For parabolic orbits, in place of the semi-major axis of the ellipse a, we use the perihelion distance q, and in place of the epoch time we use the time of perihelion passage T_{p}. In the...For parabolic orbits, in place of the semi-major axis of the ellipse a, we use the perihelion distance q, and in place of the epoch time we use the time of perihelion passage T_{p}. In the case of a body orbiting the Sun, G M is the graviational constant of the Sun, equal to 1.32712440041 \times10^{20} \mathrm{~m}^{3} \mathrm{~s}^{-2}.
