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Dynamically New Comet C/2025 D1 (Groeller) with Record Perihelion Distance

We studied C/2025 D1 (Groeller), a long-period comet with an unprecedented perihelion distance of 14.1 au, using archival observations. The data reveals that it had been active at inbound heliocentric distances $r_{\rm H} \gtrsim 20$ au. Initially, the comet intrinsically brightened at $r_{\rm H} \gtrsim 16$ au, with brightening parameters comparable to those of other long-period comets. However, observations after late 2023 showed a gradual decay, despite the inbound trajectory of the comet. To our knowledge, such behaviours have not been observed for other long-period comets at similar heliocentric distances. We speculate that this might be linked to the onset of CO$_{2}$ sublimation and/or crystallisation processes. Alternatively, the activity source might have been exhausted. The surface brightness profile of the coma indicates a steady-state mass loss, implying supervolatile sublimation as the primary driver of the observed activity. Despite changes in the orbital plane angle, the circularly symmetric coma persisted throughout the observed period, indicative of the dominance of large grains in the coma. Assuming the activity trend is independent of bandpass, we found that comet was redder than many other solar system comets. Our model-dependent constraint estimates the nucleus radius to be $\gtrsim\!0.4$ km. We performed astrometric measurements, refined the orbital solution, and derived the original and future orbits of the comet. Our N-body integration, accounting for the Galactic tide, strongly favours that the comet is dynamically new, with its previous perihelion at $\gtrsim\!60$ au from the Sun $\gtrsim\!6$ Myr ago. It is highly likely that the comet will be lost from our solar system after the current apparition.