Predict what happens to an orbit when you push in a given direction.
You are in orbit, chasing a station a few kilometres ahead. You fire your engine forward, to speed up. What happens?
Yes.Adding speed adds energy, and energy becomes altitude. The higher orbit takes longer to go round, so the chase gets worse.
Not quite.The intuition from every road and river, and orbits do not work that way. Thrust changes your orbit, not just your speed along it.
Not quite.Even a small push changes the shape of the orbit immediately.
A push does not move you. It changes your orbit — and the orbit decides where you will be an hour from now.
Put in order how you actually catch a target ahead of you.
Tap them in order — first to last.
Brake→Drop lower→Go round faster→Rise to meet it
The counter-intuitive part is step one.Yes.Slow down to catch up. Every docking mission does this, and it is the clearest sign that orbital motion follows different rules from anything on a road.
You fire your engine once. Match the direction to what it changes.
Firing forwards, along your path.
Firing backwards, against your path.
Firing sideways, out of the orbital plane.
Firing along the path at the lowest point.
Firing perpendicular to the plane at the equator crossing.
Yes.Forwards and backwards raise and lower the opposite side of the orbit. Sideways tilts the whole plane — and that is by far the most expensive manoeuvre in spaceflight, which is why launch sites are chosen for the plane they can reach.
You fire briefly at one point in a circular orbit. Slide to change the direction and see the new orbit.
ForwardsBackwardsStraight up
Altitude where you fired400 km
Altitude on the far side800 km
ForwardsThe far side rises. You now have an oval orbit with its high point opposite where you fired.
Altitude where you fired400 km
Altitude on the far side120 km
BackwardsThe far side drops. Fire hard enough and the far side is inside the atmosphere — this is how you come home.
Altitude where you fired400 km
Altitude on the far side410 km
Straight upBarely changes the altitude at all. It shifts where the high and low points sit.
Why does a burn change the altitude on the OPPOSITE side of the orbit?
Yes.An orbit is a single closed path, not a position. Changing it at one point necessarily changes it everywhere — and the largest change appears at the far side.
Not quite.The engine points along your path. The far side responds because the path is a loop.
Not quite.Earth's spin matters for launch and for ground tracks. The far-side effect happens regardless.
Move the control to see what changes.
Why is changing the tilt of an orbit so much more expensive than raising it?
Yes.A modest plane change can cost as much fuel as launching to orbit in the first place. It is why satellites launch into the plane they need and stay in it.
Not quite.Direction of thrust costs nothing extra. The cost is in how much velocity has to be redirected.
Not quite.There is essentially no drag at these altitudes. The cost is pure geometry.
A spacecraft in a 400 km orbit fires backwards until the far side of its orbit is at 120 km — inside the atmosphere. By how many kilometres did the far side drop?
km
Yes.400 − 120. A re-entry burn is a small push that moves the far side of the orbit into the air, and the atmosphere does everything after that.
Lesson complete
A push does not move you along your orbit — it changes the orbit itself.