Mini Review Investigation of the Interplanetary Transfer of Microbes in the Tanpopo Mission at the Exposed Facility of the International Space Station Yuko Kawaguchi, 1,2 Shin-ichi Yokobori, 2 Hirofumi Hashimoto, 1 Hajime Yano, 1 Makoto Tabata, 3 Hideyuki Kawai, 3 and Akihiko Yamagishi 2 Abstract The Tanpopo mission will address fundamental questions on the origin of terrestrial life. The main goal is to test the panspermia hypothesis. Panspermia is a long-standing hypothesis suggesting the interplanetary transport of microbes. Another goal is to test the possible origin of organic compounds carried from space by micrometeorites before the terrestrial origin of life. To investigate the panspermia hypothesis and the possible space origin of organic compounds, we performed space experiments at the Exposed Facility (EF) of the Japanese Experiment Module ( JEM) of the International Space Station (ISS). The mission was named Tanpopo, which in Japanese means dandelion. We capture any orbiting microparticles, such as micrometeorites, space debris, and terrestrial particles carrying microbes as bioaerosols, by using blocks of silica aerogel. We also test the survival of microbial species and organic compounds in the space environment for up to 3 years. The goal of this review is to introduce an overview of the Tanpopo mission with particular emphasis on the investigation of the interplanetary transfer of microbes. The Exposed Experiment Handrail Attachment Mechanism with aluminum Capture Panels (CPs) and Exposure Panels (EPs) was exposed on the EF-JEM on May 26, 2015. The first CPs and EPs will be returned to the ground in mid-2016. Possible escape of terrestrial microbes from Earth to space will be evaluated by investigating the upper limit of terrestrial microbes by the capture experiment. Possible mechanisms for transfer of microbes over the stratosphere and an investigation of the effect of the microbial cell-aggregate size on survivability in space will also be discussed. Key Words: Panspermia—Astrobiology—Low-Earth orbit. Astrobiology 16, 363–376. 1. Introduction 1.1. Investigation of the panspermia hypothesis and chemical evolution in space T here is no single scenario for the origin of life, nor a definite place where terrestrial life emerged. The pan- spermia hypothesis was proposed more than a century ago (e.g., Arrhenius, 1903). This hypothesis suggests the trans- fer of a single spore through interplanetary space by radio- pressure (Arrhenius, 1903; Crick and Orgel, 1972). There are arguments against the panspermia hypothesis (Davies, 1988). For example, terrestrial microbes cannot survive for long pe- riods in the space environment (Shklovskiı ˘ and Sagan, 1966; Nussinov and Lysenko, 1983). Gualtieri (1977) analyzed the trace element concentration in various living organisms and concluded that the data were strongly against a nonterrestrial origin of life as proposed by the panspermia hypothesis. However, the finding of a cell-like structural fossil in the interior of a martian meteorite ( McKay et al., 1996) recalled the panspermia hypothesis. Despite the infrequent opportu- nities for conducting space experiments, significant results have been reported by European and Russian groups (re- viewed in Horneck et al., 2010; Baranov et al., 2009; Olsson- Francis and Cockell, 2010; Rabbow et al., 2012, 2015; de Vera et al., 2012). Another line of investigation has been carried out to test the presence of terrestrial microbes at high altitude using aircrafts, balloons, and a meteorological rocket (reviewed in Yang et al., 2009a). Terrestrial microbes were isolated from air samples collected in the troposphere and stratosphere. If microbes exist at upper altitudes, it would suggest that mi- crobes might be transferred from Earth to other planets. Before the origin of life on early Earth, organic compounds were formed in a process called chemical evolution and 1 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), Sagamihara, Japan. 2 School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Hachioji, Tokyo, Japan. 3 Graduate School of Science, Chiba University, Chiba-shi, Japan. ASTROBIOLOGY Volume 16, Number 5, 2016 ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2015.1415 363