Posted January 23, 2020 by IC Rainbow
Prograde “forward” burn (blue): increase speed, raise/widen orbit and leave.
Retrograde “reverse” burn (red): slow down, lower/tighten orbit and fall on a body.
Burning at periapsis (the “peak”) moves apoapsis (opposite point) directly towards/away wrt. planet center.
The orbiter always wants to crash land onto something. So your goal is to miss the surface while falling at it.