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As part of the PRU project a new plant lighting system has been developed. System design focused on light source development, chamber optical performance improvements and electronics optimization. Central to the lighting system performance is a high density LED Light Engine, enabling increased spectral diversity, higher irradiance levels, enhanced uniformity and improved efficiency. Chamber wall surface materials were tested to minimize the vertical irradiance gradient and improve planar uniformity. Total lighting system efficiency was improved through the use of switching converter LED drive circuitry. As an alternative to the LED light source, an advanced planar fluorescent lighting source has also been developed.

The PESTO (Photosynthetic Experiment System Testing and Operation) experiment flew in the Biomass Production System (BPS) to International Space Station (ISS) on STS-110 (Atlantis) April 8, 2002, and returned on STS-111 (Endeavour) June 19, 2002, after 73 days in space. The ground control was conducted on a two-week delay at Kennedy Space Center in a BPS unit under environmental conditions comparable to ISS. Wheat (Triticum aestivum cv Apogee) and Brassica rapa cv Astroplant were independently grown in root modules for multiple grow-outs. On-orbit harvests, root modules exchanges and primings, seeds imbibitions, and gas and water samplings occurred at periodic intervals; all were replicated in ground controls. Many operations required crew handling and open access to individual chambers, allowing the exchange of microorganisms between the crew environment and the BPS modules.

A soil moisture sensor system consisting of small heat-pulse probes, a microcontroller, and software for data acquisition and signal conditioning was developed for use in space based plant growth units. The microcontroller allows the sensors to be used in a control application with minimum time demands on the control subsystem. A single digital serial link may be shared by up to 16 microcontrollers with 8 sensors each, for a total of 128 sensors. The microcontroller independently applies heat cycles to determine the current moisture level, and responds to a request from the computer with the last known value. Using the microcontroller system, repeatability testing was completed for wet 1–2 mm arcillite. The standard deviation in wet arcillite over a 16-hour period was about 3%. Software filtering can be used to reduce the standard deviation further.

The Biomass Production System (BPS) was developed to meet science, biotechnology and commercial plant growth needs in Space. The BPS is a double middeck locker equivalent payload with four internal plant chambers. The chambers can be removed to allow manipulation or sampling of specimens, and are sealed to allow CO2 and water vapor exchange measurements. Each of the growth chambers has independent control of temperature, humidity, lighting, and carbon dioxide levels. Preliminary acceptance and performance testing has demonstrated temperature control within ±1.0°C (between 20°C and 30°C) and humidity control within ±5% (between 60% and 90% RH, depending on ambient temperature and plant load). The fluorescent lighting system provides light levels between 60 and 350 μmol m−2s−1. The CO2 control system controls to the greater of ±50 ppm or ±5% (with plants, as a scrubber is not currently available).

The ASTROCULTURE™ flight unit flown as part of the SPACEHAB-03 mission on STS-63 was a complete plant growth system providing plant lighting, temperature control, humidity control, water and nutrient delivery, a CO2 control system, nutrient control using the NASA Zeoponics system, an ethylene photocatalysis unit, a control and data acquisition system, and plant video. The objective of the ASTROCULTURE™-4 experiment was to continue technological assessment of these environmental control subsystems. Plants were included in this package for the first time. Two plant species were flown, rapid cycling ‘Wisconsin Fast Plants’ (Brassica rapa), and dwarf wheat (Triticum aestivum cv. ‘Super Dwarf’). Growth and development of both plant species on orbit appeared normal and similar to that of plants grown under terrestrial conditions.

The ASTROCULTURE™ (ASC) middeck flight experiment series was developed to test subsystems required to grow plants in reduced gravity, with the goal of developing a plant growth unit suitable for conducting quality biological research in microgravity. Previous Space Shuttle flights (STS-50 and STS-57) have successfully demonstrated the ability to control water movement through a particulate rooting matrix in microgravity and the ability of LED lighting systems to provide high levels of irradiance without excessive heat build-up in microgravity. The humidity and temperature control system used in the middeck flight unit is described in this paper. The system controls air flow and provides dehumidification, humidification, and condensate recovery for a plant growth chamber volume of 1450 cm3.