How to Simulate Lunar Gravity in Matter.js

Simulating a realistic lunar landing environment in Matter.js requires modifying the physics world's default environmental constants to mirror the Moon's physical conditions. To achieve this, you must reduce the engine's vertical gravitational scale to roughly 16.6% of Earth's gravity and eliminate air resistance parameters on the spacecraft body. This guide demonstrates how to calibrate the engine's gravity vector, strip away aerodynamic damping, and apply vector-based thruster controls to create an authentic vacuum flight model.

1. Configure the Lunar Gravity Vector

By default, Matter.js initializes the world with standard gravity directed downward along the Y-axis (engine.gravity.y = 1).

The Moon's gravitational acceleration is approximately \(1.62\text{ m/s}^2\), roughly one-sixth of Earth's \(9.8\text{ m/s}^2\). To reflect this, adjust the y component of the engine's gravity property or modify its overall scale:

// Option A: Adjust the Y-component relative to default 1.0
engine.gravity.y = 0.165;
engine.gravity.x = 0;

// Option B: Scale the default gravitational constant
engine.gravity.scale = 0.001 * 0.165; // Adjust based on your simulation scale

Setting engine.gravity.y = 0.165 ensures that any free-falling body accelerates at a rate consistent with the lunar surface relative to default engine units.

2. Eliminate Atmospheric Drag

The Moon has no atmosphere, meaning objects experience zero aerodynamic drag. In Matter.js, bodies have an innate frictionAir property (defaulting to 0.01), which acts as linear damping against translational and rotational motion.

When creating the lander vehicle, set frictionAir to 0:

const lander = Matter.Bodies.polygon(x, y, 3, 30, {
    density: 0.002,
    frictionAir: 0,     // Eliminates all atmospheric drag
    friction: 0.8,       // Surface friction for landing pads
    restitution: 0.05   // Low bounciness upon touchdown
});

Matter.Composite.add(engine.world, lander);

With frictionAir: 0, the lander will maintain its velocity and trajectory indefinitely until an external force—such as thruster output, gravity, or a surface collision—acts upon it.

3. Implement Inertial Thruster Mechanics

Without atmospheric resistance, spacecraft maneuvering relies entirely on Newtonian mechanics (\(F = ma\)). Angular momentum and linear velocity do not decay automatically; stopping a rotation or translation requires an opposing counter-force.

Apply forces using Matter.Body.applyForce directed along the lander's orientation vector:

function applyMainThruster(lander, power) {
    // Determine force direction based on the lander's current angle
    const angle = lander.angle - Math.PI / 2;
    const force = {
        x: Math.cos(angle) * power,
        y: Math.sin(angle) * power
    };

    // Apply force directly at the center of mass
    Matter.Body.applyForce(lander, lander.position, force);
}

function applyRotationalThruster(lander, torque) {
    // Modifies angular velocity directly to simulate Reaction Control System (RCS) thrusters
    Matter.Body.setAngularVelocity(lander, lander.angularVelocity + torque);
}

4. Verification and Fine-Tuning

To confirm the vacuum physics implementation is correct:

1. Verify Trajectory: Fire the main thrusters diagonally, then cut power. The lander must trace a true parabolic arc influenced only by gravity, without deceleration in horizontal speed.
2. Verify Angular Momentum: Rotate the craft using RCS controls. If torque is applied once, the craft must continue spinning at a constant angular velocity until an equal and opposite torque is applied to stabilize it.