positive grid shows how positive ions eventually escape the chamber. Eventually, so many ions accumulate in the plasma layer that the repulsive force between ions exceeds the force pushing this ions away from the positive grid. Ions then pass through gaps in the grid. Once they reach the other side, the repulsive forces from both the grid and the other plasma accelerate them outward. Returning to the larger diagram, the negative grid focuses the beam of ions so that they all proceed in roughly the same direction. Finally, another electrode fires electrons at the escaping ions, preventing them from losing velocity due to the influence of the negative grid and preventing a buildup of net charge in the engine.

Typically, the type of propellant used for ion propulsion is xenon gas. The use of xenon, which is element 54 on the periodic table, has several advantages. First, it is a noble gas so it is inert and does not react chemically with other parts of the engine. Further, it is the heaviest (non-radioactive) noble gas, so the thermal effect removing electrons from the gaseous plasma is enhanced.

NASA tested the ion thruster shown above in the early 1990's. The blue glow originates from charged xenon particles.

The next post describes the applications of the ion thruster and its impact on space travel.