Apoapsis and periapsis: the high point and the low point
S01:E02
This is Orbital Radio. Out here, you’re never alone.
Here’s a riddle. Your ship is in orbit, engines off. Nothing’s pushing it. Nothing’s touching it. Up above the air, there’s nothing to slow it down. And yet, unless your orbit is a perfect circle, the speed on the navball won’t sit still. For half of every lap it climbs. For the other half it sinks. So who’s working the throttle?
Nobody. Your ship is rolling around a bowl. And every bowl has a top and a bottom.
Today on Orbital Radio, the high point and the low point. By the end of this show, you’ll know why nearly every orbit has one of each, and what Orbital Frontier calls them. You’ll know why your ship is fastest at one and slowest at the other. And you’ll know why it spends most of its time up high.
On another show, we fired Isaac Newton’s cannon from a mountain so tall that its top pokes out above the air. Fire the ball sideways at just the right speed, and it falls all the way around the world at the same height. A perfect circle.
But what if you get the speed a little wrong? Fire it a little faster, and the ball still comes all the way round. But on the far side of the world, it swings out higher. Fire it a little slower, and the far side dips lower. Either way, the ball comes back to the mountain top, again and again. The path has simply been stretched.
That stretched loop is called an ellipse. Tilt a glass of water and look down at the water’s surface. That oval is an ellipse. And here’s the clever part. The world doesn’t sit in the middle of the oval. It sits off towards one end.
Picture a long running track, with the Earth standing near one end of the field instead of in the middle. One end of the track passes close to the Earth. The other end swings far away. That’s the shape of nearly every orbit there is. One near end, one far end.
People worked this out long before anyone flew. In sixteen oh nine, the astronomer Johannes Kepler worked it out from years of careful measurements of Mars. The planets don’t go round the Sun in circles. They go round in ellipses, with the Sun off to one side.
So every loop has two special places. The far end, the highest point, is called the apoapsis. The near end, the lowest point, is called the periapsis.
They sound like Greek homework, and that’s exactly what they are. Apo means away. Peri means near. And apsis is an old Greek word for a loop, or an arch. So the apoapsis is the away point of your loop, and the periapsis is the near point.
Around the real Earth, astronomers often say apogee and perigee instead. The Sun and the Moon have special pairs of their own, too. Orbital Frontier keeps it simple, with the one pair that works around any world at all.
You’ll find them in two places. On the flight screen, look at the top left, at your altitude. Right under it is a second line with two figures on it, marked apo and per. That’s your high point and your low point, and they’re there from the moment you leave the pad.
Then tap Map. In orbit, your path is drawn as a loop, with two little diamonds on it. One is marked with the letters A and P, for apoapsis. The other is marked P and E, for periapsis. Tap a diamond, and it tells you how high that point is, and how long until you get there.
Now, the speed. Why is your ship fastest at the bottom and slowest at the top?
Think of a skateboarder in a curved bowl. They drop in from the lip, race through the bottom, roll up the far side, slow down, and come back. Going down, they pick up speed. Going up, they pay it back. Nothing is pushing, and yet the speed is always changing.
An orbit is that bowl, with the world at the bottom of it. Falling in towards the world, your ship speeds up. Climbing away from it, your ship slows down. So it rushes through the periapsis, and crawls over the apoapsis.
To watch it happen, tap the navball until the word above its speed says orbit. Then just coast. Say your loop dips to a hundred kilometres above the Earth, and swings out to a thousand. At the low point, you’d be doing about two thousand six hundred and fifty metres every second. At the high point, less than half of that.
Mission Control keeps an eye on both ends. While you’re coasting above the air, it calls each one a minute ahead. You’ll hear something like this: “Coming up on apoapsis.” Or this: “Periapsis in a minute. Lowest you will be this lap.”
So your ship trades height for speed on the way down, and speed for height on the way up. Lap after lap, and it never runs out.
Here’s the part that surprises people. Because your ship crawls at the top and rushes at the bottom, it spends most of its time up high.
Kepler found this too. Draw a line from the Sun to a planet, like the hand of a clock that stretches and shrinks. That line sweeps out the same amount of sky every day. Close to the Sun, the line is short, so it has to swing fast. Far away, the line is long, so a slow swing covers just as much.
Think of slices of a pizza, all the same size. The ones near the low point are short and wide. The ones out at the high point are long and thin. Each slice is the same amount of time.
Take that loop again, a hundred kilometres up at the bottom and a thousand at the top. One lap takes a little over an hour. More than forty minutes of it are spent above halfway up.
That’s worth knowing. Near the apoapsis, there’s time to look around, line things up, and think. Near the periapsis, everything comes at you fast. So if you need time to get something done, you’ll find it near the top.
So how stretched is your loop? Remember the tilted glass. Tip it a little, and the oval is nearly round. Tip it further, and the oval gets longer and thinner. Orbits come in every one of those shapes, and the map puts a number on yours.
Open the map, and the orbit panel at the top shows your altitude, and under it your apoapsis and your periapsis. Tap the panel, and it unfolds. At the bottom is your eccentricity, which just means how stretched the loop is.
When the two ends are the same height, the loop is a circle, and the eccentricity is zero. On a loop that’s very nearly a circle, Mission Control won’t call either end at all. On a circle, the high point could be anywhere.
Pull the two ends apart, and the number grows. Push the low point under the ground, and on the map the periapsis says impact instead of a height. That’s not an orbit any more. It’s a throw.
Stretch the loop until the number reaches one, and it never comes back at all. The apoapsis says escape instead of a height, because there’s no far end left to reach. You’re leaving that world behind.
So here’s something to try, next time you play.
Start in sandbox, your own private practice world, where a launch costs you nothing. Take the starter rocket, point it straight up, and watch the second line under your altitude. The per figure shows a height below zero, hundreds of kilometres underground. A rocket thrown straight up is on a loop too. It’s just a loop that passes through the Earth.
Next time you’re in orbit, wait for the periapsis. Point the nose at the green mark on the navball, the way you’re already going, and give it a few seconds of throttle. Watch the apoapsis climb. Then let go and coast. Watch the speed on the navball swing for a whole lap, with nobody touching the throttle. In sandbox, the rate button at the top right of the map will hurry the lap along.
And one for the map. Zoom all the way out and find Pluto’s ring around the Sun. The Sun isn’t in the middle of it. It sits off to one side, and on one side Pluto’s ring swings in so close that it crosses Squanch’s. That’s an ellipse you can see.
So, one more time. Nearly every orbit is a stretched loop, with the world near one end. The far end is the apoapsis, high and slow. The near end is the periapsis, low and fast. And your ship spends most of its time up at the high end, taking it easy.
This has been Apoapsis and Periapsis, on Orbital Radio. Go and stretch an orbit in Orbital Frontier, and enjoy the view from the top. And remember. Out here, you’re never alone.
