The fantastic game Osmos has finally come to Android. In celebration of which, here are a couple of tips for playing it well. They are based on some similar remarks by Mat Jarvis.

The first tip expands on Mat’s “Vidiian Technique”: using ejecta to push a mote that’s too big to absorb into an even bigger mote, thus cutting it down to size. My addition here is patience: use the bare minimum force necessary to get the thing moving (most likely a single push). When it gets small enough and you plow into it, it will start shrinking from your side; if this happens quickly enough, the shrinkage will make it lose contact with the larger mote that was absorbing it on the other side, so you’re no longer competing with Big Daddy for dinner.

The other tips are about orbital mechanics, and they come from reading science fiction.1 The good news is, they’re not restricted to Osmos: if you ever find yourself in freefall orbit, they’ll come in handy in real life!2

The first thing you have to get straight is the connection between your orbit and the speed you travel at. Any motes on exactly the same orbit will move with exactly the same speed. That means if your path-projection stays glued to some particular mote, you’ll never catch it. You can catch up on motes “above” you (farther from the “sun”), and ones “below” you will catch up on you (but so long as the orbits stay fairly circular, “catching up” doesn’t mean you’ll touch, just that at some point you’ll have the same angular position).

To understand how your speed depends on the size of your orbit you need to note the difference between angular speed (measured by how long it takes you to make one complete revolution around the “sun”: shorter means faster, in “rounds per second”) and speed simpliciter (measured by how long it takes to cover a fixed distance, e.g. in km/h). A high speed gives you a large orbit, which (counterintuitively) means a lower angular speed (by going faster you make a wider circle, which takes longer to complete one revolution around the “sun”).

So how do you catch up on that juicy mote sitting exactly in your orbit, tantalisingly close ahead of you? Not by jetting yourself towards it: speeding up will move you into a higher orbit, and you’ll see your target creep away from you (although you’re moving faster than you were, your angular speed has decreased so the angle between you and your target is widening).3 Instead, you expel mass towards your target: you jet directly away from it. That drops you into a lower orbit, which lets you close the angular gap; when you’re directly below your target, or perhaps a little ahead, speed up again (in the direction of your orbit) to raise your orbit to meet it.

Likewise, to catch a mote in your orbit but behind you, you speed up (jetting yourself away from it). This pushes you into a higher orbit, which is slower (in angular terms): your target catches up on your angular position, and when the time comes you slow yourself again to drop down on it from above.

The other thing to note about orbital mechanics is that they don’t work in circles but in ellipses. When you slow down you don’t, in fact, drop into a lower circular orbit. Your orbit from any point x will pass again through x (barring interruptions on the way — and only on single-attractor levels!), but if you slow down at x (starting from a circular orbit) then you’ll reach your lowest point directly across from x, on the opposite side of the “sun”. Such cometary orbits (elongated ellipses) are risky if they cut through the (roughly circular) orbits of larger motes (if your orbital periods don’t match, there’s a good chance that after some number of revolutions you’ll collide). Luckily, returning yourself to a roughly circular orbit is easy: when you reach the lowest point of your orbit, slow down again by the same amount you used to first deform your original orbit. Similarly, going to a larger circular orbit requires speeding up twice: first transforming your circle to an ellipse, then at the ellipse’s largest point (opposite your first jet) you speed up again, transforming the ellipse back to a circle. With your orbital prediction path turned on it’s very easy to see whether you’ve got this right or not.

You can use these orbital mechanics to devious effect. If the only remaining mote is bigger than you and close to the sun, you haven’t necessarily lost yet. Use your ejecta to slow it down, and it will fall into lower and lower orbits. (For best effect, time your missiles to catch it at the high point of its orbit: a change in speed has its most extreme effects at the orbital position opposite to where the change occurs, so to make its low end lower you hit it at its high end.) With a bit of care (don’t forget that these missiles change your orbit too!) you can send it to die in the sun, leaving you the winner by default.

In conclusion, as I have argued above, you should play Osmos because it’s educational. If you ever drift away from the station during an EVA, it might even save your life.

Notes:

  1. Possibly Neal Stephenson’s Anathem, although I wouldn’t swear to it — they might just be an amalgamation of a infodumps from a whole host of bad sf novels and shorts. [↪]
  2. Although things are much more complicated in three dimensions: you have to deal not just with speed but with velocity –speed plus direction– since objects orbiting in different planes can quickly spread apart or –catastrophically– come together as their orbital planes diverge and converge. [↪]
  3. This is like overtaking on a corner: you have more distance to cover, so you have to drive faster just to keep level. Only here the road is all corner, and as you go faster they handicap you by moving you to the outer lanes. [↪]