Frequently Asked Questions
This is a generalized overview of the issue of exposure to high levels of radiation in space and our plan of attack to deal with it.
Without a doubt, mitigating the negative effects of exposure to radiation in space is going to be the biggest challenge. This is complicated by the fact that not all of the spacecraft components are going to be space-rated to NASA or military requirements (nor do they need to be). However, as the STRaND-1 and PhoneSat nanosatellites successfully demonstrated, it is possible to utilize commercial off-the-shelf (COTS) components. In the case of those spacecraft, they utilized was commercially available Android powered cellphones with minimal modification. The three NASA PhoneSats launched earlier this year were only planned to spend a week in space before re-entering, but STRaND-1, launched in February is still in orbit and alive as of August 1.
It is interesting to note that, at one point, communication with STRaND-1 was lost for an extended period before unexpectedly returning in late July. This could have been due to a number of reasons, but radiation exposure is a strong possibility. Even a single charged particle can knock a spacecraft offline if it hits the CPU or memory. This is, in fact, our biggest concern.
Designers follow several routes to making their spacecraft survivable in the high-radiation environment of space. Electronic components may be designed and manufactured from scratch to be rad-hard or tolerant of exposure to electromagnetic radiation. Second, shielding may be placed around components that are not rad-hard, including using the spacecraft struture itself to provide shielding. This is one of the methods used on the International Space Station's habitable modules. Third, systems may be made radiation tolerant by making critical subsystems and components redundant.
We will put in practice all three methods where feasible. Where rad=hard components can be obtained at an acceptable cost, we will use them. The primary ADACS will be space-rated, so LunarSail shouldn't ever lose the ability to control its attitude or send and receive commands from Earth. However, some essential parts of the satellite, in particular the Raspberry Pi, Arduino, memory, etc., are not tolerant of radiation out of the box.
Without mitigating or accomodating the effects of EM exposure, we would fully expect the spacecraft to have periods of downtime in response to exposure events. The effects could include things like memory corruption and even the shutdown of computers.
To address this challenge, we will place shielding around sensitive components. Because of the mass of radiation shielding materials, we will have to shield only the components that could be affected by radiation exposure rather than, for example, wrapping the entire inside of the satellite body in a layer of lead. A side benefit of this is that we will be able to use the radiation shields to assist with thermal management.
We will also employ redundant systems, specifically the computers and memory. Each will be duplicated on the spacecraft and integrated in such a way that they are not in-line with each other to minimize the chance of a single event taking out both systems. For example, placing memory modules at right angles to each other makes it less likely that a single cosmic ray or particle will be able to corrupt both sets of memory.
During flight, backup systems will maintain an identical synchronized state to the primary system and ready to take over instantly. If one computer is taken out of commission due to a radiation hit or memory corruption, the secondary system will immediately take over. With the secondary system now in a primary role, the main system will be power-cycled, brought back into operation and error-checked. The systems will re-synchronize from the secondary to the primary and then the secondary system will become the default primary. LunarSail will be able to go through this kind of fail-over scenario repeatedly for at least a year as long as one of the systems isn't knocked out of commission permanently. Even then, there would still be usable life left in the spacecraft so long as the remaining system remains healthy.
Handling redundancy and recovery will not only be an automatic process. The whole thing will be able to be commanded from the ground as well.
Lastly, it is important to note that our requirements are much - much - lower than those of NASA, commercial satellites or the military. For one thing, we don't have human lives at stake. We also do not require 100% uptime. It is also not necessary to have reliability out to three-9's. If an image is corrupted, it's no big deal. If the spacecraft loses communication with Earth for a day, that is tolerable too. We are much more tolerant of transient events and our main concern is minimizing the chance of mission-ending anomalies. That's a very different, and more forgiving, standard than that for NASA or commercial spacecraft.Last updated:
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