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NASA's first ion probe studies planetoid; will go on to study Ceres dwarf planet, which may have a hidden ocean

Located in our solar system's asteroid belt, mid-way between Mars and Jupiter, there's an area where the dense field of asteroids grows sparse.  Orbitting in this region, known as the Kirkwood gap is a massive planetoid -- 4 Vesta.

I. Ion-Propelled Probe Produces Results

Named after the Roman goddess of home and hearth, Vesta is the second largest object in the asteroid belt, dwarfed in size only by ice-rock dwarf planet Ceres.  Vesta is slightly closer to Earth than Ceres.

The fuzzy mega-asteroid (also referred to as a planetoid) was just a mysterious fuzzy spec in astronomers' telescopes until July 2011, when the U.S. National Aeronautics and Space Administration's Dawn probe entered orbit.  Launched in Sept. 2007, Dawn is a very special probe, in that it is the first pure exploratory probe to use ion propulsion.

Dawn probe
[Image Source: NASA/JPL-Caltech/UCLA/McREL]

Now NASA's findings have been published in the prestigious peer-reviewed Science journal.

Ceres Hubble v. Dawn
Ceres -- left (Dawn photos); right (Hubble) [Image Source: NASA]

Using the size, shape, and gravitational field of the asteroid, the research team was able to estimate that the asteroid had a 220km (135 mi.) diameter metal core, much like a miniature version of the metal core found in the Earth and other planets.

Armed with this information the team has proposed a bold hypothesis -- that Vesta is one of the solar system's only surviving protoplanetoids.  The proto-planetoids formed approximately 2 million years after solids began to coalesce throughout the solar system into planets and smaller bodies.  

Vesta
Vesta is the second largest object in the asteroid belt. [Image Source: NASA]

These smaller sisters to full planets like the Earth and Mars had a similar structure, with dense elements like iron sinking to the core, and lighter elements like carbon, oxygen, and silicon, rising to the surface.  But as they had much less mass, their larger siblings -- the solar system's nine planets (or 10, if you count Eris and Pluto) devoured all but a handful of these smaller bodies.

II. Vesta: Portrait of a Survivor

But Vesta survived.

That's not to say that parts of Vesta didn't wind up getting incorporated into the Earth and Mars.  Life in the asteroid belt is full of collisions, and much material is upheaved from the surface of Vesta and sent hurling across the solar system.

Scientists have pinpointed Vesta's handouts to a special type of meteorite -- howardite-eucrite-diogenite (HED) -- which account for approximately 6 percent of the total meteorites that collide with Earth yearly.  Their source was confirmed by spectroscopy, which revealed Vesta's surface to be rich in iron and magnesium core components of the meteorites.

Vesta HED Meteors
Vesta is an apparent source of the abundant HED meteors [Image Source: Univ. of Tennessee]

The precise ratio of these elements, seen seldom else in the solar system -- except on Vesta's surface and the HED meteorites, leaves little doubt than many or all of them come from the planetoid.

The Dawn probe indicates that many of those debris may come from a pair of large collisions that left two gaping craters in the planet's southern bottom.  One of these impact craters, named Rhea Silvia, is 427 km (265 mi.) across.

Dr. David O'Brien, from the Planetary Science Institute in Tucson, Ariz. and a participant in the study, told BBC News, "Our estimate [is that] about 250,000 cubic miles [were] excavated from Rhea Silvia.  To put this into another perspective - 250,000 cubic miles is enough to fill the Grand Canyon a thousand times over."

III. Next Stop -- Ceres

The probe's exploration of Vesta is currently wrapping up.  Its last major objective for this leg of its misison will be to collect images of the planetoid's northern pole, which is currently clothed in season darkness.  With that complete, researchers should have the celestial body's surface fully mapped out.

The probe will then fire up its ion engines and cruise farther out in the belt, entering the orbit of Ceres.  That phase of the expedition is to head towards Ceres, the inner solar system's only dwarf planet.  Rich in carbon and water, the planetoid may hold a liquid ocean under its icy surface.  Where there's carbon and liquid water on Earth there is life, so expect intrigue to be high when Dawn examines Ceres for signs of liquid water.

Sources: NASA, Science, BBC News





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