Physics games often focus on the exaggeration of effects to make the visual appearance of force and motion a gratifying experience. Yet few games truly illustrate the fundamentals of the system they are representing. Gravity 2.0 is an unassuming title, with simple graphics and interface, however its design is where it shines. It distills the process of measuring angle of trajectory, force, and velocity into a series of trial and error experiments with the flight path of projectiles in space until the player achieves the orbit they seek to produce. It explores how objects move through space and behave in orbit and in the process helps us to visualize Kepler’s Laws with impressive fidelity.
Physics as an experience
Gravity 2.0 features an incredibly straightforward physics shooter; you place a turret, adjust its angle, the power with which it will fire a projectile, and let it fly. However within Gravity 2.0 the player is firing from a planet, meaning that with the right angle and force the object can either escape its gravitational pull, or gracefully enter its orbit.
(Projectiles being affected by the gravitational forces of multiple planets [Gravity 2.0])
The game can simulate planets of different sizes and gravitational forces, even including the capacity to have multiple planets affecting the trajectory of the projectile through space. This opens up the ability to slingshot the player’s projectile around other planets, in the same fashion the Apollo 8 mission used the Moon’s gravity to hurdle it back to Earth.
(Apollo 8 and Saturn S-IVB flight plan using the Moon’s gravitational pull to curve its path [history.nasa.gov])
(A projectile’s trajectory curving around the gravitational pull of a smaller celestial body [Gravity 2.0])
Elliptical orbits and eccentricity
Gravity 2.0 helps to dispel the oversimplification of our solar system that has been taught to us from a young age; with planets aligning in perfect circles expanding in size as we go to planets further and further from the sun. In reality as has been noted in Kepler’s Law of Orbits “All planets move in elliptical orbits, with the sun at one focus.”(http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html) In the case of Gravity 2.0 the planets are represented by the projectiles and a gravitational sphere from which the projectiles are fired can represent either a planetary platform or the sun.
(An example of an elliptical orbit using multiple projectiles to help visualize the object in motion [Gravity 2.0])
The exaggeration of an orbit from a more circular, even shape to a wide, thin ellipse, is known as its eccentricity. The most common objects we can observe which exhibit eccentric orbits are comets; they travel to areas far out in our solar system and return after an extended period of time to briefly fly across our night sky. The eccentricity of the earth along with its wobble produces the seasons.
(An orbit with a small amount of eccentricity [Gravity 2.0])
Retrograde and orbital periods
Orbital periods describe the amount of time an object takes to complete an orbit. From the platform of Earth we often see planets with varying orbital periods cross the different constellations some within weeks others within years. However when the earth’s orbit passes the rate of another planet it appears as if the other planet begins to travel backwards across the sky, this is known as retrograde.
Thanks to Gravity 2.0‘s macroscopic model and the quick orbital periods of its projectiles, we can observe this in-game by establishing two orbits with different orbital periods and eccentricities. As the two orbiting objects come close to each other, variations in their orbital period may place one in front of the other for a portion of their orbit only to be surpassed by the other at another point, creating retrograde phases.
(Two orbits illustrating intersecting paths and how retrograde appears outside of the planetary platform [Gravity 2.0])
(A sequence of pictures showing the path of Mars in retrograde from Nasa [http://apod.nasa.gov])
Kepler’s Laws at play
Showcasing Kepler’s Laws in gameplay, Gravity 2.0 is a refreshing addition to the genre of physics shooters. It takes the well known mechanic of launching an object and presents the player with the challenge of solving for the physics needed to acquire an intended trajectory or orbit.