The brightest of the asteroids
Vesta
Discovered in 1807, Vesta is the second most massive body in the main belt after Ceres and the brightest asteroid seen from Earth: on the best nights it can be made out with the naked eye. The Dawn spacecraft, which orbited it for more than a year, found a world with a core, a mantle and a crust, like a planet stopped halfway through its formation.
- Size
- 569 × 554 × 453 km
- Distance from Sun
- 353 M km
- Orbital period
- 3.63 years
- Rotation
- 5.34 hours
- Discovered
- 1807
- Mission
- Dawn (2011–2012)
Physical characteristics
Vesta measures 569 × 554 × 453 km: large enough to have differentiated, that is, to have separated into layers while it was still hot. Dawn's gravity measurements point to an iron-nickel core about 220 km across, wrapped in a rocky mantle and a crust of basalt, the rock of lava flows. That is why it is regarded as a protoplanet: a planetary embryo that has survived almost intact from the first few million years of the Solar System.
The south pole is dominated by Rheasilvia, an impact basin about 500 km across, nearly as wide as Vesta itself, with a central peak more than 20 km high, among the tallest known in the Solar System. The collision that carved it, about a billion years ago, blasted an enormous amount of debris into space. Beneath Rheasilvia lies an older basin, Veneneia, and a set of parallel troughs runs along the equator, a result of those same impacts.
Orbit and rotation
Vesta orbits in the inner part of the main belt, 353 million km from the Sun on average, or 2.36 astronomical units, and takes 3.63 years to complete one lap. Its orbit is only slightly eccentric and tilted 7.1° to the ecliptic. It spins once every 5 hours and 20 minutes, one of the shortest periods among the large asteroids.
Its surface reflects more than 40% of the light it receives, far more than the other large asteroids. That is why Vesta, though smaller than Ceres, is the brightest asteroid in the sky: at its most favourable oppositions it reaches magnitude 5.1 and, from a dark site, can be seen without any instrument.
Discovery and exploration
Heinrich Wilhelm Olbers discovered it from Bremen on 29 March 1807: it was the fourth asteroid known, after Ceres, Pallas and Juno. Its name, that of the Roman goddess of the hearth, was chosen by the mathematician Carl Friedrich Gauss, who had computed its orbit. For nearly two centuries Vesta remained a point of light; only in the 1990s did the Hubble Space Telescope reveal its shape and the great basin at its south pole.
NASA's Dawn spacecraft entered orbit around Vesta on 16 July 2011 and stayed until 5 September 2012, mapping its surface, composition and gravity field. Then, thanks to its ion engines, it set off for Ceres: it was the first spacecraft to orbit two different bodies beyond Earth. Its data confirmed that an entire family of meteorites comes from Vesta.
Frequently asked questions
Can Vesta be seen with the naked eye?
Yes, but only under favourable conditions. When it is at opposition and near perihelion Vesta reaches magnitude 5.1, just within the limit of the human eye under a dark sky. It is the only asteroid that manages this regularly; with binoculars it is easy to find at every opposition.
Is Vesta a dwarf planet?
No. A dwarf planet must have a shape rounded by its own gravity, and Vesta no longer does: the impact that created Rheasilvia flattened it at the south pole. Because of its layered structure it is described as a protoplanet, but the IAU classifies it as an asteroid.
Which meteorites come from Vesta?
The HEDs, short for howardites, eucrites and diogenites: about 6% of the meteorites found on Earth. Their composition matches what Dawn measured in Vesta's crust and mantle. They are fragments blasted into space by the great impacts at the south pole, and they make Vesta one of the few bodies in the Solar System of which we hold samples in the laboratory.
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