Asteroids

You fly a small ship in a field of drifting rocks. Shoot a rock and it breaks into smaller, faster pieces. Clear every rock and the next, larger field arrives. You have one ship, and it is protected by a shield that recharges and an energy reserve that refills, so a careless touch costs you shield rather than the whole run.

The view follows your ship across a large field that loops around, so you can fly in one direction forever and never reach an edge. Rocks you cannot see are shown on the radar in the bottom left corner, where the orange box marks what is on your screen, the dark dots are rocks and the orange dots are saucers.

Your ship keeps its momentum, so turning does not change where you are heading. Thrust adds speed in the direction you are pointing, and only a thrust the other way slows you down. Learning to drift and aim at the same time is most of the game.

The bubble around your ship is your shield, and it starts full at 100. Touching a rock or being hit by a saucer drains it, and the hit also shoves the ship away. A large rock costs 34, a medium rock 22, a small rock 14, a saucer shot 18 and ramming a saucer 30.

After each hit you get a short moment where nothing else can touch you, so a single collision never chains into several. If you then stay clear for two and a half seconds the shield starts to refill by itself. The bubble also shows its state: it fades as the shield drops and turns red when it is nearly gone.

Energy powers everything else. Each shot costs 7, thrusting drains 10 a second and hyperspace costs 35. It refills quickly while you coast, and slowly while you thrust. With an empty reserve you cannot shoot or jump and the thruster only works at partial power, so ration it in a fight.

Rocks come in three sizes, and the smaller they are the faster they move and the harder they are to hit. The points reward the risk.

Every so often a flying saucer crosses the board and shoots at you. The big saucer is worth 200 points and fires in random directions. The small saucer is worth 1,000 points and aims right at you, so it is far more dangerous.

The higher your score, the more often the small saucer appears. A saucer shows up some distance away, drifts in your direction and leaves again after about twenty seconds. Shoot it for the points, or put a rock between you and its line of fire.

Each level starts with more large rocks than the last, up to twenty, and they never spawn close to your ship. Saucers appear sooner on later levels.

The run ends when your shield reaches zero, and the bar turns red and starts to beep when it falls below a quarter.

Nothing is downloaded for the audio. The music is composed live in your browser from a random number that is drawn fresh every time you open the page. That number, shown on the start screen as the seed, decides the key, the scale, the tempo, the chord progression, the rhythm and the sound of the lead voice.

The pieces are built from music theory. Chords follow likely moves between the degrees of a dark scale, such as Phrygian or harmonic minor. The drums use Euclidean rhythms, which spread a number of hits evenly across sixteen steps. The melody is a random walk that lands on a note of the current chord on every strong beat, so it wanders without sounding lost, and each four bar phrase ends on the home note.

Small random variations in timing and loudness keep it from sounding mechanical. The music also listens to the game and changes section as the situation changes. In open space with no rocks near, it thins out to a slow pad with no drums. With rocks around it settles into a steady groove, and when you are firing or the rocks crowd in, the tempo rises and a snare and a pulsing bass join. A saucer brings in sharp staccato stabs in a tense scale. Low shields replace the drums with a heartbeat and a dissonant drone. Clear the field and the music brightens into a major key. Every new section re-rolls its scale, sound and rhythm, may change key, and each new level modulates, so a long game keeps moving. You and a friend will almost certainly hear different pieces, and the same seed always produces the same music.

Ship, rock and saucer sounds are calculated the same way. The engine is a continuous voice built from a sawtooth wave, a square wave an octave lower and a hiss of noise, all passed through a filter with a slow tremolo on top. Open the throttle and it swells. The faster you are going, the higher its pitch and the brighter its tone, so you can hear your speed as well as read it on the speed bar. The speaker button above the board turns everything off.

Does it save my score? Only your best score, and only in this browser. There is no account and nothing is uploaded.

Does it work on a phone? Yes. Drag to aim and use the buttons under the board to thrust, fire and jump.

Can I play full screen? Yes. Pressing Start enters full screen on its own, and the full screen button does the same at any time. Leaving full screen pauses the game.

Why did the game pause by itself? It pauses when the tab is hidden, so nothing hits your shield while you are away.