Don't think for a moment that icy, distant Pluto is alone in the ongoing astronomical game to redefine well-know bodies inside our solar system. Some astronomers classify Pluto as a dwarf planet rather than as the ninth major planet-fine, but other researchers still cling to Pluto's discoverer Clyde Tombaugh's identification of this distant world as the real McCoy, a planet. No matter, it's likely that this reclassification revolution, so near-and-dear to astronomers, will continue for years to come.
Now there's a new naming battle brewing: some astronomers are wondering how to reclassify Vesta, the solar system's largest asteroid or planetoid.
According to Dr. Tony Phillips of NASA, astronomers consider Vesta to be an asteroid because it is within the main asteroid belt-between the planets Mars and Jupiter. Yet there's something very weird about it.
Unlike its tumbling neighbors in the main belt, Vesta is no lightweight asteroid. And unlike its rocky pals (most only 100 kilometers wide and smaller), Vesta is big-530 kilometers in diameter.
That's almost planet sized or rather dwarf-planet sized.
"On March 29, 1807, German astronomer Heinrich Wilhelm Olbers spotted Vesta as a pinprick of light in the sky. Two hundred and four years later, as NASA's Dawn spacecraft prepares to begin orbiting this intriguing world, scientists now know how special this world is, even if there has been some debate on how to classify it," Dr. Phillips told Seeing Stars.
While Dr. Phillips doesn't take sides in NASA's emerging Vesta debate, Dr. Tom McCord, a NASA Dawn co-investigator, most definitely does.
"I don't think Vesta should be called an asteroid," said Dr. McCord.
"Not only is Vesta so much larger, but it's an evolved object, unlike most things we call asteroids."
Dr. McCord points to Vesta's layered structure (with a core, mantle and crust) as the reason why it's more aligned with the terrestrial planets-Mercury, Earth, Venus and Mars-than the lumpy asteroids.
"Vesta had sufficient radioactive material inside when it coalesced, releasing heat that melted rock and enabled lighter layers to float to the outside. Scientists call this process differentiation," Dr.
Phillips explained. "That's why Dawn scientists prefer to think of Vesta as a protoplanet which is the next step up from a planetoid. It is a dense, layered body that orbits the
Sun and began in the same fashion as Mercury, Venus, Earth and Mars, but somehow never fully developed."
Dr. Phillips also said "Other space rocks have collided with Vesta and knocked off bits of it. Those became debris in the asteroid belt known as Vestoids, and even hundreds of meteorites that have ended up on Earth. But Vesta never collided with something of sufficient size to disrupt it, and it remained intact. As a result, Vesta is a time capsule from that earlier era."
Asteroid, planetoid, protoplanet? How astronomers will ultimately classify Vesta is unknown, but NASA's Dawn spacecraft is about to get some up close views of this oh, so very heavenly body. Stay tuned.
Lou Varricchio, M.Sc., was a science writer at the NASA Ames Research Center in California. He maintains his space agency connections through the NASA-JPL Solar System Ambassador Program, an education outreach effort of the Jet Propulsion Laboratory.