Getting up close
Since entering into orbit around Ceres on 6th March 2015, after its
two and a half year cruise from Vesta, Dawn’s science instruments have
been revealing its nature in detail from a series of ever-closer orbits.
Following a brief period in a ‘rotation characterisation’ orbit, Dawn spi-
ralled inwards and took up its ‘Survey orbit’ 4,430km (2,750 miles) out on
6th June 2015, from which it made detailed global maps of the dwarf
planet with its framing camera and visible and infrared mapping spec-
trometer (VIR).
Following a problem with one of its ion engines, which was resolved
successfully, Dawn then descended to reach its High-Altitude Mapping
Orbit (HAMO) at 1,480 km (920 miles) in August 2015. From this altitude,
it spent a period of two months mapping Ceres at higher resolution,
again with the framing camera and VIR, before spiralling slowly inwards
once more to reach a Low-Altitude Mapping Orbit (LAMO) in Decem-
ber 2015, a mere 375km (233 miles) above the Cerean surface. From
this low altitude the spacecraft’s instruments, particularly its gamma-ray
and neutron detector (GRaND), were focused on trying to determine
the chemical composition at and just below the surface of Ceres.
DAWN’S SPIRAL TRANSFER FROM HAMO TO LAMO: This diagram shows Dawn’s patient spiral descent of 160 revolu-
tions over a two-month period towards its closest orbit around Ceres, using its ion thrusters. For clarity, the trajectory is
shown changing from blue to red as time progresses over this period. Red dashed sections are where ion thrusting was
stopped so the spacecraft could point its main antenna towards Earth. Once in LAMO, closer to Ceres than the ISS is to
Earth, the spacecraft was orbiting Ceres every 5.5 hours. Credit: NASA/JPL-Caltech
From LAMO, Dawn was also able to probe deeper down to try and
discover the dwarf planet’s internal structure. By using radiometric data
– that is, detecting tiny variations in the spacecraft’s orbit from Doppler
shifts in the radio waves of signals transmitted back to Earth - the grav-
ity field and, thus, internal distribution of mass in Ceres’ interior can be
determined. As Marc Rayman explains ‘If, for example, there is a large
region of unusually dense material, even if deep underground, the craft
will speed up slightly as it travels toward it. After Dawn passes overhead,
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