Adapted by: Alicia Vílchez Bedmar
Using Commercial Off-The-Shelf (COTS) components may be considered a “NewSpace” trend which already established strong presence in LEO satellites. But can one use COTS for space exploration?
As Plus Ultra Space is currently aiming to launch a lunar satellite constellation, Harmony, the question on our mind is if COTS components are reliable enough to support our mission. And the answer is… “Yes, but…”.
Let’s dig in.
Table of contents:
What(s) and Why(s) of COTS
Traditionally, satellites are built with parts designed for space environment, however such parts are very expensive, usually technologically outdated, and big. The idea behind COTS, simply put, is to take a state-of-the-art, high-performing part from a different industry (usually automotive) and put it inside your satellite.
On the surface, it sounds like a no-brainer: you get a higher-performing system for a fraction of the price and size of space parts. However, as the environment in space is much harsher compared to the one on Earth’s surface, there is no guarantee that the part will work as intended, or work at all.
Nowadays, there is a trend to check, screen, and pre-qualify some parts of COTS systems for space use to allow the customers to have a better understanding of parts performance and radiation tolerance levels. Sometimes such assemblies are called Space COTS.
Cislunar radiation environment and its effects
So, what is so different with the space environment? The magnetic field of Earth protects the surface from high-energy particles (i.e., radiation) and satellites have to “live” within such harsh environment. These charged particles can be divided into trapped and transient.
Trapped particles are the high-energy particles which through interactions with Earth’s magnetic field got trapped in one of the special regions, called Van Allen radiation belts. The Van Allen belts are usually subdivided into inner (with fairly stable population of photons) and outer (electrons). The transient particles come from solar events (solar wind, solar flares, corona mass ejections) and galactic cosmic radiation.
These charged-particles environment can have the following effects on a satellite:
Surface and internal charging.
Single event effects (SEE).
Total ionizing dose (TID).
Surface and internal charging is the build-up of charge on/in a satellite’s exterior/interior. Electrostatic discharges are probably the most damaging outcome of spacecraft charging, as it can cause structural damages, component degradation, and anomalies.
SEE occur from a single charged particle, which travels through the entire spacecraft’s body and deposits sufficient charge to cause an effect on the electronics. This can cause a variation of negative effects ranging from soft errors (i.e., bit flips) to latches, short-circuiting, and other potentially destructive hard errors.
TID refers to cumulative deposition of energy in materials through a satellite’s lifetime, which lead to slow degradation of the components and eventual failure.
Displacement damage refers to actual displacement of atoms from its original position in the arrangements (crystal structure) by an incoming particle through collision, which naturally leads to degradation of materials and their properties.
As you could have guessed, the chosen COST components would need to work reliably under all of these harmful effects. Let’s see what is waiting for a spacecraft trying to get into Moon orbit.
Radiation effects on a lunar orbiter
We can distinguish two phases: transfer from Earth to the Moon and Moon orbit.
The transfer to the Moon is tricky, as the spacecraft’s orbit will be changing, and it will be passing through different environments. Here the satellite will also be crossing the regions which are usually avoided: the radiation belts. Strap up for the whole range of negative radiation environment effects!
COTS components need to be protected from internal dielectric discharge and SEE for the spacecraft to pass safely through this “stage”. With TID and displacement damage, the situation is somewhat better: these effects are by nature cumulative, and, although the effects can be relatively important, the short duration of the transfer phase prevents large effects.
This is, however, very different, if one plans to go with electric propulsion and slowly spiral from Earth to the Moon. In this case, the transfer will take months and the cumulative effects will be comparable to a polar LEO satellite with 10 years’ lifetime.
The radiation environment for a lunar orbiter has also its own peculiarities. As the Moon does not have a protective magnetic field (unlike Earth), one does not need to be concerned with trapped particles. This significantly lowers the expected TID: for the lunar orbiter it would be approximately one magnitude lower than TID for LEO or GEO satellites (of course depending on the shielding).
However, as there is no protection, galactic cosmic rays and solar particles can wreak havoc unchecked. The COTS components need to be prepared to withstand SEE.
Surface charging needs to be considered as well. Although effects should be relatively small, during times when the Moon passes through the tail of Earth’s magnetosphere and the satellite is in shadow the effect can reach typical GEO levels. Internal charging usually can be ignored, as there are no steady interactions with high energy electrons. However, this changes during solar flare events.
Let’s protect our COTS!
Now, knowing the full extent of what’s waiting for our spacecraft during the push to the Moon, let’s prepare our COTS components to survive such a harsh radiation environment. The first step is shielding.
Shielding (for example with some millimeters of Aluminium) prevents charged particles from “reaching” sensitive electronic. Thus, it effectively lowers TID and provides protection against displacement damage. It is usually implemented in the spacecraft’s outer structure, however sensitive components can be additionally shielded (spot shielding) to increase protection. Don’t forget that one component of spacecraft can act as a shield for another component!
With this approach, cumulative radiation effects for a Moon orbiter can be kept almost a magnitude lower than a typical LEO satellite. Nevertheless, doing the low-thrust transfer from Earth to the Moon will push a satellite to the levels above typical LEO.
Shielding with a combination of careful choices during satellite design (conducting surface, bonding of structural elements, etc.) should be able to mitigate damage from surface and interior charging.
Soft errors by SEE can be mitigated by parity check, coding, and software-based mitigation. However, there are limits to the extent that such techniques can be integrated to already existing COTS. Hard error can be somewhat mitigated by careful choices of COTS parts, as some circuit designs are less prominent to latches, for example. Alternatively, redundancy (cold spare) could be considered a valid (but costly) option.
Will we see COTS on the Moon?
So, knowing all of this, will there be place for COTS in Moon missions? Definitely yes, — for Space COTS. The environment is challenging, especially in terms of SEE, and the high cost of lunar missions calls for a reliable solution.
Nevertheless, economical viability of COTS will definitely be needed to establish a sustainable lunar economy. Overall, as always, careful COTS part pre-selection needs to be done to ensure the fulfillment of mission requirements, and lifetime and reliability goals.