The objective of Astrosmash is to shoot down as many falling objects as possible (meteors, missiles, bombs, and UFOs) without being hit or blown up. As the game progresses, the objects become faster and more numerous, and the tempo of the game, including the game's background "music" of a single cymbal-like sound, increases to a frantic pace.
The player controls a gun, called a "laser battery" by the instruction manual, which can fire a white laser blast straight into the air to destroy incoming objects. These lasers can be fired in manual bursts, or set to auto-fire at an interval. The battery can slide horizontally along the ground, or use a risky "hyper space" maneuver to warp to a random spot on the ground.
While any falling object hitting the laser battery will cost the player a life, most objects can be avoided with little to no consequence. The only exception to this are Spinners, which will automatically cost the player a life if they reach the ground.
There are two sizes of meteors, big and small. These colorful rocks will fall at varying angles and speeds, and disappear once they touch the ground. While it isn't critical to destroy every meteor that falls, every meteor that touches the ground will cause a player's score to go down. In addition, big meteors may split into two small meteors when shot.
There are also two sizes of Spinners, big and small. Spinners are white spinning bombs that make a falling sound upon entering the screen. They are arguably the most critical enemies to destroy, as if they are allowed to touch the ground, they will destroy the laser battery and cost the player a life. Like meteors, their angle and speed vary.
Emitting a beeping sound upon entering the screen, guided missiles look like pulsing white diamonds. These missiles fall at a fast rate, homing in on the battery's location and tracking its movement. If guided missiles are allowed to touch the ground, they may slide along the ground towards the battery. The only way to avoid a guided missile once it is sliding along the ground is to use hyper space to warp to the other side of it, although it may still re-lock onto the player and continue sliding towards their location.
These will only appear once the player has reached the 4x multiplier level. UFOs move horizontally across the top of the screen, emitting a constant sonar-like beep, and will fire square bombs at the laser battery. Both the UFO and bombs can be destroyed with laser fire.
Points are scored for destroying anything that falls or flies (except UFO bombs). A player loses points for letting meteors hit the ground or dying. The pre-multiplier breakdown is as follows:
- Big meteors: 10 points
- Letting a big meteor land: -5 points
- Small meteors: 20 points
- Letting a small meteor land: -10 points
- Big Spinner: 40 points
- Small Spinner: 80 points
- Guided missile: 50 points
- UFO: 100 points
- Losing a life: -100 points
Because a player can lose points, the game keeps track of the player's current score and peak score, displaying the peak score at the game over screen. The current score affects the speed and difficulty, and also determines the multiplier level of points scored or subtracted. The color of the sky changes to signify the multiplier level a player is on.
- 1x (Black sky): 0 - 999 points
- 2x (Blue sky): 1,000 - 4,999 points
- 3x (Purple sky): 5,000 - 19,999 points
- 4x (Turquoise sky): 20,000 - 49,999 points
- 5x (Grey sky): 50,000 - 99,999 points
- 6x (Black sky): 100,000 points or more
It is possible to drop down multiplier levels due to death or letting too many meteors hit the ground.
The Atari 2600 version of Astrosmash is called "Astroblast".