[meteorite-list] Dawn Journal - January 31, 2016

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2016/01/31/dawn-journal-january-31/

Dawn Journal 
by Dr. Marc Rayman
January 31, 2016

Dear Spellbindawngs,

A veteran interplanetary traveler is writing the closing chapter in its 
long and storied expedition. In its final orbit, where it will remain 
even beyond the end of its mission, at its lowest altitude, Dawn is circling 
dwarf planet Ceres, gathering an album of spellbinding pictures and other 
data to reveal the nature of this mysterious world of rock and ice.

[Image]
Dawn captured this view of Kupalo crater on Dec. 20, shortly after beginning 
the observations from its current low altitude mapping orbit at 240 miles 
(385 kilometers). (Kupalo is a Slavic harvest deity associated with love 
and fertility.) This is a relatively young crater, as seen by its sharp, 
clear features and the paucity of overlying smaller impact craters which 
would have formed later. Bright material on the rim and walls may be salts, 
as explained last month. The crater is 16 miles (26 kilometers) across. 
Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres turns on its axis in a little more than nine hours (one Cerean day). 
Meanwhile, its new permanent companion, a robotic emissary from Earth, 
revolves in a polar orbit, completing a loop in slightly under 5.5 hours. 
It flies from the north pole to the south over the side of Ceres facing 
the sun. Then when it heads north, the ground beneath it is cloaked in 
the deep dark of night on a world without a moon (save Dawn itself). As 
we discussed last month, Dawn's primary measurements do not depend on 
illumination. It can sense the nuclear radiation (specifically, gamma 
rays and neutrons) and the gravity field regardless of the lighting. This 
month, let's take a look at the other measurements our explorer is performing, 
most of which do depend on sunlight.

Of course the photographs do. Dawn had already mapped Ceres quite thoroughly 
from higher altitudes. The spacecraft acquired an extensive set of stereo 
and color pictures in its third mapping orbit. But now that Dawn is only 
about 240 miles (385 kilometers) high, its images are four times as sharp, 
revealing new details of the strange and beautiful landscapes.

[Image]
This is an excerpt from a much more extensive animation providing a colorful 
tour of some of the highlights on Ceres. It is made with the color and 
stereo pictures Dawn collected in its third mapping orbit 915 miles (1,470 
kilometers) above the dwarf planet. Here we see Occator crater, with its 
famous bright regions. The full animation (in which both color and sound 
are exaggerated) also shows the strange, conical mountain Ahuna Mons plus 
Urvara, Haulani and Dantu (seen in more detail below) craters and more. 
The colors indicate different compositions, which may include salts and 
phyllosilicates, as explained last month. Full animation and caption. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Our spaceship is closer to Ceres than the International Space Station 
is to Earth. At that short range, it takes a long time to capture all 
of the vast territory, because each picture covers a relatively small 
area. Dawn's camera sees a square about 23 miles (37 kilometers) on a 
side, less than one twentieth of one percent of the more than one million 
square miles (nearly 2.8 million square kilometers). In an ideal world 
(which is not the one Dawn is in or at), it would take just over two thousand 
photos from this altitude to see all the sights. However, as we will discuss 
in more detail next month, it is not possible to control the orbital motion 
and the pointing of the camera accurately enough to manage without more 
photos than that.

Most of the time, Dawn is programmed to turn at just the right rate to 
keep looking at the ground beneath it as it travels, synchronizing its 
rotation with its revolution around Ceres. It photographs the passing 
scenery, storing the pictures for later transmission to Earth. But some 
of the time, it cannot take pictures, because to send its bounty of data, 
it needs to point its main antenna at that distant planet, home not only 
to its controllers but also to many others (including you, loyal reader) 
who share in the thrill of a bold cosmic adventure. Dawn spends about 
three and a half days (nine Cerean days) with its camera and other sensors 
pointed at Ceres. Then it radioes its findings home for a little more 
than one day (almost three Cerean days). During these communications sessions, 
even when it soars over lit terrain, it does not observe the sights below.

Mission planners have devised an intricate plan that should allow nearly 
complete coverage in about six weeks. To accomplish this, they guided 
Dawn to a carefully chosen orbit, and it has been doing an exceptionally 
good job there executing its complex activities.

[Image]
On Dec. 21, in its lowest orbit at about 240 miles (385 kilometers), Dawn 
flew over Dantu crater and obtained pictures with four times the clarity 
of the third mapping orbit, where we saw the entire crater. (Dantu is 
a timekeeper god who initiates the cycle of planting rites among the Ga 
people of the Accra Plains of southeastern Ghana.) The bright material 
here is at the 4:00 position, half way from the center to the rim, in 
the picture shown in November. The network of fractures may have formed 
when the ground cooled after being heated by the crater-forming impact, 
or perhaps later when other geological processes caused the crater floor 
to be uplifted. The crater is about 78 miles (126 kilometers) in diameter. 
The next picture below shows detail of another part of Dantu. The animation 
above includes Dantu (as seen from farther away). Full image and caption. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Last month, we marveled at a stunning view that was not the typical perspective 
of peering straight down from orbit. Sometimes controllers now program 
Dawn to take a few more pictures after it stops aiming its instruments 
down, while it starts to turn to aim its antenna to Earth. This clever 
idea provides bonus views of whatever happens to be in the camera's sights 
as it slowly rotates from the point beneath the spacecraft off to the 
horizon. Who doesn't feel the attraction of the horizon and long to know 
what lies beyond?

Another of Dawn's scientific devices is two different sensors combined 
into one instrument. Like the camera, the visible and infrared mapping 
spectrometers (VIR) look at the sunlight reflected from the ground. (As 
we'll see below, however, VIR also can detect something more.) A spectrometer 
breaks up light into its constituent colors, just as a prism or a droplet 
of water does when revealing, quite literally, all the colors of the rainbow. 
Dawn's visible spectrometer would have a view very much like that. The 
infrared spectrometer, of course, looks at wavelengths of light our limited 
eyes cannot see, just as there are wavelengths of sound our limited ears 
cannot hear (consult with your dog for details).

A spectrometer does more than simply disperse the light into its components, 
however. It measures the intensity of that light at the different wavelengths. 
The materials on the surface leave their signature in the sunlight they 
reflect, making some wavelengths relatively brighter and some dimmer. 
That characteristic pattern is called a spectrum. By comparing these spectra 
with spectra measured in laboratories, scientists can infer the nature 
of the minerals on the ground. We described some of the intriguing conclusions 
last month.

[Image]
On Dec. 19, Dawn's orbit took it over a different part of Dantu crater, 
showing more reflective material on the walls and floor. (This scene is 
from the right side of the crater as pictured in November.) More of the 
fractures visible in the picture above are in the upper left of this picture. 
Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

VIR does still more. Rather than record the visible spectrum and the infrared 
spectrum from a single region, it takes spectra at 256 adjacent locations 
simultaneously. This would be like taking one column of 256 pixels in 
a picture and having a separate spectrum for each. By stitching columns 
together, you could construct the two dimensional picture but with the 
added dimension of an extensive spectrum at every location. (Because the 
extra information provides a sort of depth that flat pictures don't have, 
the result is sometimes called an "image cube.") This capability to build 
up an image with spectra everywhere is what makes it a mapping spectrometer. 
VIR produces a remarkably rich view of its targets!

VIR's spectra contain much finer measurements of the colors and a wider 
range of wavelengths than the camera's images. In exchange, the camera 
has sharper vision and so can discern smaller geological features. In 
more technical terms, VIR achieves better spectral resolution and the 
camera achieves better spatial resolution. Fortunately, it is not a competition, 
because Dawn has both, and the instruments yield complementary measurements.

VIR generates a very large volume of data in each snapshot. As a result, 
Dawn can only capture and store relatively small areas of the dwarf planet 
with the mapping spectrometers, especially at this low altitude. Scientists 
have recognized from the first design of the mission that it would not 
be possible to cover all of Ceres (or Vesta) with VIR from the closer 
orbits. Nevertheless, Dawn has far exceeded expectations, returning a 
great many more spectra than anticipated. Still, as long as the spacecraft 
operates in this final mapping orbit, there will continue to be interesting 
targets to study with VIR.

Based on the nearly 20 million spectra of Ceres that VIR acquired from 
higher altitudes, the team has determined that new infrared spectra will 
provide more insight into the dwarf planet's character than the visible 
spectra. Because of their composition, the minerals display more salient 
signatures in infrared wavelengths than visible. The excellent visible 
spectra from the first three mapping orbits are deemed more than sufficient. 
Therefore, to make the best use of our faithful probe and to dedicate 
the resources to what is most likely to yield new knowledge about Ceres, 
VIR is devoting its share of the mission data in this final orbit to its 
infrared mapping spectrometer. We have many more exciting discoveries 
to look forward to!

[Image]
Dawn photographed this unnamed crater on Dec. 23. It is 20 miles (32 kilometers) 
in diameter and is between Dantu and Rao craters. (See the map here.) 
Part of this crater is shown at the bottom left of the photo of Dantu 
we saw in November. The many ridges and steep slopes here may be the result 
of the crater partially collapsing during its formation. The complex geology 
evokes an image of a flower (at least for this writer). Full image and 
caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The infrared light Ceres reflects from the sun can tell scientists a great 
deal about the composition, but they can learn even more from analyzing 
VIR's measurements. The sun isn't the only source of infrared. Ceres itself 
is. Many people correctly associate infrared with heat, because warm objects 
emit infrared light, and the strength at different wavelengths depends 
on the temperature. That calls for measuring the spectrum! Distant from 
the sun though it is, Ceres is warmed slightly by the brilliant star, 
so it has a very faint infrared glow of its own. Scientists can distinguish 
in VIR's observations between the reflected infrared sunlight and the 
infrared light Ceres radiates. In essence, VIR can function as a remote 
thermometer.

Last month, in one of Dawn's best photos yet of Ceres, we considered planning 
a hike across a breathtaking landscape. In case we do, VIR has shown we 
should be prepared for chilly conditions. Observed temperatures (all rounded 
to the nearest multiple of five) during the day on the dwarf planet range 
from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit 
(-35 degrees Celsius). (It is so cold in some locations and times, especially 
at night, that Ceres produces too little infrared light for VIR to measure. 
Temperatures below the coldest reported here actually don't register.) 
This finding provides compelling support for this writer's frequent claim 
that Ceres is really cool. In addition, knowing the temperatures will 
be very important for understanding geological processes on this icy, 
rocky world, just as we know the movement of terrestrial glaciers depends 
on temperature.

Your loyal correspondent can't - or, at least, won't - help but indulge 
his nerdiness with a brief tangent. The range of temperatures above represent 
the warmest on Ceres, given that VIR cannot measure lower values. It's 
amusing, if you have a similar weird sense of humor, that Ceres' average 
temperature apparently is not that far from what it would be for a black 
hole of the same mass. We won't delve into the physics here, but such 
a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). 
OK, enough hilarity. Back to Dawn and Ceres!

Ever creative, scientists are attempting another clever method to gain 
insight into the nature of this exotic orb. When Dawn is at just the right 
position in its orbit on the far side of Ceres, so that a straight line 
to Earth passes very close to the limb of Ceres itself, the spacecraft's 
radio signal will actually hit the dwarf planet. The radio waves interact 
with the materials on the surface, which can induce an exquisitely subtle 
distortion. After bouncing off the ground at a grazing angle, the radio 
signal continues on its way, heading toward Earth. The effect on the signal 
is much too small to affect the normal communications at all, but specialized 
equipment at NASA's Deep Space Network designed for this purpose might 
still be able to detect the tiny changes. The fantastically sensitive 
antennas measure the properties of the radio waves, and by studying the 
details, scientists may be able to learn more about the properties of 
the surface of the distant world. For example, this could help them distinguish 
between different types of materials (such as ice, rocks, sand, etc.) 
as well as reveal how rough or smooth the ground is at scales far, far 
smaller than the camera can discern. This is an extremely challenging 
measurement, and no small distortions have been detected so far, but always 
making the best possible use of the resources, scientists continue to 
look for them.

In addition to those bonus measurements, Dawn remains very productive 
in acquiring infrared spectra, photographs, gamma ray spectra and neutron 
spectra plus conducting measurements of the massive body's gravitational 
field, all of which contribute to unlocking the mysteries of the first 
dwarf planet ever discovered or explored. The venerable adventurer is 
in good condition and is operating flawlessly.

[Image]
Dawn observed Victa crater on Dec. 19. (Victa was a Roman goddess of food 
and nourishment.) The crater is 20 miles (32 kilometers) in diameter and 
so is the same size as the unnamed one shown above. Full image and caption. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We have discussed extensively the failures of two of the four reaction 
wheels, devices Dawn used to depend on to control its orientation in space. 
Without three healthy reaction wheels, the probe has had to rely instead 
on hydrazine propellant expelled from the small jets of the reaction control 
system. (When Dawn uses its ion engine, that remarkable system does double 
duty, reducing the need for the hydrazine.)

For most of the time since escaping from Vesta's gravitational clutches 
in 2012, Dawn has kept the other two reaction wheels in reserve so any 
remaining lifetime from those devices could offset the high cost of hydrazine 
propellant to turn and point in this current tight orbit. Those two wheels 
have been on and functioning flawlessly since Dec. 14, 2015, and every 
day they operate, they keep the expenditure of the dwindling supply of 
hydrazine to half of what it would be without them. (Next month we will 
offer some estimates of how long Dawn might continue to operate.) But 
the ever-diligent team recognizes another wheel could falter at any moment, 
and they remain ready to continue the mission with pure hydrazine control 
after only a short recovery operation. If a third failure is at all like 
the two that have occurred already, the hapless wheel won't give an indication 
of a problem until it's too late. A reaction wheel failure evidently is 
entirely unpredictable. We'll know about it only after it occurs in the 
remote depths of space where Dawn resides at an alien world.

Earth and Ceres are so far from each other that their motions are essentially 
independent. The planet and the dwarf planet follow their own separate 
repetitive paths around the sun. And each carries its own retinue: Earth 
has thousands of artificial satellites and one prominent natural one, 
the moon. Ceres has one known satellite. It arrived there in March 2015, 
and its name is Dawn.

Coincidentally, both reached extremes earlier this month in their elliptical 
heliocentric orbits. Earth, in its annual journey around our star, was 
at perihelion, or the closest point to the sun, on Jan. 2, when it was 
0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which 
takes 4.6 years (one Cerean year) for each loop, attained its aphelion, 
or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 
AU (277 million miles, or 445 million kilometers) from the gravitational 
master of the solar system.

Far, far from the planet where its deep-space voyage began, Dawn is now 
bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft 
continues to unveil new and fascinating secrets there for the benefit 
of all those who remain with Earth but who still look to the sky with 
wonder, who feel the lure of the unknown, who are thrilled by new knowledge, 
and who yearn to know the cosmos.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 
million miles, or 580 million kilometers) from Earth, or 1,440 times as 
far as the moon and 3.93 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take one hour 
and four minutes to make the round trip.