Shedding Light on Large Space-Based Telescopes: Modeling Stray Light due to Primary Mirror Damage from Micrometeoroid Impacts

TL;DR

This work develops a semi-analytic, computationally efficient framework to quantify stray light from micrometeoroid-induced damage on the primary mirror of a large space-based, visible-wavelength telescope designed for exoplanet characterization. By coupling Peterson BRDF digs with crater-diameter relations (Watts and MR), spatially resolved mirror and sky-background modeling, Gaia-based sky maps, and the Altruistic Yield Optimization (AYO) approach, it estimates how single high-energy impacts can degrade exoEarth yield for Habitable Worlds Observatory scenarios. The results indicate that stray light from high-energy impacts can reduce yield by roughly 30–60% in worst-case single-hit events, with substantial sensitivity to the crater-energy relation and telescope design; lower-energy events are dominated by host-star stray light unless the sky is explicitly resolved. The findings motivate mitigations such as longer telescope barrels, micrometeoroid avoidance zones, and higher-throughput coronagraphs, while also highlighting the need for more detailed impact testing and refined crater-energy modeling to support robust mission planning.

Abstract

A large space-based telescope aimed at detecting and characterizing the atmospheres of Earth-like planets orbiting Sun-like stars will require unprecedented contrast and stability. However, damage to the primary mirror due to micrometeoroid impacts will provide a stochastic, time-dependent source of stray light in the coronagraph's field of view that could significantly lengthen exposure times and reduce the expected science yield. To better quantify the impact of stray light and inform the Habitable Worlds Observatory mission design process, we present estimates of stray light in different micrometeoroid damage scenarios for a broad range of targets, and use that to find the expected decrease in science yield (i.e., the expected number of detected exoEarth candidates). We find that stray light due to micrometeoroid damage may significantly reduce yield, by 30% -- 60% in some cases, but significant uncertainties remain due to the unknown maximum expected impactor energy, and the relationship between impact energy and expected crater size. Micrometeoroid damage therefore needs further exploration, as it has the potential to reduce scientific yield, and in turn drive the development of mitigation strategies, selection of telescope designs, and selection of observing priorities in the future.

Figures (8)

• Figure 1: An example of a damage crater (represented by a 'dig' in the Peterson Peterson2012brdf model) with damage diameter $D_{mm}$. For incoming light with a direction of incidence of $\Vec{V_{i}}$, the direction of perfect specular reflection is given by $\Vec{V_{ps}}$. The direction of reflectance would be determined by the direction over which you are interested in quantifying the amount of reflected light.
• Figure 2: The maximum allowed number of impacts per unit area (N$_{D}$, m$^{-2}$) of a given impactor mass (M$_{mm}$, g) with energy E$_{i}$ (assuming a constant impact velocity of 20 km/s) to comply with the stray light requirement (Equation \ref{['eq:nonsimplifiedBRDF']}) under the assumption that the host star is the sole source of stray light for an Earth-Sun twin. We assume a V band wavelength ($\lambda$ = 0.545$\mu$m) and a primary mirror with a diameter of 6 meters. An optimistic wattsModel1993heaney1993 and pessimistic spenvis2024spenviswebsite model relating impactor mass (M$_{mm}$) to crater diameter (D$_{mm}$) is used to determine the allowed number of impacts. For the MR and Watts models, impactor masses $\gtrsim$1.1$\times$10$^{-4}$ grams (22 J) and $\gtrsim$0.18 grams (36 kJ), respectively, would violate the stray light requirement after a single hit.
• Figure 3: Cumulative incident flux from the sky background in Gaia G (top left, Photons m$^{-2}$ s$^{-1}$), Johnson's V (bottom left, W m$^{-2}$$\mu$m$^{-1}$), and Cousin's Ic band (bottom right, W m$^{-2}$$\mu$m$^{-1}$). Additionally shown is the total number of sources per bin (top right). These maps were created using the entire Gaia DR3 archive Prusti2016gaiamissionVallenari2023gaiaDR3; conversions from Gaia G band magnitudes to Johnson's V and Cousin's Ic bands used photometric transformations defined by Riello2021gaiaEDR3PhotometricTrans and zeropoint fluxes from Bessell1979cousinsriBessell1998uvb.
• Figure 4: A comparison of the stray light found when considering all sources in a target's resolved sky background (solid lines) versus only the host star (dashed lines) as a function of the micrometeoroid mass (or impact energy) for a single-hit event. From the truncated HPIC catalog Tuchow2024hpic used in this study, the 3 targets at the lowest and highest galactic latitudes are shown in the left and right columns, respectively. The top and bottom rows demonstrate the Watts and MR models, respectively, for relating impactor mass/energy to crater diameter. In all cases, considering all sources in the resolved sky background only notably increases the computed stray light for single-hit impactors of mass $\lesssim$10$^{-10}$ or impact energies $\lesssim$10$^{-5}$ J.
• Figure 5: The exoEarth candidate yield over a 2 year survey calculated by AYO versus the impactor energy from a single-hit micrometeroid strike on the primary mirror. The Watts and MR models relating impactor mass/energy to crater diameter were tested, and are shown by the blue and orange lines, respectively. The left and right panels adopt the Scenario A and F telescope designs, respectively, from Stark2024yield; note the changing y-axis scale for the separate scenarios. The horizontal dashed black line in both panels gives the baseline yield, found when assuming there is no stray light contribution from micrometeoroid damage. The largest 2$\sigma$ probability impact cases shown in Table \ref{['tab:MMImpactMasses']}for RAM or Anti-RAM direction pointing fall within the grey and teal shaded regions, respectively. If the more pessimistic MR model relating impactor mass to crater diameter is adopted, the impact cases identified for RAM direction pointing would lead to a reduction in yield of 30 -- 60%.