Interstellar Material within the Solar System Author: Timothy Swindle, PhD, Professor, Lunar and Planetary Laboratory, University of Arizona Abstract Background: One of the primary goals of interstellar missions is to directly observe what conditions apply in stellar systems other than our own. However, it is worth asking whether there are interstellar materials within our own system that can add to our understanding of other systems, and whether they can be of use in developing interstellar missions. Abstract Objectives: I evaluate three types of interstellar material that have been observed within the Solar System and discuss their potential utility for the goals of interstellar missions. Abstract Methods: Review of recent scientific literature. Abstract Results: The best-studied type of interstellar material available is presolar grains within meteorites, grains grains that survived the formation of the Solar System intact and were incorporated into rocks 4.5 billion years ago. Unfortunately, they typically formed as condensates in outflows of dying stars, so they do not reflect current conditions. In recent decades, interstellar dust grains have been discovered to be streaming through the Solar System. The most detailed analyses come from the dust collector on the Cassini spacecraft, but give only minimal chemical information. The Stardust comet coma sample return mission probably succeeded in returning samples of interstellar dust, but they are so small (<1 micrometer) that they are difficult to find, and difficult to separate from the collector material in analyses. The most promising avenue is probably the study of larger objects like 1I/2017 U1 ‘Oumuamua (100 m in longest dimension), which was discovered by an asteroid survey in 2017. Several ideas for missions to future interstellar visitors of this type have been suggested. Because they are likely to spend no more than a few weeks in the inner Solar System, even a flyby mission would likely require a spacecraft already launched and waiting. Because the encounter velocities are so high (‘Oumuamua had a geocentric velocity of >60 km/sec), there are significant propulsion issues for a rendezvous mission. Abstract Conclusions: As asteroid surveys improve and new programs come online, more macroscopic interstellar objects will be discovered passing through the Solar System, but studying them poses significant challenges.