Why Do Asteroids and Comets Break Up?

As an asteroid comes crashing down into Earth, it burns up in the atmosphere. It is destroyed by the heat produced by the friction between the asteroid surface and the atmospheric particles. But that is not the only way that an asteroid can break up. In this article, let’s explore how asteroids and comets, the minor planets in the Solar System, can get shattered into pieces (or even completely disappear).

Obviously, a planetary impact is one way that an asteroid can break up. In fact, it’s one of the most violent ways that it could be destroyed, as it doesn’t just break apart into a few fragments. It destroys the asteroid to the point that only traces of the physical process are left, or maybe it disappears without a trace at all (the chemical footprints are still there, but they are too dispersed in the atmosphere to detect).

What happens to the traces of the asteroid often depends on the size of the asteroid and the thickness of the atmosphere of the object being impacted. If the planet involved does not have an atmosphere (like Mercury), the object can travel straight to the surface. The object is destroyed, but the sheer force of the impact also displaces material near the impact site, creating an impact crater. That’s why objects with no atmosphere have so many impact craters.

But if the planet has an atmosphere (like Earth), the asteroid must pass through the atmosphere to make it to the surface. And it doesn’t always do so, as the heat produced by the friction between the atmospheric particles and the asteroid’s surface can overwhelm the material of the asteroid and break it up into smaller fragments, or even burn them up completely. But if an asteroid is large enough, it makes it to the surface even though it needs to pass through an atmosphere, as there would be too much material to burn up.

But other than planets, there are other asteroids orbiting the Sun in the Solar System. Therefore, collisions between asteroids should be possible. And while it is theoretically possible, realistically it happens very rarely these days, as the asteroids are dispersed across the Solar System with a very low density. But in the past when the Solar System was just forming, this often happened. These collisions, if they occur at high speeds, can obviously destroy the object via very obvious physical means. But if these collisions occur at low speeds, asteroids can form contact binaries (where two objects become attached without destroying each other), and is the basis of planetary formation via accretion.

But impact is not the only way that an asteroid or comet can get destroyed. Sometimes, it can also spin so fast that the centrifugal force overcomes the gravitational attraction of the asteroid, causing the object to fragment.

One reason that the asteroid may spin too quickly is that it is constantly being pushed by the reradiations on the surface of the asteroid. When the asteroid receives sunlight, it heats up, and some of the energy is reradiated back into space as blackbody radiation, or reflected back into space directly. Photons carry a small amount of momentum, so as the light gets reradiated, the asteroid is actually pushed a little bit. Over time, this results in a change in angular momentum of the asteroid, and can thus alter its rotation.

But a prerequisite for the YORP effect to apply is that the asteroid must be of irregular shape. If it is a perfect sphere, all the reflected or reradiated light rays would intersect the center of mass, if we extrapolate them indefinitely. This would not cause any change to the momentum. Therefore, irregularities on the asteroid’s surface are necessary for the YORP effect to apply — and that’s also a significant characteristic of almost all asteroids in the Solar System.

The YORP effect can cause all kinds of changes to an asteroid’s rotation, from spinning down to making it tumble with irregular rotation patterns. But what we’re interested in is when the asteroid spins up. For each object, there’s a limit to how fast it can spin before the centrifugal force starts to overpower the gravitational force and ejects mass out of the object. For some combinations (involving surface patterns and the axis of rotation, and so on), the YORP effect can actually spin the asteroid up to the point where the rotational speed exceeds this limit, causing it to break up into many pieces. This is especially prevalent in rubble-pile asteroids, which are nothing more than loosely bound collections of rocks of different sizes held together by gravity.

This can even cause comet-like activity in the asteroid, making a spectacular sight in the images of the object.

Examples of asteroids breaking up likely due to rotational spin-up includes:

1. 6478 Gault
2. 311P/PanSTARRS
3. 331P/Gibbs

What we’ve been talking about so far is mostly about asteroids. But what about comets? Cometary activity can also result in the destruction of a comet. Comets are abundant in volatile materials (ices), that can evaporate fairly easily as comets approach the Sun. Obviously, they are also vulnerable to impact-related and rotational break-ups, the really interesting prospect is that comets might be particularly fragile due to its composition.

As a large proportion of a comet’s interior is volatile material, it’s easy for these materials to vaporize when the comets are close to the Sun. Oftentimes, there are vents on the surface of the comet nucleus to send these gases off to space, making up the coma and the dust tail of the comet. But if some of these gases cannot be released, the gas pressure instead builds up inside the comet. When the internal pressure overcomes the forces holding the surface of the comet together, it can result in an explosive release of these gases, also known as a cometary outburst. In some cases, cometary outbursts can send large fragments of a comet off into space (sometimes with sizes comparable to the original comet nucleus itself), considering that comets might already be more fragile than most asteroids on the inside.

There are many cases of these comets splitting up near perihelion without any encounters with massive planets. An example is 73P/Schwassmann–Wachmann, which began fragmenting in 1995. There are some comets that have been lost after being seen breaking up as well, such as the comet 3D/Biela.

Moreover, tidal forces experienced by objects in close approaches can also cause them to disintegrate. Specifically, the object might reach its Roche limit with the planet or star as it passes close by it. This is the limit where the tidal forces acting on the object — the difference between the gravitational forces on each side of the object, is equal to the gravitational and rigid-body forces binding the object together. Once the object gets closer to the massive body after reaching the Roche limit, the tidal forces become too strong to handle, causing the object to split up.

There is a type of comet called sungrazing comets — comets that approach very close to the Sun at perihelion, getting within millions if not thousands of kilometers of the Sun’s surface. At this distance, the comet experiences very strong tidal forces. Coupled with the pressure of the volatile vapors trying to escape, sungrazer comets are very fragile and often do not survive the trip that takes them close to the Sun’s surface.

In the impact cases where the asteroid gets completely destroyed, it leaves its mark as an impact crater or as negligible chemical footprints in the atmosphere of that planet, without sending any significant fragments into orbit. However, in other cases where the fragments are ejected at low speeds, such as from rotational spin-up, this can result in very interesting patterns between the fragments of these objects.

If the object is elected at a velocity that allows it to go into orbit (but not escape the gravitational field of the asteroid), the fragment could orbit the asteroid. This results in a binary asteroid configuration, where two asteroids orbit each other.

But if it is ejected fast enough that it could just escape the gravitational field of the original asteroid, the fragments would orbit the Sun in similar orbits. In that case, the group of asteroids (at least when the group size is 2) is called an asteroid pair.

And when the group size is larger, due to the breakup involving a cometary outburst instead of rotational spin-up or other factors, it could even lead to a meteor shower. Thousands of fragments orbit the Sun in very similar but slightly different orbits, and over time the slight differences in the orbital period can scatter the small fragments all across the orbit, creating an elliptical of these fragments in the Solar System. When these belts intersect Earth’s orbit, Earth will pass through a lot of these small fragments every year, causing meteor showers. The fragments are too small for the event to be considered a major break-up or disintegration — but it’s a very significant observation coming from a similar phenomenon.

In this article, we’ve explored the reasons why asteroids and comets might break up and disintegrate. Despite seemingly being rigid bodies composed of solid material, they could actually be quite fragile, especially if the object concerned is a rubble-pile asteroid. These objects might break up due to impact, due to rotational spin-up, cometary activity, and also tidal forces, creating a wide variety of interesting sights in our Solar System, such as binary asteroids and asteroid pairs. While it is difficult to conclusively determine the cause of breakup for asteroids and comets (and each case requires a separate analysis), those reasons are some of the main factors causing these objects to disintegrate.