Vesta is the asteroid belt's second largest object, but figuring out what it is has driven astronomers crazy. Is it an asteroid? A planet, perhaps a protoplanet? Whatever the answer, Vesta might just be the solar system's most important object.
Our current planet classification system isn't perfect, and it's still more than a little controversial with astronomers and laypeople alike - just look at the recent (flawed) assertion that Earth might need to be demoted for ample evidence of that - and yet it's not terrible at dividing the solar system's many bodies into some broadly useful categories.
Really, it's simple enough: objects that orbit the Sun and are massive enough that their gravitational force makes them spherical are either planets or dwarf planets, and those that are also able to clear their orbital neighborhood of debris are considered full-blown planets. Under those definitions, there are eight planets alongside five dwarf planets - the trans-Neptunian objects Pluto, Haumea, Makemake, and Eris, along with Ceres in the asteroid belt. Everything else that orbits the Sun is now considered a Small Solar System Body, or SSSB, although the term "minor planet" was used until recently to describe them.
This is where Vesta enters the picture, awkwardly straddling the line between planet and SSSB. It's much bigger than almost any other object in the asteroid belt - while its neighbors are almost all 100 kilometers wide or less, Vesta's diameter is 530 kilometers. At 267 quintillion kilograms, it accounts for about 9% of the total mass of the asteroid belt (Ceres accounts for 32%). If it is just an asteroid, it's a very unusual one.
That said, none of this would be enough to consider Vesta anything more than a large asteroid - it's not massive enough to be spherical, and it's quite a bit smaller than the five dwarf planets. The trouble starts when you peer below the surface of Vesta. Unlike almost all asteroids, Vesta is what's known as an "evolved object", meaning it has a distinct core, mantle, and crust that makes it much more like Earth or Mars than its asteroid brethren.
Objects become evolved if they have sufficient radioactive material inside them at the time of formation. This creates heat that melts rock and, in turn, sorts the object into distinct layers, with the lightest rocks perched at the top to form the crust while the heaviest rocks sink inside to become the core. We know all this happened on Vesta because we detected basalt on the planet back in 1972, meaning the planet must once have melted.
Under the circumstances, calling Vesta a minor planet or SSSB seems to sell it shamefully short, even if it can't be considered a dwarf planet like its neighbor Ceres. One possibility is to consider it a protoplanet, a leftover planetary embryo that could have formed into something bigger if events had unfolded differently.
The ancient solar system was full of protoplanets roughly Vesta's sized - our planet is formed from the union of a few of these - but for whatever reason Vesta was never able to team up with another object and start the full formation process. Most likely, Jupiter's gravitational force was too great for Vesta and its fellow asteroid belt protoplanets - its bigger sibling Ceres and the slightly smaller Pallas - to come together into a planet.
Whatever we want to call it, Vesta is one of the most important objects in the solar system. Like most other large objects in the solar system, it's been bombarded by lots of smaller rocks in the last four billion years. But, crucially, it was never hit by something big enough to disrupt it completely, leaving its surface a largely undisturbed relic of the solar system's earliest days. In fact, it might well be the only body in the solar system for which that's the case.
That's why NASA is currently sending the Dawn probe to Vesta for a year of exploration. Team member Christopher Russell explains:
"This gritty little protoplanet has survived bombardment in the asteroid belt for over 4.5 billion years, making its surface possibly the oldest planetary surface in the solar system. Studying Vesta will enable us to write a much better history of the solar system's turbulent youth."
So then, asteroid, protoplanet, or SSSB - whatever the classification, it doesn't really matter that much when Vesta can reveal some of the solar system's most ancient secrets. Dawn will spend twelve months orbiting Vesta. Plans include careful mapping of its mostly uncharted terrain, analysis of its surface composition, and close study of its gravitational forces, which should reveal more about its internal structure.