Nasa confirms its Dawn spacecraft is in orbit around dwarf planet Ceres
It has taken the satellite 7.5 years to reach
its destination, and it will spend the next 14
months mapping the diminutive world.
Ceres is the first of the dwarf planets to be
visited by a spacecraft.
It should tell us something about the origins
of the Solar System.
Dawn had been chasing down its quarry at a
relatively slow pace and so was being
captured very gently by the gravity of Ceres.
At closest approach, the separation between
Dawn and Ceres will be roughly 40,000km.
"That's about 10 times closer than the Moon
is to the Earth," explained Robert Mase, the
Dawn project manager at Nasa's Jet
Propulsion Laboratory (JPL).
"Over the next month, we'll re-shape the
orbit and get ready to begin the prime
science phase."
The intention is to progressively lower the
orbit until the probe is just a few hundred km
above the surface.
Similar narrative
The satellite turned up at Ceres having
previously visited asteroid Vesta.
Both objects reside in the belt of rocky debris
that circles the Sun beyond Mars.
Of the two, Ceres is the bigger at 950km
across; Vesta has a diameter of 525km.
The pair should tell a similar story, says Dr
Carol Raymond, the mission's deputy principal
investigator at JPL.
"Both Ceres and Vesta, we believe, are proto-
planets. They were on their way to forming
larger planetary embryos and they were the
type of object that merged to form the
terrestrial planets," she told the BBC's
Inside Science programme.
"But these two stopped before they reached
that evolutionary stage, and so they are
essentially these intact 'time capsules' from
the very beginning of our Solar System; and
that's really the motivation for why Dawn is
going there to explore them in detail."
Dwarf status
Researchers think Ceres' interior is
dominated by a rocky core topped by ice that
is then insulated by rocky lag deposits at the
surface.
A big question the mission hopes to answer is
whether there is a liquid ocean of water at
depth. Some models suggest there could well
be.
The evidence will probably be found in Ceres'
craters which have a muted look to them.
That is, the soft interior of Ceres has
undoubtedly had the effect of relaxing the
craters' original hard outline.
One big talking point has dominated the
approach to the object: the origin and nature
of two very bright spots seen inside a 92km-
wide crater in the Northern Hemisphere.
The speculation is that Ceres has been struck
by something, exposing deeply buried ices.
These will have vaporised on the airless
world, perhaps leaving behind highly
reflective salts.
Ceres is classed as a dwarf planet in the
nomenclature, and Dawn takes the honour of
being the first spacecraft to visit such a body.
The second encounter will occur in a matter
of months when Nasa's New Horizons probe
makes a close flyby of Pluto.
The far more distant Pluto was demoted from
full planet to dwarf status at an international
astronomy meeting in 2006.
