Development and Testing of Harpoon-Based Approaches for Collecting Comet Samples
Comets, having bright tails visible to the unassisted human eye, are considered to have been known about since pre-historic times. In fact 3,000-year old written records of comet sightings have been identified. In comparison, asteroids, being so dim that telescopes are required for observation, were not discovered until 1801. Yet, despite their later discovery, a space mission returned the first samples of an asteroid in 2010 and two more asteroid sample return missions have already been launched. By contrast no comet sample return mission has ever been funded, despite the fact that comets in certain ways are far more scientifically interesting than asteroids. Why is this? The basic answer is the greater difficulty, and consequently higher cost, of a comet sample return mission. Comets typically are in highly elliptical heliocentric orbits which require much more time and propulsion for Space Craft (SC) to reach from Earth and then return to Earth as compared to many asteroids which are in Earth-like orbits. It is also harder for a SC to maneuver safely near a comet given the generally longer communications distances and the challenge of navigating in the comet's, when the comet is close to perihelion, which turns out to be one of the most interesting times for a SC to get close to the comet surface. Due to the science value of better understanding the sublimation of volatiles near the comet surface, other contributions to higher cost as desire to get sample material from both the comet surface and a little below, to preserve the stratigraphy of the sample, and to return the sample in a storage state where it does not undergo undesirable alterations, such as aqueous. In response to these challenges of comet sample return missions, the NASA Goddard Space Flight Center (GFSC) has worked for about a decade (2006 to this time) to develop and test approaches for comet sample return that would enable such a mission to be scientifically valuable, while having acceptably low risk and an affordable cost. A harpoon-based approach for gathering comet samples appears to offer the most effective way of accomplishing this goal. As described below, with a decade of development, analysis, testing and refinement, the harpoon approach has evolved from a promising concept to a practical element of a realistic comet sample return mission. Note that the following material includes references to videos, all of which are contained in different sections of the video supplement identified in the references. Each video will be identified as "SS##", where "SS" means the supplement section and "##" will be the number of the section.