As shown in the center panel of this screenshot from NASA's livestream of the James Webb Space Telescope's launch, the telescope briefly lost altitude during its launch trajectory (and this was preplanned):

enter image description here

Why was the launch trajectory chosen to (briefly) move the telescope closer to Earth? It seems to me that losing altitude could give the telescope additional kinetic energy, but that extra energy would inevitably be lost as the telescope regains the lost altitude. Does it have something to do with the Oberth effect?

  • 1
    $\begingroup$ might this question be more fitting for Space Exploration? $\endgroup$
    – ACuriousMind
    Dec 25, 2021 at 13:34
  • $\begingroup$ Yes, I suspect the guys at the Space Exploration SE will know a lot more about this than we will. $\endgroup$ Dec 25, 2021 at 13:44
  • $\begingroup$ I can't keep track of all of these different Stack Exchange communities anymore. $\endgroup$
    – tparker
    Dec 25, 2021 at 13:47

1 Answer 1


The upper stage of Ariane 5 has very little power, the thrust to weight ratio is around 0.25 at engine ignition, depending on the payload. At that low of a TWR, the spacecraft tends to lower in altitude a bit before it starts to raise again. This allows the rocket to take maximum advantage of the Oberth effect, and minimize gravity losses, allowing for a most effective trajectory.