Disintegration Update | Alpha 19 | Now Available

Universe Sandbox ² - Many Moons

Run Steam to update to Alpha 19, or buy Universe Sandbox ² via our website or the Steam Store.

Please note, this is an update for the desktop mode only. We will now be focusing on an Alpha 19 VR update. In the future, these two modes will be merged and won’t require separate updates.

In Alpha 19, simulations are more dynamic, realistic, and exciting. Why? 


1. Tidal Forces Now Tear Apart Planets

Alpha 19 Roche

When one body gets too close to another body of higher mass, tidal friction will begin to heat it up. And if it’s close enough, Roche limit fragmentation will tear it apart, leaving a trail of fragments. 

A major theory for the formation of Saturn’s rings is that the rings were once a moon orbiting Saturn, before its orbit took it within the Roche limit and it was torn apart.

Home → Open → Physics tab

Home → Tutorials → 11- Tidal Forces


2. Planets Are Now Vaporized by High Temperatures & Supernovae

Universe Sandbox ² - Supernova Mars

Whether by a supernova or extreme tidal heating, bodies now lose volatile material as their temperatures rise. The required temperature and type of volatile loss factors in the body’s material composition and mass. 

There are also new visuals for supernovae, and their appearance varies based on the type of the star.


3. Improved Performance, Appearance, & User Control for Particles & Fragments

Universe Sandbox ² - Alpha 19

A new performance budgeting system knows when to let particles and fragments stick around, and when to get rid of them to keep everything running smoothly and accurately. 

Particles and fragments are now much more varied in appearance, from hot to cold, rocky to gaseous. They can also now be individually- or group-selected, to easily watch, move, or delete them, or even convert them into bodies. 


4. Explosions Look More Epic & Cause “Shockwaves”

Universe Sandbox ² - Alpha 19 Explosion

Combined with the improvements made to the particle & fragment system, explosions look a whole lot better in Alpha 19.

Also, explosions now simulate an expanding bubble of exploded material, which can collide with other planets, heating and ripping them apart.

Home → Open → Explosions tab


5. You Can Record Animated GIFs

Easily & instantly create animated GIFs like the ones above of planetary movement & destruction.

Press F9 to Start/Stop Video Capture

Or click the Video button in the bottom right for Start/Stop Video Capture and Video Capture Settings. These settings can also be found through Home  Settings → Video.


6. “New” Labels, Stats, & Sorting Options for Sims

Universe Sandbox ² - New Sim Labels

All sims new to Alpha 19 have a “New” label which is replaced with a check mark once the sim has been opened.

You can hover over the sim tile to see sim stats, like when you last opened it, and you can now sort the sim list using the options next to the search bar.


7. New Object Models

Universe Sandbox ² - Alpha 19 New Models

We’ve improved the models for the Saturn V 3rd Stage rocket and New Horizons probe, and added The Great Pyramid of Giza and a new police box object.

Home → Open → Human Scale tab → Human Sized Objects

Add  → Objects tab


8. New Music

Two new, original tracks by our composer, Macoubre.


9. And Hundreds More Improvements & Bug Fixes

With nearly an 8-page list of new features, improvements, and fixes, this is our biggest update by far. We’re very excited with all of the changes and would love to hear what everyone thinks!

Check out the full list of new features, improvements, & bug fixes in What’s New.


For the latest Universe Sandbox ² news, follow us on Twitter and Facebook.

Help Us Test the New Alpha 19 Preview

Universe Sandbox ² - Explosions

Thank you for your patience while we get Alpha 19 in top shape. We’ve hit a few snags in development, but it’s now looking better than ever, and we’re very excited to let everyone try it out.

We’d appreciate it if you could test the new preview version of Alpha 19 and let us know what you think.

Once you’ve opted in, please follow the start-up instructions and run the performance test.

Thanks for testing and for your patience!

Follow us on Facebook and Twitter for the latest Universe Sandbox ² news.


How to Opt In to the latest Alpha 19 preview

1. Right-click on the game title in your Steam Library
2. Click on ‘Properties’
3. Select the ‘Betas’ tab
4. Set the dropdown menu to ‘alpha19preview’
5. Close the Properties window
6. Steam will now update Universe Sandbox ² to the preview version
7. Once updated, launch Universe Sandbox ²

What’s New & Improved in Alpha 19 x5

• Improved tidal heating & Roche fragmentation
• Improved explosion effects
• New fragment system for cold/hot, rocky/gaseous fragments
• Improved volatiles
• Many, many bug fixes

Check out the new tidal forces and explosions effects:
• Home > Open > Physics tab
• Home > Open > Explosions tab

Updated 2016 Roadmap

We’re now halfway through 2016, and that means it’s time for an update on the 2016 roadmap we shared at the beginning of the year.


What’s Completed?


1. Virtual Reality

This was a big one for us. Our recent blog post gathered some of the really amazing feedback we’ve been receiving about Universe Sandbox ² in VR.

We’re very proud of the results, and look forward to continually improving the VR experience. Check out the trailer below.


2. Tidal Forces

The new tidal heating and Roche limit effects add another element of realism to Universe Sandbox ², and they also look really cool. In Alpha 19, bodies now heat up and can be torn apart by tidal forces if they approach other bodies closely enough.

Universe Sandbox ² - Roche Limit Moon


3. New Object Models

You’ve probably heard of the Great Pyramid of Giza, but have you ever seen it floating through space? Below is a preview of some of the pyramid blocks which will be available, but of course we’re also adding in the whole pyramid.

Universe Sandbox ² - Pyramid Blocks

We’ve also added improved models for the New Horizons probe and the third stage of the Saturn V rocket, as well as a new, shiny police box. See more.


4. Bug Fixes

Our new Quality Assurance team member, Steven, has been busy testing, tracking down bugs, and assuring that when those bugs are squashed, we’re not inadvertently breaking anything else.

With Steven’s help, and the help of our new developer, Jonathan, we’ve been able to fix a big list of long-standing issues. For a complete list of what’s been fixed, see our release notes.



What’s Next for 2016 and Beyond?


1. Alpha 19

We’re rounding the corner on Alpha 19, which should include the tidal forces, new models, and bug fixes mentioned above, as well as some beautiful new supernova visuals.

Universe Sandbox ² - New Supernova


2. VR Update

Right now the VR version is separate from the desktop version, so Alpha 19 will only be a desktop update. But we’ve been looking at feedback on the VR version, and we plan to address some of these issues with a small VR update after we release Alpha 19.

This VR update should include:

  • A new tutorial to better explain controls & mechanics
  • Save and Load
  • A bunch of bug fixes


3. Alpha 20

Alpha 20 is looking like it’ll be a very big and exciting update.

Here’s what we hope to include in Alpha 20:

  • Automata (more planetary details)
    • Increased spatial resolution for a planet’s data means that it’s possible to affect values locally
    • For example, this should help with local temperatures, like impact and laser heating; terrain deformation; and, further down the line, life simulation
  • Total body fragmentation
    • Combined with the new tidal forces and explosion effects in Alpha 19, total body fragmentation should be a very impressive effect
  • Merged VR and desktop versions
    • After merging we’ll be able to release updates for VR and desktop simultaneously, and more easily integrate features for both versions
    • Complete feature parity for the VR interface will take longer, but this is an important step in that direction
  • Rewritten user interface
    • Allows for scaling the interface for higher resolutions and…
    • Language localization (coming later)


4. Alpha 21 & Beyond

After Alpha 20, we’ll be setting our sights on some even bigger projects:

  • Stellar evolution
    • The new model supports 16 star types, versus the previous 5
    • The result should be more dynamic and accurate properties for stars, as well as smoother transitions between types
    • If we’re lucky, we’ll have this ready for Alpha 20, but a safer bet is Alpha 21
  • Life simulation
    • There’s a lot of planning to do on this, but the Automata tech coming in Alpha 20 should help pave the way for the beginnings of life simulation
  • Space megastructures
    • These accidentally made it into an experimental build of Alpha 19, and if you had a chance to see them, you know that they’re very cool, but they still need a lot of work before they are more interactive and less CPU-hungry
  • Mobile
    • We just hired a dedicated mobile developer, Dave Nelson, who will be working full-steam on a mobile version of Universe Sandbox ²
    • This is still in preliminary phases and we don’t have a timeline for release yet
  • Steam Workshop support
    • Being able to share saved simulations and objects will open up the creative potential even more
  • Language localization


We’re very happy with the progress we’ve made so far in 2016. And we’re very excited for what’s in store for the rest of 2016. We hope you share our enthusiasm for exploring this crazy and ambitious project.

Follow us on Facebook and Twitter for the latest news.

Get instant access to Universe Sandbox ² on Steam Early Access.


Help Us Test the New Alpha 19 Preview

Universe Sandbox ² - Roche Limit Glow

We’re getting very close to officially releasing Alpha 19.

But before we can do that, we need help testing the latest preview version, Alpha 19 x3.

If you’ve already opted in to the preview version, Steam will automatically download this update.

Opt in to the Alpha 19 preview:

  1. Right-click on the game title in your Steam Library
  2. Click on ‘Properties’
  3. Select the ‘Betas’ tab
  4. Set the dropdown menu to ‘alpha19preview’
  5. Close the Properties window
  6. Steam will now update Universe Sandbox ² to the preview version
  7. Once updated, launch Universe Sandbox ²

Since the last preview version, Alpha 19 x2, we’ve improved the tidal heating and Roche limit effects, made further improvements to explosions, added new Physics sims and a tidal forces tutorial, added a new Milky Way background, and fixed a ton of bugs.

For a full list of what’s new, improved, and fixed since Alpha 18, see our release notes: What’s New.

When testing, please pay the most attention to the new tidal forces and explosion effects. You can share issues with us in-game via Home > Send Feedback, or on our forums: Official Forum | Steam Forum

The team is in Amsterdam this week for the Unite Europe 2016 conference. Once we’re all back we’ll work on fixing any remaining issues and polish up Alpha 19 for release.

Thanks for testing and for your patience!

New Spacecraft Models Coming in Alpha 19

Universe Sandbox ² - 3 New

New models in Alpha 19. Left to right: New Horizons, Saturn V 3rd Stage, Police Box

Alpha 19 is not yet officially available. You can try an opt-in preview version of Alpha 19 via Steam:

  1. Right-click on the game title in your Steam Library
  2. Click on ‘Properties’
  3. Select the ‘Betas’ tab
  4. Set the dropdown menu to ‘alpha19preview‘
  5. Close the Properties window
  6. Steam will now update Universe Sandbox ² to the preview version
  7. Once updated, launch Universe Sandbox ²


In Alpha 19 we’ve added in new models for 3 different kinds of spacecraft: the New Horizons Probe, the Third Stage of the Saturn V rocket, and a police box.

These models were made by Toby Halter for Universe Sandbox ².  Older alpha versions of Universe Sandbox ² had models for both the New Horizons Probe and the Saturn V 3rd Stage, but Toby completely remade these, adding a lot more detail and texture to make them much truer to the real spacecraft. And thanks to Georg, our shader wizard, for working his magic on these models and implementing them in Universe Sandbox ².


1. Third Stage of the Saturn V Rocket

Universe Sandbox ² - Saturn V Third Stage Close-Up

This model is based on the third stage of the Saturn V rocket which was used in the 1969 Apollo 12 mission, the second to land astronauts on the moon.

The simulation in Universe Sandbox ² refers to the discovery of what was suspected to be an asteroid orbiting Earth in 2002. After more observation, however, astronomers concluded the most likely explanation was that this object was not an asteroid, but rather the third stage of the Saturn V rocket used in Apollo 12. It was likely flung out of Earth’s orbit in 2003, but there’s a good chance it’ll show up again in a few decades.

Check it out in the included sim: Home > Open > Third Stage of the Apollo 12 in Orbit of Earth in 2003.

Or you place it in any simulation: Add > Objects tab > Third Stage of the Apollo 12.


2. New Horizons Probe

Universe Sandbox ² - New Horizon Probe

The New Horizons probe is most famous for giving us our first close look at Pluto, which it approached in July, 2015. Now that it’s sent us detailed pictures of Pluto and its moons, it continues on its way toward the very distant Kuiper Belt.

The most notable change between this model and the old one is the shiny, gold foil wrapped around it, which serves as a insulating blanket for the grand piano-sized probe.

Check it out in the included sim: Home > Open > New Horizons Pluto Encounter in 2015.

Or you place it in any simulation: Add > Objects tab > New Horizons.


3. Police Box

Universe Sandbox ² - Police Box

As everyone knows, there’s nothing like a police box for traveling through space… or time.

You can add the police box to any simulation: Add > Objects tab > Police Box.


4. What’s Coming

Universe Sandbox ² - Pyramid Blocks Early Model

Toby is going to continue making models for Universe Sandbox ², but we’ve decided to take a break from spacecraft for our next model. Wouldn’t it be pretty cool to have a gigantic pyramid floating through space? The image above is a selection of work-in-progress bricks which will lay the groundwork for an Egyptian-style pyramid.

And while you may not have the urge to pilot a pyramid, you’re probably wondering if you’ll ever be able to control the other nice looking spacecraft in Universe Sandbox ². Well… we’re talking about it.



Follow us on Twitter and Facebook for the latest Universe Sandbox ² news.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Universe Sandbox ² Alpha 19 Preview

Universe Sandbox ² - Third Stage Saturn V

The new model for the third stage of the Saturn V rocket.

We’re still working hard to get Alpha 19 polished and ready to go. We hope to have it ready in the next few weeks.

Until then, you can read about what’s coming in Alpha 19 below and try out a preview version by manually opting in via Steam:

  1. Right-click on the game title in your Steam Library
  2. Click on ‘Properties’
  3. Select the ‘Betas’ tab
  4. Set the dropdown menu to ‘alpha19preview
  5. Close the Properties window
  6. Steam will now update Universe Sandbox ² to the preview version
  7. Once updated, launch Universe Sandbox ²


1. Roche Limit & Tidal Heating

In this simulation, the Moon is orbiting Earth within the Roche limit and is being torn apart by tidal forces.

In this simulation, the Moon is orbiting Earth within the Roche limit and is being torn apart by tidal forces.

When a small celestial body approaches a larger body at a distance inside its Roche limit, it begins to stretch and disintegrate due to tidal forces. Essentially, the gravitational force acting on the closer side of the smaller body is stronger than the force acting on the far side. Outside of the Roche limit, these tidal forces are still present and can heat bodies in close orbits.

In Universe Sandbox ² Alpha 19, you can see these effects when a satellite tightly orbits its primary body, or when a star gets too close to a massive black hole, or even if two identically-sized bodies are in a close binary orbit. First, the secondary body will heat up, then if it’s close enough, it will begin to disintegrate and lose mass by producing a stream of particles. Often times these particles will begin orbiting and form a nice glowing ring around the primary body.


2. Models for Saturn V 3rd Stage, New Horizons probe, & Police Box

Universe Sandbox ² - Updated New Horizons Model

We’ve put in improved models for the third stage of the Saturn V rocket, which was used in multiple lunar missions, and the New Horizons probe, which has famously given us a closer look at Pluto. We’ve also added in another very important spacecraft, the police box.  These new and improved models were made for Universe Sandbox ² by Toby Halter, who is continuing to make models for us. It’s not possible to pilot or control these spacecraft… but we’re talking about it.

To check these out, you can load up the following sims:
– Third Stage of the Apollo 12 in Orbit of Earth in 2003
– New Horizons Pluto Encounter in 2015

Or you can place them in any simulation: Add > Objects tab.


3. Improved Explosion Power

Universe Sandbox ² - Earth Moons Explode

We’ve improved both the visuals and the simulated effects for the explosion power.

As you can see in the screenshot above, using Explode on a body will send out a “shock wave” which will affect surrounding bodies as well, causing a chain of destruction. Realistic destruction, of course.


4. Simulation Quality Setting

You can now adjust a Simulation Quality setting in Home>Settings>General tab. This will adjust the maximum number of attracting and non-attracting bodies, which in turn will affect the number of particles/fragments produced and how long they last. Set it to “Auto” to determine the best settings for your hardware.

In the future we hope to have this setting adjust dynamically with what is happening in the simulation, in order to constantly find the best balance between performance and simulation experience.



We still have some bugs to sort through and some polish to add, so hold tight. We’ll let everyone know once Alpha 19 is available.

The laser, which is already part of the VR mode, is still being integrated with the additional surface detail feature. We expect these features to be ready for Alpha 20.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Follow us on Twitter and Facebook for the latest Universe Sandbox ² news.

VR is Now Available for Universe Sandbox ²

Universe Sandbox ² - Earth Explosion VR

For the past few months our highest priority has been adding in HTC Vive/SteamVR support for Universe Sandbox ². The team’s hard work has paid off: we now have a fully-functional virtual reality version which allows you to walk and fly around your simulations, melt planets with a giant laser, then grab them and hurl them off into nothingness with a flick of the wrist.

The best way to experience the amazing sense of scale and immersion in Universe Sandbox ² VR is to put on a headset and check it out for yourself. But if that’s not possible, then here’s a trailer for Universe Sandbox ² VR, featuring user-created VR gameplay videos courtesy of Brad Lynch and Count Lorek of BMcD Gaming.

VR is Free for Owners of Universe Sandbox ²

Universe Sandbox ² now includes both the desktop version and the new VR mode. So anyone who buys Universe Sandbox ² will also receive Universe Sandbox ² VR, and anyone who already owns Universe Sandbox ² can access the VR mode at no extra charge.

If you own Universe Sandbox ² on Steam and have access to a Vive, just run Steam to update. Then launch Universe Sandbox ² and select “Launch Universe Sandbox ² in Steam VR Mode” in the popup dialog box.

Universe Sandbox ² - Uranus VR

Lots of Love for Universe Sandbox ² VR at GDC

Thanks to Valve, we were able to demo our VR version of Universe Sandbox ² in the Steam booth two weeks ago at GDC, the world’s largest professionals-only video game conference.

We received great feedback from everyone who stopped by for a demo, with many agreeing that it was one of the most compelling VR experiences they had tried.

PC Gamer assistant editor Tom Marks said Universe Sandbox ² “was one of the coolest thing [he’s] tried on Vive so far.” Check out the video below for his discussion of Universe Sandbox ² VR:

Scott Manley, a popular YouTuber whose videos combine science and games, was one of the first to stop by for a demo:

Here’s a small write-up from Rock Paper Shotgun editor Graham Smith: Universe Sandbox ² Now Has a Virtual Reality Mode

In another article about VR, Graham mentions Universe Sandbox ² as one of the few upcoming VR games which he thinks “look spectacular.”

And here’s an older article from UploadVR, based on an interview with Dan, creator and project lead on Universe Sandbox ²: Literally Play Pool with the Planets in Universe Sandbox ² VR

Universe Sandbox ² - Planets VR

What’s Next for VR?

We still have work to do, but we’ve already got an enjoyable and impressive VR experience. Beyond adding polish to the interface, we also have to work on getting the best performance possible in order to keep up with VR requirements of 90 frames per second. With VR it’s crucial to meet this requirement in order to keep the risk of motion sickness at a minimum.

We’ll continue to work on the VR version as well as the desktop version, and we’ll release updates when they’re ready. We don’t support Oculus Rift yet, but we’re looking into adding this in once the Touch is available.

You can buy Universe Sandbox ² on our website or on the Steam Store.

Follow us on Twitter and Facebook.

Did We Say VR? We Meant RV! Cruise Around Space with RV Mode


RV photo credit: Greg Gjerdingen (CC License)

RV Mode is Coming Soon to Universe Sandbox ² (sorry, it’s RV, not VR mode)

First, we must apologize for this typo which was made many, many times in many of the posts here and on our promotional material. While we’re not sure who was responsible for editing these posts, we’re in the process of hiring someone now who can find the culprit and immediately fire them. We will then immediately fire this new employee as well.

So to be clear, we meant to say RV mode, not VR mode.  We’re talking recreational vehicles here, not virtual reality. Yep, RVs, those huge clunky things you can use to take the whole family on the greatest ever cross-country road trip, with no need for rest stops because inside of that RV you’ve got a bathroom, plenty of room to pace back and forth, and, if you’re lucky, even a satellite TV.

Step Inside the RV of Your Dreams… In Space

While a VR mode for Universe Sandbox ² does sound pretty cool (we’ll look into this), the RV mode which we’ve actually been focusing on for the past few months is something which we think you’ll agree is way cooler.

Just imagine: You load up a simulation of our solar system and engage RV Mode. Suddenly you’re sitting in the cushy front seat of your very own recreational vehicle, feeling like royalty on the Intergalactic Highway. You kick it into high gear and blaze through space at 60 miles per hour (that’s about 96 km/h if you’re in Europe), looking out the huge, panoramic windshield as you pass the gas giants of our solar system. What’s that, not interested in Jupiter’s moons? No problem, take a break and check out the kitchenette. Explore the mini-fridge. Explore the contents of the pot on the hot-plate. Are you picking up a signal on the satellite TV? If so, explore the 200+ channels of premium content. 

Of course, while making this RV Mode we haven’t forgotten about the spirit of the sandbox. So yes you can still experiment and get answers to fantastical what-if questions like, “What if I put the hot tub jets [yes you can unlock a hot tub] on full blast? How much sooner will I run out of gas?” or “What if I open the windows?” or “What if I drive this thing right into the sun?”

Why RV Mode for Universe Sandbox ²?

Good question. But have you considered “Why not?”

Truth be told, this may seem like it’s coming out of left field for a space and gravity simulator, but since day one it’s been part of the dream for Universe Sandbox ². Dan Dixon, creator and project lead on Universe Sandbox ², pointed out the obvious reason: “The driving force behind this project has always been about giving the player the freedom and tools to do whatever they want. And what embodies freedom more than cruising around the universe in an RV?”

So forget about exploring your Universe Sandbox ² simulations in full 3D and waving your hands around to make planets collide. The real frontier of next-generation gaming lies in the powerful and inspiring potential of the RV, the cutting edge of unwieldy, carbon-friendly (and family-friendly) automobiles. 

Again, sorry for all those typos.

Exploring Some Lesser-Known Features

Maybe you’ve noticed that there are a lot of features in Universe Sandbox ².  As we continue development, we work on ways to make all of these features discoverable and easy-to-use.

But some remain a little hidden right now. So here’s a list of some of the features we see commonly requested, and which are actually already in Universe Sandbox ².


1. You can fly the camera around your simulation using the WASD keys.

  • Use the WASD keys on your keyboard to freely move the camera around
  • Click and drag the mouse to point the camera in a different direction
  • You can use this to focus the camera on a point in your simulation instead of a specific body


2. Want to see moons and rings in our solar system? The default sim doesn’t have these because they tend to slow things down, but you can load sims which do.

  • Search for “moon” or “ring” in Home > Open to find simulations which have the moons and rings of our solar system
  • These sims require a lot of accuracy to keep moons and rings in tight orbits, so they tend to run slower than a standard simulation

Universe Sandbox ² - Jupiter Moons


3. Or you can add rings yourself to any body in a simulation.

  • Select a Body > Add > Rings tab > Select a preset > Add Ring

Universe Sandbox ² - Mars Rings


4. Universe Sandbox ² has a database of over 50 thousand known objects.

  • Use the search bar in the Add panel to find less common objects
  • Add > Search bar (top right)

Universe Sandbox ² - Search


5. You can land the camera on the surface of a body.

  • Click twice to focus on a body > Press C on your keyboard
  • Press C again to return to the normal camera view
  • Getting dizzy? Properties > Motion tab > Rotation Lock (bottom of list)


6. You can create randomly-generated solar systems.

  • Home > Create > Systems

Universe Sandbox ² - Random Solar System


7. Earth and Mars both have climate simulation.

  • Select Earth or Mars > Properties > Climate tab > Enable Climate
  • Learn more about how we simulate climate in our Climate post


8. Accidentally delete the sun? No worries, you can Undo!

  • Press Ctrl+Z on your keyboard to undo your last action
  • This is still a work-in-progress feature and does not undo every action


9. You can change the frame of reference for trails.

  • Select a body > Properties > Actions tab > Set as Trail Center
  • Return to default: Actions tab > Clear Trail Center
  • For example, instead of seeing how bodies in our solar system move in relation to the sun, you can see how they move in relation to Earth, as shown in the screenshot below

Universe Sandbox ² - Trail Centers


10. You can view and customize keyboard shortcuts for many commands.

  • Home > Settings > Input


11. You can hide the interface at any time for a more cinematic view.

  • Press Tab on your keyboard to hide the interface
  • Press Tab again to make it return


12. You can change a simulation’s starting date, have it start paused, and adjust other advanced starting parameters.

  • Home > [Open or Create] > Hover over a preset and click the pencil icon

Universe Sandbox ² - Starting Parameters


13. Bodies and trails can be colored according to their velocity, mass, or acceleration.

  • View > Colors

Universe Sandbox ² - Colors

Working Through the N-Body Problem in Universe Sandbox ²

Orbits of major planets and all possible dwarf planets in our solar system. (from Universe Sandbox ²)

Orbits of major planets and all possible dwarf planets in our solar system. (From Universe Sandbox ²)

What is the n-body problem?

The n-body problem can be defined as “the problem of predicting the individual motions of a group of celestial objects interacting with each other gravitationally.” Or, in a gravitational system of n bodies (where n can be any number), where will they all be after one year?

It’s helpful to frame this in contrast to the two-body problem, which looks at the motion of just two objects interacting with each other. For example, you can look at the Earth and Moon as a two-body problem. The Earth pulls on the Moon quite a bit, keeping it in orbit, and the Moon pulls on the Earth just a little bit.

The issue here is that the Moon is not affected gravitationally by just the Earth; it is also being pulled by the Sun, and Jupiter, and every other object in space. The same is true when looking at the Sun and Earth: the Sun is not the only object pulling on Earth. So to account for all of these gravitational forces, you need to use an n-body solution.

The problem of the n-body problem in Universe Sandbox ²

In Universe Sandbox ², every object is simulated as part of an n-body problem. Unfortunately, when solving for many objects, or n objects, you can’t just jump forward in time without getting massive errors. There’s simply no way around this. Solving an n-body problem requires calculating how each object affects each other object every step of the way. Errors will still happen, but taking smaller steps reduces them.

By default, the simulations in Universe Sandbox ² try to set an accuracy which prevents orbits from falling apart due to error. This means setting a maximum error tolerance for each step and also making sure the total error doesn’t reach an upper limit.

If you crank up the time step, the simulation then has to take fewer, larger steps. This means the potential for greater error. And the greater the error, the more likely it is that an orbit, which otherwise would be stable, falls apart. Moons crash into planets, Mercury gets thrown out of the solar system — things like that.

This isn’t what most people want in their simulations. But at the same time, most people also don’t want a limit on how fast they can run their simulation. This is a problem.

An imperfect solution

So how can we get around this problem? How can we accurately simulate thousands of objects while still allowing for large steps forward in time? For example, what if you wanted to simulate our solar system on a time scale of millions of years per second so that you could see the evolution of our Sun?

One solution proposed by Thomas, our physics programmer, is to allow for a special mode within simulations running at high time steps. This mode (which of course could be toggled) would collapse the existing n-body simulation into a series of 2-body problems: Moon & Earth, Earth & Sun, Europa & Jupiter, Jupiter & Sun, etc.

Solving a 2-body problem is much easier than solving an n-body problem. Not only is it faster computationally, but there is also a relatively arbitrary difference between figuring out where the two objects will be in one year and where they’ll be in a million years — it still requires just one calculation. So if you collapse an n-body simulation into a series of two-body problems, the simulation could take one big step forward, instead of taking the small steps needed for calculating it as an n-body problem.

The results won’t be entirely accurate, as this method would effectively ignore all gravitational influences outside of the main attractor. As mentioned before, calculating Earth’s orbit by looking at how it interacts with just the Sun is not accurate, as Earth is also affected by every other body. The Sun, however, is the most significant factor by far, because it is much more massive than any other object in our solar system. The other, much smaller forces tend to have little effect overall in non-chaotic systems. So while it’s not correct, it’s close enough when simulating something relatively stable like our solar system.

This isn’t a perfect solution. But we think it could be an improvement over the current system and its limitations, which leave you with the choice of either destabilizing the orbits with massive errors, or waiting days for the simulation to advance the millions of years needed for the Sun to evolve. Neither is particularly interesting.

When is this coming to Universe Sandbox ²?

Not anytime soon.

This solution is just a proposed idea right now, and is not a high priority for us, as we already have a big list of exciting features planned. But we think it is useful to understand the complexity of accurately simulating the motions of hundreds to thousands of objects interacting gravitationally.

This is especially a challenge when attempting to do this in real-time on a home computer, which is why researchers run numerical simulations on supercomputers which take days to complete.  With Universe Sandbox ², we’re exploring new territory and working through problems which haven’t been solved before. And this is a big part of why we love making it.