Oddly coherent, this issue (May 2024, vol. 685) of Astronomy & Astrophysics:
Poggiali, G. Fossi, L. Wargnier, A. et al. Grain size effects on the infrared spectrum of mineral mixt… A14 202347681
Alvarez-Candal, A. The multiwavelength phase curves of small bodies – Phase coloring A29
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Pfeifer, M. Agarwal, J. Marschall, R. et al. Dynamics and potential origins of decimeter-sized part… A136
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We look at asteroids through telescopes (regular, optical/IR- not planetary radars) and they’re (almost always) unresolved. Dots. One pixel. It’s a tribute to ingenuity and diligence and persistence and deep diving that we can pull information out of that one pixel. And this month, A&A hits it. First, pulling color information out of that pixel, via spectroscopy. Of course, that one body in that one pixel is not homogeneous; what’s its makeup, its mineral composition, possibly including contaminating collisions? Disentangling the colorimetry and originating composition (“spectral interpretation”) is hardly settled science, particularly for some asteroids of certain (weak or flat) spectra, ones that have never been seen close-up by a spacecraft to provide ground truth, or just mixed-origin bodies that don’t fit our pigeonholes.
Second, we can track asteroids (and comets… and astero-comets) across an orbit, and across the sweeping illumination angles. A phase curve is generated as we view westward, outward (anti-Solar), then eastward as the target passes through one apparition. As it does so, the brightness changes, even for one pixel. Part of that is simply the target going from crescent, to full-disk illumination, to the other crescent, yet that can’t explain it. We also know rocky, airless bodies have “phase effects” as illumination goes from low angle (from its point of view), to high, to the other low angle. This can tell us of the properties of the surface: grain sizes, some mineral compositions, and unfortunately, second-order effects of grain-composition-mineral-sizes. Again, not yet completely settled. Though we do know we can pull certain asteroid types from the data; and in general, dark (low-albedo) asteroids have a different phase curve than high-albedo ones, to first-order.
And let’s not forget lesser objects shed from greater, parent objects. We can look at a target and check its polarization; even one pixel can tell us of its ‘meta-roughness’ (presence of a cloud of shed particles). In this case (Comet 67P/Churyumov-Gerasimenko), a probe got more- far more- than one pixel, with one polarization value. “Chury” then serves- at minimum- as a proxy or reference or ground truth for how we look at, oh… every other comet (and astero-comet). …And our comet models/simulations; considering how hard it is to get probes to these elliptical, inclined orbits, we know a lot less about these targets than we’d like at this point in the ‘Space Age.’