Knowledge Zone · Card 6 of 13
Kepler's ellipses
An orbit is not an altitude, it is an ellipse, and it takes two numbers to say which one: the periapsis, its lowest point, and the apoapsis, its highest. Kepler's first law is that this is the only shape a two-body orbit can have — a circle is just the ellipse where the two numbers agree. The badge on the launch screen prints both, which is why it says Parking orbit 157 × 633 km rather than a single height.
The useful consequence is where a burn lands. Add speed at one end of an ellipse and you do not move that end at all; you move the other one. This is why the game cuts the engine the instant the periapsis clears 150 km: from there on, every further metre per second the Scout has goes into the far end of its orbit and nowhere else.
You can watch that happen in one slider. Move only Start pitch-over at, from 0.1 km to 20 km, and the badge's first number stays where it was — 158, 157, 159, 151 — while the second walks from 475 km to 3 444. Two hundredfold more of one slider, and the ship is still coming back to the same place. The whole transfer to Mars is the same move on a larger stage: one burn on Earth's orbit, and only the far end of the arc moves out to Mars'.