Mass is an unchanging attribute that is different for each ship in Starblast.io. The mass determines how much impact a ship will have when colliding with another ship or with an asteroid. Ships with higher mass values will generally deal more collision damage and take less. Having high mass is very helpful in ramming, both in ramming asteroids for Gems and ramming enemy players. If a heavy ship like an O-Defender can manage to ram a small low-mass ship like the A-Speedster, it will deal a large amount of knockback and give the heavy ship the upper hand. Mass usually comes with slow ship speed, and vice versa. The Shadow X-1 has high speed but small mass, and the Aries has high mass but low speed. This is not always the case, but it's the majority of the time. Note that a large size is not always synonymous with high mass, as if you are the H-Mercury, even a smaller ship like the Condor can knock you around!