Sky and Telescope - April 2015 - 26

Asteroid Adventure

SOURCE: NASA \ JPL-CALTECH, S&T: GREGG DINDERMAN

High-altitude mapping orbit
910 miles
1,470 kilometers

Survey orbit
2,700 miles
4,400 kilometers

Low-altitude
mapping orbit
230 miles
375 kilometers

First full characterization
8,400 miles
13,500 kilometers

SCIENCE ORBITS Dawn will conduct four diﬀerent orbits
around Ceres, spiraling down from the outermost to the innermost over a few months.

What we do know about Ceres is that it is large, it is
unmistakably round, and it is brighter in some places
than in others. Those three facts make it very likely that
Ceres has an exciting story to tell.

Looping an Asteroid

DECEMBER: NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA;
JANUARY: NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA / PSI

Dawn's science plans for Ceres are very similar to the survey it performed at Vesta, with a few tweaks. Dawn began
its approach observations of Ceres in January 2015. The
very ﬁrst images Dawn acquires during approach will be
similar in quality to Hubble's of this little world, and they
will only get better from there.
As Dawn approaches, it will take nine sets of photos
of Ceres while also searching nearby space for possible
moons. These will include perform two "rotation characterizations," where it will watch Ceres through one
complete 9-hour day, imaging the entire globe with both
camera and spectrometer in visible and infrared bands.

December 1, 2014

April 2015 sky & telescope

Dawn will arrive in its ﬁrst science orbit in late April,
a polar one at an altitude of 13,500 kilometers (8,400
miles), with a leisurely orbital period of 15 days. That's far
enough away for Ceres to still ﬁt comfortably within the
Framing Camera's ﬁeld of view, around 700 pixels across.
Dawn will watch Ceres rotate through three complete
Cerean days, thoroughly characterizing its three-dimensional shape. Once its orbit takes it to Ceres' nightside,
it will repeat the observation, capturing what will be an
exciting set of images of Ceres in a crescent phase.
It will take a month for the gentle pressure of Dawn's
ion engines to shift it down to its survey orbit altitude of
4,400 km, where the spacecraft will spend three weeks
acquiring global maps with camera and spectrometer.
Another six weeks of orbit adjustment will take it to its
high-altitude mapping orbit, circling Ceres once every 19
hours at an altitude of 1,470 km. A 12-orbit cycle will carry
Dawn over every inch of Ceres' surface. From this highaltitude orbit, Dawn will cover Ceres six times - once
looking straight down, and the rest at a variety of angles
in order to measure what may be subtle topographic rises
and falls.
Finally, Dawn will perform its last orbital shift,
descending to a low-altitude mapping orbit only 375
kilometers above the surface, about the same as the
International Space Station's orbit above Earth. Here, its
GRAND spectrometer will be able to most strongly sense
the sparse neutrons emanating from Ceres' surface; over
time, their energies and locations will tell the Dawn team
about the distribution of chemical elements.
While GRAND is mapping the surface, Dawn will
also stay in nearly continuous contact with NASA's Deep
Space Network. From tiny shifts in Dawn's orbital speed
as it circles Ceres, the mission team will be able to map
Ceres' gravity ﬁeld, looking for clues to its internal
structure. Dawn will also acquire image data - the
highest resolution of the mission - during the lowest
orbit, but the focus of this phase is composition and
gravity mapping.
In low-altitude mapping orbit, Dawn will prolong its
hydrazine supply by switching to a "hybrid mode" of
pointing, where it will use its two remaining functional

January 13, 2015

LAND HO! Ceres
was a dot to
Dawn's Framing Camera at a
distance of three
Earth-Moon separations ( far left).
When the distance
closed to match
the Moon-Earth
system, Dawn
saw mottling and
a bright spot.

Sky and Telescope - April 2015

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