Speed of a planet in an elliptical orbit with semi major axis 'a' about the sun of mass M at a distance 'r' from the sun is

if major axis is ‘a’ then mean distance is also a and total energy is $-\frac{GMm}{2a}$

at a distance r if the velocity is v then kinetic energy + potential energy = -$-\frac{GmM}{2a}$

$$\begin{gathered} \frac{1}{2}m{v}^2-\frac{ \\ GMm}{r}=-\frac{GMm}{2a} \end{gathered}$$

$$\begin{gathered} v^2=2\left(\frac{GM}{r}-\frac{GM}{2a}\right) \\ v=\sqrt{GM\left(\frac{2}{r}-\frac{1}{a}\right)} \end{gathered}$$