macpuzl
By Chuck McPartlin
The fourth asteroid, discovered in 1807, is known as 4 Vesta. It was named after the Roman goddess of home and hearth by the mathematician Johann Carl Friedrich Gauss. Although it was the fourth discovered, it is the second largest asteroid, and is also the brightest of them all. It reflects 43% of visible light that hits it, compared to just 12% for our Moon. Because of this, it is sometimes visible to the unaided eye when it is at its closest to Earth, and that is happening now.
On dark evenings between June 8 and June 22, head for a spot with a dark southern horizon, like the beach, and see whether you can spot it. It won’t be easy unless the sky is pretty dark. Binoculars will make it a lot easier if you can’t eyeball it. Vesta will move noticeably from one night to the next, and it will be at its brightest on June 19. Star charts and tips for finding Vesta may be found at the Vesta Visibility link to Sky & Telescope referenced below. Smartphone planetarium apps like SkySafari can also help you find Vesta, as shown here for 1 AM PDT June 19, 2018.
Vesta is considered a planetesimal, similar to the original building blocks of the major planets. It got big enough that it differentiated; its components melted and stratified to form a dense core surrounded by a rocky mantle and thin outer crust. Processes such as volcanism and maybe even erosion and sedimentation by running water have occurred in Vesta’s past. It was considered a planet until the 1850s, when it was demoted to minor planet status.
NASA’s Dawn spacecraft, currently in low orbit around the dwarf planet Ceres, spent about a year mapping and studying Vesta, between July 2011 and September 2012. Here is one of the resulting images. Dawn is a slow but efficient spacecraft powered by a Xenon ion propulsion unit, and is the first to have gone into orbit around two solar system bodies (excluding Earth) in succession.
That sequence of three adjacent craters to the upper left was initially named The Snowman, but I see Igor from Young Frankenstein. Hump? What hump?
As you can see, Vesta is somewhat walnut-shaped. It doesn’t currently have enough mass for its own gravity to squish it into hydrostatic equilibrium. It appears to have suffered a very large impact around a billion years ago that modified its initial, probably spheroidal shape, carving out a huge impact basin at its south pole called Rheasilvia. This huge crater has a diameter that is 95% of the mean diameter of Vesta, which would equate to a 7,600 mile wide crater on Earth.
The Rheasilvia impact excavated an estimated 1% of the original mass of Vesta, and sent fragments throughout the asteroid belt, and even to Earth. About one out of every sixteen meteorites found so far, totaling 1200 samples, seem to have originated on Vesta. These meteorites are classed as Howardites, Eucrites, or Diogenites, separated into three classes based on their composition, and thus the layer of Vesta where they originated.
Vesta is roughly 340 by 290 miles across, making it about 1/7 the diameter of our Moon. It is denser than any of the moons in our solar system, and accounts for about 9% of the entire mass of the asteroid belt. An accurate estimate of its size and shape was determined back in 1991 when it occulted a background star, and it was the first asteroid to have its mass determined, based on close approaches of smaller asteroids. It orbits in the inner part of the asteroid belt between Mars and Jupiter, and takes 3.6 years to circle the Sun. Its rotational period is just over 5 hours.
Closeup images of Vesta courtesy of NASA/JPL and the Dawn Mission.
