[meteorite-list] Dawn Journal: June 30, 2014

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 1 Jul 2014 13:09:43 -0700 (PDT)
Message-ID: <201407012009.s61K9hNi012691_at_zagami.jpl.nasa.gov>


Dawn Journal
By Marc Rayman
June 30, 2014

Dear Mastodawns,

Deep in the main asteroid belt, between Mars and Jupiter, far from
Earth, far from the sun, far now even from the giant protoplanet Vesta
that it orbited for 14 months, Dawn flies with its sights set on dwarf
planet Ceres. Using the uniquely efficient, whisper-like thrust of its
remarkable ion propulsion system, the interplanetary adventurer is
making good progress toward its rendezvous with the uncharted, alien
world in about nine months.

Dawn's ambitious mission of exploration will require it to carry out a
complex plan at Ceres. In December, we had a preview of the "approach phase,"
and in January, we saw how the high velocity beam of xenon ions will let the
ship slip smoothly into Ceres's gravitational embrace. We followed that
with a description in February of the first of four orbital phases (with the
delightfully irreverent name RC3), in which the probe will scrutinize the
exotic landscape from an altitude of 8,400 miles (13,500 kilometers). We
saw in April how the spacecraft will take advantage of the extraordinary
maneuverability of ion propulsion to spiral from one observation orbit to
another, each one lower than the one before, and each one affording a more
detailed view of the exotic world of rock and ice. The second orbit, at an
altitude of about 2,730 miles (4,400 kilometers), known to insiders
(like you, faithful reader) as "survey orbit," was the topic of our
preview in May. This month, we will have an overview of the plan for the
third and penultimate orbital phase, the "high altitude mapping orbit"

(The origins of the names of the phases are based on ancient ideas, and
the reasons are, or should be, lost in the mists of time. Readers should
avoid trying to infer anything at all meaningful in the designations.
After some careful consideration, your correspondent chose to use the
same names the Dawn team uses rather than create more helpful
descriptors for the purposes of these logs. What is important is not
what the different orbits are called but rather what amazing new
discoveries each one enables.)

It will take Dawn almost six weeks to descend to HAMO, where it will be
910 miles (1,470 kilometers) high, or three times closer to the
mysterious surface than in survey orbit. As we have seen before, a
lower orbit, whether around Ceres, Earth, the sun, or the Milky Way
galaxy, means greater orbital velocity to balance the stronger
gravitational grip. In HAMO, the spacecraft will complete each loop
around Ceres in 19 hours, only one quarter of the time it will take in
survey orbit.

Dawn's spiral descent from survey orbit to the high altitude mapping
orbit. The trajectory progresses from blue to red over the course of the
six weeks. The red dashed segments are where the spacecraft is not
thrusting with its ion propulsion system (as explained in April).
Credit: NASA/JPL

In formulating the HAMO plans, Dawn's human colleagues (most of whom
reside much, much closer to Earth than the spacecraft does) have taken
advantage of their tremendous successes with HAMO1 and HAMO02 at Vesta. We
will see below, however, there is one particularly interesting difference.

As in all observation phases at Ceres (and Vesta), Dawn's orbital path
will take it from pole to pole and back. It will fly over the sunlit
side as it travels from north to south and then above the side in the
deep darkness of night on the northward segment of each orbit. This
polar orbit ensures a view of all latitudes. As Ceres pirouettes on its
axis, it presents all longitudes to the orbiting observer. The mission
planners have choreographed the celestial pas de deux so that in a dozen
revolutions, Dawn's camera can map nearly the entire surface.

Rather than mapping once, however, the spacecraft will map Ceres up to
six times. One of Dawn's many objectives is to develop a topographical
map, revealing the detailed contours of the terrain, such as the depths
of craters, the heights of mountains, and the slopes and variations of
plains. To do so, it will follow the same strategy employed so
successfully at Vesta, by taking pictures at different angles, much like
stereo imaging. The spacecraft will make its first HAMO map by aiming
its camera straight down, photographing the ground directly beneath it.
Then it will map the surface again with the camera pointed in a slightly
different direction, and it will repeat this for a total of six maps, or
six mapping "cycles." With views from up to six different perspectives,
the landscape will pop from flat images into its full three
dimensionality. (As with all the plans, engineers recognize that complex
and challenging operations in the forbidding, unforgiving depths of
space do not always go as intended. So they plan to collect more data
than they need. If some of the images, or even entire maps, are not
acquired, there should still be plenty of pictures to use in revealing
the topography.)

In addition to acquiring the photos, Dawn will make other measurements
in HAMO. During some of the cycles, the camera will use its color to
glean more about the nature of the surface. The visible and infrared
mapping spectrometer will collect spectra to help scientists determine
the composition of the surface, its temperature, and other properties.

Exquisitely accurate radio tracking of the spacecraft in its orbit, as
indicated by the Doppler shift (the change in frequency, or pitch, as
the craft moves toward or away from Earth) and by the time it takes
radio signals to make the round trip from Earth, allows navigators to
determine the strength of the gravitational tugging. That can be
translated into not only the mass of Ceres but also how the mass is
distributed in its interior. In August, when we look ahead to the fourth
and final science phase of the Ceres mission, the low altitude mapping
orbit, we will explain this in greater detail.

Although still too high for anything but the weakest indication of
radiation from Ceres, the gamma ray and neutron detector will measure
the radiation environment in HAMO. This will yield a useful reference
for the stronger signals it will detect when it is closer.

There is a noteworthy difference between how Dawn will operate in HAMO
and how it operated in HAMO1 and HAMO2 at Vesta and even how it will
operate in survey orbit at Ceres. In those other orbits, whenever the
spacecraft flies above the hemisphere in sunlight, it keeps its sensors
pointed at the surface, and whenever it is over the night side, it
points its main antenna to Earth. At Vesta, where each HAMO revolution
took just over 12 hours, this meant that about every six hours, it had
to execute a turn. Were it to follow the same plan at Ceres with a
19-hour HAMO period, when it passed over the north pole, it would begin
aiming its scientific instruments at the dwarf planet. When it reached
the south pole 9.5 hours later, it would rotate to point its antenna to
Earth. Another 9.5 hours after that, when it reached the north pole
again, it would pivot to bring the alien terrain back into its sights.

If the robot had its full complement of functioning reaction wheels,
that is what it would do in HAMO. Reaction wheels are similar to
gyroscopes, and by electrically changing the speed at which they spin,
the probe can turn or stabilize itself. The mission was designed to use
three reaction wheels, so the ship was outfitted with four. Two are no
longer operable. While such a loss could be devastating for some
spacecraft, the Dawn flight team has devised highly innovative solutions
to accomplish all of the original, ambitious objectives, regardless of the
condition of any of the wheels, even the two that are (currently) still
healthy. Key to Dawn's success will be conserving hydrazine, the
conventional rocket fuel that it can use to accomplish turns. Dawn's
controllers are taking care with every soup??on of the precious
propellant, stretching the supply to allow the mission to complete its
bold plans. When the hydrazine is exhausted, Dawn's expedition will

Turning so often in HAMO, keeping up with the frequent transitions
between flying over the illuminated surface and the surface in the
darkness of night, would be unaffordable without reaction wheels, a
profligate use of the irreplaceable hydrazine. Instead, it is
significantly more efficient to turn less often, allowing the spacecraft
sometimes to wait patiently for half an orbit as its instruments stare
at the undetectably dark land beneath it and sometimes to maintain its
antenna pointing at Earth, even when it is passing over features it
otherwise could see. It will see them on other loops however. With this
strategy, Ceres can be mapped extensively in HAMO without consuming an
excessive amount of hydrazine.

In each mapping cycle, Dawn will make two and a half or three and a half
revolutions peering at Ceres, storing images and other valuable data
onboard. (The specific duration varies from one cycle to another.) Then,
with its memory full, it will turn so it can beam some of its precious
findings to distant Earth while it is on the night side of Ceres. That
will not be long enough to completely empty the memory but will be
sufficient to make room for more data, so after half an orbit, it will
turn back to resume its observations. It will follow this pattern for
one full cycle, with the dozen passages over the day side providing
enough opportunities to complete one map. Then it will devote two and a
half revolutions, or two full days, to transmitting the rest of its
scientific treasures for the benefit of all those on Earth who ever look
to the sky with wonder.

So over the course of 14 complete circuits around Ceres in 11 days, the
spacecraft will turn only six or eight times. Ever the responsible
conservationists, the team developed all the details of this plan to
acquire as much data as possible with the minimum expenditure of hydrazine.

It will take more than two months to carry out all the HAMO activities,
with the spacecraft making more than 80 orbital loops. This continues
the trend in which the explorer will spend more time in each successive
orbital phase than in the ones before. It will complete its assignment
in survey orbit in 22 days, during which it will circle Ceres seven
times. As we will see in August, the final orbital phase will last even
longer than HAMO and include many more revolutions.

Each phase of the mission at Ceres will reveal exciting new insights
into a relict from the dawn of the solar system. That same solar
system's complex ballet happens to be playing out now in a way that
affords terrestrial observers a nice view of Ceres, Vesta, Mars and the
moon. (It also affords Cerean observers a nice view of Vesta, Mars, the
moon, and Earth, but that will be described in more detail in the
special Cerean local edition of this log.) We wrote in March
about the alignment and provided a chart you can still use to locate
Vesta and Ceres with a small telescope or even good binoculars. On July
5, Ceres and Vesta will appear to be separated by only one third the
diameter of the full moon, even as these distant worlds are 0.57 AU (52
million miles, or 85 million kilometers) from each other. In Earth's
skies, Mars and the moon (both of which are closer to Earth) will not be
far away, all of them in Virgo.

Although even the most powerful telescopes are quite insufficient to
show it, when we turn our mind's eye to the sky, with its greater visual
acuity, we can discern one more object in this lovely arrangement of
gleaming celestial jewels set against the backdrop of the incomparable
blackness of the universe. A probe from Earth, a robotic ambassador to
the cosmos, on a long and daring expedition, is in transit from Vesta to
Ceres. Even as those terrestrial observers enjoy the view, Dawn is
patiently making its way through the interplanetary void to a world that
has been glimpsed only from afar for more than two centuries. Soon it
will undertake a new phase of its extraordinary mission, promising
exciting new knowledge and surprising new insights. Engaged in one of
humankind's grand adventures, we extend the best we have within
ourselves to reach far, far beyond our humble home.

Dawn is 5.6 million miles (9.0 million kilometers) from Ceres. It is
also 2.24 AU (208 million miles, or 335 million kilometers) from Earth,
or 825 times as far as the moon and 2.20 times as far as the sun today.
Radio signals, traveling at the universal limit of the speed of light,
take 37 minutes to make the round trip.
Received on Tue 01 Jul 2014 04:09:43 PM PDT

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