Zero-Gravity Mechanics

Zero-Gravity Mechanics

One of the defining features of The Expanse: Osiris Reborn is its focus on 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. Chiao spent over 229 days in space across four missions and provided detailed feedback on movement, breathing, eating, pressure changes, and the psychology of spacewalks. 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. CTO Alexey Drobyshevsky led the effort to rewrite substantial portions of the engine's movement and physics code. This was one of the most technically challenging aspects of the entire project.

The core problem was that UE5 assumes a flat plane for character movement. Walking, jumping, falling, aiming, and camera orientation all reference a global "up" vector. In zero-g, that vector does not exist. Every system that interacts with physics needed to account for the absence of a gravity direction: player movement, AI pathfinding, how debris scatters after an explosion, how projectiles travel, and how the camera orients itself relative to the player. Drobyshevsky's team ended up treating character movement in zero-g as a "series of small jumps," with each step being a calculated push off a surface rather than a traditional walking animation.

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. The boots clamp the player to the floor (or wall, or ceiling) and provide 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 of being held to the surface by magnetism rather than weight.

The magnetic boot vibrations serve a secondary gameplay purpose. According to the sound design team, vibrations transmitted through the boots provide tactile feedback that substitutes for sound in vacuum environments. The player can "feel" nearby activity through these vibrations: footsteps of approaching enemies, machinery operating in adjacent compartments, or explosions reverberating through the station's hull.

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. The camera auto-adjusts based on surface angles, so if the player walks from the floor onto a wall using magnetic boots, the camera smoothly rotates to keep the player's current surface oriented as "down." This prevents disorientation while preserving the sense that the player is walking on a vertical surface.

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 limited, and miscalculating a push can send the player drifting in unintended directions.

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.

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, especially at longer ranges.

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 in Vacuum

In real space, sound does not propagate because there is no medium 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. Audio Director Sergey Eybog described the approach in a March 2026 developer diary: the game is "not an astronaut simulator, but a cover shooter," so the team had to find a balance between scientific accuracy and gameplay readability.

In vacuum, external sounds are not truly silent. Instead, feedback comes through several channels. Breathing inside the spacesuit is always audible. Vibrations from magnetic boots, nearby explosions, and weapons fire transmit through physical contact with surfaces. Radio communications from companions and allies come through the helmet speakers, sometimes affected by EM interference that distorts the signal. Spacesuit warning systems provide audio cues for suit integrity, oxygen levels, and incoming threats.

Weapons in vacuum send "tremors" rather than traditional gunfire sounds. The player hears the mechanical action of their weapon through suit 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.

Blood and Visual Effects in Vacuum

Blood behaves differently in vacuum. Rather than pooling or splattering as it would in gravity and atmosphere, blood in the game's vacuum environments forms floating spheres and drifting patterns that reflect actual fluid behavior in microgravity. The visual effects team created unique blood animations specifically for zero-g combat to reinforce the alien feeling of fighting in space.

NASA Consultation Details

Leroy Chiao's consultation went beyond general movement advice. He provided specific feedback on topics including how the body moves when pushing off from surfaces (it does not move the way most video games depict), what breathing feels like in a sealed suit during physical exertion, the psychological experience of looking out into open space during an EVA, how pressure changes affect the body when transitioning between pressurized and unpressurized areas, and the sensation of eating in microgravity (crew members crave spicy food because reduced blood flow to the head dulls taste and smell). Several of these details were incorporated into environmental storytelling, ambient animations, and gameplay feel.

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.

Additional Details From the Closed Beta

Hands-on time with the closed beta filled in the texture of how zero-gravity play actually feels in motion. The sections below fold that footage into what was previously established about the engine work, the magnetic boots, and the audio philosophy.

Two Classes of Zero-G Gameplay

The beta exposes two distinct flavors of zero-G traversal, and they are structured differently. The first and most prominent is extravehicular activity along a station's exterior hull, where the player wears magnetic boots and walks across the outside of the structure as if it were a floor. Gravity orientation in these sections is arbitrary and contextual. Enemies can be locked to a ceiling relative to you, angled at roughly forty-five degrees along a curved section of hull, or standing on what your camera is treating as a wall, and combat genuinely plays out in three dimensions. The camera dynamically reorients as you cross curved surfaces, smoothly rotating to keep your current footing oriented as down so that aiming stays readable even as the world rolls around you.

The second class is fully free zero-G without magnetic boots. In the beta these sequences are shorter, more scripted, and largely on-rails. The player is in vacuum with no anchor point, and the path through the encounter is narrowly defined. Deviating from the safe route in these moments is punishing and often lethal in a single misstep, which makes them feel closer to a tense set piece than an open playground. Together the two classes give vacuum sections a rhythm: longer, more exploratory hull walks broken up by brief, high-risk zero-G passages where a bad push can end the run.

Soundscape in Vacuum

Traditional gunfire and explosion sound effects are intentionally muted while the player is in vacuum. Instead of the crack and boom you hear inside pressurized sections of the station, combat comes through mostly as vibrations transmitted through the suit and as pings bouncing off nearby structures and cover. Impacts pound through the chest rather than slap against the ears, and you register a nearby explosion first as a visible shockwave and a thump felt through the mag-boots, then as the debris it flings outward. The effect is deliberate, called out in the developer's own beta overview as a way to make vacuum combat feel distinctly more dangerous and muted than interior combat, and in practice it nudges players toward reading the battlefield visually rather than tracking targets by ear.

This is continuous with the broader audio philosophy covered in sound design, and with the use of suit vibrations as a tactile substitute for sound. The vacuum soundscape is not silence. It is a shifted mix in which radio chatter, breathing, and mechanical action through the suit become the loudest channels.

Lethality and Physics

Vacuum combat has its own set of physics tells that are not present in pressurized fights. A few are worth calling out in detail:

• Drifting corpses. Enemies killed in vacuum lose their magnetic boot lock the moment they die. Their bodies detach from the hull, tumble free, and drift off into open space. The kill is effectively permanent in a visual sense as well as a mechanical one, because the body is no longer anywhere you can walk to.

• Free-floating blood. Blood sprays outward in spheres and droplets and hangs in the vacuum as free-floating particles rather than splashing onto surfaces. It drifts with whatever momentum it had at the moment of the hit, which reinforces the alien feel of the space sections without resorting to silence.

• Debris cloud cover. All cover is destructible, and when a piece of destructible cover is blown apart in vacuum the fragments do not settle. The area showers in slow-moving metal shrapnel that stays in view instead of falling to the ground, briefly obscuring sightlines and giving both sides improvised temporary cover.

• Incoming grenades and micro-rockets. Enemies throw grenades and fire wrist-mounted micro-rockets that arc through zero-G rather than trace a traditional ballistic parabola. The player has to actively dodge incoming debris and projectiles while maneuvering, not just strafe inside cover, because a round that misses does not simply fall away.

This physics layer sits on top of the cover-based fundamentals described in combat and gameplay. Staying exposed in a vacuum fight is even less survivable than inside the station, because your cover can disintegrate in one hit and there is no ground to take you to safety if you lose your mag-boot grip.

Environmental Exploits in Vacuum

Vacuum is also where the game's environmental interactions look their most dramatic. Pressurized containers and external fuel tanks attached to the hull are marked targets: shooting them creates massive explosions that fling nearby enemies off the station, stripping cover and sometimes ending an encounter in a single trigger pull. These are a particularly satisfying application of the contextual cues covered under the exploit system.

The ship-support exploit takes this a step further. While the player is out on the hull, Zafar can be called in to operate the point-defense cannons on the Gemini. The PDCs tear through the station exterior itself, not just through enemies standing on it. They open new angles of attack by cutting holes through walls, turn whole sections of the hull into drifting debris fields, and visibly shatter the cover enemies were relying on. The effect is clearly keyed to scripted beats in the beta mission rather than available at will, but it shows how the ship is meant to feel like an active participant in vacuum engagements rather than a static backdrop.

Choice Moments in Spacewalks

Spacewalks are not purely combat sequences. At least one of them includes a branching approach to an airlock that interacts with the larger choices and consequences system. The recommended path is to wait for Zafar to run a distraction on an overhead threat and then enter through the airlock he points out, which is the cleaner, quieter route. A second airlock is visible nearby and can be rushed instead. The rush is faster, but it triggers an alarm that forces the player into a side objective to disable the station's response, and it sours the relationship with Zafar, who voices disapproval over the radio and carries that grudge forward.

Some vacuum loot and routing is also skill-gated rather than choice-gated. Successful engineering and perception checks during hull exploration can open a side path through the exterior that feeds into a hidden loot room, which is a parallel to the station-interior branching described in the larger choices writeup. Pinkwater Security crews patrolling the hull may be present or absent depending on earlier decisions made inside the station, so the tone of a given spacewalk can shift substantially between runs.

Cinematic Polish Caveat (Beta)

One beta caveat is worth recording alongside the rest. During in-engine cinematics that layer on top of zero-G sequences, some impact and environment sounds were missing in the build testers played, even though real-time combat audio worked as intended. The design intent for vacuum combat is clearly the muted, vibration-forward mix described above, and the moment you regained control and returned to active play the mix would come back. The dropouts were isolated to cutscene layers rather than gameplay and are the kind of polish item the closed beta exists to surface. This is noted here so that the article's description of vacuum audio tracks the design target rather than any single build's cutscene issue.