Planetary Defense Potential Hazardous Object Mass Determinations

Previous research has investigated gravimetry for fast flybys of targets greater than 250 meters in diameter. These studied showed that it was possible to reduce the uncertainty of a target’s purely by optical tracking measurements of released retroreflectors flying past the body. However, acquisition time of the target and realization of target relative state uncertainty was not fully taken into account. For example, the studies mentioned assumed that the smaller targets could be acquired at the same distance as larger targets, allowing for a certain amount of target relative state uncertainty reduction and ample time to release retroreflectors with this reduce state uncertainty. This, however, is not the case. A recent IRAD invested the possibility of mass reduction incorporating the acquisition time, and it was found to be impractical to acquire the target (< 200 meter diameter), launch retro reflectors (including slewing maneuver and b-plane targeting), and perform any required divert maneuvers. An illustration of a target flyby with retroreflectors can be seen in Figure 1.

This work aims to revisit hypervelocity asteroid flybys with the prime focus on target volume uncertainty reduction, which includes reducing the uncertainty of the spacecraft’s state with respect to the target. Doing so will allow for a better understanding of the target’s trajectory as well as reduce the mass bounds by providing more accurate volume measurements and an estimation of density via spectral type classification. More specifically, work herein will implement a standard filter but incorporate image limb and center points for asteroid state and scale estimation. Additionally, various scenarios will be investigated to see how the concept reduces size/volume uncertainty and state. Figure 2 illustrates a two-spacecraft scenario, which could be considered to reduce target state uncertainty to allow for closer approach distances, obtaining higher resolution images and better volume estimates.

Additionally, a recent idea (by the PI) to leverage out-of-focus images to help in range estimation may be used in the estimation filter. This concept needs to be further investigated, but it involves mapping the level of blurring of an image to a range, which requires an instrument to be designed to infinity, having a hyperfocal point. However, this out-of-focus design will reduce the camera resolution at close distances to the asteroid. At this time, only range estimates with be considered to determine what level of range uncertainty will be required to decrease the size uncertainty from using pure image measurements.

Understand asteroid size uncertainties for fast flyby scenarios. Reducing the size uncertainty will reduce the uncertainty on the asteroid's mass, which helps in deflection mission design or civil defense plans. Additionally, the project will help in the understanding of asteroid intercept geometries that may result in greater uncertainty reduction in the target's state and size. Furthermore, including range and range uncertainty will provide design requirements for possible ranging techniques, possibly using only images

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