The Surface Properties of Two Dark Comets

The Dark Comets are a recently recognized class of Near-Earth Object which experience non-gravitational accelerations, beyond what can be explained by the Yarkovsky effect, though without an accompanying detection of cometary activity such as the presence of a coma or dust tail. A clearer understanding of what processes drive the Dark Comets to behave the way they do would not just shed light on the orbital evolution of the NEOs – and thus the potential risks they might pose – but also the nature of the non-gravitational acceleration seen on the first Interstellar Object, 1I/'Oumuamua.

We present multi-filter photometric observations of two Dark Comets, 1998 KY26 and 2003 RM, taken with the 4.3m Lowell Discovery Telescope throughout 2023 and 2024. The two objects, despite being an order of magnitude different in size and having different kinds of non-gravitational accelerations (e.g. transverse vs. perpendicular to the orbital plane), both have D-type reflectance spectra – like some outer main belt asteroids and most cometary nuclei. In other words, the reflectance properties of their surfaces are comet-like, as is their orbital evolution. While the lack of detected cometary mass loss remains puzzling, their properties are broadly consistent with traditional comets approaching dormancy.

KY26 is also the target of Hayabusa-2's extended mission, so we will also comment on how our observations might help plan for the spacecraft's encounter with the body later in the decade.