Radius (r_n) vs. nucleus density (ρ) plane for the nucleus of Comet Borisov. The upper limit from the fitted profile, r_n < 0.5 km, is shown as a horizontal black line. The nongravitational motion lower limit (Equation (2)) is shown as a red line. Point A shows r_n > 0.2 km, the radius limit inferred if the density is a comet-like ρ = 500 kg m⁻³. Solutions in the “sweet spot” yellow triangle satisfy both the profile and nongravitational constraints. No solutions exist for densities ρ < 25 kg m⁻³ (point B). Very low (fractal) densities like that proposed for the nucleus of 1I/’Oumuamua (point C, with ρ = 10⁻² kg m⁻³) are specifically excluded. The blue dashed line shows Equation (3) computed with f_A = 1.