Innovative Interstellar Explorer

Innovative Interstellar Explorer was a NASA "Vision Mission" study funded by NASA following a proposal under NRA-03-OSS-01 on 11 September 2003. This study focused on measuring the interstellar medium, the region outside the influence of the nearest star, the Sun.[1] It proposes to use a radioisotope thermal generator to power ion thrusters.[1]

Early concept art for the explorer.

The project is a study of a proposed interstellar precursor mission that would probe the nearby interstellar medium and measure the properties of magnetic fields and cosmic rays and their effects on a spacecraft leaving the Solar System.[2] Mission launch plans analyzed direct, one planet, multi-planet, and upper-stage trades.[3] As a concept study, a number of technologies, configurations, and mission goals were considered, leading to the choice of a spacecraft propelled with ion engines powered by a radioisotope thermoelectric generator (RTG).[4] The focus was getting a spacecraft launched by about 2014, achieving 200 AU by the year 2031.[5]

A variety of strategies were assessed, including using launch windows (not counting backups) for a Jupiter assist in 2014, 2026, 2038, and 2050—about every 12 years.[3] The launch opportunity for the 2014 window passed,[6] but for example it could have resulted in a Jupiter flyby by early 2016 and then go on to reach 200 AU by 2044.[3][2] With an ion drive, a speed of about 7.9 AU/yr could be attained by the time its xenon propellant was depleted, enabling a travel distance of 200 AU by 2044 and perhaps 1000 AU after one hundred years from launch.[3] Different launch times and configurations have various timelines and options.[3] One configuration for launch saw the use of a Delta IV Heavy and for the upper stages a stack of Star 48 and Star 37 leading to various gravity assist options.[4] Another launch stack that was considered was the Atlas V 551 with a Star 48.[4]

In 2011, the study's primary author gave an update to website Centauri Dreams, giving a retrospective on the mission and its feasibility since its publication in 2003.[7] By that time, some of earliest launch windows were no longer feasible without a ready spacecraft.[7] Some retrospectives were the advantages and potential of solar sails, but the need for them to be more advanced for a mission, and also the utility of a radioisotope propulsion (REP) for such a mission.[7] REP was the combination of using an RTG to power an ion drive.[7]