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One Step Closer to Tomorrow |
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Propulsion Systems |
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MASA’s Exoplanetary Mission |
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Any chosen propulsion system must be capable of successfully transporting the spacecraft from Earth's orbit into the star system Epsilon Eridani. Epsilon Eridani is located approximately 10.5 light years away from earth with a star about 0.85 times the mass of Earth’s sun. The propulsion system must be a feasible solution to interstellar travel, long lasting, and have a high specific impulse (Isp) to minimize the mass of the combined system while attaining the highest practical travel velocity. The specific impulse is essentially a measure of fuel efficiency for rockets.
Possible propulsion options |
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Disadvantages: Practically achieving a specific impulse approaching c with a mass annihilation rocket would not be feasible given that most of the energy would be dissipated in high energy gamma rays or neutrinos difficult to convert into thrust. The production and storage of antimatter on a spacecraft would itself prove a near impossible task. Recently, physicists at CERN were able to store 38 anti-hydrogen atoms for 170ms which is a great achievement but nowhere near the timeframe necessary for interstellar travel. Inertial Confinement Fusion: Advantages: This fuel source offers a very high Isp (105-106 s) and has had some limited success in small scale testing. The fuel source for fusion, deuterium, is relatively easy to obtain. The fusion products have high velocities and some could be directly converted to thrust. The lasers used to ignite and fuse the matter are pulsed and only need to be on for nanoseconds at a time. Fusing pellets of deuterium at a different rates would allow for variable thrust. Disadvantages: There have not been many great breakthroughs in this technology over the past few decades to make inertial confinement fusion a viable source of power (ability to produce more power than is input). The lasers necessary for ignition of the fusion products are in the petawatt range. Also, the fusion reaction with the lowest ignition temperature (least energy needed for fusion) is a deuterium-tritium fusion. Tritium would need to be manufactured onboard due to its short half-life (12 years), while the primary deuterium-deuterium reaction would require 6 times the ignition temperature. The fusion reactions emit high energy neutrons which are difficult to capture, would require massive shielding components, and would degrade the structural integrity of the reactor and spacecraft over time. Magnetic Confinement Fusion: Advantages: This fuel source offers a high Isp (104-106 s) and the fuel, deuterium, is relatively abundant. This fusion reactor design, as evidenced in the ITER, promises to output 10 times the energy it consumes. Disadvantages: Magnetic confinement is well ahead of inertial confinement fusion in experimental results, but is not yet near commercial success. Along with the disadvantages of the inertial confinement fusion rocket, magnetic confinement requires continuous use of power in order to contain the produced plasma. The predominant design for magnetic confinement utilizes a toroid of superconducting wires to confine the plasma. This makes direct conversion of energetic plasma particles into thrust a difficult challenge. Ion Thruster: Advantages: Ion thrusters have already been tested and used for space exploration. The newest design, NEXT, being developed by NASA's Glenn Research Center is showing a maximum specific impulse over 4000 s. This is ten times larger than chemical rockets currently used for most space missions. This design is nearing mission readiness and with some new developments in materials and chemical engineering, it is not inconceivable that ion thruster could reach Isp of 10000 s in the next few decades. Disadvantages: Unlike the aforementioned propulsion systems, and ion thruster does not produce its own power. An external power source must be used. When departing from Earth a large Radioisotope Thermoelectric Generator (RTG) could be used to accelerate the spacecraft. Before arriving at Epsilon Eridani, the spacecraft would need to be decelerated in order to enter an orbit around the star. 238Pu RTGs, with a half-life of 87.7 years, would be unable to power the ion thruster after the long voyage. A source such as a nuclear fission reactor would have to be utilized for power at the final stage of deceleration.
Other sources of thrust such as a chemical rocket or solar sail are not considered because they do not meet the criteria needed. Chemical rockets have a very low Isp compared to the alternatives. Solar sails are impractical because they would require Giga- or Terrawatt lasers continuously shining from Earth at a solar sail on the order of kilometers in diameter attached to the satellite. To minimize mass, this sail would be very delicate and would probably not survive the trip through space intact. Based upon this information, an ion thruster has been selected as the primary propulsion system for this expedition. |
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Trajectory Overview After assembly of the spacecraft in a Low Earth Orbit (LEO), the spacecraft will be thrust into an eccentric orbit around Earth. The orbit will be such that Earth can be used as a gravitational slingshot to reach Jupiter. Once the slingshot past Earth is made, the ion thruster will be enabled. Jupiter will then be used to gain more velocity relative to the Sun by using it as a gravitational slingshot to escape the solar system at a velocity of 18.5 km/s (escape velocity from Sun at Jupiter). This phase of the journey will take roughly 5 years. |
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source propelled out of the ion drive to provide thrust. During this period, power from the RTG will occasionally be diverted to a star tracking system to determine the position of the satellite in space and verify its trajectory toward Epsilon Eridani. Over the course of the 200 years, the spacecraft should be oriented on a trajectory accurate enough to reach Epsilon Eridani. The trajectory used for the voyage at this stage will have been precalculated on Earth to account for the motion of the stars and hypothesized solar winds and particle densities in deep space. The spacecraft will cruise toward Epsilon Eridani with systems shut down at a velocity of 33 km/s for approximately 96,000 years until a small directed solar array detects enough photon density over a period of time to generate a voltage and engage a hydrazine-based power generator. This generator will be used to generate a small neutron source, possibly by accelerating alpha particles into beryllium, capable of initiating fission in a small reactor. Once this reactor is on, it will be able to power the electronics used for star tracking and power the ion drive. Its power output will be moderated by boron control rods. The fission reactor will be an adapted version of the Hyperion Power Module (HPM). The HPM is designed to weigh less than 50 metric tons and generate 25 MW of power. Our reactor would operate at a lower power than the HPM design and would have a much longer operational life of over 200 years. The ion drive will direct and decelerate the spacecraft to enter the star system. Once again, the xenon containers are released from the spacecraft as they are depleted in order to minimize the mass of the spacecraft and increase its deceleration rate. |
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The spacecraft will decelerate for approximately 150 years to enter the Epsilon Eridani system. During this time, the star tracking system will be used to align the spacecraft with its orbital entry point. Roughly 24 tons of xenon will be spent to decelerate the spacecraft to a velocity compatible with the bandwidth of the relay satellite. The entry point will be approximately 6E12 m from the star (the distance from the Sun to Jupiter) and will have a velocity of approximately 0.34 km/s heading normal to the direction of the star. This trajectory will put the satellite into an elliptical orbit around the star with a period of 100 years and an eccentricity of 0.99. While in the system, the satellite will use star tracking to locate the solar system and orient its dish antenna toward Earth for data transmission. |