Figure 1. SOHO being prepared for acoustic testing, with the anti-particle protective bag partly deployed over the spacecraft
As with all satellites, extreme care had to be taken to limit the effects of both particulate and molecular contamination on SOHO's instruments. An additional complication in SOHO's case was that the twelve instruments were supplied by separate institutes as 'free issue' to ESA and its Contractors, making the cleanliness responsibilities rather diffuse. Some of the sensors are relatively insensitive to contamination, but some would be seriously affected by particles settling on optical surfaces and others by condensible molecular deposits. Some sensors have cold detectors for optimum noise reduction and so would be highly sensitive to organic contamination.
The range of wavelengths of interest with SOHO extends from 11 to 950 nm, with signal levels of up to 'one Sun'. In SOHO's nominal Sun-staring mode of operation, any organic material deposited on cold windows or mirrors would stand a high chance of photo-polymerisation, which would adversely affect their transmission or reflectance.
The first precautionary step taken by the Project was to obtain 'cleanliness budgets' from each of the Experimenters and to consolidate these as an overall budget for the spacecraft. For this it was necessary to calculate the potential impacts on each instrument of contamination emanating from all of the instruments and the spacecraft hardware, both during ground integration and test activities (including contributions from clean rooms, environmental test facilities and launch preparations) and during flight.
One main design solution for the instruments was to incorporate continuous gaseous nitrogen purging at low rates for the critical sensor volumes. This led to an onboard distribution system fed from a 'purge cart' during the ground testing, and via a special connector to the launcher until lift-off. The quasi-sealed nature of the instrument sensor boxes, isolation of optics from electronics, and the use of the gas purging virtually precluded the risk of the sensor optics and detectors being contaminated from external sources.
To comply with the overall cleanliness budget, it was decided to space-condition (vacuum-bake) certain hardware. This included the Payload Module structure, harness, Multi-Layer Insulation (MLI) and the Optical Surface Reflector (OSR) panels (due to 2 square metres of OSRs being mounted very close to the aperture plane of most instruments and the use of silicone adhesives for OSR fixation). The instrument providers were required to space-condition their MLI and external harnesses, but not their internal hardware, although those with critical cleanliness requirements did choose to bake much of this.
The more critical sensor boxes were assembled under high-quality clean-room conditions (class 100 clean benches or class 1000 rooms; see accompanying panel), but it is impractical to integrate and especially test spacecraft of SOHO's size in such conditions. Most of the spacecraft integration was done in clean rooms of class 10000 or better, partly under class 100 downdraft clean units. Such a unit was used at Intespace in Toulouse (F) in preparing for the environmental tests, and was supplemented by the use of a low outgassing plastic cover suspended from a crane for acoustic and vibration test exposures, and for final preparation for entry into the thermal-vacuum chamber. Instrument doors were only opened under clean conditions.
Additional precautions were taken for the thermal-vacuum testing. The chamber was baked at 100 deg C prior to the test, using Quartz Crystal Monitors (QCMs) to measure the outgassing rates and to help identify the main materials deposited. The pumpdown was partly conducted with instrument purging and the shroud temperatures, Sun (simulated) intensity and OSR panel onboard heating were coordinated in pumpdown and recovery with the objective of keeping the OSR panel and the front surfaces of the sensors warmer than the shrouds at all times. For most of the test duration, the shrouds were at very low temperatures (e.g. - 190 deg C), so that spaceraft contamination was only likely to occur during recovery; this was modified from the usual sequence by ensuring that the shroud temperatures did not exceed -75 deg C until the pressure was greater than 1 millibar, with the objective of limiting migration of contaminants deposited on the shrouds at -180 deg C to the spacecraft's exterior during this phase. This extended the test by 12 hours but, as expected, subsequent witness-plate examination showed negligible deposits on the OSR panel and other external surfaces. Instrument doors were closed during pumpdown and recovery and so the internal optics are very unlikely to have been contaminated by the chamber or spacecraft outgassing.
Many of the planned launch-preparation activities (about to start at the time of writing) will take place in a clean-air tent in a room already offering better than class-10000 particulate control. During fuelling and other work necessarily conducted outside this tent, an overhead cover will protect the spacecraft from falling - i.e. relatively dense - particles, and the fairing will be cleaned to an equivalent standard. For the final 18 calendar days until launch, SOHO will be under the Atlas launcher's fairing, which will be conditioned with class-5000 air, and the instrument purge will be maintained throughout. A few sensors will be exposed to this environment, the 'red-tag covers' having been removed at encapsulation. These units have been determined by the Principal Investigators to be insensitive to the expected deposits.
In the first days after launch, instrument doors may be 'cracked' open to speed outgassing without admitting sunlight, but full opening is not recommended during the first month, to minimise the risks of in-flight contamination before reaching halo orbit four months after lift-off.
In the light of all of the above precautions, it is believed that the risk of any instruments being contaminated from either the spacecraft or the other instruments is extremely low, and that SOHO should not experience the rapid degradation seen on many previous Sun-staring missions.
Dust particles are ever present; just look across a narrow beam of sunlight in your house and you will see hundreds, dancing in the convection currents. These are still present in the 'clean rooms' in which spacecraft are tested, but in much lower concentrations. The quality of the air in these rooms is specified in terms of the number of particles of 0.5 microns or larger occurring per cubic foot (class 100 has about 3.5 particles per litre of air). The fact that you see particles dancing means that many are large but of such low density that they do not settle. Clean rooms therefore use a relatively high laminar air flow to entrain and sweep such particles into filters.
For spacecraft, the number of particles falling onto and remaining on critical surfaces is much more important than the number per unit volume. Hence sensors that allow this 'fallout' to be counted have been used on SOHO. Fallout is promoted by turbulence (even caused by the spacecraft itself) in otherwise laminar air flow, and by the presence of particle generators (such as people!) situated upstream of sensitive hardware. People working in clean rooms therefore need to follow strict disciplines and to wear appropriate clothing (releasing very little 'lint'). For class-100 clean rooms, full 'bunny suits' are needed, with just the eyes exposed. Class 1000 is compatible with faces being exposed (but beards covered), while class 100 000 allows clean coats and overshoes.
If you buy a new car, look at the light scattered by the 'fogged' interior surface of its windscreen in low-angle winter sunshine. This fogging results from a heavy deposit of plasticisers, emitted by the car's new carpets and seat covers, etc. This is the plastic age, and spacecraft too use many different plastics, many of which contain volatile materials to provide added flexibility, discharge static, and serve as lubricants in sheet or roll form.
Volatility is increased in vacuum, and as temperature increases significant quantities of such organics can form a local cloud around a spacecraft. If a critical surface such as a lens, mirror or detector happens to be colder than the emitter, then it is possible for the volatile components to condense onto that surface and cause filtered obscuration, light scattering and loss of detector performance.
Other organic contaminants that may be present include unreacted potting agents, solvents, oils from lubricants and from human skin, and silicones used as parting agents. Sunlight falling on organically contaminated surfaces can cause chemical reactions, often stabilising and darkening them to form a brown tarry substance and destroying the critical optical properties required by experimenters.
Deposition rates in vacuum can be measured using Quartz Crystal Monitors (QCMs), especially if the crystal temperature can be cooled below that likely to apply to sensitive surfaces. As a rule of thumb, a temperature difference of 60 deg C corresponds to two orders of magnitude difference in acquisition rates, so that a test lasting 100 h can indicate the effect of a year's space exposure. For those materials found to have high contaminant potential, vacuum-baking can remove the majority of the more loosely bound molecules before integration into critical assemblies.
Apart from QCMs, so-called 'witness plates' may be used to accumulate organics in clean rooms, and infrared spectroscopy can be used to quantify, and possibly identify the nature of the deposits and hence pinpoint the sources.
Figure 1. SOHO being prepared for acoustic testing, with the anti-particle protective bag partly deployed over the spacecraft
Figure 2. View of the upper face of the SOHO Payload Module prior to thermal-balance/ thermal-vacuum testing, showing mainly particle sensors on the right and the OSRs of the Sun shield pierced by the apertures for the Sun sensors
ESA Bulletin Nr. 84.