Interstellar Heliopause Probe


Loeb H. W.1*, Schartner K. 1**, Dachwald B. 2***, Ohndorf A. 3****, Seboldt W. 3*****

1. Justus-Liebig-Universität,
2. Aachen University of Applied Sciences,
3. German Aerospace Centre DLR,



There is common agreement within the scientific community that in order to understand our local galactic environment it will be necessary to send a spacecraft into the region beyond the solar wind termination shock. Considering distances of 200 AU for a new mission, one needs a spacecraft traveling at a speed of close to 10 AU/yr in order to keep the mission duration in the range of less than 25 yrs, a transfer time postulated by European Space Agency (ESA). Two propulsion options for the mission have been proposed and discussed so far: the solar sail propulsion and the ballistic/radioisotope-electric propulsion (REP). As a further alternative, we here investigate a combination of solar-electric propulsion (SEP) and REP. The SEP stage consists of six 22-cms diameter RIT-22 ion thrusters working with a high specific impulse of 7377 s corresponding to a positive grid voltage of 5 kV. Solar power of 53 kW at begin of mission (BOM) is provided by a light-weight solar array. The REP stage consists of four space-proven 10 cm diameter RIT-10 (radio-frequency ion thruster) that will be operating one after the other for 9 yrs in total. Four advanced radioisotope generators provide 648 W at the beginning of mission (BOM). The scientific instrument package is oriented at earlier studies. For its mass and electric power requirement 35 kg and 35 W are assessed, respectively. Optimized trajectory calculations, are based on our «InTrance» method. The program yields a burn out of the REP stage in a distance of 79.6 AU for a usage of 154 kg of Xenon propellant. With a hyperbolic excess energy C3 = 45.1 km2/s2 a heliocentric probe velocity of 10 AU/yr is reached at this distance, provided a close Jupiter gravity assist adds a velocity increment of 2.7 AU/yr. A transfer time of 23.8 yrs results for this scenario requiring about 450 kg Xenon for the SEP stage, jettisoned at 3 AU. We interpret the solar and SEP/REP as a competing alternative to solar sail and ballistic/REP. Omitting a Jupiter fly-by even allows more launch flexibility, leaving the mission duration in the range of the ESA specification.


radio-frequency ion thruster; solar electric propulsion; interstellar heliopause probe; mission strategy; radioisotope-electric propulsion

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