|Fig. 1: Illustration of Prometheus Project DSV exploring one of Jupiter's icy moons. (Source: Wikimedia Commons)|
Indisputably, space proudly stands as one of the most complex and extensive phenomenas known to man. Considering the fractionally minimal grasp we have on the celestial bodies and properties which govern space, scientists are constantly looking for innovations to truly explore its depths. Thus, was the inception of NASA's Prometheus Project. Essentially, NASA looked to address two very pertinent goals in order to achieve mission success: firstly, the Prometheus Project would have to push space exploration bounds by being a Deep Space Vehicle (DSV) which could efficiently, safely, and reliably handle systematic/robotic investigation beyond our solar system utilizing a Nuclear Reactor with electric propulsion (NEP). Upon the latter's success, the vehicle would have the mission objective of collecting observational data on Jupiter's icy moons such as Europa, as depicted in Fig. 1, which is suggested to have an ocean of liquid water which very well could be sustaining alien life. However, the project was not without its limiting constraints - if proven successful given the immense technological/technical challenges, the Prometheus Project's "capability would enable a new era of space exploration through increased spacecraft maneuverability and unprecedented amounts of on-board electrical energy."  Sadly, the task proved to be too overwhelming in light of funding and higher priorities for the NASA Agency. Upon successfully completing Phase A (Missions and Systems Definition) the project was discontinued on October 2nd, 2005 from entering into Phase B (Preliminary Design). Thus, the following gives insight on the two major nuclear power methods that were being investigated and decided between had the project continued on.
Just like any intricate space mission, there are always numerous mechanisms at play. Working to identify how to tackle such new technological objectives, NASA identified 7 areas that would require serious advancement before any evident progress could be made: the reactor, energy conversion, heat rejection, electric propulsion, high-power telecommunications, radiation hardened components, and low-thrust trajectory tools.  The topics of interest for our particular discussion regard how effectively the NEP system operates and furthermore how it propels itself through deep space which then inevitably includes the reactor, the electric propulsion, and such related entities.
|Fig. 2: This was the NERVA project. A nuclear reactor undergoes nuclear fission to produce energy which is surrounded by various mechanisms such as the control drum which is an electropneumatic device that converts an electrical input command signal into rotational output motion or the reflector which slows the speed of fast neutrons to utilize their thermal energy for reactions. (Source: Wikimedia Commons)|
The first reactor concept was comprised of several other components that would make up what is known as a Nuclear Thermal Rocket (NTR) which creates thrust by taking in a given element and creating some built up pressure and force by expanding and heating the working fluid element. Simply put, this energy is what provides the energy for electrical propulsion of the vehicle. However, what makes the NTR advantageous compared to a typical chemical reactor, and ultimately an ideal candidate for Project Prometheus, is their ability to maximize the same components of a chemical rocket due to their efficient specific impulse (ISP). The ISP, being the energy density per unit mass of fuel, will fission elements such as Uranium-235 (commonly used by NASA) to provide thermal energy for propellant heating and moreover electric energy generation.  NASA, partnering with NEXIS and HiPEP, concluded that the energy source would require ISP values ranging from 6000-9000 with efficiencies greater than 65%.  To achieve higher ISP levels, a rocket must utilize a lower molecular weight to fuel ratio which is sacrificed at the expense of "heavier" chemical compounds and reactions in chemical rockets. Since a NTR uses nuclear fission to spontaneously produce heat and energy as oppose to chemical rocket's dependence on certain chemical compounds, the ISP is significantly lower with a wider variety of low molecular weight options elements to chose from (e.g Hydrogen). Consequentially, these spacecrafts can accommodate a much heavier payload as a compensation factor while still supplying electricity; this is the "Bimodal mode" NTR which supplies electrical energy when not being used for thrust and coincides very closely with the Nuclear Engine for Rocket Vehicle Application (NERVA) project, shown in Fig. 2, which was NASA's first use of a nuclear rocket for exploration in the 1960's.
|Fig. 3: A pictorial depiction of an ion thruster as described above. (Source: Wikimedia Commons)|
The next worthy candidate that was under heavy consideration from NASA was the Ion Propulsion Thruster. The way these devices work is somewhat complicated as seen in Fig. 3. The device starts by taking the propellant and ionizing it - a high energy electron with its negative charge bombardes the propellant with a neutral charge. Consequentially, the propellant responds to this massing of negative charges by discharging its electrons, thus producing positively charged ions creating what is known as a "no overall electric charge" - formally, plasma. The unique properties of plasma is what makes this such an efficient and unique system: playing with the differences in voltage potentials and Coulomb's Law, scientist are able to have positively charged ions migrate through the thruster to eventually produce speed. Very strong magnets are used to prevent electrons from reaching positively charged discharge walls - the new positive ions move along two different aperture grids until they reach the aft end of the thruster. A high positive voltage is applied to what is known as the screen grid (aperture 1) to cast the ions further down the thruster until the positive ions are quickly accelerated to a negatively charged accelerator grid (aperture 2) sending thousands of positive ions out for thrust to reach speeds up to 90,000 mph. Due to the fact the spacecraft is at such a high positive voltage compared to its surroundings, it is enclosed in what is called a "plasma screen" to prevent electron collection on the positively biased surfaces of the system.  A neutralizer then extracts these excess electrons so that the net charge of the exhaust does not become negative causing this flow of positive ions to be attracted back inside the spacecraft and produce reduce thrust and erosion. This well researched process is what allows the ion thruster to hold so much value with efficiencies between 65-90% seen in NASA's work with the Deep Space 1 project. Further investigation was indeed necessary, however, as an ion thruster has several drawbacks with minimal acceleration and low thrust capabilities due to space charge issue created by ions within the thruster.
Despite the disappointment of Project Prometheus not taking off, serious innovations forward are currently in place. This project worked to lay the groundwork for some very serious space ventures in exploring our galaxy and what's beyond which has been implemented since Bush's 2004 "bold vision for U.S space exploration". Mars has a very similar day cycle, pressure atmosphere and weather system as Earth making it a very viable candidate for human sustainability - we just need the resources to go further than the moon. Humans curiosity is burning after events such as the Mars Pathfinder or even the 1984 Antartica Martian meteorite ALH84001 discovery implicating Martian fossils. With some more intensive iterations and creative genius, Project Prometheus is far from over.
© William Mangram. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. Taylor, "Prometheus Project: Final Report," Jet Propulsion Laboratory, 982-R120461, October 2005.
 S. K. Borowski, "'Bimodal' Nuclear Thermal Rocket (BNTR) Propulsion for Future Human Mars Exploration Missions," NASA/CP-2004-212963/VOL1, November 2003.
 D. M. Goebel and I. Katz, Fundamentals of Electric Propulsion: Ion and Hall Thrusters (Wiley, 2008) pp. 429-446.