This post is part of a series of updates on our mission planning process, which began with Mission Planning Updates, Part 1: Trajectory.
Micrometeoroids and orbital debris (MMOD) are small objects in space, the possibility of collision with which poses a hazard to spacecraft. Relative velocities between objects in space can be in excess of 20 km/s. Just about anything can be dangerous at such incredible speeds; a speck 1 mg in mass has about as much energy at 30 km/s (450 J) as a 9 mm handgun bullet fired on Earth.
MMOD damage is a concern for all spacecraft, not only for their own sake, but also because a spacecraft lost to MMOD damage can become a derilect or debris, creating a new hazard to avoid for future spacecraft.
The relevant standards we need to comply with are found in NASA-STD 8719.14 and NPR 8715.6A, and we have written an Orbital Debris Assessment Report as required by them. We have also completed an End of Mission Plan (EOMP) and Planetary Protection Plan (PPP), but those are topics for another post.
The MMOD environment depends heavily on the region of space one considers–for example, human-made debris such as paint chips are a much greater concern in Low Earth Orbit, where they are distressingly common, than they are near the Moon.
Because our deployment options begin past the geostationary orbit distance, and we never get closer to the Earth than that, we mainly have to worry about micrometeoroids and not human-made debris.
Efforts have been made to quantify the micrometeoroid environment in the inner solar system, resulting in tools such as NASA’s Micrometeoroid Engineering Model (MEM), which is what we have used for our analysis. Software for this purpose can be obtained on request from NASA’s Meteoroid Environment Office.
The expected flux of micrometeoroids on a spacecraft at a given moment depends mainly on the surface area of the spacecraft (a larger one such as the ISS will naturally be struck more often), the environment where the spacecraft is orbiting, and the threshhold of size and velocity for what one considers a potentially disabling meteoroid. Fortunately for us, our spacecraft are very small targets, which improves our odds.
We have ran MMOD analysis for multiple different size thresholds and are confident that our spacecraft will survive in the cislunar MMOD environment for the duration of the mission and beyond. Example: For the entire mission, we computed odds of 0.04% chance of being struck by a micrometeoroid of 1 mg mass or larger moving 20 km/s relative to the spacecraft.
The odds are increased to as much as 0.8% across the entire mission for a 0.1 mg g or larger micrometeoroid, but the smaller objects included in this chance are much less dangerous. Any new trajectories we develop will not travel through any different regions of space, so we do not expect these results to change in future runs.