In 2007, NASA launched the Dawn spacecraft towards the main asteroid belt between Mars and Jupiter to study the asteroid Vesta and the dwarf planet Ceres. Dawn arrived at the former target last July and has already made some astounding discoveries. Not only does the small body have surface features and signs of differentiation, it doesn’t really seem to be an asteroid at all. Dawn went to study Vesta the asteroid but it arrived to find Vesta the protoplanet.
NASA chose Vesta and Ceres for the Dawn mission because each has different characteristics that can teach us about our solar system and the history of the Earth’s formation. Ceres has a primitive surface with water-bearing minerals and may possess a weak atmosphere while Vesta is dry with signs of resurfacing characteristic of the terrestrial planets of the inner Solar system. Dawn was designed to go into orbit around both bodies and take measurements with its visible camera, visible and infrared mapping spectrometer, and gamma ray and neutron spectrometer. Radiometric and optical navigation data will add gravity field and internal structure measurements to the scientific return. The rationale behind studying both bodies with the same instrumentation is so mission scientists can compare the two and measure the different evolutionary path each took. This will in turn enable them to create a better overall picture of the early Solar System.
The Dawn spacecraft successfully entered into orbit around Vesta last July 16. As data started flowing back to Earth, the previously accepted concept of Vesta began to change. Vesta, which is about 178 miles wide, 173 miles long and 139 miles deep (by comparison the Moon is nearly 300 times more massive), has surface features like mountains, hills, cliffs, and craters. It also shows signs of differentiation, evidence that its core was once molten and allowed heavier material like iron to sink towards the centre. Its iron core could even give the rocky body a very weak magnetic field.
In short, Vesta is sufficiently complex and has a number of planetary characteristics of terrestrial planets that mission scientists think it’s actually a protoplanet, a planet that started to form but never made it to completion, and not an asteroid.
Adding to the wealth of information is that Vesta is incredibly well preserved. “Vesta is like a time probe we haven’t had in the solar system before,” said Dawn lead scientist Christopher Russell from the University of California. “We have a body that formed very, very early so we know from Vesta what conditions were like back then. We would not have learned that from a body like the moon or Earth. We would not be able to go back that far in time,” he said.
The records on its surface have helped the Dawn science team confirm long-held theories about Vesta’s history. They’ve figured it dates back to within 300 million years of the beginning of the Solar System, which was about 4.6 billion years ago. Similar records on Earth have been erased by erosion, winds, and active geology while the Moon has fallen victim to asteroid impacts. Analysis of craters on the moon date back a little more than 3 billion years; earlier evidence was obliterated during a period of heavy bombardment.
But Vesta isn’t without the marks of cosmic collisions. Cameras that have surveyed about 80 percent of its surface and revealed a heavily cratered northern hemisphere and a surprisingly smooth southern hemisphere marked by two giant impact craters partially layered on top of one another. This layered crater is thought to be the source of two groups of meteorites found on Earth. “There is evidence in the meteoritic record to give us co-incident dates with the two impacts that we’ve dated on Vesta,” Russell said.
Scientists also found rings around the impact basins, a phenomenon that may be related to Vesta’s iron core. “We haven’t done simulations of this so we don’t really know yet,” Russell said. “I would guess that the iron core sort of holds Vesta in place and the (impacting body) rings around that.”
So if Vesta has all the makings of a planet, why didn’t it ever form? The same reason the asteroid belt has remained a ring of debris for so long: Jupiter. “Jupiter started to act like a spoon in a pot, stirring up the asteroid belt and the asteroids started bumping into one another. If they’re just out there gently orbiting and everything is going smoothly, then without Jupiter in the picture, they would gather mass and get bigger and bigger and bigger. But with Jupiter there, stirring the pot, then the asteroids start bumping into one another and breaking apart, so nothing grew in that region, but started to shrink,” Russell said.
The formal decision on whether or not Vesta should be reclassified as a protoplanet or some other planetary type falls to the International Astronomical Union, the same body that determined Pluto was a dwarf planet. Whatever happens, whether Vesta becomes a controversial body in a discussion of taxonomy or not, it’s certain to generate a strong reaction. Particularly from Pluto-hugger types who hope to see everything vaguley planet-like classified as a full planet. Mike Brown, notorious for finding the trans-Plutonian object Eris that led to Pluto’s reclassification, tweeted this about Vesta: “Yup. Vesta has some of the same properties that planets do. But, then, so does the rock I am smashing my forehead into repeatedly.”
Vesta’s possible reclassification doesn’t really mean anything more than a change in status. Like Pluto and Eris, a protoplanet isn’t any less scientifically interesting than a “proper” planet. And with the wealth of well-preserved history on Vesta, this much closer record of the solar system is certainly worth going back to study. For now, Dawn is getting ready to leave Vesta this August and making its way over the Ceres, but there are still a couple of months for the spacecraft to unravel a little more of Vesta’s mysteries.