The dawn of next-generation, minor-body exploration
Since the late 1990's, we have been refining mission designs and making observations to search for easily accessible asteroids as a part of the MUSES-C program. Through the "Minor-body Exploration Forum (MEF) activity" with more than 200 members all over Japan, we have then devised the next mission plans for more than four years. In March of 2004, we proposed establishing the "Minor-body Exploration Working Group" to the Space Science Committee of JAXA/ISAS and were given approval.
The core-member meeting of young scientists and engineers of this WG is held weekly in preparation for the first general meeting at the end of May, 2004, and the establishment of five sub-groups: Orbiter Science; Sampling Improvement and Sample Analysis; Surface Science Package; Mission Design; and Navigation and Guidance Control. In subsequent detailed consideration, we expect to establish a mission with both high scientific value and engineering feasibility, and minimal development lead-time and cost, based on lessons learned from the design, tests, and operation of HAYABUSA. Below, I will introduce two typical mission schemes we have so far envisioned. (For more details, please refer to: http://www.asexploration.com/mef/mef_report/mef_report.html.)
Mission Plan 1: Multiple rendezvous and sample returns for NEOs (Near Earth Objects) in known spectral types
Asteroids are classified into about a dozen of spectral types based on their reflectance spectrum observations. Statistical study reveals that S, E, M and V types exist in the most inner region of the current asteroid belt, while C and P types are in the more outer region. D-type is distributed mostly in the outer most region of the belt as well as the Jovian Trojan region. This suggests that primitive materials had not been mixed completely along the heliocentric (radial) distances at least between Mars and Jupiter, after the formation of the early solar system. Accordingly, this mission intends to send one or two spacecraft to rendezvous with several near-earth asteroids belonging to the major spectral types, conduct global mapping and surface/internal structure investigation, and then return samples to the Earth. Since HAYABUSA will visit an S-type asteroid, new candidate targets are: C-type, undifferentiated asteroids, the second most abundant class following the S-type, that are believed to be the parent bodies of carbonaceous chondrite meteorites and cosmic dust; M, E, and V types that are appropriate for studying thermal-differentiation inside their parent bodies; and D and P types that are believed to be treasuries of life precursors. We aim to complete the work of correlating classifications of asteroids and meteorites/cosmic dust, and recompile spatial distribution of materials in the early solar system. Mission design of sample returns from multiple asteroids requires the evaluation of a combination of many parameters, including the number of launchers and spacecraft, the number of asteroids to be visited by each spacecraft, from various aspects such as scientific premises, engineering requirements, cost, and risk.
Mission Plan 2: Family mission
"Asteroid Family" a group of objects with similar orbital elements in the main asteroid belt, is thought to have been formed by the catastrophic disruption of planetisimals by impacts. In this plan, we aim to elucidate, by visiting several asteroids within the same family, the internal structure of their parent bodies, the process of impact disruption and re-aggregation, and the physical and chemical processer of the differentiation. The spacecraft will first visit either:
(1) 3 to 4 S-type asteroids of the Koronis family, one of the three major families, in 3 to 6 years, or;
(2) 2 asteroids with different spectral types belonging to the Nysa-Polana family in 3 years. This family is thought to be the locale of the recent impact disruption of two asteroids since these asteroids belong to the same family but have diverse spectral types.
The spacecraft will then release their intelligent impactors with autonomous navigation capability to crash into the surface of each asteroid, collect ejecting samples from several meters under the surface and carry them back to the Earth. This mission is focused on the quest for the internal structure study of asteroids, one of three themes of next-generation minor-body exploration already mentioned, but the challenge is that there are several new engineering developments to be made.