The Roche Limit

Introduction

The Roche limit is what I like to think of as a death threat to celestial objects. You’ll see why at the end of the article.

What is the Roche limit?

The Roche Limit was first conceptualised by the French astronomer Édouard Roche, he is mainly known for his contributions in celestial mechanics.

To start, I’ll illustrate the idea with an example, I feel like that will be a lot more conducive towards understanding what this actually is. Despite the example being milked so much, I still feel like it’s a great example.

Let’s take the planet Saturn. Saturn is most probably known for the rings that surround the planet.

… Why do these rings even exist? We know for a fact that Gravity likes to clump things together in space, that’s exactly how the star lifecycle advances in the first place and it’s also the reason why many celestial objects are nearly perfect spheres (Not exactly but it’s a pretty good approximation). So why doesn’t Gravity just clump these rings together to form a spherical object?

Well, it’s because of the Roche limit! The Roche limit is essentially this distance from a large massive body which a satellite can enter without being affected by the tidal forces of another body.

What are tidal forces? Tidal forces are essentially forces that “stretch” a celestial object. Tidal forces are the reason for tides.

Let’s take a look at the earth and moon for example.

The moon’s gravitational force pulls on the earth. As only one side of the earth, roughly speaking, is in line with the moon, only one side is going to face a greater force. As a result, one side of the earth is sort of compressed while the other side expands. Along with the fact that the earth rotates on its own axis, we experience high tides and low tides. The part of the earth which expands faces high tides while the part that is compressed faces low tides.

We essentially go through the high tide sections and low tide sections twice in one rotation, hence there are two high tides in a day, and two low tides in a day. The Moon also experiences tidal forces due to the Earth. The reason it stays a Moon is because its gravity kind of “counters” the tidal forces

So what does this have to do with the Roche limit? Well this idea of tidal forces is experienced by every planetary system. Let’s go back to Saturn. If we were to imagine that Saturn had no rings and we placed a moon there, far from its roche limit, there would be tidal forces acting on the moon, its gravity however would again counteract the tidal forces.

However, as you move the moon closer to Saturn, the tidal forces start becoming much larger than its gravity, this causes the moon to be stretched out and compressed in all directions which would basically cause it to explode. If you go past the roche limit, closer to Saturn, these exploded particles kind of compress into these rings.

Shoemaker Levy

Shoemaker-Levy 9 acts as another example of how the roche limit affects celestial objects. This comet was observed colliding with Jupiter in 1994 (This collision was what led to us realising that Jupiter is acting as a barrier of sorts to prevent comets from reaching us, thanks big guy!)

However, the observed comet was kind of broken down and fragmented, this was because the comet had previously gone beyond Jupiter’s roche limit, thus the tidal forces had broken up the comet into smaller fragments.

Why don’t humans disintegrate then?

Technically we’re way past the Earth’s roche limit, so why don’t we just disintegrate as a moon would? Well, we’re extremely tiny compared to the Moon. Hence, we can’t really be stretched or compressed because there isn’t really much of a difference between the gravitational force all throughout our body.

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