It wasn't so long ago that asteroids were considered whirling rocks confined mostly to the flattened region between the orbits of Mars and Jupiter and comets were dirty ice balls arriving with diverse inclinations from a cloud of comets at the very edge of our solar system. Comets were comets and asteroids were asteroids. Things were simple - life was good.
Astronomers and scientists like to put celestial objects into categories, or boxes, by their common physical and orbital characteristics. Three decades ago, the known stony asteroids moved about the sun in roughly the same plane and in the same direction as the planets. We put the asteroids into one box. Into another box went the icy comets whose orbits had very diverse orbital inclinations with many of them in orbits called retrograde orbits that carried them about the sun in a direction opposite to that of the planets.
Recently, Mother Nature has kicked over these boxes and demonstrated that the line between comets and asteroids is no longer clearly drawn.
Running Out of Gas
In 1950-51, the American astronomer Fred Whipple put forth his dirty ice ball model for the cometary nucleus. According to Whipple, the solid heart of a comet was a collection of ices (mostly water ice) with embedded dust particles. As an icy comet passes by the sun, the ices vaporize and the entrained dust particles are released to form the impressive dust tails that are pushed back in the anti-solar direction by the pressure of sunlight and shine by reflected sunlight. From time to time, the Earth passes through these trails of cometary dust and we are treated to a meteor shower (or storm). This was an easily understood picture of the cometary aging process but it was Whipple himself who pointed out in 1983 that the orbit of the Geminid meteor stream particles was very similar to that of a recently discovered asteroid, rather than a comet. This asteroid, called Phaethon, is in a rather eccentric, comet-like orbit, and it is entirely likely that Phaethon, as well as a few others like it, is a defunct comet that has now lost its ices and hence its ability to expel the gas and dust that are characteristic of comets. This comet has literally "run out of gas". Once a comet has lost much of the ices that hold the dust and carbon-based materials together, it can lose its structural integrity and split into many pieces, or disintegrate completely into a meteor trail of particles.
In 1977, Chiron, the first of many so-called Centaur objects, was discovered in the outer solar system near the regions defined by the orbits of Saturn and Uranus. In 1989-90, after behaving itself for more than 10 years, it began to throw off dust and gas and show some comet-like qualities. Chiron was the first of five objects to be given the double designation of a comet and an asteroid. It is now known as the ninety-fifth periodic comet (95P) and the two-thousand-and-sixtieth numbered asteroid (2060). Hence we have 95P/Chiron = (2060) Chiron. Another Centaur object with a dual asteroid and comet designation is 174P/Echeclus. It was an asteroidal object when first discovered but more careful subsequent observations detected its comet-like atmosphere. Comet/asteroid 107P/Wilson-Harrington is yet another object with a split personality. It showed cometary activity in 1949 but has revealed no such activity since. The fourth object with a dual designation is 133P/Elst-Pizarro, which has a nearly circular orbit in the main asteroid belt between the orbits of Mars and Jupiter but showed a temporary, comet-like dust tail in 1996. Comet/asteroid 176P/LINEAR is the most recent object to receive a dual designation and it too is circling the sun in the asteroid belt. Moreover, there are currently 18 inactive asteroids in eccentric, comet-like orbits about the Sun and a handful of active comets in asteroid-like, nearly-circular, orbits within the main asteroid belt. The poor attempts of astronomers to sort these objects in separate boxes are just not working out.
A Diverse Population
Comets can appear asteroidal when they lose their ability to outgas, either by depleting their volatile ices or perhaps by having their ices fully shielded from sunlight by an inert overlying crust. These ex-comets would have the fragile and weak structure of active comet rather than the rocky structure of most asteroids. Moreover, the vast majority of asteroids are catastrophic collisions fragments of once larger bodies, some of which were composed of dark carbon-based materials and others that had original silicate mantles and nickel-iron cores. Some of these collision fragments re-agglomerated into loosely bound "rubble-pile" structures that are held together by little more than their own gravity. Thus, the diversity range among the asteroid population is enormous; they can run the gamut from wimpy ex-cometary fluff balls, to loose rubble piles, to shattered silicate rocks, all the way to slabs of solid nickel-iron.
Bad News - Good News
The bad news in all of this is that, in the unlikely event that one of these diverse near-Earth objects is found on an Earth threatening trajectory, the extraordinary diversity of the population would mean that a single deflection technology could not be used for all types of objects. The good news is that their very proximity to Earth makes some of them quite accessible. Some are easier to land upon and return from than the moon itself. The near-Earth object population is rich in natural resources including metals, minerals, and water ices. In the coming decades, as we continue our steps off the shores of Earth into the interplanetary sea, the materials for inner solar system space structures are awaiting us - as well as the resources necessary to sustain life. Water can be broken down into hydrogen and oxygen, the most efficient type of rocket fuel. One day, comets may serve as the fueling stations and watering holes for interplanetary travel. When we have discovered and tracked the vast majority of these near-Earth objects, their risk to Earth will be vastly diminished and their potential as natural space resources will be dramatically increased. Such is the foe/friend duality of this diverse population of comets and asteroids in the Earth's neighborhood. These small bodies of the solar system may be a short-term risk, until we can find and track most of them. Our increasing understanding of this diverse population suggests their potential for the future of interplanetary travel and colonization is virtually unlimited.
Last Updated: 29 November 2012