FIGURE 1 Meteoroid and orbital debris detection platforms and capabilities. SOURCE: John Lyver, NASA, presentation at the Workshop to Identify Gaps and Possible Directions for NASA’s MMOD Programs, March 9, 2011.
Another obstacle facing NASA is that even for particles that are centimeters in size, the agency lacks a process for determining how particle shape plays a role in the damage that can be caused by an impact. Methods to incorporate particle shape into impact models have not been validated, and hence the application of a safety factor in spacecraft design techniques may result in uncertainty to both weight and physical dimensions of a spacecraft designed to operate in the MMOD environment.
A third obstacle cited by panelists as adding uncertainties in design is that experimental facilities for testing spacecraft damage, including via hypervelocity testing, generally employ spheres or simple shapes made from aluminum. Moreover, typical impact speeds at these test facilities do not exceed speeds of 7-8 km/s, whereas the average relative impact velocity of orbital debris particles in low Earth orbit (LEO) is around 9 km/s, and orbital debris and meteoroids can reach relative impact velocities of up to 15 km/s and 70 km/s, respectively.1 Although ultra-high-velocity testing facilities exist, they are more expensive than facilities with conventional high-velocity-testing techniques, and they employ particles on the average of several microns in size. Thus extrapolation from such experiments, which do not duplicate debris shape or material properties, introduces even more uncertainties due to a mismatch of collision velocity.
Panelists did point out that the space shuttle and some spacecraft parts returned from orbit, for example solar panels from the Hubble Space Telescope (HST), have exhibited debris damage that can be analyzed, although the conditions of impact (debris shape, size, and impact velocity) are seldom known with precision.
1 Eric Christiansen, “Hypervelocity Impact Technology (HVIT) Group,” presented at the NRC Workshop to Identify Gaps and Possible Directions for NASA’s MMOD Programs, March 9, 2011.