Fig. 1. Section of the truss of the International Space Station with attached EXPRESS Pallets. v = flight direction.
The International Space Station (ISS) will offer external accommodation of payloads outside the pressurized modules on elements provided by the USA, Russia and Japan. The American truss structure will be equipped with four accommodation sites for external platforms, attached with the 'EXPRESS Pallets'. The EXPRESS Pallet concept allows accommodation of individual experiments on adapters which can be installed and exchanged by the external robotics system of the Space Station. On one of these adapters, the Space Exposure Biology Assembly (SEBA) may be accommodated. SEBA is a multi-user exposure facility for space biology. SEBA is scheduled for flight in 2001.
SEBA will be dedicated to experiments in the fields of
exobiology, chemical evolution, radiation biology and
radiation dosimetry. The concept of SEBA is partly derived
from existing ESA exposure facilities which have successfully
flown in the past, such as the 'Exobiology and Radiation
Assembly' (ERA) and BIOPAN. ERA was a box-shaped facility
with a deployable lid that contained sample trays for exposure
to solar UV. In 1992/93, ERA completed an 11-month flight on
the EURECA free-flyer. BIOPAN, an exposure platform shaped
like a circular container, was designed for medium-duration
flights on Russian retrievable satellites. BIOPAN has
completed two 2-week missions, in 1992 and in 1994, the
latter carrying six experiments in fields of radiation
biology, exobiology and radiation dosimetry. A third BIOPAN
flight is scheduled for 1997.
The SEBA facility will allow the scientific community to
continue, extend and improve the type of research that has
been carried out on ERA and BIOPAN.
SEBA Express Pallet Accommodation on ISS Truss
The four EXPRESS Pallets are attached to the starboard truss of the Space Station. In the Space Station's nominal (Local Vertical/Local Horizontal) flight attitude, two EXPRESS Pallets are nadir oriented and two pallets are zenith oriented. SEBA would be installed on one of the two zenith oriented pallets (depicted in Fig. 1) on one of the three adapters allocated to ESA for the early utilization opportunity.
Fig. 1. Section of the truss of the International Space
Station with attached EXPRESS Pallets.
v = flight direction.
SEBA Mechanical Configuration on Express Pallet
The configuration of SEBA on the EXPRESS Pallet is shown in Figure 2. The SEBA facility is planned to be composed of two independent experiment units (EXPOSE and Matroshka), a Coarse Pointing Device (CPD), a central Standard Payload Computer (SPLC), a Power Distribution Unit, a mechanical support structure as interface to the EXPRESS Pallet, an interconnecting harness, and thermal hardware. The remaining free areas on the carrier platform will be reserved for self-standing add-on experiments. The envelope of SEBA is defined by the footprint of a single EXPRESS Pallet Adapter (ca. 1.20 x 1.05 m²); the height may be up to 1.25 m. The mass of SEBA will be about 150 kg. In addition, max. 20 kg is allocated to add-on experiments.
Fig. 2. Preliminary configuration of SEBA.
SEBA will be a multi-user facility, featuring two independent experiment units: 'EXPOSE' and 'Matroshka'.
EXPOSE is designed for experiments in photobiology and photoprocessing. The UV-exposed experiments require variable periods with (preferably vertical) incidence of sun light on the experiment samples. In order to achieve appropriate sample orientation towards the sun and sufficient exposure duration, accumulated over the mission, EXPOSE will be mounted on a dedicated sun pointing device.
For the Matroshka experiment unit a new concept was chosen. The scientific objective of this unit is to simulate the radiation absorption characteristics of a human body, which allows measurements of the dose depth distributions of different components of the space radiation field to which crew members are exposed during extravehicular activities.
In addition to the above described experiment units, 'Add-on experiments' can be installed on the SEBA payload assembly platform. These experiment units may be used for space biology and dosimetry or for other science disciplines. The experiments are conceived as self-standing hardware that could be supplied by the science community. Power and simple data and command interfaces can be provided to these experiments by the SEBA system.
The experiment units and all elements of the SEBA infrastructure, including the electronics subsystem, will be mounted on an EXPRESS Pallet Adapter.
EXPOSE
EXPOSE is a multi-user experiment unit. It has a box-shaped structural assembly (preliminary dimensions: footprint 65 x 46 cm², height 15 cm) which houses a sample tray in its upper section. The tray is fix-mounted in the structure. Defined exposure duration will be achieved by a motor-operated shutter system, which can be command controlled by timeline, pointing status information and/or ground command. The mechanical configuration of EXPOSE is shown in Figure 3, which depicts EXPOSE on a two-axis pointing device.
Fig. 3. EXPOSE Unit mounted on Coarse Pointing Device
Shading of the sample tray will be prevented. The tray provides square recesses (pockets with footprint of ca. 8 x 8 cm², approximately 16 in total) to accommodate the experiment samples. The pockets, individually covered by a quartz windows, may be hermetically sealed or exposed to space vacuum. The experiment samples can thus be maintained under pressure or in vacuum. Neutral density and bandpass or cut-off filters can be installed to control the incident UV intensity and wavelength. Dark control samples or dosimetry packages to measure the space radiation field can be placed in the lower part of the sample pockets. The temperature of the sample tray will be stabilized at a predefined setpoint in the range from -10 °C to +40 °C by controlled heating. Sensors will monitor the incident solar UV light and internal gas pressure.
Matroshka
Matroshka is a phantom which shall simulate the upper part of a human body consisting of trunk and head. The body will be made from tissue- and bone-equivalent materials. Like EXPOSE, Matroshka is conceived as a multi-user experiment unit. It will be equipped with user-provided passive and/or active detectors for ionizing radiation, placed at different internal locations and at the surface. The objective of Matroshka is to measure absorbed doses of space radiation not only at the surface of the body, i.e. skin doses which are obtained with conventional personal dosimetry, but also doses deposited at different locations inside the human phantom, e.g. at the positions of dedicated organs. With Matroshka, dose depth distribution measurements inside the body can be performed over a depth range of 0.01 to 14 cm. In addition, biological dosimetry of UV light at the site of the eyes will be possible with Matroshka as a user-provided experiment.
In order to simulate the human body for radiation dosimetric purposes an appropriate configuration and composition of the phantom will be chosen, as well as appropriate methods for measuring ionizing radiation. The body of Matroshka will be as similar as possible to a reference human body with respect to mass, size, atomic composition and distribution. A possible approach for the composition of the human phantom is to use e.g. a block of tissue-equivalent plastic with bone-equivalent minerals embedded at the positions of major bones (Figure 4). Materials known as Shonka plastic, composed of polyethylene and nylon, loaded with fine dispersed mineral powders, could be used as tissue- and bone-equivalent materials. However, the material properties, such as the long-term structural stability in space need further investigations, and the results may affect the final design of the Matroshka.
The phantom provides a system of internal channels which house individual user-provided experiment packages. This permits placement of detector sets at the positions of organs of interest or measurements of dose depth distributions over the entire range of absorption depth. User-provided experiments may consist of passive experiment packages (e.g. solid state nuclear track detectors, radiation sensitive crystals, polymers) or active experiments (e.g. ionization chambers). In order not to disturb the tissue equivalence of the phantom by experiment packages more than un-avoidable, active experiment packages are physically divided into two parts: the sensor electronics inside the phantom and a data acquisition electronic with possible data reduction outside the phantom. The SEBA central Standard Payload Computer (SPLC) will send the science data of the active experiment packages to ground for analysis.
Fig. 4. Configuration of Matroshka
The model (Figure 4) shows the outer dimensions of the pure technological model of the phantom. Covers (not shown) are foreseen which are equivalent to a space helmet and suit. The temperature of Matroshka shall be allowed to float within a range of -10 °C to +40 °C by appropriate thermal design. Matroshka specific control and interface electronics are housed in a separate box. The overall height of Matroshka is at least 80 cm. No specific orientation is required for Matroshka in space. It is planned to place the unit in a fixed position on the SEBA carrier plate.
Add-on Experiments
The remaining area on the SEBA EXPRESS Pallet Adapter (Figure 2) may be used for self-standing 'Add-on experiments'. This opportunity may be used for e.g. another radiation package, material exposure experiments or technology experiments.
The SEBA facility will be able to host three different categories of experiments, i.e. sun-exposure experiments in the EXPOSE unit, radiation detectors in the Matroshka unit, plus add-on experiment packages which are envisaged to be defined by the experimenters.
As SEBA is still under detailed design study at the time of writing, the experiment interfaces of EXPOSE and Matroshka have not yet been fully defined. The figures given below are all preliminary.
Sample Accommodation within EXPOSE
The EXPOSE tray will be outfitted with 16 experiment compartments (also known as "pockets"). The entire EXPOSE unit is mounted on a sun-pointing device to obtain optimal solar irradiance into the pockets. Each pocket can be shielded against the sun by means of an electrically operated shutter. The surface area of each pocket is ca. 80 x 80 mm², the depth is ca. 35 mm. Inside each pocket, an experiment package (or sample carrier ) can be inserted. The sample carrier is a box-like container with a quartz window at the top, allowing solar UV to enter inside. The atmosphere contained inside the sample carrier is at the experimenter's option: it can be vacuum, air, inert gas, (simulated) Martian atmosphere and so on. In a typical EXPOSE experiment, samples inside the sample carriers will be arranged in two layers, one at the top exposed to solar light, the other at the bottom in darkness, the latter serving as an internal, in-flight reference. EXPOSE will be provided with temperature sensors, pressure sensors, UV sensors etc. The environmental history of the EXPOSE experiments will be completely recorded and downlinked. Figures 6a and 6b show half of the tray and how the EXPOSE pockets and sample carriers may look like.
Fig. 6a. SEBA EXPOSE Sample Accommodation
Fig. 6b. SEBA EXPOSE Sample Accommodation
Experiment Package Accommodation within Matroshka
Installed on the SEBA platform, Matroshka is so to say engaged in a permanent EVA (extra-vehicular activity, or "space walk"). Cavities inside the Matroshka will accommodate experimenter-provided radiation packages, which can be linked to SEBA-provided power lines and/or data recorders. Active as well as passive detector packages may be installed in the phantom, with a maximum envelope per package of 5 x 5 x 5 cm³. The Matroshka data electronics will be contained in a box, located next to the phantom. The recorded data of the Matroshka active experiment packages can be downlinked to Earth during flight. The accommodation of the experiment packages in Matroshka is illustrated in Figure 7.
Fig. 7. SEBA Matroshka Experiment Package Accommodation
Add-on Experiments
The size of the SEBA platform allows the installation of additional, experimenter-provided scientific equipment next to EXPOSE and Matroshka. A mass of up to ca. 20 kg will be allowed, a footprint and height has not yet been defined. Power and a simple data and command interface (analog, discrete and serial RS422 interfaces) will be offered by the SEBA system at the experimenter's option.
The electrical set-up shows the interfaces to the EXPRESS Pallet and is given in figure 8. The SEBA Power Distribution Unit provides electrical power to the subunits (EXPOSE, Matroshka and the add-on experiments) within the SEBA system. It will be connected to the 28 VDC power bus of the EXPRESS Pallet.
The SEBA Control Unit functions as the central data handling system and interface between the SEBA experiment units and the EXPRESS Pallet. It executes overall control functions, such as data and command reception, decoding or formatting, processing, intermediate storage and distribution. The link with the EXPRESS Pallet computer for the reception of commands and delivery of housekeeping and science data, to be downlinked via telemetry, is established via the MIL Std. 1553B data interface.
The experiment units are equipped with local controllers and software to execute specific functions such as motor drive control, heater current control, and acquisition and preprocessing of housekeeping and science data, including data generated by active detector systems within Matroshka. Data and power interfaces will also be offered to the 'Add-on experiments'.
Fig. 8. SEBA electrical configuration and external
interfaces to the EXPRESS Pallet.
Integration of experiment samples requiring late access would be performed at the Kennedy Space Center prior to launch. The EXPRESS Pallet Adapter, with the installed SEBA, will be uploaded on the Space Shuttle with the EXPRESS Pallet. The first launch is planned for 2001. All in-orbit handling and installation of SEBA will be carried out with the completely integrated EXPRESS Pallet Adapter or EXPRESS Pallet by means of Space Station robotics. SEBA will remain in space for one to three years, depending on NASA/ESA mission agreements. SEBA will be designed for autonomous operations in orbit, but essential functions can also be controlled from the ground by uplinked commands. During flight telemetry science data (e.g. from active detectors), will be received on the ground in regular intervals.
At the end of the mission the facility will be returned to earth, and the experiment samples and detector packages will be retrieved for scientific post-flight analysis.
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