Panel a: Portion of the domain of definition of the initial orbital parameters є, r₀, and r_cir,p involved in the Keplerian two-body problem (shaded in grey). The blue and brown surfaces depict limiting cases. Panel b: A closer view of the lower boundary of this domain (brown surface). It allows for a greater appreciation of the values of the initial orbital energy, r_cir,p, below which the initial relative velocity, ν₀, is not defined.