by Bruce Moomaw
Cameron Park - August 1, 2000 - The fuss over whether to continue calling the little world of Pluto a "planet", or switch to calling it an big ice asteroid (or even a giant comet nucleus), has drawn a great deal of public attention in recent years.
But in the April 20 "Nature", Dr. Don Yeomans points out that astronomers will soon be faced with a new naming problem that dwarfs the Pluto controversy -- for the distinction between "asteroids" and "comets" has now started to blur seriously.
Up to now, "comets" have been defined as small icy objects ("comet nuclei") surrounded by a visible coma (a cloud of dust and gas) boiled off them by the heat of the Sun as they approach it from the outer Solar System -- while all small Solar System objects without a coma have been called "asteroids", on the grounds that they must be made of rock rather than ice.
But now this distinction is breaking down -- because our telescopes are powerful enough that they can routinely see small objects so far out in the Solar System that they have no comas even if they are made of ice.
Chiron -- the big "asteroid" discovered between the orbits of Saturn and Uranus in 1977 -- was the first of these; just a few years after its discovery, as it moved closer to the Sun, it suddenly developed an unmistakable coma, and thus received double designation as a comet.
Since then, the Kuiper Belt -- the huge region beyond Neptune, containing millions of icy objects ranging from a few hundred km in diameter down to only a few km -- has been discovered.
They're all too cold to possess comas, but the vast majority of them are definitely made of ice rather than rock -- in fact, they are comet nuclei.
More "Centaurs" like Chiron -- similar icy objects that have wandered inside Neptune's orbit -- have also been discovered.
And to complicate matters further, it turns out not to be safe to assume that every object in the far outer Solar System IS ice rather than rock.
Yeomans in Nature points out that recent computer simulations show that as much as three percent of Kuiper Belt objects are likely to be rocky asteroids that formed in the outer fringes of the Asteroid Belt -- but then, at some point over the eons, flew close enough to Jupiter to be catapulted by its gravity into the outer Solar System.
And the same is possible for the far more distant Oort Cloud, made of objects that formed in the realm of the giant planets and then flew close enough to one of them during the following eons to be flung into incredibly distant orbits taking them up to thousands of times farther from the Sun than Pluto - an appreciable part of the distance to the nearest star.
Indeed, the first such provable "outer asteroid" (my own term) may have been discovered in 1996; like many of the comets, its orbit is incredibly elongated, suggesting that it has probably wandered back into the region of the outer planets from the far reaches of the Oort Cloud -- but although it's now close enough to the Sun that it should definitely be showing a coma, it has none.
Since then, at least three more "outer asteroids", with the elongated orbits of regular comets but with no trace of a coma, have been discovered -- and two of them are orbiting the Sun backwards, a trait previously limited entirely to occasional comets.
So if you can't be sure whether a small object you find in the outer Solar System would develop a coma or not if it was close to the Sun, what do you call it -- an asteroid or a comet?
The naming conventions, after all, are completely different: asteroids are given whatever name their discoverer wishes (preceded by a number), while comets are named AFTER their discoverers (preceded by a "P").
This problem is holding up all attempts to name any of the Kuiper Belt objects; instead, they're all retaining their initial post-discovery labels such as "1996 PW".
To make matter worse, we're now confirming what astronomers have suspected for decades -- some of the "asteroids" in the inner Solar System are really dried-out comets, jockeyed (like some still-active comets) into orbits completely in the inner Solar System by a chance encounter with Jupiter, and exposed to the Sun's heat so many times since that all their ices have evaporated, leaving only a lump made out of the dust and rock mixed with the comet's ice.
Two recently discovered "asteroids" have turned out to have been seen years earlier while they still had comas and were labeled as comets -- that must have been their very last gasp of outgassing before they "died".
And IR spectral measurements have raised the suspicion that some of the other "near-Earth" asteroids closer to the Sun than the Asteroid Belt are also dried-out comet nuclei -- although the indications are still that most near-Earth asteroids really are authentic rocks from the Asteroid Belt, nudged by chance into the realm of the inner planets by Jupiter's long-distance gravitational tuggings.
Finally, over the past couple of years, meteorites have been found still containing significant traces of water trapped inside them -- which means that "Far from being the dry rocky bodies they were once thought to be, it would seem that some asteroids, along with with comets, might be significant sourcees of water" -- especially those in the outer fringes of the Asteroid Belt, or the "Trojan" asteroids that hover in the Lagrange points of Jupiter's orbit.
So all our old categories are evaporating.
Not only can we no longer draw a clear boundary between "planets" and "asteroids" [especially since the odds are excellent that the Kuiper Belt and Oort Cloud hold a fair number of undiscovered objects ranging in size from the biggest asteroid Ceres (914 km diameter)to as large as Pluto (2274 km) and possibly even as big as Mercury (4,878 km,] but we can no longer distinguish at all convincingly between "asteroids" and "comets".
You could even make a case for calling Pluto any of the three!
At some point, the International Astronomical Union will finally have to grasp this nettle and change the way we name newly discovered Solar System objects -- which may to some extent require changing the long-established official titles we've given to the asteroids and comets (and even planets!) we've already discovered.
But while this is a mess for the nomenclaturists, it's yet another facet of our growing knowledge of the solar system and the great variety of objects orbiting the Sun.
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