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http://dawnblog.jpl.nasa.gov/2016/04/29/dawn-journal-april-29-2/

Dawn Journal
by Dr. Marc Rayman
April 29, 2016

Dear Glutdawnous Readers,

The distant dwarf planet that Dawn is circling is full of mystery and
yet growing ever more familiar. Ceres, which only last year was hardly
more than a fuzzy blob against the stars, is now a richly detailed world,
and our portrait grows more elaborate every day. Having greatly surpassed
all of its original objectives, the reliable explorer is gathering still
more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share
in the sumptuous feast Dawn has been serving.

One of the major objectives of the mission was to photograph 80 percent
of Ceres' vast landscape with a resolution of 660 feet (200 meters)
per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution
of 120 feet (35 meters) per pixel.


This image of Haulani Crater uses color pictures Dawn acquired during
its third mapping orbit at an altitude of 915 miles (1,470 kilometers).
We saw the crater from the same altitude in black and white here. This
false color picture highlights differences in composition or other properties that your eye would not be able to detect. In this color scheme, blue
is associated with geologically young material, consistent with the description

of the black and white image as showing a young crater. It is easy to
see that the surrounding region was affected by the formation of the crater. (The last picture below shows the area around another crater that was
altered by an impact.) Also note the variation in terrain within the crater, including a prominent ridge in the center. The crater is 21 miles (34 kilometers) in diameter. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

This example of Dawn's extraordinary productivity may appear to be the
limit of what it could achieve. After all, the spaceship is orbiting at
an altitude of only 240 miles (385 kilometers), closer to the ground than
the International Space Station is to Earth, and it will never go lower
for more pictures. But it is already doing more.

Since April 11, instead of photographing the scenery directly beneath
it, Dawn has been aiming its camera to the left and forward as it orbits
and Ceres rotates. By May 25, it will have mapped most of the globe from
that angle. Then it will start all over once more, looking instead to
the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring
out the full three dimensionality of the alien world. Dawn already accomplished

this in its third mapping orbit from four times its current altitude,
but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.

[Image]
Dawn captured this view of Oxo Crater on Jan. 16 from an altitude of 240
miles (385 kilometers). Although it is a modest six miles (10 kilometers) across, it is a particularly interesting crater. This is the only location
(so far) on Ceres where Dawn has clearly detected water. Oxo is the second brightest area on Ceres. Only Occator Crater is brighter. Oxo also displays
a uniquely large "slump" in its rim, where a mass of material has
dropped below the surface. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is also earning extra credit on its assignment to measure the energy
of gamma rays and neutrons. We have discussed before how the gamma ray
and neutron detector (GRaND) can reveal the atomic composition down to
about a yard (meter) underground, and last month we saw initial findings
about the distribution of hydrogen. However, Ceres' nuclear glow is
very faint. Scientists already have three times as much GRaND data from
this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving
a longer exposure, making its nuclear picture of Ceres brighter and sharper.

In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn
about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength
of the dwarf planet's gravitational tug depends on the distance, even
finer measurements of how it varies from location to location are possible
in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they
had anticipated. With additional measurements, they may be able to squeeze
out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.


Dawn took this picture on Feb. 8 at an altitude of 240 miles (385 kilometers). Prominent in the center is part of a crater wall, which shows many scars
from subsequent impacts, indicating it is old. Two sizable younger craters with bright material, which is likely some kind of salt, are evident inside the larger crater. Compare the number and size of craters in this scene
with those in the younger scene below showing an area of the same size.
Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn has dramatically overachieved in acquiring spectra at both visible
and infrared wavelengths. We have previously delved into how these measurements

reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra
and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January
that VIR has such a narrow view that it will never see all of Ceres from
this close, so it is programmed to observe features that have caught scientists'
interest based on the broad coverage from higher altitudes.

[Image]
Dawn took this picture on Feb. 16 (eight days after the picture above)
at an altitude of 240 miles (385 kilometers). It shows a region northwest
of Occator Crater, site of the famous bright region (which may become
one of the most popular tourist destinations on Ceres). (You can locate
this area in the upper right of the mosaic shown last month.) Compare
the number and size of craters in this scene with those in the older scene above showing an area of the same size. There are fewer craters here,
because the material ejected from the impact that excavated Occator resurfaced the area nearby, erasing the craters that had formed earlier. Because
Occator is relatively young (perhaps 80 million years old), there has
not been enough time for as many new craters to form as in most other
areas on Ceres, including the one shown in the previous picture, that
have been exposed to pelting from interplanetary debris for much longer.
Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn's remarkable success at Ceres was not a foregone conclusion. Of
course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was
further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices
aren't actually lost. We know precisely where they are. But given that
one stopped functioning in 2010 and the other in 2012, they might as well
be elsewhere in the universe; they don't do Dawn any good.) Without
three of these units to control its orientation in space, the robot has
relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming
Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.

In the meantime, Dawn is continuing its intensive campaign to reveal the
dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would
have been impossible without ion propulsion.) The spacecraft provided
us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.

For an interplanetary traveler, terrestrial days have little meaning.
They are merely a memory of how long a faraway planet takes to turn on
its axis. Dawn left that planet long ago, and as one of Earth's ambassadors
to the cosmos, it is an inhabitant of deep space. But for those who keep
track of its progress yet are still tied to Earth, on May 3 the journey
will be pi thousand days long. (And for our nerdier friends and selves,
it will be shortly after 6:47 p.m. PDT.)

By any measure, Dawn has already accomplished an extraordinary mission,
and there is more to look forward to as its ambitious expedition continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346
million miles, or 558 million kilometers) from Earth, or 1,455 times as
far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour
and two minutes to make the round trip.

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