an idea of the scale of this feature. The event sent monumental seismic shock waves through Vesta which caused
many faults and troughs around its equatorial regions, as
well as blanketing the southern half of the protoplanet
with a thick layer of debris. This explains the less cratered
nature of the south in contrast to the northern regions.
Beyond the rim of Rheasilvia, the rest of Vesta’s
tormented surface is no less spectacular, and the
stories it tells of the distant past will gradually be deciphered by Dawn’s science team. The spacecraft
broke free from Vesta’s sphere of influence in September 2012 to head out towards its next destination,
but its mission there is far from over. Its sensors have
returned sufficient data on this unique world to keep
mission scientists busy for decades, together with
the continuing study of meteorites here on Earth.
Onward to Ceres
The Dawn Mission Trajectory. Credit: NASA-JPL
Having begun to unlock the secrets of one new
world, we look forward to discovering another unique
member of our Sun’s family of worlds. Ceres, named
for the Roman goddess of corn and harvests, grows
ever more clear in Dawn’s cameras, becoming a
whole new world for us. This will be, to quote Dr. Rayman, ‘an ambitious and exciting exploration of the
alien world ahead… an intriguing and mysterious
orb that has beckoned for more than two centuries.…. Our goal is to develop that faint smudge of
light amidst the stars into a richly detailed portrait.’
A major source of heating at Vesta will have been impacts, much more frequent during the accretion process
when there was a lot more material to be swept up in
the inner solar system. There is much evidence of these
impacts, both on its heavily cratered and fractured surface, and also in fragments originating from them, many
Since its discovery on 1st January 1801 by Giuseppe
of which end up falling to Earth and other terrestrial
Piazzi, Ceres has been considered firstly as a planet, then
bodies as meteorites. An estimated 6% of all meteorites
an asteroid (or minor planet) and, since 2006, as a dwarf
observed on our own planet come from Vesta. Analysis
planet. However we classify it today, with a mean diamof materials found in them
eter of 952km (591 miles) it
provides supporting eviis the largest body between
dence for differentiation in
the Sun and Pluto yet to be
the growing protoplanet.
visited by a space probe,
On Vesta itself the scale
and appears to be very
and violence of impacts
different from Vesta and
early in its history is borne
most other bodies in the
out by at least seven cramain asteroid belt. It has a
ters of over 150km in diamdensity closer to that of the
eter. Most impressive is the
big icy moons of the giant
scar of an impact which
planets, like Ganymede
may have come close
(1.93 g/cm3) or Titan (1.88
to smashing Vesta apart,
g/cm3). Ceres is expected
the giant crater named
to present, beneath a thin
Rheasilvia. The impactor
dusty crust, a differentiated
was probably as much
structure, but one with a
as 50km (30 miles) across,
water ice mantle some
and the resulting basin in
100km (60 miles) thick surVesta’s south polar region
rounding a rocky, silicate
is 500km (300 miles) across,
core. The presence of ice is
12km (7.5 miles) deep and
expected to have created
is distinguished by a central
differences in the dwarf
peak which rises to almost
planet’s surface features,
25km (15 miles) above the
when compared to those
basin floor. The image on
of Vesta, due to the more
Vesta’s battered surface as imaged by Dawn’s framing cameras
(full mosaic). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
the preceeding page gives
flexible nature of ice than
30
30
www.RocketSTEM .org