Vesta: the asteroid that could have been a planet
New data on Vesta asteroid revealing its complex structure and turbulent history were finally made public. Obtained by NASA's Dawn mission, the first ever spacecraft to orbit an asteroid, they confirm earlier suggestions on Vesta's origin and nature and pose new questions for planetary scientists. The findings show that Vesta could have possibly become a real planet had it not been located in the vicinity of Jupiter.
A batch of articles published in Science magazine marked the completion of the first part of NASA's Dawn mission, targeted at two small bodies of the Solar System believed to be remnants from the first stages of its formation. According to the current theory, Earth and Mars, as well as Venus and Mercury, formed from much smaller bodies through accretion. However, the process was not at all smooth with Jupiter in the vicinity, whose gravity interfered with the formation of one more planet leaving a belt of asteroids instead of one body. This asteroid belt, known as the main belt, now hosts numerous small bodies, Vesta being one of the largest (with radii from 289 to 229 km) and one of the most massive among them. It was believed that Vesta is a protoplanet – a body formed from much smaller planetesimals, which could later have become a planet.
Launched in 2007, Dawn spacecraft spent almost a year (since mid-2011) orbiting Vesta from different altitudes (210 km the minimal one) and examining it with a number of instruments.
This study brought a detailed map of Vesta’s surface and its mineral composition, as well as new data on its gravitational field that could be used to define the body’s inner structure. This information confirmed that Vesta is a differentiated body, i.e. that it is structured into core, mantle and crust, thus differing from lesser asteroids and small bodies that had not undergone internal changes. It seems that Vesta was large enough for its interior to melt, so that heavier matter could form a core, presumably of an average radius of 107 to 113 km. Then, numerous craters and ‘scratches’ on the surface revealed the differences in the mineralogy of the body dependent on the depth, which supports the idea that Vesta once had a magma ocean beneath the crust.
Moreover, the study confirmed that some of the meteorites found on Earth could be of Vestan origin (so-called “Howardite-Eucrite-Diogenite” meteorites), which makes Dawn mission the first to trace the matter on Earth to space.
Dawn has also provided data to determine the age of Vesta’s depressions, namely the largest one – Rheasilvia basin (500 km wide and 19 km deep). It overlaps the older Veneneia basin (400 km wide) in the southern hemisphere. Both of them are about 1–2 billion years old. The latter of them might have been caused by a violent impact that led to the formation of many smaller asteroids, and those that eventually reached the Earth. However, most of Vesta’s craters formed earlier than these two, partly resembling the lunar surface and Earth-like planets.
All of the evidence confirms the idea that Vesta is a missing link between undifferentiated smaller asteroids and ‘large’ terrestrial planets, or a protoplanet – a body that would have possibly become a larger one, had it been not in the vicinity of the Jupiter.
Dawn spacecraft currently sits near Vesta, gradually ascending from the low-altitude orbit (around 210 km since December 2011) since an additional time of 40 days was allocated to the mission. However, in August 2012 it will bid farewell to Vesta and head towards Ceres, the second target body of the mission. Larger than Vesta (with average radius of about 470 km) and the largest in the main belt, Ceres is unlike Vesta in many ways. For instance, its observational characteristics are not that diverse. Earlier studies showed that water might have contributed to Ceres’ formation and slowed its differentiation, hence the search for subsurface water deposits would be an important part of the future investigations. No asteroid family is associated with Ceres.
The initial idea of the Dawn mission designers was to study these two apparently different bodies with the same number of instruments, thus encompassing the scope of events which happened to the early Solar System. The rendezvous with Ceres is planned for February 2015 and the end of the primary mission is scheduled for July 2015.
The hunt for the most primitive bodies of the Solar System remains one of the most interesting and challenging tasks, both scientifically and technically. For instance, Japanese mission Hayabusa that apparently managed to deliver some samples of the Itokawa asteroid to Earth, has suffered several technical failures and was literally saved by the Earth operators. If Russian Phobos-Grunt had succeeded, it would have combined both the advantages of distant (like Dawn) and in situ measurements (which have never been performed before) with sample return, thus providing Earth laboratories with extraterrestrial matter.
Meanwhile there are two more spacecraft en route to smaller bodies of the Solar System. The first to get to the target is the European Rosetta mission, which will meet the comet Churyumov-Gerasimenko in 2014 and send a lander to the surface. Underway to much more faraway asteroid belt is another NASA spacecraft New Horizons, that is on path to Pluto (encounter is expected in 2015) and further to the objects of the Kuiper belt in the outskirts of the Solar System. While asteroids in the main belt are mostly rocks, metal, and ice, the Kuiper belt’s bodies are believed to be largely frozen volatiles. However, it also hosts debris from the earliest stage of the Solar System formation. One may presume that comparing one to the other may bring invaluable information on the origin and evolution of the Solar System.
Dawn's success once again revives the failure of Phobos-Grunt and poses a question – whether it is worth repeating the mission. Such operations take time – the idea for Dawn appeared around a decade ago – and their life is not always smooth. Dawn had been cancelled and postponed several times before it reached the launch pad. Even if the project is approved, its timeline might be much longer than expected. However, the task of studying the Solar System will most likely be top priority for years to come, so that Phobos, a likely asteroid captured by the planet, would be a good target all the same.