Zero-Gravity Mechanics
One of the defining features of The Expanse: Osiris Reborn is its commitment to authentic zero-gravity gameplay. The Expanse universe is famously grounded in real physics (with a few exceptions), and Owlcat Games took that seriously enough to hire NASA astronaut Leroy Chiao as a consultant. Implementing convincing zero-g required significant rewrites to Unreal Engine 5, which was not designed from the ground up to handle gameplay where "down" does not exist.
Engine Modifications
Unreal Engine 5 assumes a consistent gravity direction in most of its physics and gameplay systems. Character controllers, projectile trajectories, navigation meshes, and even camera behavior all default to a world where gravity pulls everything in one direction. To make zero-gravity sections feel right, Owlcat had to rewrite substantial portions of the engine's movement and physics code.
This was not a minor undertaking. The studio has described it as one of the most technically challenging aspects of the entire project. Every system that interacts with physics, from the player character's movement to AI pathfinding to how debris scatters after an explosion, needed to account for the absence of a gravity vector.
Magnetic Boots
When exploring space stations, ships, and other structures in zero-gravity, players use magnetic boots to walk along surfaces. This is a direct nod to how characters in The Expanse books and TV show navigate in microgravity. Magnetic boots clamp the player to the floor (or wall, or ceiling) and provide a sense of stable footing.
Walking with magnetic boots is slower and more deliberate than standard movement in gravity. The boots make a distinct clanking sound with each step, reinforcing the feeling that you are held to the surface by magnetism rather than weight. Players can disengage their boots at any time to push off and float freely, which is useful for reaching areas that cannot be walked to or for gaining a positional advantage in combat.
Free-Floating Movement and Thrust
When not magnetically attached to a surface, the player moves by applying thrust. Small thrusters on the player's suit allow for directional movement in all six degrees of freedom: forward, backward, left, right, up, and down. Thrust is not unlimited, and miscalculating a push can send the player drifting in a direction they did not intend.
The developers have emphasized that miscalculating thrust can be fatal. Push off a surface too hard in the wrong direction and you might drift into an exposed area where enemies have a clear shot. In the worst case, a badly timed thrust in open space near a station exterior can send the player tumbling away from any surface to grab onto. Movement in zero-g rewards careful, measured inputs rather than frantic button-mashing.
Grenades with Thrusters
Standard grenades do not work well in zero gravity because there is no arc. A thrown grenade in microgravity will travel in a straight line forever unless it hits something. To solve this, grenades in the game are equipped with small thrusters that allow them to adjust course mid-flight. Players can influence the grenade's trajectory, curving it around cover or redirecting it toward a cluster of enemies.
This mechanic adds a layer of skill to grenade usage that does not exist in traditional cover shooters. In normal gravity, a grenade follows a predictable parabolic arc. In zero-g, the thruster-equipped grenades follow player-guided paths, making them simultaneously more versatile and harder to use effectively.
Coriolis Force
In rotating space stations (which simulate gravity through centripetal force), the game models the Coriolis effect. This means that projectiles fired inside a spinning station do not travel in perfectly straight lines from the shooter's perspective. The rotation of the station causes a slight deflection that experienced players will need to account for.
This is a detail pulled straight from the physics of The Expanse universe. In the books and show, characters aboard rotating stations like Ceres or Eros can observe Coriolis effects in everything from poured liquids to thrown objects. The game translates this into a gameplay mechanic that subtly affects aiming and projectile behavior in rotating environments.
Sound Design in Vacuum
In real space, sound does not propagate because there is no medium (like air) to carry the waves. The Expanse has always respected this fact, and Osiris Reborn follows suit. When the player is in a vacuum, the sound design changes dramatically. External sounds are heavily muffled or absent entirely, replaced by the player character's breathing, suit creaks, and vibrations transmitted through physical contact with surfaces.
Gunfire in vacuum sounds completely different from gunfire in a pressurized station. The player hears the mechanical action of their weapon through their suit's vibrations, but the actual report of the shot is absent. Explosions produce a visible flash and shockwave of debris but no boom. This creates an eerie, tense atmosphere during zero-g combat encounters that sets them apart from fights in pressurized environments.
When the player is inside a pressurized area, sound returns to normal. The transition between pressurized and vacuum environments is handled dynamically, with sound gradually fading as atmosphere vents or returning as the player enters an airlock.
Combat in Zero-G
Zero-gravity combat encounters play very differently from standard firefights. The cover system still functions (players can use magnetic boots to anchor behind objects), but the addition of free-floating movement and three-dimensional threat angles changes the tactical calculus completely. Enemies can approach from above, below, or any direction, and the player needs to manage their orientation as well as their position.
Destroyed cover in zero-g creates floating debris that can obscure sightlines or serve as improvised obstacles. The dynamic, chaotic nature of zero-g fights is one of the things that distinguishes Osiris Reborn from other third-person shooters.