Asteroids may be lumped together as a collection of boring, cratered rocks, but the more scientists study them, the more interesting they become. Vesta and Ceres, like many, are in classes by themselves and very different from each other. An amazing mission was devised so both could be reached and orbited by the same spacecraft. Dawn was designed, cancelled, restored, and finally launched on September 27, 2007 on an epic journey of discovery.
Exploring asteroids is important because the planets did not simply form overnight. Each planet formed gradually starting with dust that condensed out of the solar nebula. The dust grains gradually increased in size to form chunks of rock and metal, which continued to grow further by crashing into others, which again increased their mass. Their accumulated gravity further increased the frequency of collisions, thus perpetuating their growth. Eventually the collisions decreased as encountering debris became more infrequent. The remaining planetesimals were gravitationally scattered with most eventually settling in stable orbits between Mars and Jupiter, which is now know as the Asteroid Belt.
The existence of asteroids was not known until January 1, 1801 when Giuseppe Piazzi discovered Ceres. It was thought at the time that a planet remained undiscovered between Mars and Jupiter so a search was made; Ceres seemed to fit the bill, but it was rather small. Then Pallas, Juno, and Vesta were discovered and also classified as planets each with their own planetary symbols. After the discovery of Vesta there were no other objects discovered for 38 years. Then in 1845 new asteroids were discovered at a rapid pace. By 1851 there were 15 small planets along with the eight original, which included Earth. These tiny worlds were then reclassified as asteroids, meaning star-like, because they appeared like stars in even the largest telescopes. The asteroids were thought to be the remains of a large planet that was destroyed. It is now known that they are actually the left over remains of the building blocks of the planets that never found their way to be incorporated into one them.
Vesta is the third largest asteroid at 326 miles in diameter and the second most massive after Ceres. It was discovered by Heinrich Wilhelm Olbers on March 29, 1807 and named after the virgin goddess of home and hearth from Roman mythology. Vesta is the brightest asteroid seen from Earth getting as bright as magnitude +5.1 and the only asteroid that shows any color through a telescope, appearing lemon yellow. It orbits about 240 million miles from the Sun and takes 3.63 years to go around it once. Vesta is tilted 28º on its axis and a day lasts just over 5.3 hours. Much was known about the characteristics of Vesta before Dawn arrived thanks to the powerful optics of the Keck and Hubble Space Telescopes. Vesta was known to be of volcanic origin and the telescopes fuzzily revealed what appeared to be a huge impact crater that blew out the south polar region and contained a towering central peak. Vesta appeared like a badly deformed potato.
Dawn dramatically resolved the huge impact crater that is now known as Rheasilvia. It is 314 miles across, 96% the diameter of Vesta. The floor of the crater is about 8 miles deep and the rim rises 3-8 miles above the surrounding terrain with a total surface relief of about 16 miles. The central peak rises about 13 miles above the lowest part of the crater floor. The impact responsible excavated about 1% of Vesta and scattered the fragments into space. Dawn has confirmed that this is the source of the howardite-eucrite-dioginite (HED) meteorites found on Earth. This means scientists and meteorite collectors have a sample of Vesta in their hands without the expense of a sample return mission. The impact penetrated deep into Vesta through several layers of crust, and possibly into the mantle, which contains olivine. This huge crater overlies part of another huge crater, Veneneia, that is older and 245 miles across. Vesta has several other interesting craters such as the “snowman craters” that are a trio of craters arranged from largest to smallest creating an image of a snowman. There are craters with massive landslides within them and others with bright streaks and flows, possibly created from the heat of impact or exposure of fresh ice when the soil slumped away. A few craters have unusually dark streaks that appear to be from impacts of carbonaceous chondrite meteorites. Vesta is heavily covered with craters of all sizes and ages.
The majority of the equatorial region of Vesta is girdled by a series of concentric troughs such as Divalia Fossa that encircles the equator and is 6-12 miles wide, 300 miles long, and has a depth of 3 miles. This chasm easily dwarfs the Grand Canyon. Another set of troughs are found further north and are thought to be large-scale fractures caused by the impacts that created the huge Rheasilivia and Veneneia craters. These are some of the largest chasms in the Solar System. These troughs can only form if the object is differentiated and has a core, which proves that Vesta is differentiated. This means it was hot enough early on for the heavier elements such as iron and nickel to settle towards the center and form a core wrapped by a mantle and crust above. There was enough radiation early on within the rocks to create the intense heat necessary for differentiation to occur. Vesta may have a nickel-iron core about 135 miles in diameter, a rocky olivine mantle, and a surface crust. Vesta cannot be classified as a dwarf planet because it is not round due to the huge impact craters. Vesta was already too cold at the time of the huge impacts for it to eradicate the craters and round itself.
Dawn is a historic spacecraft using ion propulsion, which allowed it to be the first spacecraft ever to orbit two different worlds. Its mission will end February 2016, but an extended mission is possible. Hopefully, NASA will continue Dawn’s mission orbiting dwarf planet Ceres, but presently there are no plans to send it elsewhere. Vesta with its differentiated interior and Ceres with its bright salt deposits have shown that asteroids are not a collection of boring rocks and worthless debris. These are important worlds that are the true leftover building blocks of the Solar System and the rock on which we live.