Energy and Heating | ScienceLog #1

Jupiter orbiting a mere 0.04 AU from the Sun, heating quickly under the intense stellar energy it receives at this distance.

It’s Getting Hot in Here…

One of the many important astrophysical processes that Universe Sandbox simulates is the changing temperature of an object as it is warmed by nearby stars and other sources of heat. Thanks to our new Surface Grids feature, introduced in Update 24, Universe Sandbox can now simulate the heating of each point on an object’s surface, to create a 2D map of a planet or moon’s surface temperature.

In addition to the Surface Grids simulation in Update 24, we also added new properties and tools related to heat and temperature in Update 25, so we wanted to take this opportunity to explain what makes planets get so hot (or cold!), and how you can use Universe Sandbox to explore the flow of energy through your objects.


Go with the Flow: Energy Flow and Temperature

So what makes the temperature of an object change? It all comes down to energy. An object like a planet or moon is continuously absorbing energy from its surroundings (like the heat from nearby stars) and radiating energy out into space. If the object is absorbing more energy than it is radiating away, that extra energy is used to raise the temperature of the object. On the other hand, if the object is radiating more energy than it’s receiving, that lost energy causes the object’s temperature to drop.

Universe Sandbox simulates the temperature of an object based on the flow of energy into and out of the object. You can see the data related to this “Energy Flow” in the Surface tab in the object’s properties panel. The first two properties, Energy Absorption Rate and Energy Radiation Rate, show the speed at which the object is gaining and losing energy. The Heating Rate tells you how fast the object’s surface temperature is expected to change based on this energy flow. If the object is absorbing more energy than it’s radiating, the Heating Rate will be positive, and the object will heat up. If it’s radiating more energy than it’s absorbing, the Heating rate will be negative, and the object will cool down.

Try experimenting with properties like the object’s Average Albedo or Surface Heat Capacity to see how they affect the energy flow rates and surface temperature (or check out our Energy Flow guide in Home > Guides > Tutorials > 14 – Energy and Heating).

The Earth in the Solar System, with the Energy Flow section displayed in its properties panel.


Heat Wave: Sources of Heat Energy

What are these sources of energy that can heat an object in Universe Sandbox? Energy from stars is the major source of heat in most simulations. These heat sources are directional: they only heat the part of the object’s surface facing the star. Heating from supernova explosions is also directional, not to mention extremely powerful.

The Earth, heated by a recently exploded Sun. The directional heating from the supernova causes the side of the Earth facing the supernova to receive all the heat energy. Eventually, the Earth absorbs too much energy, too fast, and it is vaporized away.

Other sources of heat come from all directions at once, or from inside the object, so the heat energy is evenly distributed over the object’s surface. For example, objects with atmospheres are heated by energy that the atmosphere radiates back down towards the surface. (This is the mechanism that causes the greenhouse effect leading to the climate crisis here on real-life Earth.)

All these contributions to the heating of an object are listed in the Energy Flow section, and can be seen by expanding the Energy Absorption Rate property (by selecting the list icon on the right side of the property).


Temperature Simulation in Two Dimensions

The properties in the Energy Flow section are used to estimate the change in the object’s Average Surface Temperature, a single value that represents the temperature of the object as a whole. The Surface Grids feature also allows us to simulate this energy flow and heating process at every point on an object’s surface. You can see the object’s 2D temperature map at the top of the Surface tab. Hovering over a pixel on the map will display the temperature at that point.

This temperature map is especially useful for seeing the effects of directional heating. For example, selecting Tidally Lock in an object’s Motion tab will change the object’s rotation period such that one side of the object always faces its star and the other always faces away from the star. If we tidally lock the Earth, the hemisphere facing away from the Sun will get so cold that the ocean freezes over, while the side facing the Sun gets uncomfortably warm. Even though the Earth as a whole is receiving the same amount of energy from the Sun, the conditions on the surface, simulated by Surface Grids, have changed a lot!

A tidally-locked Earth, spinning so slowly that one side always faces the Sun and the other always faces away. The “night” side gets so cold that it freezes over, while the “day” side continues to heat in the constant, direct sunlight.

This blog post is the first in our new series of ScienceLog articles, intended to share the science behind some of Universe Sandbox’s most interesting features. If you would love to learn about the real-life science powering our simulator, please stay tuned and let us know what you would like to read about next.

To join our community discussions, please join us on our Steam Forum and our official Discord community.

SPH Fluid Simulation | DevLog

Video: Simulating a planetary collision using a new method called smoothed-particle hydrodynamics (SPH).

Hopefully by now you’ve had time to check out Surface Grids & Lasers | Update 24 of Universe Sandbox. If you haven’t, time to get out from that rock you’ve been living under and start terraforming all those other rocks floating through space.

We plan to continue to add to the Surface Grids feature with even more detailed surface simulation through next year and beyond. Surface Grids is a massive new feature that changes a lot with the core simulation of objects in Universe Sandbox, and so far we’ve just scratched the surface of what it can do. We’re excited to explore its possibilities even more.

But right now, let’s turn our attention to something our physics developer, Alexander, has been working on. Introducing… smoothed-particle hydrodynamic fluid simulation. Let’s just call it SPH for now.

SPH is NOT included in Universe Sandbox yet. This is a behind-the-scenes look at a feature that we are still working on.


What is SPH and how does it work?

For a deep dive into the mechanics of SPH, check out this paper from our very own physics developer, written back in 2010 (interestingly, not written in relation to Universe Sandbox, but for another project that was similar in many ways — there’s a reason why we hired him many years ago to help build this new version of Universe Sandbox, and it had more to do with relevant experience than it did with his propensity for typos… *wink*).

Or if you’re curious about SPH, but perhaps not curious enough to read 35 pages on it, here’s a crash course:

SPH is a computational method commonly used for modeling fluids (though it can also handle solids). That might make you think that we’d use this for simulating something like water flow on a planet’s surface, but “fluid” here actually has more to do with simulating much larger objects.

On an astronomical scale, many of the objects you can simulate, like stars and galaxies, behave like fluids. This is also true for planets, whether it’s a gas giant or a rocky planet with, or even without, a molten core. And even in the case of large chunks of solid rock colliding with each other, there is such intense temperature and pressure that the materials behave more like fluid rather than rigid solids: they’ll stretch and distort and be torn apart, rather than splinter, crack, and shatter.

So in short, SPH will help create more detailed, realistic simulations of collisions, fragmentation, and formation of different types of objects in Universe Sandbox.

How? First, the material, such as a planet, is broken into a number of “particles” that each have properties such as mass, temperature, velocity, and position. You can see these particles clearly in any of the videos in this post.

But the “smoothed” part of “smoothed-particle hydrodynamics” means that these particles are just sample points of what is actually a continuous material, where they each contribute to the properties at a given point based on a weighted, smoothed, average. Together, they describe the properties that exist at any given point in a flow of material, but they themselves are not the material. Think of it like buoys in an ocean: the buoys will each monitor the properties at their location, and they are distinct from the continuous fluid, ie the ocean, that they are monitoring. So for the future of SPH in Universe Sandbox, the current debugging visuals, where you can see individual particles, will ideally be replaced by something that better represents the continuous fluid that is actually being modeled.

By tracking how each of these particles move, and more importantly how they move in relation to their neighbors, you can calculate pressure and viscosity (friction) at any point in
the fluid. And then you can estimate how this will move over time under different forces. Combine this with gravity and you start to see a simulation with emergent behavior that matches what we observe in real life.


Why SPH?

Because you get accurate simulation with emergent behavior, rather than disparate modeling of phenomena that needs to be stitched together. For example, with SPH, material will collect under the influence of gravity, but it will not all fall to the center of mass. Instead, as more material collects, the pressure increases and starts pushing out material, preventing a total collapse. The result is a spherical shape, and not because we specifically told it to become a sphere, but because that’s what happens when you simulate physics on a more granular level.

Or look at the case of Roche fragmentation, where a moon may be torn apart from the gravity of its host object “pulling” more on its near side than its far side. In our current simulation, pre-SPH, where we model how single points of mass move purely under the influence of gravity, we need special handling to calculate when and how this should happen, according to analytical models. But with SPH, this phenomenon just happens as the result of forces acting on the moon.

Why SPH specifically and not another method? When simulating space, there is more literal space than there is simulated material. SPH is great for handling cases like this where material is sparse. Other methods instead require simulating each point of space, seeing how each of these points changes (versus tracking only points specifically in a material), which would be very slow for anything like entire star systems.

Universe Sandbox is a unique physics simulator because we aim to make it an accessible, real-time, interactive experience. When compared to non-real-time simulations run on supercomputers, this presents a lot of limitations, and SPH is not immune to these. The biggest issue we will need to navigate as we continue development is the resolution of the model — to be really accurate and demonstrate smaller, local changes, you need a lot of sample points. But each point comes at the cost of a good chunk of computing power. So as with all features in Universe Sandbox, we’ll need to find a balance, with enough points to model things in interesting ways, but not so many that it becomes a slideshow.


So… what does it do?

Technical explanations are fun (…did I get that right?), but what you really want to know is what does this SPH thing mean for me and my planets? That’s also answered above: it will help create more detailed, realistic simulations of collisions, fragmentation, and formation of different types of objects in Universe Sandbox. But what you really, really want is a bunch of videos of this is in action. Understandable.

Quick disclaimer: SPH is a feature in its early stages of development. Visuals are for debugging purposes. Anything shown many not be representative of how it’ll appear and behave when included in an official Universe Sandbox update.

Two equal-sized bodies showing pulsating behavior as pressure and gravity tries to find a balance.

Two earths spinning the same direction and colliding. The result is a combined body with non-zero angular momentum from the individual momentums adding together in the same direction.

Increasing the density and speed of Mars before it impacts Earth and shoots right through it.

The existing simulation “Earth & Moon x25 Offset” showing all Earths collapsing and combining.

Results of the Moon fragmenting around Earth.

Want to see more? Check out the full video from our physics developer, Alexander, on YouTube

So in short, SPH will improve or make possible simulations of the following:

  • Total fragmentation
  • Tidal deformation and Roche fragmentation
  • Accretion disks / object formation from debris
  • Giant-impact hypothesis (moon formation!)

And in the longer term, we hope to apply it elsewhere, including more accurate galaxy collisions and star formation.


What’s Next

As you can see, SPH is already working pretty well within Universe Sandbox. But you can also see that it’s not exactly integrated with everything else yet. The visuals right now are intended solely for debugging purposes, and the transition from our standard planet visuals to the SPH particles is a little rough. Making visuals that look more like molten planets being torn apart will definitely take time, but we have some ideas in mind that we’re excited to explore.

The visuals are just one component of what we’ll need to work on to integrate SPH with Universe Sandbox. Making it work with other complex aspects of the simulation, like the new Surface Grids feature, will be its own can of worms. But we’re no strangers to technical challenges. And since we think SPH is worth experimenting with on its own, we hope to release an early version of it using the debug visuals and let you turn it on if you’re interested in checking it out. We don’t know when this will happen yet, but hopefully not too long into next year.

And hopefully before then, we’ll have a small update ready that will add some oft-requested color customization…


Saturn’s New Moons | Update 23.2

Run Steam to download Update 23.2, or buy Universe Sandbox via our website or the Steam Store.

Introducing the new Moon Champion of the Solar System, with a total of 82 known moons, it’s the great ringed gas giant Saturn!

Take a tour through the discoveries of Saturn’s moons, from the first discovered moon, Titan, in 1655, to the latest discovery of 20 new moons in October 2019:

Home > Guides > Science > History of Saturn’s Moons

With 82 moons, Saturn now has the most known moons, surpassing the previous record holder Jupiter and its 79 known moons.

This update also includes a refresh of our database and Saturn simulations to add its new moons, plus a few smaller fixes and improvements.

Check out a full list of What’s New in Update 23.2


Dark Matter & Galaxies in Universe Sandbox

You may notice that our new galaxy model (added in Update 23, released on June 25, 2019) no longer includes those bright red dots. The dots were how we represented dark matter in the old galaxy model (pre-Update 23), but we’ve decided not to include dark matter in the new model, for a number of reasons.

Short Explanation

Here’s the TL;DR explanation of why we removed dark matter in our new galaxy model:

Dark matter is a theoretical particle proposed to explain the unexpected motion of stars in galaxies. Due to performance constraints, our simplified galaxy dynamics model can’t simulate these complex orbits, so we’ve decided to remove dark matter from our simulations for now.

If you’re looking for a more in-depth explanation, keep reading!

Left: Spiral galaxy with dark matter (pre-Update 23). Right: Spiral galaxy in Update 23.

What is dark matter?

No one knows for sure what dark matter is, or even if it exists! But a number of different observations of our universe have revealed stars and galaxies moving under the gravitational influence of more mass than we can see. This hints at the presence of some kind of matter that affects stars and other bodies via gravity, but that can’t be observed directly. This proposed “dark matter” doesn’t produce light, but it also doesn’t block it, or we would be able to see it silhouetted against brighter stars and galaxies in the background (like we can see dust in the Milky Way).

We don’t know of a type of particle that has mass but that doesn’t interact with light, but a few ideas have been proposed. It may be a new type of particle that we haven’t discovered yet, and several ongoing experiments are trying to directly detect such a particle. Some scientists argue that dark matter does not exist at all, and that the “missing mass” in astronomical observations simply indicates that our mathematical description of gravity is not yet complete.

What does this have to do with galaxies?

Spiral galaxies were one of the first examples of the missing mass problem. Astronomers discovered the problem while calculating the “rotation curve” for these galaxies: a plot of the velocity of a star orbiting in the galaxy, versus the distance of that star to the center of the galaxy. The speed at which an object orbits in space is related to the mass of everything inside its orbit, and the distance to the center of the orbit. In the Solar System, nearly all of the mass inside a planet’s orbit is made up of the mass of the Sun, so the difference in speeds of planet orbits is due mostly to their distance from the Sun. Thus, the rotation curve of planets in the Solar System starts with the high speed of Mercury’s orbit, and then drops off as you move outwards to Venus, Earth, and the rest of the planets.

But in a galaxy, most of the mass is distributed among the stars that make up the galaxy, so stars farther from the center are orbiting more mass than stars closer in. We can estimate the distribution of mass based on the stars that we see, and predict a slightly more complicated curve: First, the velocities of orbiting stars should increase as you move away from the center, as more and more mass is enclosed by the orbit. But eventually, the extra mass inside the orbit won’t be enough to make up for the increased distance from the center, and the velocities will start to decrease again. The predicted curve has a sort of hump shape, with a long, decreasing tail.

Rotation curve of the galaxy M33. The yellow and blue dots indicate the data, while the dashed line represents the curve you would expect based on the amount of visible mass in the galaxy. Instead, the velocity increases with distance, indicating that more mass is present than we can see. Credit: Mario De Leo

But when astronomers actually measure these velocities and create rotation curves of spiral galaxies, the curves don’t drop off with distance. Instead, the velocities get faster and faster as you move outwards, with stars on the outer edges moving so fast that you would expect them to fly off, pulling the galaxy apart. One explanation for this discrepancy is that some kind of unseen mass (“dark matter”) may be present in spiral galaxies, keeping those stars gravitationally bound to the galaxy despite their high speeds.

Dark matter in Universe Sandbox

Since Universe Sandbox is at its core a gravity simulator, we tried to show the influence of dark matter in our previous galaxy model. For a given galaxy, we would calculate the distribution of dark matter that we would expect based on real observations of galaxy rotation curves. Specifically, we used what’s called the Navarro-Frenk-White (NFW) profile, after the astronomers who identified the distribution. We simulated the dark matter as points of mass scattered through the galaxy, and displayed them as bright red dots (because dark matter is invisible, we wanted to make it clear that we weren’t showing what dark matter “really” looks like!).

This model would give the “right” distribution of dark matter in a galaxy, but it couldn’t reproduce the most important feature of dark matter in galaxies: the rotation curve. This is because of the way that galaxy simulation works in Universe Sandbox.

How galaxies are simulated in Universe Sandbox

In both the old and the new versions of our galaxy model, we represent the galaxy as a collection of non-attracting particles orbiting a single attracting body, the black hole at the center. Each particle represents a cloud of gas, dust, and stars, which we call a nebula. This means that to our physics engine, the nebulae have zero mass, and the only gravity in the galaxy comes from the black hole.

But wait, earlier we said that the mass in a galaxy is spread out among all the stars in the galaxy, instead of being concentrated in the center like the Solar System. Why don’t we make all the nebulae into attracting particles? This would certainly make the motion of the galaxy more accurate, but in any gravity simulator, the number of attracting particles significantly affects performance. (You can see this for yourself by opening a simulation with a lot of attracting bodies, like Earth & 50 Moons.) To make galaxies look as good as they do, we need to use hundreds or even thousands of nebulae. A simulation with a thousand attracting particles would run extremely slowly even on a very powerful gaming computer. So instead, we used a simplified model of non-attracting nebulae orbiting an attracting black hole.

In the old version of galaxies, nebulae moved on circular orbits around the black hole, and the initial structure of a galaxy, whether it was a spiral or elliptical, would quickly lose its distinctive shape. In our upgraded version, nebulae are given specific orbits to allow the galaxy to hold its shape over time. The presence of another attracting body besides the black hole will pull the galaxy out of shape. (You can watch this happen in any galaxy collision simulation, or just by adding multiple galaxies to one of your own simulations!) During the development of this upgrade, we realized that adding attracting particles to represent dark matter would make it difficult to maintain the shape of spiral and elliptical galaxies for the same reason.

Because we are using a simplified galaxy model, we can’t reproduce the galaxy rotation curves we would expect either with or without dark matter. Instead, the rotation curves for our galaxies look more like the Solar System’s: the velocities of the nebulae drop off quickly as you move outwards from the center. Since this model can’t demonstrate the major effect of dark matter in galaxies, we decided to remove it for now.

We are hoping that a future version of galaxies will use computational methods like Smoothed-Particle Hydrodynamics (SPH) that will allow us to simulate hundreds to thousands of attracting nebulae orbiting the galaxy. This even more accurate model will be able to produce realistic galaxy rotation curves, and at that point, we’ll add dark matter back in so users can see its observable effect. In the meantime, we hope you enjoy our improved, interactive galaxy model!


Universe Sandbox at the American Astronomical Society Conference

Super Bowl of Astronomy

In early January we gathered some of our team in Seattle, Washington to show off Universe Sandbox at the 233rd meetup of the American Astronomical Society (AAS).

We’ve attended other conferences before that focus on video games, like PAX, but AAS gave us an opportunity to show Universe Sandbox to a different crowd. If you are a researcher, educator, science journalist, or student in the world of astronomy, then AAS is the go-to conference, what some call the “Super Bowl of Astronomy.” And while the government shutdown meant that hundreds of NASA employees who planned on attending couldn’t go, there was still plenty of folk there who had never heard of Universe Sandbox and wanted to learn more.


Come for the Collisions, Stay for the Accurate Mass Loss

Drawing people into our booth was helped a bit by two gigantic TVs showing off some of the usual Universe Sandbox scenarios — you know the ones: Earth melting, stars exploding, moons ripping apart under massive tidal stress.

But what made many attendees stick around and talk to us was the fact that what we were showing not only looked great, but it was also based in science. Universe Sandbox: Come for the fiery collisions, stay for the accurate mass loss when Ceres makes a near pass of a white dwarf!


Communicating with Universe Sandbox

In talking to AAS attendees, we hoped to show the potential for using Universe Sandbox for education and visualizations. While most Universe Sandbox players know and appreciate how useful it can be as an educational tool, we want to make sure it gets used in actual classrooms. We believe Universe Sandbox makes it quick and easy to demonstrate astronomy and physics concepts with intuitive and interactive experiments. But don’t take our word for it — here’s astronomy YouTuber Scott Manley with a similar message.

And beyond the classroom, it’s just as quick and easy to use Universe Sandbox for creating visualizations for research, lectures, and articles. There are more sophisticated tools for gathering data with the accuracy needed for research, but there’s nothing quite as convenient as Universe Sandbox for then using the data to create a visual representation, as shown here with the discovery of exoplanets around our nearby star Wolf 1061.

If you’re an educator, a researcher, or are otherwise curious how you can use Universe Sandbox for science communication, please get in touch!


New Year, New Limits of our Solar System for New Horizons

Happy New Year!

While we celebrate one more trip of our beautiful planet around the Sun, the spacecraft New Horizons sets a record for traveling to the most distant object in our Solar System ever visited, 2014 MU69, nicknamed “Ultima Thule.” This object is currently 1 billion miles beyond Pluto, or more than 43 AU from the Sun, which means it is more than 43 times the distance between the Earth and the Sun. New Horizons is expected to make its closest approach to Ultima Thule shortly after midnight EST January 1, 2019.

Check out the flyby in Universe Sandbox:

Home > Open > New Horizons Ultima Thule Encounter in 2019


New Limits for New Horizons

After the record-setting 2015 flyby of Pluto and its moons, the New Horizons spacecraft continued its journey through the outer reaches of the Solar System. In that same year, NASA selected a new target for New Horizons to observe: a Kuiper Belt object discovered by the Hubble Space Telescope the year earlier, known as 2014 MU69. Unofficially named Ultima Thule in 2018 based on a public vote, this object will be the most distant ever visited by a human spacecraft (breaking the record New Horizons itself set when it flew past Pluto).

The team says it hopes to set a new target for New Horizons once it passes Ultima Thule. With plenty of remaining fuel and equipment and instruments that remain in good condition, New Horizons is all set to head toward another distant object in the Kuiper Belt, arriving sometime in the 2020s, the team said.


Simulation Limitations

Simulations in Universe Sandbox are not perfect representations of reality. Rather, they’re meant to provide a visual — and as a result, a more intuitive understanding — of what is happening farther away than we can see or even imagine. With that in mind, there are a couple of limitations currently in this simulation:

1 –  Trajectory

The trajectory shown is according to the NASA Jet Propulsion Laboratory’s orbital predictions as of September 2018. Additional maneuvering with thruster burns is expected, which would change the final trajectory. New Horizons will make an approach much closer than is represented in the simulation: it should pass about 3,500 km from 2014 MU69. Once actual trajectories have been recorded, we will update the simulation.

2 – Shape

Previous observations show that 2014 MU69 is likely not spherical, but rather cigar-shaped. Researchers suspect that Ultima Thule may even be two separate bodies that are either orbiting very closely as a binary or actually touching each other, which is called a contact binary. We should know more once New Horizons sends back data from its flyby! Right now, Ultima Thule is represented in Universe Sandbox as just a single, spherical body.


Other Far Out Objects

Update 22.1 of Universe Sandbox added three other simulations that feature very distant objects in our Solar System.


1 – Voyagers 1 & 2 in Interstellar Space

In November 2018, more than 40 years after its launch, and long since trips past Jupiter, Saturn, Uranus, and Neptune, the Voyager 2 probe entered interstellar space. It now joins its twin, Voyager 1, in exploring beyond our Solar System. They are expected to continue to send back data until they run out of power in 2025.

Home > Open > Voyagers 1 & 2 Start 2019 Outside the Solar System


2 – 2018 VG18, “Farout”

On December 17, 2018, astronomers announced the discovery of the most distant known object in the Solar System, 2018 VG18. Nicknamed “Farout” (can you guess why they chose that name?), the trans-Neptunian object is currently around 120 AU (1 AU is the distance from the Sun to the Earth) from the Sun. While this object is the most distant ever observed, there are other known objects, like Sedna and the Goblin (see below), that have orbits that take them much farther from the Sun.

Farout’s orbit shown in this simulation is a preliminary estimate; its distance means it will take years of observation before its precise orbit is known.

Home > Open > 2018 VG18: The Most Distant Object in the Solar System


3 – 2015 TG387, “The Goblin”

On October 1, 2018, astronomers announced the discovery of the trans-Neptunian object 2015 TG387, which they nicknamed “The Goblin.” It was observed at about 80 AU from the Sun, but because of its extremely elongated orbit, it likely travels to a distance of more than 2300 AU at its farthest point.

Home > Open > 2015 TG387: A Goblin at the Edge of the Solar System


The Extremes of Our Solar System | Update 21.3

Run Steam to download Update 21.3, or buy Universe Sandbox ² via our website or the Steam Store.

This is a small update that features a new Parker Solar Probe model and new simulations exploring extremes in our Solar System:

Skim past the Sun with the Parker Solar Probe. The probe was launched in August and now has 24 trips around the Sun planned for its 7-year mission. Each year its orbit will take it closer to the Sun as its instruments capture data that will help us better understand our resident star. Its closest approach will bring it within 8.86 solar radii, or 3.83 million miles, of the Sun’s surface, more than 7 times closer than any previous spacecraft.

Home > Open > The Parker Solar Probe
Home > Open > The Parker Solar Probe’s Closest Approach to the Sun
Add > Objects > Parker Solar Probe

And ride along with New Horizons as it continues through the far reaches of our Solar System past Ultima Thule. After the probe’s flyby of Pluto and its moons in 2015, NASA selected the Kuiper Belt object Ultima Thule as its next target. When New Horizons makes its closest approach on January 1, 2019, Ultima Thule will become the farthest object ever visited by a spacecraft.  

Home > Open > New Horizons Ultima Thule Encounter in 2019


Plus: what if our Sun was replaced with one of the largest known stars in the universe, the red supergiant Betelgeuse?

Home > Open > Solar System with Betelgeuse Instead of the Sun

This update also includes an improvement to the accuracy of positions for moons and other objects in the Solar System Now & Real Time simulation, plus a few other smaller improvements and bug fixes.


Check out a full list of What’s New in Update 21.3

Please report any issues on our forums (local forum | Steam forum) or in-game via Home > Send Feedback.

See Asteroid 2012 TC4’s Trip Past Earth

On October 12, 2017, asteroid 2012 TC4 passed Earth early in the morning, flying above Antarctica at around 1:42am EDT.
See its close approach in Universe Sandbox ²:
Home > Open > Historical > 2012 TC4 Passes Earth on October 12, 2017
The roughly house-sized asteroid passed just a bit beyond the orbits of communications satellites, well within the Moon’s orbit. This wasn’t close enough to pose any threat, but its orbit was slightly changed by Earth’s gravity, as you can see in the GIF above from Universe Sandbox ². Learn more on NASA’s website
For the latest Universe Sandbox ² news, follow us on Twitter and Facebook.

Cassini’s Limitations in Universe Sandbox ²

It’s been two weeks now since we said goodbye to the Cassini spacecraft. After its 20-year-long mission in space, it was running low on fuel, so NASA directed it toward Saturn in order to eliminate the risk of it contaminating the moons. On September 15, 2017,  Cassini sent its final image back to Earth before disintegrating in Saturn’s atmosphere.

To commemorate this event, we released Ciao, Cassini | Update 20.2 which featured a simulation of Cassini’s final hours and collision with Saturn.

See Cassini’s final hours in Universe Sandbox ²:

Home > Open > Core/Historical > Cassini collision with Saturn on September 15, 2017
Home > Tutorials > Science > What Is Cassini’s Grand Finale?

Simulation Limitations

We had initially been wary of including a simulation of this event, as there are several limitations in Universe Sandbox ² that reduce the realism of this particular simulation. In the end, however, we decided it was still an informative and interesting visualization of an event that is very important to science and the world.

But instead of brushing these limitations under the rug, we’d like to take a closer look at them.

1 – Atmospheric Entry

We love collisions in Universe Sandbox ². But just because an object is on a collision course with a planet, that doesn’t mean it should actually collide with that planet in every scenario. If it’s small enough, then it should instead disintegrate in the planet’s atmosphere. This is what happened to Cassini as it entered Saturn’s atmosphere.

Universe Sandbox ² doesn’t yet simulate the forces associated with atmospheric entry, like atmospheric drag and aerodynamic heating. So instead, small objects will simply collide with planets. We’re very interested in simulating these effects in Universe Sandbox ², not just for objects like Cassini, but also for meteoroids that should burn up in an atmosphere, creating meteors or “shooting stars.”

Meteoroid meteor meteorite.gif
An animation from Wikipedia of a meteoroid losing material as it enters Earth’s atmosphere


2 – Rocket Engines & Spacecraft Flight

Cassini didn’t stay in orbit around Saturn for 13 years, weaving through its rings and doing close flybys of its moons, all by the grace of gravity. No, its trajectory was carefully planned by engineers and frequently adjusted with its rocket engines. But while Universe Sandbox ² has models of various spacecraft, it doesn’t yet include any way to simulate or control propellant. You can manually change an object’s velocity, but you can’t apply a force to it in the same way that a rocket engine would.

We’re still a while away from controlling thrust on spacecraft and flying them through simulations in Universe Sandbox ², but we’re taking steps in that direction with our work on rigid body physics. This new tech will better simulate the physics of smaller-scale objects, such as spacecraft, allowing for precise collisions and parts that can be attached with breakable joints and other constraints.


3 – Minimum Size for Impact Visuals

When the Shoemaker–Levy 9 fragments collided with Jupiter, observers from Earth could see marks on Jupiter that were more easily visible than the famous Great Red Spot. Some of these fragments were larger than one kilometer in diameter. The Cassini spacecraft, on the other hand, was only about 6.8 meters high and 4 meters wide. Its disintegration in Saturn’s atmosphere should not be easily visible, and even if it were to collide with Saturn instead of disintegrating, it should not leave a large impact mark. [Our Cassini update added a quick fix for this problem to disable an impact mark for its collision.]

Unfortunately, in Universe Sandbox ², impact marks have a minimum visual size. This minimum size is much larger than it should be for small asteroids and human-sized objects. The alternative is to not show any impact mark at all, but that can also be unsatisfactory. There are technical reasons for this limitation, but let’s not worry about those, because there’s good news…

We are currently working on a new system which will allow for impact marks and craters of any size, no more minimum. And on top of that, the new system will be a lot faster, too, and scale to the system resolution, which should help lower-end hardware a lot.

Work-in-progress screenshot of the new impact visuals in Universe Sandbox ²/span>

Opportunities, Not Limitations

The first rule of self-criticism (which we didn’t follow above) is that it’s necessary to throw things in a positive light. Drawbacks, weaknesses, limitations — they’re all opportunities for improvement! Often times these simulations of historical events are great tests of the simulation. When we put in the known properties and velocities for a set of bodies, does the simulation play out similarly to how it did in observed reality? If something is different, how can we improve the simulation in order to achieve the same results?  But if instead it’s remarkably similar, as it sometimes is, then we can give ourselves a nice pat on the back and sit back and relax… Until we load a different simulation and notice another opportunity for improvement. We often say that building a universe simulator is never a completed job. A big chunk of that lies within our commitment to continuously notice where we can improve, and then take the necessary steps to make these improvements.

The above items are all on our to-do list, though we can’t promise when they will be added to Universe Sandbox ². Learn more about what we’re working on in our 2017 Roadmap.

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

Ciao, Cassini | Update 20.2

Run Steam to download Update 20.2, or buy Universe Sandbox ² via our website or the Steam Store.

November 3: Updates 20.2.3 and 20.2.4 are small updates which add two sims from the latest Vsauce video as well as a few improvements and bug fixes.

November 2: Update 20.2.2 is a minor update which includes a number of improvements and bug fixes.

October 5: Update 20.2.1 a minor update which adds a Cassini spacecraft model, Quicksave & Quickload (F5 & F9), and a number of smaller improvements and fixes.


On September 15, 2017, the world says goodbye to the Cassini spacecraft as it ends its historic mission with a final plunge into Saturn.

During its 13 years orbiting Saturn, Cassini made a number of invaluable discoveries about the planet, its rings, and its moons.

We now know that massive geysers covering the south polar region of the moon Enceladus shoot icy particles into space, forming most of Saturn’s E-ring and hinting at a massive, subsurface ocean. And we now know that the surface of Titan, Saturn’s biggest moon, shares a surprising number of characteristics with Earth, including dunes, mountain ranges, rivers, lakes, and seas.

With its array of sophisticated instruments, Cassini watched as storms raged on Saturn and seasons changed. It discovered eight new moons, provided insight into the behavior of its famous rings, and completely changed our understanding of its magnetosphere. The mission has been extended twice and now Cassini has been in orbit nine years longer than originally intended. Its fuel is nearly gone and so, to prevent possible contamination of any of the moons, its course has intentionally been set for disintegration in Saturn’s atmosphere.

So now we say ciao, Cassini. Thanks for all of your work.

And thanks to all of the hard-working scientists from NASA, the European Space Agency, and the Italian Space Agency who made the Cassini mission possible.

See Cassini’s final hours in Universe Sandbox ²

Home > Open > Core/Historical > Cassini collision with Saturn on September 15, 2017
Home > Tutorials > Science > What Is Cassini’s Grand Finale?

This update also includes a number of smaller improvements and bug fixes.

Check out the full list of What’s New.


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