Asteroid Vesta - Images from the Dawn Spacecraft

With the New Horizons spacecraft hurtling toward its 2014 encounter with Pluto, and with the Dawn spacecraft now at its most up-close and personal encounter with Vesta, we are in the process of learning scads of information about two objects that are among the poorest understood and least explored bodies in the Solar System.

Before NASA's Dawn settled into orbit around the asteroid Vesta—the second largest object in the Main Asteroid Belt, after the Dwarf Planet Ceres—we knew very little about it. That it is mega-mountain of rock 330 miles across that rotates rather quickly in space and is slightly egg-shaped, these things we knew—but not much more.

What Dawn has revealed to us, however, is a tiny world with unexpected complexities, inside and out.

Inside, Vesta's anatomy may not be unlike Earth and the other Terrestrial planets, which all developed cores heavy with iron and mantles and crusts made of lighter silicate rocks when they were young and molten. This "differentiation" occurs for the same reason that gold particles sink to the bottom of a gold-pan as a prospector shakes the water-sand slurry back and forth: the gold is denser, the sand lighter, so the materials separate.

Outside, Vesta's surface offers amazing landscape vista opportunities for a future robot lander or astronaut: complex topography of valleys, cliffs, troughs, ridges, and a huge mountain, with elevation differences deviating above and below the global average elevation by as much as 15 miles—that's three Mount Everests, or two Marianas Trenches!

Parts of the surface resemble some of the basaltic formations of cooled lava in Hawaii, suggesting that, long ago, there may have been active volcanoes on Vesta, spewing out lava to shape the young surface.

What a sight it must have been—and it makes me smile when I think about the children's book "The Little Prince." My favorite part of that story was the description of how the Prince, on his little asteroid world (which was only twenty or thirty feet across, I'd guess), cooked his meals on a frying pan held over a miniature volcano, which he made sure to keep clean and functional with a periodic cleaning using a giant Q-tip….

All of these revelations—the core/mantle differentiation, complicated geography, possible tectonic features, and signs of past volcanism–have prompted some scientists to ask, should Vesta be reclassified as a Dwarf Planet, along with Ceres, Pluto, and the others thus dubbed?

I have on my desk at work a letter from a 3rd Grader. It starts, "I think Pluto should be a planet (not a Dwarf Planet)…." The letter continues in richer detail and quite a bit of passionate defense of Pluto, but I was struck by the fact that this 3rd Grader was, at the time Pluto was originally "demoted," three years old. (And some thought the Pluto controversy would end with the previous generation of kids….)

But it did get me wondering. If Dawn has changed our view of Vesta from a mere large asteroid to something maybe worthy of promotion to Dwarf Planet, what might New Horizons do to our current view of Pluto? I'm not suggesting the International Astronomical Union will reinstate Pluto as a planet when we get our first up-close images of its surface—after all, no matter what Pluto's surface may hold in store for us, this Dwarf Planet can't meet one of the three conditions for planethood: being massive enough to clear the region of space in which it revolves. Alas, Pluto shares its orbital space with other objects.

But I fully expect that New Horizons will change our perspective on Pluto, as Dawn is doing for Vesta. The more we learn of the rich details of mysterious places like these, the more, I think, we regard them as "worlds"—regardless of their classification as asteroid, dwarf planet, or planet.

Vesta, image from NASA's Dawn spacecraft

Ion thrusters full! Set us into a standard orbit, Mr. Sulu….

Well, I don't know if any of the helms-persons at NASA are named Sulu, but we have indeed achieved orbit—that is, NASA's Dawn spacecraft around the large asteroid Vesta.

I wrote about Dawn and Vesta not long ago, before the spunky little ion-driven robot arrived there. Since then, Dawn has reached its first destination, 117 million miles from Earth, entering a 9,900 mile orbit around Vesta on July 15th. Science observations are expected to begin in early August, but already Dawn has sent back wonderful preliminary images showing details never before seen.

Vesta's surface may bear features and materials among the oldest in the Solar System. Already we can see that Vesta is pock-marked and scared by impacts incurred over the eons. Similar to how a forensic scientist may determine the sequence of events that occurred at a crime scene by studying the physical evidence left behind, the scars and residues on Vesta will help paint a picture of conditions throughout the Solar System's history.

Almost as cool as its science mission is Dawn's propulsion system. To use a term from a certain Smith and Jones movie, it's "the New Hotness." Technology first demonstrated on NASA's Deep Space 1 spacecraft, Dawn's engine is the first solar electric ion propulsion system used on a purely scientific spacecraft. Using electrical power generated by solar panels, Dawn's engine ionizes xenon atoms and accelerates them with an electric field, squirting them out the back of the engine to produce thrust–similar to a balloon-powered car or rocket toy propelled by spurting air. And though a conventional chemical rocket can produce much stronger thrust, Dawn's ion drive, operating with high efficiency and over longer periods of time, achieves up to 10 times the velocity change for an equivalent amount of propellant.

(As a sign of the technological times, in one episode of the original Star Trek series, Scotty was awe-stricken by an advanced alien spacecraft that used ion propulsion. Ironic; what today's space explorers wouldn't give for warp drive….)

Dawn will spend a year orbiting and studying Vesta before it moves onto its second target, Ceres, to harvest its secrets.

Vesta is now the largest known asteroid in our Solar System. It was second fiddle to Ceres for a long time, but back in 2006 when Pluto got "demoted" to dwarf planet status, Ceres' status also changed—promoted or demoted, take your pick. Sure, Ceres is now in the more exclusive club of the dwarf planets, but it's the smallest of that group, whereas when it was an asteroid, it was the largest, going from big fish in big pond to junior member of the upstairs office team….

So what's Vesta like—what we know about it at the moment, anyway? Vesta is a mega-mountain of rock and dust, somewhat lumpy and potato-shaped, but approximating a spherical object with a mean diameter of about 330 miles–roughly the distance from Oakland to Los Angeles as the ion-driven robot flies. In terms of surface area, Vesta has about twice the real estate as the entire state of California!

Sounds pretty big—and it is—but you'd still need over 20,000 Vestas to make one planet with the mass of the Earth. And if you stood on the surface of Vesta, you'd weigh little more than 2% what you weigh on Earth. Myself, I'd weigh in at a tad under 5 pounds. Presumably that means I could jump a hundred feet into the sky and land again safely.

I don't know about the science, but Vesta sounds like a fun place to me!

Illustration from The Little Prince

We are soon to explore a new world, one that we haven't seen up close before: the asteroid Vesta. NASA's Dawn spacecraft, which has been conducting maneuvers for the past four years to get into position to orbit Vesta, will arrive at the asteroid in early August—a la Star Wars, "Almost there…." What will we find, and why are we even interested in what amounts to a mega-mountain of rock hurtling through space?

I've been intrigued by asteroids since I read The Little Prince in childhood. I loved the whole idea of the Solar System filled with uncounted miniature "personal planets," each with one occupant and all the solitude and spare time one might wish. Never mind that the asteroids in that story (not much bigger than houses, if even that) are too small for hydrodynamic equilibrium to shape them into spheres or to hold onto any sort of atmosphere….

Much attention has been given to the planets as good places to explore, but the smaller bodies orbiting the Sun have a lot to tell us. They are composed of materials that may have been altered very little, if at all, since the earliest times in the formation of the Solar System. They also bear the marks—features of the processes of their formation, scars of collisions—of the eons of their existence.

Dawn's mission to Vesta–and ultimately on to the larger Ceres—might be characterized as a Solar-System-scale geological expedition. Much as geologists may probe a deep layer of rock on Earth to learn about our planet's geological past and evolution, Dawn will be probing Vesta and Ceres to learn about the Solar System's formation and development.

Vesta and Ceres—classified as asteroid and dwarf planet, respectively—are the two largest protoplanets, each quite different from the other in potentially informative ways. Vesta is a dry airless body, while Ceres may contain water (ice) and maybe even a thin atmosphere. Being protoplanets, both objects went through a process of differentiation early in their history, forming a core and a crust not unlike planets do.

As we understand the formation of the Solar System today, both from exploration of our own and observation of other planetary and pre-planetary systems at different stages of development, it all started with the Protosolar Nebula: the cloud of gas and dust that began the long gravitational collapse that ultimately gave birth to the Sun, planets, asteroids, and comets. Little by little, dust particles in the nebula stuck together, forming larger and larger clumps. Eventually, gravity began to play a stronger role in the clumping process, with the larger clumps pulling in more and more dust and neighboring clumps, snowballing ever larger.

Clumps that reached sizes of a mile or so across earned the title "planetesimal," and when planetesimals grew into objects hundreds of miles across, they graduated to the rank of protoplanet. This career ladder climbing went on until most of the raw material had been swept up. At this point, if the snowballing process had produced a Sun-orbiting object that was large enough to pull itself into a spherical shape and also "dominate" the region of space it was in (according to International Astronomical Union requirements), then it could be called a planet. If not, other classifications, like dwarf planet or small Solar System body, might apply…if you have a complaint, register it with the IAU….

In the case of Vesta and Ceres, which are on the order of 300 and 600 miles across, roughly and respectively, their career aspirations in planetary stature building are thought to have been crushed by the formation of Jupiter—perhaps as how a type-A personality with powerful career ambitions might derail a coworker by grabbing all the promotions….

What world might have been if Vesta and Ceres had finished their work and not ended up in the Mail Room of the Solar System? Well, if you took all of the material present in the Main Asteroid Belt today and brought it together into one object, orbiting the Sun between Mars and Jupiter, you would have a sphere probably no bigger than 900 miles across—less than half the diameter of Earth's Moon—so at best it might rate as a dwarf planet.

However, it is believed that most of the original mass of the Main Belt has been lost, scattered to other places in the Solar System by that ambitious Jupiter…so who knows what this world that never was might have been if circumstances had been different?

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