Fluorescence imaging of microbe-containing particles shot from a two-stage Light-gas gun into an aerogel.

We have proposed an experiment (the Tanpopo mission) to capture microbes on the Japan Experimental Module of the International Space Station. An ultra low-density silica aerogel will be exposed to space for more than 1 year. After retrieving the aerogel, particle tracks and particles found in it will be visualized by fluorescence microscopy after staining it with a DNA-specific fluorescence dye. In preparation for this study, we simulated particle trapping in an aerogel so that methods could be developed to visualize the particles and their tracks. During the Tanpopo mission, particles that have an orbital velocity of ~8 km/s are expected to collide with the aerogel. To simulate these collisions, we shot Deinococcus radiodurans-containing Lucentite particles into the aerogel from a two-stage light-gas gun (acceleration 4.2 km/s). The shapes of the captured particles, and their tracks and entrance holes were recorded with a microscope/camera system for further analysis. The size distribution of the captured particles was smaller than the original distribution, suggesting that the particles had fragmented. We were able to distinguish between microbial DNA and inorganic compounds after staining the aerogel with the DNA-specific fluorescence dye SYBR green I as the fluorescence of the stained DNA and the autofluorescence of the inorganic particles decay at different rates. The developed methods are suitable to determine if microbes exist at the International Space Station altitude.

Four-Year Operation of Tanpopo: Astrobiology Exposure and Micrometeoroid Capture Experiments on the JEM Exposed Facility of the International Space Station.

The Tanpopo experiment was the first Japanese astrobiology mission on board the International Space Station. It included exposure experiments of microbes and organic compounds as well as a capture experiment of hypervelocity impacting microparticles. We deployed three Exposure Panels, each consisting of 20 Exposure Units that contained microbes, organic compounds, an alanine UV dosimeter or an ionizing radiation dosimeter. The three Exposure Panels were situated on the zenith face of the Exposed Experiment Handrail Attachment Mechanism (ExHAM) that was pointing in zenith direction toward space, which was attached on a handrail of the Japanese Experiment Module (Kibo) Exposed Facility (JEM-EF) outside the International Space Station. The three Exposure Panels were one by one retrieved and returned to the ground after approximately 1, 2, and 3 years of exposure to the space environment. Capture Panels, each of which contained one or two blocks of amorphous silica aerogel, were exposed to collect hypervelocity impact microparticles. Possible captured particles may include micrometeoroids, human-made orbital debris, and natural terrestrial particles. Each year, Capture Panels containing from 11 to 12 aerogel blocks were attached to the three faces of the ExHAM (pointing to zenith, ram, and port); they remained in place for about 1 year and were then returned to the laboratory. This process was repeated three times, in total, during 2015-2018. Additional exposure of a Capture Panel facing ram was conducted between 2018 and 2019. Once the aerogel blocks were returned to the laboratory, they were encapsulated in dedicated transparent plastic cases and optically inspected by a specially designed microscopic system. Once located and recorded, hypervelocity impact signatures were excavated one by one and distributed for further detailed analyses. The apparatus, operation, and environmental factors of all the Tanpopo experiments are summarized in this article.

Mineralogy and petrology of comet 81P/Wild 2 nucleus samples.

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.