Roche limit

The Roche limit is an important idea in space science that helps us understand when objects in orbit break apart. It tells us the distance from a big space body, like a planet or star, where a smaller object held together only by its own gravity will start to fall apart. This happens because the big body's gravity pulls on the smaller one in a way that stretches and tears it apart.

When a smaller object gets too close to the big body, the big body’s gravity pulls harder on the side that is closer to it. This difference in pulling force is called tidal force. If the tidal force becomes stronger than the smaller object’s own gravity holding it together, the object can break into pieces.

Inside the Roche limit, these pieces of material spread out and often form rings around the big body. Outside the Roche limit, the material can come back together and form a solid object again. The exact distance of the Roche limit depends on how big the smaller object is and how dense both objects are.

The idea of the Roche limit was first worked out by a French astronomer named Édouard Roche in 1848. It helps scientists explain why we see rings around some planets and why some objects in space break apart when they get too close to a large body.

Explanation

The Roche limit is the distance from a big space object where a smaller object, like a moon, can break apart. This happens because the big object pulls on the smaller one more strongly on the side that is closer to it. If this pulling force is stronger than the gravity holding the smaller object together, the smaller object can break up.

Some moons and artificial satellites can stay together even inside their Roche limit if they are held by forces other than gravity. But weaker objects, like comets, can break apart when they come too close to a big object. Because of this, most planetary rings are found inside the Roche limit of their planets. However, some rings come from material released by moons due to volcanic activity or impacts. Other reasons comets might break apart include heat, pressure inside them, or spinning too fast.

Determination

The Roche limit is the closest distance a small object can orbit a big object without breaking apart. This distance depends on how strong the small object is. A very strong object might stay in shape until tidal forces pull it apart. A very weak, loose object might stretch and break apart more easily.

For a strong, solid object, we can make a simple guess for the Roche limit. This guess works best for round objects and ignores some details to keep things easy. The guess shows how far the small object can be before pieces start to pull away from it.

For weaker objects, like those held together loosely, the Roche limit is a bit different. One real example is a comet. When comet Shoemaker–Levy 9 got too close to Jupiter, it broke into pieces because it was inside Jupiter’s Roche limit.

Modern revisions and exceptions to the Roche limit

A 2023 study of the rings of Quaoar suggested that some common ideas about the Roche limit might need updating. The study used observatories and telescopes such as CHEOPS, Instituto de Astrofísica de Canarias, the High Energy Stereoscopic System in Khomas-Namibia, and Reedy Creek Observatory.^: 239 

Usually, the Roche limit is thought to be the distance where material can no longer stay together and forms rings. However, Quaoar's rings are much farther out than where rings are expected to exist. One idea is that the rings are made of icy particles that bounce off each other and stay moving instead of sticking together. Another idea is that Quaoar is not perfectly round, which changes the gravity around it and helps keep the rings from forming larger objects.^{: 239  : 242  : 241} 

Instead of just depending on tidal forces, the study suggests that the energy in Quaoar's rings is influenced by what the particles are made of and how they move around Quaoar.