Orbital is a deceptively simple iPhone game. It only costs $1.99. Its controls and concept are incredibly basic. But that's how some of the best games are made. Add to it a scoring system and a competitive leaderboard, and Orbital breeds addiction.

Orbital is played with the iPhone held vertically. The background is open space with a bevy of stars. The bottom of the screen features a line, and underneath it a crudely-designed cannon rotates constantly. The gamer's personal high score is displayed on the left of the cannon, with the current score to the right.

Players tap the screen when the cannon reaches the optimal trajectory, and it fires a ball up the screen, followed by a trail of multi-colored sparks, purely for aesthetics. The ball is fired with the same velocity every time. When the ball stops moving, it expands until there is something to stop it from doing so. That something can be the side walls, top of the screen or other expanded circles. The ball can also ricochet off any of these objects, depending on the angle at which it strikes.

The goal is to eliminate the very balls being shot into the top portion of the screen. This is done by repeatedly striking them with other balls. Each circle stays in the same place once expanded, until it is eliminated. A number appears in the center of each circle, showing how many more hits are required to make it disappear from the board.

The catch is that any time a shot ricochets past the line at the bottom of the screen, it is game over. So players must plan their shots with extreme care, taking into consideration the angles at which the shot will bounce. As the board gets more and more cluttered, getting multiple hits per shot is key, as well as preventing any shots from expanding too large directly in front of the cannon, blocking the shots. Keeping the pieces small and keeping a clear path to the top to slow momentum helps keep shots away from the deathline.

Again, the concept seems simple, but one mistake can lead to game over. Keeping that in mind, scoring between 5 and 10 on the first couple playthroughs doesn't seem too shabby, but then looking at the leaderboard shows some gamers near 100 points on the basic mode, and it puts things in perspective. And to put those scores in perspective, eliminating one circle equals one point.

The rules above describe the single-player "Pure" mode completely, but gamers also have the choice of playing "Gravity" mode, in which each expanded circle creates its own field of gravity proportionate to its size. So instead of the ball traveling in a straight line, only affected by directly hitting objects or walls, the ball's trajectory is affected by the gravity and pulls in different directions accordingly. Rather than making things harder, the gravity actually helps curve the shots in interesting ways and pull them in, slowing the momentum that often sends them back across the deathline. It also makes subsequent shots ideally closer to another circle, keeping its expansion small. The highest score (at the time of writing) in this category, is closer to 500.

Since the game's release, the 1.2 update added a third mode, free of charge, called "Supernova." In the newest mode, the cannon has a laser guide to help players mark the first shot. Instead of rotating on its own, players press and hold, move to the desired position and release to fire the cannon. The circles take more hits to destroy in this mode, but a new "Chain Reaction" feature means its destruction causes an explosion that affects nearby circles. One high scorer in this mode has already reached 1,000 points.

All of these modes are also available in a multiplayer version, which pits two players against one another on the same iPhone. The playing surface moves to the middle ground, with a cannon on each side for each player. The goal in this mode is not to score the most points, but stay alive. Gamers try to strategically shoot balls that will expand and block the other player's shots, forcing them to ricochet back into their own deathline. It's a rather brilliant adaptation of the single-player mode, and gamers can choose to play the best of 1, 3 or 5 rounds, in any of the same modes from single-player.

The nicest touch to the multiplayer mode, though, is the option to flip or keep the orientation. One mode does not flip the screen, making it ideal for players who want to sit across from one another like in a chess match, place the iPhone on the table and have the screen stay in their perspectives as they take turns launching shots at one another. The alternative is letting the game flip the screen on each turn for players passing the iPhone.

In the scheme of things, Orbital is a simple game, but earns that right by creating an addictive formula at a cost of only $1.99. It's an easy game to pick up when just a few minutes are available and then come back to it later, but the multiplayer mode also makes it something worth sitting down to play at length with a friend.