[meteorite-list] Dawn Journal - June 29, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/06/29/dawn-journal-june-29/

Dawn Journal 
by Marc Rayman
June 29, 2015

Dear Evidawnce-Based Readers,

Dawn is continuing to unveil a Ceres of mysteries at the first dwarf planet 
discovered. The spacecraft has been extremely productive, returning a 
wealth of photographs and other scientific measurements to reveal the 
nature of this exotic alien world of rock and ice. First glimpsed more 
than 200 years ago as a dot of light among the stars, Ceres is the only 
dwarf planet between the sun and Neptune.

Dawn has been orbiting Ceres every 3.1 days at an altitude of 2,700 miles 
(4,400 kilometers). As described last month, the probe aimed its powerful 
sensors at the strange landscape throughout each long, slow passage over 
the side of Ceres facing the sun. Meanwhile, Ceres turned on its axis 
every nine hours, presenting itself to the ambassador from Earth. On the 
half of each revolution when Dawn was above ground that was cloaked in 
the darkness of night, it pointed its main antenna to that planet far, 
far away and radioed its precious findings to eager Earthlings (although 
the results will be available for others throughout the cosmos as well). 
Dawn began this second mapping campaign (also known as "survey orbit") 
on June 5, and tomorrow it will complete its eighth and final revolution.

The spacecraft made most of its observations by looking straight down 
at the terrain directly beneath it. During portions of its first, second 
and fourth orbits, however, Dawn peered at the limb of Ceres against the 
endless black of space, seeing the sights from a different perspective 
to gain a better sense of the lay of the land.

[Image]
The brightest spots on Ceres. The largest is about four miles (seven kilometers) 
across. While a picture is worth a thousand words, "wow" might summarize 
this picture pretty well. The same spots can be seen on the limb in a 
picture below. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

And what marvels Dawn has beheld! How can you not be mesmerized by the 
luminous allure of the famous bright spots? They are not, in fact, a source 
of light, but for a reason that remains elusive, the ground there reflects 
much more sunlight than elsewhere. Still, it is easy to imagine them as 
radiating a light all their own, summoning space travelers from afar, 
beckoning the curious and the bold to venture closer in return for an 
attractive reward. And that is exactly what we will do, as we seek the 
rewards of new knowledge and new insights into the cosmos.

Although scientists have not yet determined what minerals are there, Dawn 
will gather much more data. As summarized in this table, our explorer 
will map Ceres again from much closer during the course of its orbital 
mission. New bright areas have shown up in other locations too, in some 
places as relatively small spots, in others as larger areas (as in the 
photo below), and all of them will come into sharper focus when Dawn descends 
further.

[Image]
There is bright material easily visible inside and around the crater near 
the upper right. Did the powerful impact that excavated the crater deposit 
bright material that it brought from elsewhere in space, excavate bright 
material from underground or create the conditions that subsequently caused 
some material to become bright? The reason for the greater reflectivity 
is not yet known. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

In the meantime, you can register your opinion for what the bright spots 
are. Join more than 100 thousand others who have voted for an explanation 
for this enigma. Of course, Ceres will be the ultimate arbiter, and nature 
rarely depends upon public opinion, but the Dawn project will consider 
sending the results of the poll to Ceres, courtesy of our team member 
on permanent assignment there.

In addition to the bright spots, Dawn's views from its present altitude 
have included a wide range of other intriguing sights, as one would expect 
on a world of more than one million square miles (nearly 2.8 million square 
kilometers). There are myriad craters excavated by objects falling from 
space, inevitable scars from inhabiting the main asteroid belt for more 
than four billion years, even for the largest and most massive resident 
there.

The craters exhibit a wide range of appearances, not only in size but 
also in how sharp and fresh or how soft and aged they look. Some display 
a peak at the center. A crater can form from such a powerful punch that 
the hard ground practically melts and flows away from the impact site. 
Then the material rebounds, almost as if it sloshes back, while already 
cooling and then solidifying again. The central peak is like a snapshot, 
preserving a violent moment in the formation of the crater. By correlating 
the presence or absence of central peaks with the sizes of the craters, 
scientists can infer properties of Ceres' crust, such as how strong 
it is. Rather than a peak at the center, some craters contain large pits, 
depressions that may be a result of gasses escaping after the impact. 
(Craters elsewhere in the solar system, including on Vesta and Mars, also 
have pits.)

[Image]
Several craters here have central peaks. The largest also has a ridge 
at the center. Note other intriguing geological structures, including 
the terraced walls of that crater and the contrast between the smooth 
area in the top half of the picture and the more rugged terrain at the 
bottom. The picture below overlaps the top of this view. Image credit: 
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

Dawn also has spied many long, straight or gently curved canyons. Geologists 
have yet to determine how they formed, and it is likely that several different 
mechanisms are responsible. For example, some might turn out to be the 
result of the crust of Ceres shrinking as the heat and other energy accumulated 
upon formation gradually radiated into space. When the behemoth slowly 
cooled, stresses could have fractured the rocky, icy ground. Others might 
have been produced as part of the devastation when a space rock crashed, 
rupturing the terrain.

[Image]
Several long canyons are evident in this view. The large crater that extends 
off the bottom of the picture is in the center of the picture above. Also 
notice the bright spots, just visible on the limb at upper left. The first 
picture above shows them from overhead. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

Ceres shows other signs of an active past rather than that of a static 
chunk of inert material passing the eons with little notice. Some areas 
are less densely cratered than others, suggesting that there are geological 
processes that erase the craters. Indeed, some regions look as if something 
has flowed over them, as if perhaps there was mud or slush on the surface.

In addition to evidence of aging and renewal, some powerful internal forces 
have uplifted mountains. One particularly striking structure is a steep 
cone that juts three miles (five kilometers) high in an otherwise relatively 
smooth area, looking to an untrained (but transfixed) eye like a volcanic 
cone, a familiar sight on your home planet (or, at least, on mine). No 
other isolated, prominent protuberance has been spotted on Ceres.
limb with conical mountain above and to the right of center plus a few 
other bright areas

[Image]
The conical mountain is above and to the right of center. With the solar 
illumination from the top of the picture, note how crater walls are brighter 
on the bottom (facing the sun) and darker on the top (shaded by the ground 
they sink into). The cone stands out because it is brighter on the top 
(facing the sun), and the opposite side is in the shade. This view also 
show several bright spots and larger areas. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

[Image] 
The conical feature in the previous picture is visible here on the limb 
at bottom center. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

It is too soon for scientists to understand the intriguing geology of 
this ancient world, but the prolific adventurer is providing them with 
the information they will use. The bounty from this second mapping phase 
includes more than 1,600 pictures covering essentially all of Ceres, well 
over five million spectra in visible and infrared wavelengths and hundreds 
of hours of gravity measurements.

The spacecraft has performed its ambitious assignments quite admirably. 
Only a few deviations from the very elaborate plans occurred. On June 
15 and 27, during the fourth and eighth flights over the dayside, the 
computer in the combination visible and infrared mapping spectrometer 
(VIR) detected an unexpected condition, and it stopped collecting data. 
When the spacecraft's main computer recognized the situation, it instructed 
VIR to close its protective cover and then power down. The unit dutifully 
did so. Also on June 27, about three hours before VIR's interruption, 
the camera's computer experienced something similar.

Most of the time that Dawn points its sensors at Ceres, it simultaneously 
broadcasts through one of its auxiliary radio antennas, casting a very 
wide but faint signal in the general direction of Earth. (As Dawn progresses 
in its orbit, the direction to Earth changes, but the spacecraft is equipped 
with three of these auxiliary antennas, each pointing in a different direction, 
and mission controllers program it to switch antennas as needed.) The 
operations team observed what had occurred in each case and recognized 
there was no need to take immediate action. The instruments were safe 
and Dawn continued to carry out all of its other tasks.

When Dawn subsequently flew to the nightside of Ceres and pointed its 
main antenna to Earth, it transmitted much more detailed telemetry. As 
engineers and scientists continue their careful investigations, they recognize 
that in many ways, these events appear very similar to ones that have 
occurred at other times in the mission.

Four years ago, VIR's computer reset when Dawn was approaching Vesta, 
and the most likely cause was deemed to be a cosmic ray strike. That's 
life in deep space! It also reset twice in the survey orbit phase at Vesta. 
The camera reset three times in the first three months of the low altitude 
mapping orbit at Vesta.

Even with the glitches in this second mapping orbit, Dawn's outstanding 
accomplishments represent well more than was originally envisioned or 
written into the mission's scientific requirements for this phase of 
the mission. For those of you who have not been to Ceres or aren't going 
soon (and even those of you who want to plan a trip there of your own), 
you can see what Dawn sees by going to the image gallery.

Although Dawn already has revealed far, far more about Ceres in the last 
six months than had been seen in the preceding two centuries of telescopic 
studies, the explorer is not ready to rest on its laurels. It is now preparing 
to undertake another complex spiral descent, using its sophisticated ion 
propulsion system to maneuver to a circular orbit three times as close 
to the dwarf planet as it is now. It will take five weeks to perform the 
intricate choreography needed to reach the third mapping altitude, starting 
tomorrow night. You can keep track of the spaceship's flight as it propels 
itself to a new vantage point for observing Ceres by visiting the mission 
status page or following it on Twitter @NASA_Dawn.

As Dawn moves closer to Ceres, Earth will be moving closer as well. Earth 
and Ceres travel on independent orbits around the sun, the former completing 
one revolution per year (indeed, that's what defines a year) and the 
latter completing one revolution in 4.6 years (which is one Cerean year). 
(We have discussed before why Earth revolves faster in its solar orbit, 
but in brief it is because being closer to the sun, it needs to move faster 
to counterbalance the stronger gravitational pull.) Of course, now that 
Dawn is in a permanent gravitational embrace with Ceres, where Ceres goes, 
so goes Dawn. And they are now and forever more so close together that 
the distance between Earth and Ceres is essentially equivalent to the 
distance between Earth and Dawn.

On July 22, Earth and Dawn will be at their closest since June 2014. As 
Earth laps Ceres, they will be 1.94 AU (180 million miles, or 290 million 
kilometers) apart. Earth will race ahead on its tight orbit around the 
sun, and they will be more than twice as far apart early next year.
trajectory

[Graphic]
Earth's and Ceres' orbits will bring them to their minimum separation 
on July 22. Earth?s orbit is shown in green and Ceres? is in purple. 
Dawn?s interplanetary trajectory is in blue. Compare this figure with 
the ones depicting Dawn and Earth on opposite sides of the sun in December 
2014 and showing Dawn equidistant from Earth and the sun in April 2015. 
Credit: NASA/JPL-Caltech

Although Dawn communicates regularly with Earth, it left that planet behind 
nearly eight years ago and will keep its focus now on its new residence. 
With two very successful mapping campaigns complete, its next priority 
is to work its way down through Ceres' gravitational field to an altitude 
of about 900 miles (less than 1,500 kilometers). With sharper views and 
new kinds of observations (including stereo photography), the treasure 
trove obtained by this intrepid extraterrestrial prospector will only 
be more valuable. Everyone who longs for new understandings and new perspectives 
on the cosmos will grow richer as Dawn continues to pioneer at a mysterious 
and distant dwarf planet.

Dawn is 2,700 miles (4,400 kilometers) from Ceres. It is also 2.01 AU 
(187 million miles, or 301 million kilometers) from Earth, or 785 times 
as far as the moon and 1.98 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 33 minutes 
to make the round trip.