[meteorite-list] Dawn Journal - February 28, 2014

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sat, 1 Mar 2014 18:29:31 -0800 (PST)
Message-ID: <201403020229.s222TVJo014434_at_zagami.jpl.nasa.gov>

P
http://dawnblog.jpl.nasa.gov/2014/02/28/dawn-journal-february-28-2014/

Dawn Journal
Dr. Marc Rayman
February 28, 2014

Dear Ardawnt Readers,

Continuing its daring mission to explore some of the last uncharted worlds
in the inner solar system, Dawn remains on course and on schedule for
its rendezvous with dwarf planet Ceres next year. Silently and patiently
streaking through the main asteroid belt between Mars and Jupiter, the
ardent adventurer is gradually reshaping its orbit around the sun with
its uniquely efficient ion propulsion system. Vesta, the giant protoplanet
it unveiled during its spectacular expedition there in 2011-2012, grows
ever more distant.

[Image]
Following its gravitational capture by Ceres during the approach phase,
Dawn will continue to use its ion propulsion system to spiral to RC3 orbit
at an altitude of 8,400 miles (13,500 kilometers). Credit: JPL/NASA

In December and January, we saw Dawn's plans for the "approach phase"
to Ceres and how it will slip gracefully into orbit under the gentle control
of its ion engine. Entering orbit, gratifying and historic though it will
be, is only a means to an end. The reason for orbiting its destinations
is to have all the time needed to use its suite of sophisticated sensors
to scrutinize these alien worlds.

As at Vesta, Dawn will take advantage of the extraordinary capability
of its ion propulsion system to maneuver extensively in orbit at Ceres.
During the course of its long mission there, it will fly to four successively
lower orbital altitudes, each chosen to optimize certain investigations.
(The probe occupied six different orbits at Vesta, where two of them followed
the lowest altitude. As the spacecraft will not leave Ceres, there is
no value in ascending from its fourth and lowest orbit.) All of the plans
for exploring Ceres have been developed to discover as much as possible
about this mysterious dwarf planet while husbanding the precious hydrazine
propellant, ensuring that Dawn will complete its ambitious mission there
regardless of the health of its reaction wheels.

All of its orbits at Ceres will be circular and polar, meaning the spacecraft
will pass over the north pole and the south pole, so all latitudes will
come within view. Thanks to Ceres' own rotation, all longitudes will be
presented to the orbiting observer. To visualize this, think of (or even
look at) a common globe of Earth. A ring encircling it represents Dawn's
orbital path. If the ring is only over the equator, the spacecraft cannot
attain good views of the high northern and southern latitudes. If, instead,
the ring goes over both poles, then the combined motion of the globe spinning
on its axis and the craft moving along the ring provides an opportunity
for complete coverage.

Dawn will orbit in the same direction it did at Vesta, traveling from
north to south over the side illuminated by the distant sun. After flying
over the south pole, it will head north, the land directly beneath it
in the dark of night. (The orbit geometry is designed to ensure that the
spacecraft always remains in sunlight, regardless of the lighting at the
surface.) When it travels over the north pole, the terrain below will
come into daylight and the ship will sail south again.

Dawn's first orbital phase is distinguished not only by providing the
first opportunity to conduct intensive observations of Ceres but also
by having the least appealing designation of any of the Ceres phases.
It is known as RC3, or the third "rotation characterization" of the Ceres
mission. (RC1 and RC2 will occur during the approach phase, as described
in December.) What RC3 lacks in the catchiness of its name it more than
makes up for in the incredible coolness of what it will accomplish.

During RC3 in April 2015, Dawn will have its first opportunity for a global
characterization of its new residence in the asteroid belt. It will take
many pictures and record visible and infrared spectra of the surface,
which will help scientists determine its composition. In addition to learning
about the appearance and makeup of Ceres, these observations will allow
scientists to establish exactly where Ceres' pole points. The axis Earth
rotates around, for example, happens to point very near a star that has
been correspondingly named Polaris, or the North Star. [Note to editors
of local editions: You may change the preceding sentence to describe wherever
the axis of your planet points.] We know only roughly where Ceres' pole
is from our telescopic studies, but Dawn's measurements in RC3 will yield
a much more accurate result. Also, as the spacecraft circles the behemoth,
navigators will measure the strength of Ceres' gravitational attraction
and hence its overall mass.

RC3 will be at an orbital altitude of about 8,400 miles (13,500 kilometers).
>From there, the dwarf planet will appear eight times larger than the moon
as viewed from Earth, or about the size of a soccer ball seen from 10
feet (3.1 meters). At that distance, Dawn will be able to capture the
entire disk of Ceres in its pictures. The explorer's camera, designed
for mapping unfamiliar extraterrestrial landscapes from orbit, will see
details more than 20 times finer than we have now from the Hubble Space
Telescope.

[Image]
Four views of Ceres as it rotates, as seen with the Hubble Space Telescope,
are the best we have. The brightest feature has been exaggerated here.
Dawn's pictures in RC3 will show detail more than 20 times finer. Credit:
NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell
University), and L. McFadden (University of Maryland, College Park)

Although all instruments will be operated in RC3, the gamma ray and neutron
detector (GRaND) will not be able to detect the faint nuclear emissions
from Ceres when it is this far away. Rather, it will measure cosmic radiation.
In August we will learn more about how GRaND will measure Ceres' atomic
composition when it is closer.

It will take about 15 days to complete a single orbital revolution at
this altitude. Meanwhile, Ceres turns once on its axis (one Cerean day)
in just over nine hours (more than two and a half times faster than Earth).
Dawn's leisurely pace compared to the spinning world beneath it presents
a very convenient way to map it. It is almost as if the probe hovers in
place, progressing only through a short arc of its orbit as Ceres pirouettes
helpfully before it.

When Dawn is on the lit side of Ceres over a latitude of about 43 degrees
north, it will point its scientific instruments at the unknown, exotic
surface. As Ceres completes one full rotation, the robot will fill its
data buffers with as much as they can hold, storing images and spectra.
By then, most of the northern hemisphere will have presented itself, and
Dawn will have traveled to about 34 degrees north latitude. The spacecraft
will then aim its main antenna to Earth and beam its prized findings back
for all those who long to know more about the mysteries of the solar system.
When Dawn is between 3 degrees north and 6 degrees south latitude, it
will perform the same routine, acquiring more photos and spectra as Ceres
turns to reveal its equatorial regions. To gain a thorough view of the
southern latitudes, it will follow the same strategy as it orbits from
34 degrees south to 43 degrees south.

When Dawn goes over to the dark side, it will still have important measurements
to make (as long as Darth Vader does not interfere). While the ground
immediately beneath it will be in darkness, part of the limb will be illuminated,
displaying a lovely crescent against the blackness of space. Both in the
southern hemisphere and in the northern, the spacecraft will collect more
pictures and spectra from this unique perspective. Dawn's orbital dance
has been carefully choreographed to ensure the sensitive instruments are
not pointed too close to the sun.

Although it is not the primary objective of the measurements, team members
are working to determine whether observations from the vantage point of
the night side of RC3 might shed more light on the recent fascinating
detection of water vapor around Ceres by the Herschel Space Observatory.
Whether the water is lofted into space by ice sublimating on the surface
or by geysers or cryovolcanoes ("cold volcanoes," which may be active
on this small, frigid world of rock and ice far from the sun) is not yet
known. Scientists do not even know whether any water vapor will still
be there when Dawn is. Even if it is not, it may be that signs of water
will be evident on the surface from other measurements. We will discuss
this intriguing possibility more in the December 2014 log.

Dawn's controllers will take advantage of the flexibility afforded by
ion propulsion to guide the spacecraft into whatever part of the RC3 orbit
turns out to be most efficient, based on details of the trajectory as
it closes in on Ceres. So, for example, if it spirals down to RC3 over
the unlit side, its observations of the day hemisphere will first be in
the north, then the equator, then the south. But if it arrives in RC3
over the low northern latitudes on the side lit by the sun, it will begin
its observations over the equator and then continue in the south. After
it flies north over the other side and then returns to the half of Ceres
that is in daylight, it will be ready to conclude RC3 by collecting its
northern hemisphere data. The flight team has formulated the plan so that
the activities can be executed in whatever order is most natural. The
schedule will be finalized during the approach phase, and readers may
rest assured that the answer will be presented in these logs.

If all goes according to plan, which is never assured when undertaking
challenging tasks in a forbidding, distant, alien environment that has
never even been visited by a flyby spacecraft for an initial reconnaissance,
Dawn will collect in excess of 1,000 pictures and several million spectra
in RC3. After that rich bounty is securely on Earth, it will resume ion
thrusting to lower its altitude to the next orbit. We will discuss the
spiral descent in April and that second observation phase in May.

Dawn's first inspection of Ceres in RC3 promises both to provide tremendous
advancements in our knowledge and whet our appetites for its subsequent
examinations. The most massive resident of the main asteroid belt was
also the first one to be discovered. Yet for the more than two centuries
since then, our glimpses from afar have shown little more than a fuzzy
round dot. That distant orb, shining among the stars, has intrigued us
for so long. When finally its invitation for an ambassador from Earth
is answered next year, the secrets it has held since the dawn of the solar
system will begin to be revealed. The rewards for the long and challenging
journey will be new insights, new understanding, and new fuel for the
fires that burn within everyone who feels the passion to explore.

Dawn is 14 million miles (22 million kilometers) from Ceres. It is also
1.76 AU (163 million miles, or 263 million kilometers) from Earth, or
725 times as far as the moon and 1.77 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take
29 minutes to make the round trip.
Received on Sat 01 Mar 2014 09:29:31 PM PST