Figure 6.1: Quasi-steady µg Isocountours around the Centre of Gravity of the Space Station.
The orbital environment contains a large number of naturally occurring and man made objects which might impact the ISS and the payloads attached to it. Objects range in size from a mass of less than 10(exp -6)g to objects with a dimension of much more than 1m across. Most of the debris is located at altitudes less that 2000km. Meteoroids, naturally occurring space objects which nearly all originated from comets and asteroids have an individual size equal to or less than 0.1 mm. There is more total mass in debris objects with sizes equal to or more than 1mm than there is total mass of meteoroids, making man-made objects more hazardous than meteoroids.
The average velocity of these orbiting objects relative to a spaceraft at the altitude of the ISS is 19km/s. Data taken with the Long Duration Exposure Facility (LDEF) indicated that there was a 7 to 1 difference in probability of impact in the ram direction compared with the wake direction for constant particie size and 18 to 1 for constant crater size. Meteoroids were thought to be the primary constituent of the flux of objects striking that spacecraft
The basic natural environment of the Space Station is a well characterised ionospheric atmosphere consisting of a mixture of neutral and ionized gas molecules at a pressure of 10(exp -8) Torr and a charged carrier density of 10(exp 5) cm-³. The electromagnetic radiation environment is dominated by the radiation received from the Sun, unattenuated by atmospheric absorption. The corpuscular radiation environment primarily consists of Van Allen Belt radiation which at the Space Station altitude is only encountered in a limited region, the South Atlantic Anomaly (SAA).
For the low frequency regime in free drift the atmospheric
drag and gravity gradient forces
determine the residual mirogravity vector. The Space Station
altitude is such that the overall
drag deceleration of the c/g is generally at or below 10(exp -6)g.
With regard to gravity gradient
forces, the isocontours shown in Figure 6.1. indicate that for
the locations of the pressurized
modules the disturbances are of the same low order. The Space
Station centre of gravity will
of course vary during the Assembly phases as new elements are
added. Figure 6.2. shows
disturbances caused by crew motion and rotating space station
equipment. The environment
shown is for the payloads in the US Lab. and the actual
environment on the EXPRESS Pallet
is tbd.
Each individual instrument shall be designed so as not to
generate disturbances of the
microgravity environment over the operational phase. The
instrument acceleration levels
shall be verified by analysis or test.
The instrument shall be designed so as to minimise the impact, during its operation in orbit, of any disturbance to the microgravity environment of the ISS and the EXPRESS Pallet in particular.
Figure 6.1: Quasi-steady µg Isocountours around the Centre of Gravity of the Space Station.
Figure 6.2: High frequency (vibrations and sporadic impulse) disturbances of µg
environment.(predicted for the interior of the US Lab).
The strength of all instrument structures and associated equipment shall be based on the flight loads multiplied by suitable design safety factors, taking into account applicable cases (such as whether metallic/non-metallic materials, non-pressurised etc.) and the related flight and ground safety load definitions.
These and related requirements for stress analysis, stiffness, material compatibility, fracture control etc. shall be considered in line with what is required in the applicable documentation.
Contamination arises from the Space Station itself and the proximity operations of the logistics vehicles. This is either through venting or outgassing, the use of thrusters or the shedding of particulate matter from materials exposed to the space environment. The Space Station is intended to have what are termed quiescent periods, nominally of 30 days duration, when there is no active addition to the induced contamination and non-quiescent periods, when there are additions and, as a consequence, certain external payloads may have to cease operation and perhaps be protected until the next qubescent period. The contamination environment is characterized by:
Direct space station external surface to external surface fiow of outgassed material is the only major contributor to molecular deposition during quiescent periods. This can be reduced by vacuum baking of certain extemal materials. In non-quiescent periods, the main contributors are the following:
The last two sources cause only gaseous contamination - only cryogenic surfaces are vulnerable. Trace constituents are tbd.
The MCD depends on the frequency and duration of non-quiescent periods. Outside of NSTS proximity operations, vacuum system venting by the attached pressurized laboratories will cause irregular non-quiescent periods of short duration. The condensate water vents will cause iarger non-quiescent periods once every few days. Combined contributions from all the ECLSS (CO2) vents and from module leakage are still being studied, but it is thought that the currently foreseen levels of CO2 in the Space Station environment could possibly lead to additional non-quiescent periods.
It is expected that through stringent materials and processes control the rate of particulate shedding will be low. The level is tbd.
Such contamination can affect the Space Station and payload performance and thus directly or indirectly affect the payloads in a number of ways. Molecular and particulate deposition, alone or in combination with solar ultraviolet radiation and atomic oxygen, can alter the thermo-optical properties of surfaces, which in turn degrades thermal control performance. A high MCD in the Field of View (FoV) of optics, or local contaminants forming deposits on optical surfaces, can degrade performance by absorbing and/or scattering radiation, thereby reducing the signal intensity, increasing the background noise and/or producing interference patterns. Contamination can also affect solar array power performance by absorbing particular wavelengths of solar radiation that would otherwise pass through the solar cell or by adversely affecting thermal control . Methods to minimize contamination include prelaunch contamination control, including proper material selection and cleaning, proper vent configuration and placement and scheduling Space Station operations, such as the proper timelining of vent use so as to ensure predictable periods of quiescence and non-quiescence. This is likely to be particulariy important in the case of water emissions which can affect observations at infra-red wavelengths.
Other more specific induced environment aspects of the Space Station and its payloads, which might need to be taken into account by specific users, include the following:
The EXPRESS Pallet does not provide for active cooling of the EPAs. The required thermal conditions for experiments must be achieved via passive cooling.
The baseline which shall be adopted by instruments is that heat flows at the EXPRESS Pallet Adapter interface are to be minimised, limit tbd. Hence all power is to be radiatively dissipated.
Radiators couid be installed on the sides of the EPA facing space (tbc).
Battery-fed stay alive power for heaters is available for the payload during the in orbit installation period (see Section 7).
SP1201