Bagnall Beach Observatory

MPC Observatory Code 433

Asteroids and Comets

Many people have an understanding of the Solar System described by William K. Hartmann as the “nine-planets gestalt”. In this approach our knowledge of the Solar System is dominated by the Sun and the nine major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Occasionally people with this knowledge of the Solar System will be aware of an asteroid belt between Mars and Jupiter, in which orbit a collection of smaller rocky bodies called asteroids or minor planets. They will also usually be aware that from time to time, mysterious objects called comets appear in our skies on a journey around the Sun. However, these other bodies are usually regarded as an unimportant oddity.

In recent decades, the “nine-planet gestalt” has been losing its grip on our mindset because of the discovery of many other bodies in the Solar System and the implications of their existence for our distinctions.

We have tended to draw distinctions and establish categories based on the appearance these objects have for observers. Thus the major planets were large, spherical bodies which appeared as disks through our telescopes; minor planets were described as asteroids, meaning “star-like”, because of their resemblance observationally to a star; and comets were further distinguished because of their comae and tails. However, the new approach is beginning to suggest that these distinctions have limited value. First, because the distinctions have become blurred, and second, because categorisation based merely on appearance may blind us to an understanding of the formation and development of the Solar System. In the new approach, the division between minor planets and comets is no longer so clear, and a major planet like Pluto has its status as a major planet under threat.

The asteroid belt between Mars and Jupiter was discovered in the 19th century. Observers were looking for a planet in that region and discovered instead a multitude of smaller, rocky objects that they dubbed the “vermin of the skies”. As knowledge of the region grew it was realised that the presence of Jupiter made this region an area of gravitational instability in which some of these “minor planets”, as they were more properly called, were being flung either out of the Solar System, or into an orbit much closer to the Earth. The geological record showed that occasionally these “Near Earth Asteroids” collided with the Earth and caused massive changes to the development of life on Earth.

However, a further development took place in the 20th century. Another region of bodies was discovered orbiting beyond Neptune. These were dubbed “Trans-Neptunian Objects” or “Kuiper-Belt Objects” because of their location in the Edgeworth-Kuiper Belt beyond Neptune. This region also had its casualties, with some of these bodies being flung into orbit between Saturn and Uranus where they became known as the “Centaurs”; or being flung further out into a region now called the Oort Cloud.

The further out such objects initially formed, the more likely they are to have a composition including various ices. Consequently, Trans-Neptunian objects are thought to have a much higher volume of ice than the “Main Belt” objects orbiting between Mars and Jupiter. Trans-Neptunian Objects are considered asteroids. However, when they are drawn into an orbit closer to the Sun, heat from the Sun begins to create the characteristic coma of a comet. This was seem when the Centaur, 2060 Chiron, previously classified as an asteroid, drew closer to the Sun at perihelion and began to develop a comet’s coma.

The distinction between major planet and minor planet (or asteroid) has also become a little blurry. The discovery of Kuiper Belt Objects, many of them quite large, has led to the realisation that Pluto is merely the largest Kuiper Belt Object discovered to date.  It’s closest rival amongst the KBOs had been estimated to be around 900 kilometres in diameter, as compared to Pluto’s 2000 kilometre size. Not only is there a similarity in size, but the orbit of Pluto is inclined at a much greater angle to the ecliptic, the plane in which the other major planets, orbit. This characteristic is also typical of other Trans-Neptunian Objects, as illustrated in the  graphic here. This has led to suggestions by some that Pluto should be downgraded to a minor planet. The issue has now become even more controversial with the discovery of a KBO which is bigger than Pluto. Should Pluto be downgraded to a KBO, or should the new body be regarded as the 10th planet?

Another distinction frequently made is between the major planets, their moons and the minor planets. Space exploration and larger telescopes have led to the discovery of many more moons orbiting the major planets, and some moons orbiting the minor planets. However, moons like those of Mars give every indication of being captured asteroids, and some of the moons of the larger planets are larger than some of the smaller major planets.

It thus appears there is a continuum between these bodies that can only be understood by unravelling the sequence of formation in the primordial Solar System, and the ongoing gravitational interactions which make it what it is today.

The Role of Asteroids

It appears that asteroids and comets are the basic building blocks of the Solar System. Beginning as dust in the proto-planetary disk, from which the Sun also formed, the dust gradually accumulated into asteroids and comets. As time went by, collisions between asteroids led to their accumulation into planets; and the constraining of their orbits into the ecliptic plane.

Collisions with Earth

Our encounter with such solar system bodies is usually from afar. Occasionally, however, the orbits into which they have been thrown put them on a collision course with the Earth. Initially this was how the Earth itself formed. However, ongoing collisions have caused mass extinction events in the geologic past and may present a threat to life today.

Solar system objects like minor planets (asteroids) and comets are sometimes placed in orbits which put them on a collision course with the Earth. When the object is large, say over 1 kilometre in size, the outcome of such a collision could be catastrophic. The geological record of the Earth shows that such catastrophic collisions have occurred in the past. It is believed that such a catastrophic collision brought about the demise of the dinosaurs some 65 million years ago. For this reason, there are several Near Earth Object observation programs conducted by astronomers around the globe in order to warn of any impending collision, and hopefully to make it possible to take evasive action. Such programs involve vigilant observations of NEOs. In a small way, Bagnall Beach Observatory participates in such ventures through the contribution of observations of NEOs to the Minor Planet Centre.

Collisions with large objects are rare (fortunately). However, many smaller objects collide with the Earth on a nightly basis. These may be small asteroids or the rubble left in the train of a comet. They are usually referred to as meteoroids while in space, meteors as their descent through Earth’s atmosphere heats them into balls of flame, and meteorites if they survive vaporisation in their descent.

Most meteors are vaporised before landing, but those that do survive the descent may leave craters from the impact. Although most traces of impacts are eroded by the constantly changing face of the Earth’s surface, Australia has many meteorite craters which have been documented by Alex Bevan and Ken McNamara in their booklet, Australia’s Meteorite Craters, available from the Western Australian Museum. The Museum also displays a number of meteorites as seen below.

Building blocks of the Solar System

Br. Giuseppe Piazzi of the Theatine Order discovered the first asteroid between Mars and Jupiter on January 1, 1801. 

Image credit:

Palermo Astronomical Observatory

Meteorites on Display at the

Museum of Western Australia.

 

Above: My wife, Rosemary, places her finger in one of the many pockmarks of this meteorite. All the meteorites show the effect of heating by descent through the atmosphere.

 

Left: This meteorite was fashioned by its descent into something resembling a stealth fighter.

 

Meteorites bring us into direct contact with solar system objects from beyond our own planet.