Chapter 8: Interplanetary Bodies

A photograph of Halley's comet taken in 1986.

One ongoing theme in astronomy is the realization that observationally different celestial events are actually tied to a single phenomenon. One of the earliest examples may be the "discovery" that the morning star and the evening star are both actually apparitions of the planet Venus. A few more interesting examples are the unification comets, meteor showers, the Tunguska event, and maybe even the end of the Bronze Age! In this example, comets are seen in the sky; they sometimes leave behind trails of dust and particulate in the Earth's orbital path that can cause meteor showers, and occasionally comets (or at least chunks of comets) can enter our atmosphere. We suspect that a chunk of a comet is responsible for the Tunguska event in 1908 over Siberia, and there is also some research that indicates that it is possible that comet Encke broke up several thousand years ago, and a chunk of this may have hit the fertile crescent, forming Umm al Binni Lake and bringing to an end the Bronze Age (this is highly speculative research, and still falls in the category of really neat, but unconfirmed). It actually turns out that sky phenomena and planetary devastation are likely related to small interplanetary bodies, including not only comets but also asteroids.

 

This is something of a revolutionary change in thinking. A generation ago, scientists considered interplanetary bodies only a minor curiosity. Today, we're realizing that they affect planet histories in general, and the evolution of life on Earth in particular. Our presence on Earth depends in large part on a history of impacts by interplanetary debris. Beyond just helping us understand the periodic extinctions that have occurred throughout history, these bodies also contain many clues to help us learn about the origin of our solar system.

Asteroid Ida with its tiny moonlet Dactyl

Interplanetary bodies range in composition from icy to rocky and metallic. The exact name an object has depends on both its composition and its orbit. For instance, icy objects with trans-Neptunian orbits or orbits beyond Neptune are Kuiper Belt Objects while icy objects that plunge through the inner solar system are called comets. On the other hand, rocky and metallic objects are generally called asteroids, or more specifically called (among other things) Near-Earth Objects or Main Belt Asteroids based on if the orbit is in the asteroid belt or the inner solar system.

 

Gerard Kuiper

Diagram of the Oort Cloud

As with so many words, there are historical reasons for these names. When the Sun warms the ices of a comet, the ices change directly from solid into gas and evaporate away into space, or sublime. This gives comets their fuzzy, luminous "tail" of gas and dust particles. To ancient people, the tail looked like long hair blowing in the wind. The name "comet," therefore, comes from the Greek word "coma," for hair. The less excitingly named Kuiper Belt objects are named after their discoverer, Gerard Kuiper.

An asteroid, however, has no gas or tail and appears in a telescope like a faint star. Its name derives from the Greek root "aster," for star. But the light we see from an asteroid, just like from a comet, is all reflected from the Sun — they are cold chunks of rock and ice that emit no light of their own.

Until the last century or so, comets and asteroids were considered completely different phenomena. They're made of different materials, and they orbit on very different paths through the solar system. But today we realize that both comets and asteroids are examples of interplanetary debris left over from the period of planet formation. Smaller bits of leftover debris (both icy and rocky) are also related. This debris comes in all sizes, from microscopic grains to bodies a few meters across.

As the Earth orbits the Sun, it periodically collides with some of this debris. When a piece of space debris hits the atmosphere or surface of a planet, it's typically traveling at 10 to 40 kilometers per second (about 22,000 to 90,000 miles per hour!). Since kinetic energy is proportional to velocity squared, a massive object has tremendous kinetic energy. What happens next is an excellent example of the transformation of energy from one form to another. Hitting a planet's atmosphere at this speed creates friction between the object and the air (Remember, the frictional force is also related to velocity). The friction causes the projectile to slow down and heat up. In this process, its kinetic energy is transformed into thermal energy. (The melted rubber when you brake hard and your car skids to a stop is another situation where kinetic energy is turned into heat.) The incoming object heats up and begins to glow. Due to the shock of hitting the atmosphere and the sudden increase in temperature, it may break into many pieces. This is also why a spaceship re-entering the atmosphere from orbit heats up, making re-entry a critical and dangerous procedure.

156 bolides were detected on a single (pointed) photographic plate of the all sky fish-eye photographic camera during the Leonid meteor shower in 1998 at Modra observatory. The exposure time was 4 hours.

Scientists distinguish between the pieces of interplanetary bodies that reach the Earth's atmosphere and the fraction of them that actually hit the Earth. Meteors, or so-called "shooting stars," are typically pea-sized and smaller particles that burn up in the atmosphere and do not hit the ground. Meteorites are larger rocky or metallic bodies, or pieces of them, that survive passage through the atmosphere and hit the ground. Thousands of meteorites have been collected and studied, many of which you can see in museums and planetariums. They are free samples of the distant reaches of the solar system. The words meteor and meteorite come from the same root as the word meteorology, or the study of weather. For hundreds of years, people thought that shooting stars were purely terrestrial phenomena that originated in the Earth's atmosphere.

 

When scientists study a meteorite, they recognize that it is just a fragment of something larger and try to deduce the nature of the object it came from. This larger object is called the parent body. Studies of meteorites prove that most of their parent bodies are asteroids (but occasionally, they have actually originated from the Moon or Mars). Asteroids collide with each other, as well as with planets, throughout geological time. The biggest collisions disrupt the asteroids and leave fragments drifting in space. Some of these fragments are perturbed onto paths that cross the Earth's orbit, and a few eventually reach the ground as meteorites. By studying both the mineral composition of meteorites, as well as the ratios of any gases trapped inside the meteorite, it is sometimes possible to determine a meteorite's parent body.

The inner solar system.

Asteroids are generally concentrated in the region between the orbits of Mars and Jupiter. This group of asteroids is called the main belt. Most comets travel originate either in the Kuiper Belt or in the much more distant Oort Cloud. Most meteor storms are caused by the bits of debris dislodged from comets and left behind as they sweep through the Solar System. The debris is strewn unevenly along the entire path of the orbit so when the Earth crosses the path it creates a meteor shower at the same time each year. The unevenness of the debris in space means the intensity of the meteor shower is variable and hard to predict.

 

The comet and asteroid parent bodies, along with the pieces of them that reach the Earth, can trace their origins to the birth of the Solar System 4.6 billion years ago. As the planets formed, the Solar System was filled with innumerable small, pre-planetary bodies, ranging up to 1,000 kilometers across. "Planetesimal" is a generic term used to refer to these pre-planetary bodies, without specifying whether they are icy or rocky. Thus, comets, asteroids, and their fragments all descended from the original planetesimals that formed the planets. The Sun contains 99.85% of the mass of the Solar System. Jupiter accounts for 0.1%, and all the other planets together are another 0.04%. All the various interplanetary bodies amount to no more than 0.01%, or 1 part in 10,000, of the Solar System mass. Yet they can have spectacular effects on the Earth, and on life itself.