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Teach Celestial Collisions
with Universe Sandbox ²

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Theory

Celestial Collisions

Today we will take a look at celestial collisions (also known as celestial impacts). What would happen if the Moon hit the Earth?

Teacher Resources

This lesson has the students try out celestial impacts in the simulator - i.e. objects in space hitting each other. This phenomenon will be looked at in detail, and has the added bonus of teaching the students the workings of the simulator simultaneously.

  • Theory: This lesson familiarizes your students with celestial collisions and provides a quick introduction to orbital mechanics.
  • Play: The students complete tasks in the game that focus on celestial collisions.
  • Share & Discuss: These topics focus on orbital mechanics, such as tidal forces and the Roche limit.

Play

Tasks #1

To complete today's tasks, you will need to open specific simulations. How do you search for simulations in Universe Sandbox 2?

Show Notes

In the main menu, select "Open", type what you are looking for and press enter. Alternatively, you can browse through the different categories manually.

To complete these tasks, repeatedly play the slow motion impact Earth & Moon simulation (In the main menu, select "Open", then "Collisions" and finally "Earth & Moon I Slow Motion"). Click on Earth and look at its data to complete these tasks.

What happens to the oceans? Why? What is the boiling point of water?

Show Notes

The oceans are evaporated due to the extreme heat that occurs from the impact. Since the boiling point of water is 100 degrees Celsius and the impact causes the temperature to climb up to 4000 degrees, all of the oceans are completely evaporated. Where does this heat come from? Since the impact stops the speed of the Moon, the energy is transferred into heat, causing this extreme heat.

Is life possible on Earth after the impact?

Show Notes

No. To see this statistic in-game, select the planet’s materials tab and scroll down to the life likelihood value.

Tasks #2

To complete these tasks, select the Earth & Close Moon simulation (underneath the “physics” tab).

Look up the concept of tidal forces online. How do they work?

Show Notes

The tidal force is a force that is the secondary effect of the force of gravity; it is responsible for the phenomenon of tides. It arises because the gravitational force exerted by one body on another is not constant across it: the nearest side is attracted more strongly than the farthest side.

Look up Roche limit. What does it mean?

Show Notes

It is “the distance within which a celestial body held together only by its own gravity will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.” (Source)

How does the Roche limit relate to this simulation?

Show Notes

In this simulation, the Earth’s tidal forces exceed the Moon’s own gravitation and the Moon is within Earth’s Roche limit. This is why the Moon is shattered and the fragments enter Earth’s orbit (some fragments actually impact the planet but most will simply circle Earth).

How does gravity work and how does it influence Earth and Moon in this specific simulation?

Show Notes

All matter has a gravitational pull, and the greater the mass, the greater the field of gravity that the object has. Though gravity has a limitless range, its effects become drastically weaker the further away objects are - this is why in real life the Moon is in the sweet spot where it orbits the Earth i.e. is affected by Earth’s gravity, but the Moon is not within Earth’s Roche limit so the Moon is not torn apart by Earth’s tidal forces.

Tasks #3

To complete these tasks, select the Earth & Two Close Moons simulation (underneath the “physics” tab).

How does the Roche limit work here?

Show Notes

Essentially the inner moon is within Earth’s Roche limit, whereas the outer moon is not. The fragments that are created from the destruction of the inner moon end up in different places: some will hit Earth, some will orbit it and others hit the outer moon.

Some of the inner moon’s fragments hit the outer moon, some Earth. Why?

Show Notes

Even though the Earth is a greater mass than the outer moon, the outer moon has a stronger gravity in its own vicinity in relation to the gravity that the Earth projects over there. This means that fragments closer to Earth crash into it, while ones closer to the outer moon crash into it.

The outer moon continues to orbit Earth. Why? Why isn’t it destroyed?

Show Notes

Because the outer moon remains outside Earth’s Roche limit but is still close enough to Earth to be affected by its gravity.

Experiment with the different presets in Universe Sandbox 2 in the physics tab. Try launching different-sized objects (bodies) towards Earth. Can life survive smaller impacts?

Try building your own solar system!

Show Notes
  • To launch objects at Earth, start with the solar system (under the “core” tab) and then select (on the bottom) add and launch.
  • To see whether or not Earth is still livable after launching something at it, select it and look at its statistics. In the materials tab, you can see the life likelihood statistic - if the value is above zero, life is still possible (just not necessarily human life!).
  • Building your own solar system is a little complex and will be an activity on later lesson.

How to Play

As you launch the game, you can immediately start playing with the Solar System.

Gameplay screenshot

To access other simulations at any time, press Esc. Then, select Open to load any of the simulations included in the game.

Gameplay screenshot

After you've clicked Open, click on any of the simulations to load them.

Gameplay screenshot

Share & Discuss

Share & Discuss

What are tidal forces?

Show Notes

The tidal force is a force that is the secondary effect of the force of gravity; it is responsible for the phenomenon of tides. It arises because the gravitational force exerted by one body on another is not constant across it: the nearest side is attracted more strongly than the farthest side.

What is the Roche limit?

Show Notes

“The distance within which a celestial body held together only by its own gravity will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.” (Source)

What happens when objects hit planets such as Earth?

Show Notes

This depends on many factors, such as size, mass and composition of the object that strikes Earth. With sufficiently large objects, such as the Moon, the enormous amount of energy (the Moon’s mass and speed) is released as heat when the object is stopped. This heat is great enough to have severe consequences, melting the surface of the Earth, killing all life and evaporating the oceans. Smaller objects can have severe consequences as well, for example the asteroid that ended three quarters of life on Earth as it hit, ending the Cretaceous period.