Caution: space!
Space is a particularly harsh environment. What can we find there?
- Solar wind: it is a stream of plasma coming from the outer atmosphere of the Sun - the corona. The solar wind is mainly composed of electrons and protons and has an average speed of 400 km/s. While the Earth's magnetosphere and atmosphere deflect and dissipate most of the radiation carried by the solar wind, outside this protective bubble the spacecraft can easily be damaged.
- Galactic Cosmic Rays (GCR): they are produced outside the Solar System as a stream of high-energy charged particles, mostly protons. GCR can be extremely energetic and cause damage to the spacecraft's electronics.
- Radiation belts: In the inner magnetosphere of the Earth electrons and protons become trapped by the magnetic field lines and form the Van Allen radiation belts. The inner belt contains energetic electrons and protons and extends approximately from 0.01 to 1.5 ER. The altitude of the outer belt is about 3-10 Earth radii and the trapped particle population is mainly composed of electrons with some ions.
- Other hazards: electromagnetic radiation, debris and micrometeoroids, extreme temperatures...
Weather forecast
SPENVIS [11] is ESA's Space Environment Information System that models "the space environment and its effects, including the cosmic rays, natural radiation belts, solar energetic particles, plasmas, gases, and 'micro-particles'". It can be accessed free of charge on the corresponding SPENVIS web-site operated by the Belgian Institute for Space Aeronomy.
SPENVIS enables the user to define a spacecraft trajectory and calculate solar proton, trapped proton and electron fluxes and fluences, ionising and non-ionising radiation doses, damage equivalent fluences for solar cells, single event upset rates, spacecraft surface and internal charging, micrometeoroid and space debris collision risks and other parameters related to the space environment.
The study presented here focuses on some aspects of AXIOM's radiation environment, namely solar and trapped particles, GCR, radiation dose and subsequent damage to solar arrays. As a first approximation to AXIOM's operational environment, a circular equatorial orbit of 50 Earth radii (318,550 km) was chosen to represent the Lissajous L1 orbit. It accounts for the motion of the L1 point around the Earth while the much smaller motion of the spacecraft around the L1 point does not contribute to variations in its radiation environment.
Raise your shields
Charged particles cause ionising damage to materials and electronics which can be quantified by measuring radiation doses. Radiation dose is usually expressed in terms of the absorbed dose which is a measure of the energy absorbed by a unit mass of the exposed material. It is common to choose silicon as the reference material because it is present in most semiconductor devices. Radiation dose is measured in joules per gram of the irradiated material. Other common dose units are the rad (1 rad = 10-5 joules/g) or the gray (1 Gy = 10-2 rad).
Ionising radiation consists of particles that possess sufficient energy for detaching electrons from atoms or molecules, thus ionising them. It does irreversible damage to semiconductors and insulators and may cause the devices under exposure to eventually fail. Total ionising dose (TID) measures the absorbed radiation dose in a material over a certain period of time. Mitigating its effects requires assessing the total exposure over the mission's lifetime, quantifying the radiation hardness of the electronic devices and deciding on the acceptable level of risk. Electronic devices can be shielded from radiation by slabs of aluminium or other metals but the thickness of shielding needs to be studied to provide sufficient protection. [12]
The following image shows a graph of TID as a function of aluminium shielding for the different orbits considered for AXIOM, as well as for the orbits of two other space missions — THEMIS [7] and Cluster [8] (for comparison).
 Total ionising dose in silicon as a function of aluminium shielding The circular orbits of 30 and 60 ER essentially have the same radiation environment as the L1 Lissajous orbit because they are all well away from the Earth's radiation belts. Therefore, only the curve corresponding to the L1 orbit is represented on the graph. It is clear from the simulation that these three orbits provide the best radiation environment for AXIOM. The polar 6-30 Earth radii orbit is only second best because the spacecraft has to go through the outer radiation belt. The polar 2-30 orbit has a significantly harsher radiation environment because it reaches the inner radiation belt as well, and the same comment applies to the THEMIS orbit. Cluster's TID is also high, and this is probably due to the high inclination of its orbit (90o). All in all, the orbit that has been chosen for AXIOM does not seem to present a big challenge in terms of radiation shielding since other similar missions have been operating successfully in much harsher conditions.
This study provides a mechanism for selecting the depth of shielding that needs to be implemented on AXIOM once the radiation hardness of the on-board electronics is known or, conversely, selecting the appropriate components once a spacecraft platform providing a particular amount of shielding is chosen. According to Fortescue et al. (2003) [1], the typical thickness of a spacecraft is between 2.5 and 3.8 mm which gives for the L1 orbit a radiation dose between 4.7 and 3.2 krad. For comparison, most commercial electronic parts start having problems above 2 krad. If we want to use semi-hard parts, TID must be kept below 20 krad. This corresponds to a minimum shielding of 0.5 mm of aluminium and is easily achievable even with appropriate design margins.
 TID for the L1 orbit with indication of the typical shielding range and the failure regions of commercial and semi-hard components
Assess the damage
Solar cells are subject to permanent degradation of their electrical properties when exposed to corpuscular radiation. Radiation can damage solar arrays by penetrating through the cover glass (front side) or the substrate (back side). In both cases the solar cell output (short-circuit current, open-circuit voltage, output power) is reduced. However, the cover glass and the substrate shield the cells from radiation and their thickness has an impact on the damage that the solar array will sustain.
To facilitate simulation and testing of the solar cell degradation, the total damage due to electrons that would be sustained by a solar array in orbit over a particular time span (e.g. mission lifetime) is expressed in terms of the equivalent 1 MeV fluence which is the number of normally-incident, mono-energetic 1 MeV electrons per unit area that would produce the same degradation on the array. Similarly, the damage caused by protons of various energies is expressed as an equivalent fluence of 10 MeV protons. Moreover, the 1 MeV and 10 MeV fluences relate to each other by a damage conversion factor. Typically, one 10 MeV proton will cause the same amount of damage as 3,000 1 MeV electrons. The exact value depends on the cell type and may range from 2,000 to 7,000 for silicon-based cells. [13]
In SPENVIS, the EQFLUX program calculates 1 MeV electron and 10 MeV proton damage equivalent fluences using data from the solar and trapped particle models. The 10 MeV proton fluence is then converted into the equivalent 1 MeV electron fluence (by using a proton/electron damage factor of 3,000) and added to the total electron fluence. We assume that the coverglass is made of fused silica with 2.20 g cm-3 density (which is usually the case) and single junction GaAs cells which have a better efficiency than the standard silicon-based cells. We only focus on estimating the degradation of the output power since the degradation of short-circuit current and open-circuit voltage is approximately proportional and can be obtained by applying damage ratios. The results are given in terms of the equivalent 1 MeV electron fluence as a function of the coverglass thickness for different orbits. THEMIS and Cluster orbits are also included for comparison, as previously.
 1 MeV electron fluence as a function of the coverglass thickness for different orbits for GaAs cells The curves are consistent with the TID graph obtained previously. It is interesting to notice that the damage does not decrease significantly for coverglass thickness higher than 300 microns. It does on the other hand decrease a lot for up to a hundred microns. This is the reason why the typical coverglass is from 15 to 25 microns thick. The disadvantage of having a big coverglass is threefold:
- It increases the mass of the solar array
- It absorbs more solar radiation and hence less energy reaches the cells and less is converted into electricity
- The secondary particles produced in a thicker coverglass cause greater displacement damage which in some cases may become more important than ionising damage.
Once the equivalent electron fluence has been established, the damage assessment is almost over. We can then use solar array manufacturer's data to compute the decrease in the output parameters that corresponds to the calculated fluence.
| 
+44 (0)1483 204100 - Copyright © 1999-2005 UCL
