[meteorite-list] Dawn Journal - April 29, 2017

[Ron Baalke]

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

Dawn Journal 
by Dr. Marc Rayman
April 29, 2017

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.

[Haulani Crater in Enhanced Color]
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. 
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.

[Oxo Crater at LAMO]
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. 
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 LAMO Image 60]
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. 
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.

[Dawn LAMO Image 69]
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. 
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.