This concept is based on the idea of spacecraft being propelled through space by electric energy, with the power source options being solar cells or a nuclear-electric reactor.

The major drawback of these designs are the low thrust to weight ratios, meaning their acceleration is weak and they are incapable of escaping Earth's gravitational field or moving large payloads.
This is offset, however, by the performance of the engine once in space, which can leave the chemical rockets far behind due to its far superior specific impulse. In other words although the chemical rockets have high thrust to weight ratios the propellant is quickly exhausted at a low specific impulse. The electric propulsion system can run continuously for many months or even years so that, despite the low thrust, they may ultimately build up to a higher total impulse (specific impulse x propellant mass) and hence much greater velocities.

Ion Engine

Courtesy NASA

The best electric propulsion system we have is the ion engine, which obtains the highest degree of conversion of electric power into thrust, highest specific impulse (around 3000secs) and the longest operational lifetime.
This is the system of choice to power Deep Space 1.

This engine has its origins back around 1960 but still fulfils two of NASA's current objectives, i.e. significant reductions in trip time and initial mass, enabling lower cost, faster planetary missions.

The engine is known as an ion engine because it works on the principle of the ionisation of the propellant gas. This ionisation is usually produced by means of electron bombardment. The electrons are emitted from the cathode by heating and an electric charge accelerates the electrons towards the anode and into the discharge chamber.

Deep Space 1

Courtesy NASA

The propellant gas is injected into this chamber, which is magnetised to increase collision probability between the propellant gas atoms and the electrons. As electrons collide with the xenon atoms, electrons are stripped off of the xenon atoms, resulting in the atom now being of a positive charge, or ionised. These ions are very excited and move about very quickly.
Then high voltage metal grids at the back of the chamber actually produce the thrust, by exerting an electrostatic pull on the ions, causing them to accelerate through the grid. As they pass through the ions reach a velocity of 31.5 km/sec and are focused into an ion beam before finally being exhausted out of the back of the spacecraft.

It is important to note that in the final stage a neutraliser collects and injects the excess electrons into the ion beam. This prevents the spacecraft from charging to a large negative potential.

Deep Space 1 is the NASA New Millennium Program first spacecraft, it produces 0.09 Newtons of thrust and has a specific impulse of 3300secs (over 32,000m/s), with a service life of 8000 hours. The propellant gas in Deep Space 1 is xenon, which is as good as any gas for this function.
With patience, the ion propulsion system on DS1 imparts about 3.6 km/s to the spacecraft.

Microwave Ion Engine

It is worth noting that Japan is working on a microwave ion engine. This design uses microwaves to excite the electrons in the propellant gas. The ion beam is again formed by the ions being focused and accelerated by electrostatic forces.

Ion propulsion is very useful for propulsion systems on planetary missions, as it is able to build up to very high velocities, far greater than chemical propulsion systems. There are disadvantages of course, these include high power requirements and, due to low thrust levels, long service requirements.
Any electric propulsion system must also carry an electric power supply, which means that the power system is of greater mass than chemical propulsion, though the propellant mass saving for long distance flight (due to higher specific impulse) outweighs this.
Today's power systems consist of huge solar panels and so the efficiency will fall off as the craft moves further away from the Sun.

Today's work on ion engines is generally based on low power engines, this is due to the inefficiency of current solar-electric power systems.
This can not remain so indefinitely if we are to venture deeper into the solar system, perhaps transporting payloads as well.
Such missions will clearly require huge power levels to efficiently transport significant payloads, probably in the order of megawatts. By comparison our current model, Deep Space 1, produces around 2.5 kW.

• Improvements in Solar Electric Power (SEP). The only foreseeable method to improve the performance of our current system is to improve the efficiency of solar electric power systems. Nanotechnology could make this happen, but we do not know how long it will be until this technology is developed.
  So in the near term the only viable option is…
• Nuclear electric propulsion (NEP). Even with the immediate proposals power levels could be several 100 kW and the future could see developments to increase this into the megawatt category.

Nuclear Electric Propulsion

Courtesy NASA

In a nuclear electric propulsion system, heat from a nuclear reactor is converted to electrical energy either by direct thermoelectric or thermionic conversion. It has been recognised as a possible technology to allow human exploration of the solar system since the 1960's. It could also provide a cost-effective system for space commerce.

NEP provides higher power, thrust, and specific impulse than an SEP system, although the thrust still pales to insignificance compared to our chemical rockets.
For a comparison specific impulse of chemical rockets is about 400secs, Deep Space 1 uses SEP and produces around 3300secs, NEP on the other hand could produce specific impulse as high as 13,000secs.

Due to the high power NEP systems produce a sharing of power between the engine and the instruments is possible. When instruments are not needed all power can be delivered to the thrusters, but when readings are needed thrusters can be turned down and power re-routed to the instruments. This can lead to possibilities for substantial savings of weight.

One key advantage NEP will always have over SEP is the fact that it does not depend on solar energy, thus may operate in deep space.

The mission capabilities and cost performances of a NEP system can not really be estimated until we have a working model.

The main problems in NEP design are the development of efficient, high-power (megawatt-class) electric thrusters and the development of low specific mass power plants. There is also concern that the particles and fields instruments would be unable to operate in the vicinity of the ions being expelled from the thruster.
The major drawback, however, is the production of nuclear radiation and therefore the need to shield the crew, passengers and payload from both the radiation and also the massive heat from the reactor. This results in increased mass and the need for careful control of the reactor to prevent dangerous accidents. Designs are based around placing the reactor and those requiring protection at opposite ends with radiation shielding between them.

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