Tonight, a NASA Probe Will Orbit Massive Belt Asteroid, For the First Time

Enough with the lasts at NASA these days — here’s a first for you. The Dawn spacecraft is about to become the first probe to orbit a main-belt asteroid.

After speeding through the solar system for four years with ion thrusters, Dawn is now poised to enter orbit around Vesta later tonight, where it will circle for a year and study the giant space rock’s composition.

Vesta’s gravitational pull is expected to capture Dawn around 10 p.m. Pacific time/ 1 a.m. Saturday Eastern time, according to NASA.

Although it lives in the asteroid belt between Mars and Jupiter, thee monstrous, 330-mile-wide Vesta is really more of a protoplanet than an asteroid. Most space rocks in its neighborhood are a third of its size, and Vesta has a rocky composition like that of Venus, Earth and Mars, with a core, mantle and crust.

It’s distinct from the other 540,000-odd objects that are considered “minor planets,” the objects that orbit the sun but aren’t planets are comets. But it’s too small to be considered a dwarf planet, unlike Ceres — the Dawn spacecraft’s next destination in 2015, by the way.

Scientists think that Vesta could have been the fifth rocky planet, behind Mars, but it never properly coalesced. It’s probably the fault of Jupiter, which knocked away other objects that Vesta could have collided with to form a larger, denser orb. Vesta has actually been minimized through collisions with other objects, which have knocked off bits of it to form Vestoids, as this NASA Science story explains.

Dawn is designed to study several key features at Vesta, which scientists hope will help explain how it formed — or did not form, as it were. It will map the asteroid’s terrain, gravity field and interior structure, partly by scrutinizing a huge crater at its south pole which is currently illuminated by sunlight.

After a year of observations, Dawn will fire its ion thrusters again and make its way to Ceres for arrival in 2015.