Guides › Sandbox Mode
Sandbox Mode
Switch the Solar System from clockwork to live gravity, then change anything and see what physics does with it.
From clockwork to live gravity
While you're exploring, the planets move like clockwork. Each one follows its published orbit, calculated directly from the date. That's accurate and efficient, but it means nothing you add can affect them. A black hole parked next to the Sun couldn't move a planet that's on rails.
The first time you add something in Sandbox Mode, the Solar System is released. Every body is given its exact current position and velocity, so nothing jumps, and from then on everything pulls on everything else through Newton's law of gravity. The sandbox works in the same units as the published orbits (astronomical units, days and solar masses), so a released planet carries on along the same orbit it was already following, until you disturb it.
Under the hood, the simulation uses a leapfrog integrator. It's the same kind of method astronomers use for long orbit simulations, because energy doesn't slowly creep up or leak away over millions of orbits, and running time backwards retraces the same path. The time step shrinks automatically during close encounters so that a fast asteroid can't skip through a planet between two steps. If your device can't keep up, the clock slows down rather than the physics getting sloppy.
The tools
| Key | Tool | What it does |
|---|---|---|
| 1 | Select | Click anything to select it and lock the camera on. |
| 2 | Add | Pick an object from the tray, then click to place it or drag to throw it. |
| 3 | Grab | Drag any body, even the Sun. Let go while moving to fling it. |
| 4 | Explode | Blow a body apart. |
| 5 | Delete | Remove a body. |
| F | Throw | Throw another of whatever you last added (an asteroid to start with) at the crosshair. |
| H | Heat view | Paint every world by its temperature. |
| Ctrl+Z | Undo | Every add, throw, edit and explosion can be undone (and redone with Ctrl+Y). |
Hold Shift while throwing for ten times the speed. Right-drag and scroll to orbit and zoom around whatever you're locked onto. Press / to search for anything in the universe and go there without leaving the sandbox.
What you can add
The tray covers the full range of objects in the universe, from small rocks to supermassive black holes:
- Small bodies: a Tunguska-sized rock, an Apophis-sized asteroid, a larger asteroid, and a comet.
- Worlds: Moon-sized, Mars-sized, an ice moon, an Earth-like planet, a super-Earth and a gas giant.
- Stars: a brown dwarf, a red dwarf, a Sun-like star and a blue giant.
- Stellar remnants: a white dwarf and a neutron star.
- Black holes: a micro black hole, a stellar black hole and a supermassive black hole.
The inspector
Select anything and the inspector shows its orbit, speed, escape speed, temperature and makeup, all updated live. You can change its mass, radius, speed, day length and composition. Click a value to type an exact amount, such as "2 Earths" or "10 km/s". The consequences follow from the physics. Spin a planet fast enough and it flattens, then flies apart. Add enough mass to a gas giant and its core ignites as a star.
When things collide
When two bodies meet, the outcome depends on their masses, speeds and the angle of impact, as it does in real collision research. Depending on those numbers you might see:
- Accretion: the smaller body is absorbed and the larger one grows.
- Hit-and-run: a glancing blow where both bodies survive, a little damaged.
- Partial erosion or catastrophic disruption: a body loses part or all of itself as debris.
- A new ring: debris inside a planet's Roche limit can't clump into a moon, so it spreads into a ring.
- Tidal disruption: a star that strays too close to a black hole is torn apart.
- Stellar events: merging neutron stars produce a kilonova, and a white dwarf pushed over its mass limit explodes as a Type Ia supernova.
Each event opens a short report explaining what happened and the physics behind it. The sandbox also notices slower changes, such as a planet being captured by another star, a moon being stolen, or a world thrown out of the Solar System altogether.
The forecast
The forecast panel shows what's about to happen. Gravity Lab runs a copy of your universe ahead of the real one, with the same physics and the same collisions, and lists the events it finds. Because the copy plays out its own collisions, it can predict chain reactions. For example, a black hole swallows the Sun, so the Earth loses its star, its orbit opens up, and it drifts away. Gravity with many bodies is chaotic, so the further ahead a prediction is, the less certain it is. The forecast says so.
The 14 ready-made scenarios
Open the Library to jump straight into a famous "what if":
Impacts
- What if Apophis hit? The 340-metre asteroid that will safely pass Earth in April 2029 comes down in the Atlantic instead.
- The day the dinosaurs died: the 10 km Chicxulub impact, on today's Earth.
- Tunguska, 1908: a stony body about 60 metres across that exploded in the air over Siberia.
- Theia: how the Moon was made: the glancing blow that likely created our Moon.
- Stop the Moon: stop the Moon in its orbit and it falls to Earth in about four and a half days.
- A black hole through Earth: a black hole with the mass of a small mountain, smaller than an atom, fired through the planet.
The Solar System
- Give Earth rings: send the Moon inside Earth's Roche limit.
- Jupiter becomes a star: add 85 times more mass and its core fuses hydrogen.
- Delete the Sun: its light and its gravity keep arriving for eight more minutes.
- Earth escapes: a push of 13 km/s takes Earth out of the Solar System for good.
Visitors
- A black hole passes through: a 10-solar-mass black hole crosses the Solar System at 60 km/s.
- A star flies by: a red dwarf passes just beyond Saturn's orbit.
- A twin Sun: a second Sun circling 25 AU out.
- Neutron stars collide: a kilonova a third of a light-year away.
Galaxies too
Sandbox Mode also includes a galaxy collider. Add spirals, dwarfs and giant ellipticals, then drag and throw them into each other. See When galaxies collide for the physics.
Tips
- Lock onto the Earth before throwing something at it. The camera keeps up with it while it orbits.
- Slow the clock down (,) just before an impact to watch it frame by frame.
- If an experiment goes wrong, Ctrl+Z takes it back one step at a time.