Puzzler: Why higher solar phase angle (SPA) allows for many more feasible FastKD (kinetic deflection) communications satellite “hijacking” missions?

Question

Space.com's advert-laden Repurposed communications satellites could help save humanity from an asteroid impact ends with:

The study results were presented at the Planetary Defence (sic) Conference 2021.

The paper in the first link says:

The activity consequently assesses the feasibility of modifying a commercial spacecraft platform in order to perform asteroid kinetic deflection in the shortest possible time. Moreover, the necessary prerequisites to enable the challenging "build and launch of a Kinetic Impactor (KI) deflection system extremely fast".

The selected approach is outlined by "hijacking" an existing commercial platform with minimal adaptations and the addition of a pre-developed Kinetic Deflection (KD) module, providing in particular GNC impact capabilities, in order to convert it to an asteroid deflection mission.

Slide 4 of the presentation in the second link says:

For short warning scenarios: higher allowed Solar Phase Angle (SPA) at impact is required to achieve high deflection performance. And: SPA largely affects the launch opportunities: higher SPA results in many more feasible missions and thus increases the mission flexibility & deflection capabilities. → TIR NAC needed for greatest mission flexibility/applicability!

and I don't understand the reference to, and the importance of SPA at all.

Question: Why does higher solar phase angle (SPA) allow for many more feasible FastKD (kinetic deflection) “Hijacking” missions launching communications satellites as impactors to deflect NEOs that would otherwise hit Earth?

Answer 1

I don't understand the orbital mechanics aspects of this statement:

higher allowed Solar Phase Angle (SPA) at impact is required to achieve high deflection performance

But, interpreting:

SPA largely affects the launch opportunities: higher SPA results in many more feasible missions

As a greater quantity of feasible missions rather than greater feasibility of missions allows a relatively simple explanation.

Solar phase angle is an optical imager constraint. The asteroid target needs to be illuminated by the Sun in order for the proposed Narrow Angle Camera (NAC) to detect it and perform the terminal guidance (like with DART).

For reference, DART's maximum allowable SPA is $60°^1$ which seems to have shrunk from an earlier limit of $75°^2$. Visually, it is clear why this limits the quantity of feasible missions:

There is a lot of geometry excluded because of this constraint which is why the paper then states:

This imposes the use of a thermal infrared sensor to acquire the whole asteroid regardless of its illumination state.

Slide 4 of the presentation states also states this:

TIR NAC needed for greatest mission flexibility/applicability!

Where TIR = Thermal Infrared (according to the handy Chrome extension NASA Acronyms). The thermal imager opens up the rest of the available geometry because it does not need the asteroid to be illuminated by the Sun to see it, relying on the asteroid's own thermal radiation (this would not work for SPAs approaching 180°, but the imager is narrow-angle so the critical angle is exceedingly close to 180° and thus not a big issue).

The paper also states this:

The need for Visible Camera is still TBC, depending on TIR sensor capabilities for far range detection to be studied in future activities with suppliers.

Which could be why the SPA remains a variable constraint in the mission design.

References (aside from those in question):

1. Sarli, B. et al. "NASA Double Asteroid Redirection Test (Dart) Trajectory Validation and Robustness," AAS/AIAA Space Flight Mechanics Meeting (2017) (NTRS ID: 20170001428)

2. Atchison, Justin & Ozimek, Martin & Kantsiper, Brian & Cheng, Andrew. (2016). Trajectory Options for the DART Mission. Acta Astronautica. 123. (DOI: 10.1016/j.actaastro.2016.03.032, available on ResearchGate)