[meteorite-list] Dawn Journal - January 31, 2017

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_01_31_17.html

Dawn Journal
Dr. Marc Rayman
January 31, 2017

Dear Prodawns, Neudawns and Elecdawns,

A deep-space robotic emissary from Earth is continuing to carry out its 
extraordinary mission at a distant dwarf planet. Orbiting high above Ceres, 
the sophisticated Dawn spacecraft is hard at work unveiling the secrets 
of the exotic alien world that has been its home for almost two years.

Dawn's primary objective in this sixth orbital phase at Ceres (known 
as extended mission orbit 3, XMO3 or "this sixth orbital phase at Ceres") 
is to record cosmic rays. Doing so will allow scientists to remove that 
"noise" from the nuclear radiation measurements performed during the eight 
months Dawn operated in a low, tight orbit around Ceres. The result will 
be a cleaner signal, revealing even more about the atomic constituents 
down to about a yard (meter) underground. As we will see below, in addition 
to this ongoing investigation, soon the adventurer will begin pursuing 
a new objective in its exploration of Ceres.

[Ikapati Crater Image]
Dawn took this picture of Ikapati Crater on Jan. 24, 2016, from an altitude 
of 240 miles (385 kilometers), which is orbit 4 in the figure below. (Ikapati 
is an ancient Tagalog goddess whose name means "giver of food.") The 31-mile 
(50-kilometer) crater is geologically young, as evidenced by its clear, 
strong features. Note the difference in topography between the crater 
floor in the top half of the picture, with its many ridges, and in the 
bottom, which is smoother. The fractures run in different directions as 
well. Ikapati is at 34°N, 46°E on the map below. Full image and caption. 
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

With its uniquely capable ion propulsion system, Dawn has flown to orbits 
with widely varying characteristics. In contrast to the previous five 
observation orbits (and all the observation orbits at Vesta), XMO3 is 
elliptical. Over the course of almost eight days, the spacecraft sails 
from a height of about 4,670 miles (7,520 kilometers) up to almost 5,810 
miles (9,350 kilometers) and back down. Dutifully following principles 
discovered by Johannes Kepler at the beginning of the 17th century and 
explained by Isaac Newton at the end of that century, Dawn's speed 
over this range of altitudes varies from 210 mph (330 kilometers per hour) 
when it is closest to Ceres to 170 mph (270 kilometers per hour) when 
it is farthest. Yesterday afternoon, the craft was at its highest for 
the current orbit. During the day today, the ship will descend from 5,790 
miles (9,310 kilometers) to 5,550 miles (8,930 kilometers). As it does 
so, Ceres' gravity will gradually accelerate it from 170 mph (273 
kilometers per hour) to 177 mph (285 kilometers per hour). (Usually we 
round the orbital velocity to the nearest multiple of 10. In this case, 
however, to show the change during one day, the values presented are more 
precise.)

As we saw last month, the angle of XMO3 to the sun presents an opportunity 
to gain a new perspective on Ceres, with sunlight coming from a different 
angle. (We include the same figure here, because we will refer to it more 
below.) Last week, Dawn took advantage of that opportunity, seeing the 
alien landscapes in a new light as it took pictures for the first time 
since October.

[Dawn XMO2 Image 10]
This illustrates (and simplifies) the relative size and alignment of Dawn's 
six science orbits at Ceres. We are looking down on Ceres' north 
pole. The spacecraft follows polar orbits, and seen edge-on here, each 
orbit looks like a line. (Orbits 1, 2 and 6 extend off the figure to the 
lower right, on the night side. Like 3, 4 and 5, they are centered on 
Ceres.) The orbits are numbered chronologically. The first five orbits 
were circular. Orbit 6, which is XMO3, is elliptical, and the dotted section 
represents the range from the minimum to the maximum altitude. With the 
sun far to the left, the left side of Ceres is in daylight. Each time 
the spacecraft travels over the illuminated hemisphere in the different 
orbital planes, the landscape beneath it is lit from a different angle. 
Ceres rotates counterclockwise from this perspective (just as Earth does 
when viewed from the north). So higher numbers correspond to orbits that 
pass over ground closer to sunrise, earlier in the Cerean day. (Compare 
this diagram with this figure, which shows only the relative sizes of 
the first four orbits, with each one viewed face-on rather than edge-on.) 
Click on this image for a larger view. Image credit: NASA/JPL

Dawn takes more than a week to revolve around Ceres, but Ceres turns on 
its axis in just nine hours. Because Dawn moves through only a small segment 
of its orbit in one Cerean day, it is almost as if the spacecraft hovers 
in place as the dwarf planet pirouettes beneath it. During one such period 
on Jan. 27, Dawn's high perch moved only from 11°N to 12°S latitude 
as Ceres presented her full range of longitudes to the explorer's 
watchful eye. This made it very convenient to take pictures and visible 
spectra as the scenery helpfully paraded by. (The spacecraft was high 
enough to see much farther north and south than the latitudes immediately 
beneath it.) Dawn will make similar observations again twice in February.

As Dawn was expertly executing the elegant, complex spiral ascent from 
XMO2 to XMO3 in November, the flight team considered it to be the final 
choreography in the venerable probe's multi-act grand interplanetary 
performance. By then, Dawn had already far exceeded all of its original 
objectives at Vesta and Ceres, and the last of the new scientific goals 
could be met in XMO3, the end of the encore. The primary consideration 
was to keep Dawn high enough to measure cosmic rays, meaning it needed 
to stay above about 4,500 miles (7,200 kilometers). There was no justification 
or motivation to go anywhere else. Well, that's the way it was in 
November anyway. This is January. And now it's (almost) time for 
a previously unanticipated new act, XMO4.

Always looking for ways to squeeze as much out of the mission as possible, 
the team has now devised a new and challenging investigation. It will 
consume the next five months (and much of the next five Dawn Journals). 
We begin this month with an overview, but follow along each month as we 
present the full story, including a detailed explanation of the underlying 
science, the observations themselves and the remarkable orbital maneuvering 
entirely unlike anything Dawn has done before. (You can also follow along 
with your correspondent's uncharacteristically brief and more frequent 
mission status updates.)

[Ceres Map]
This map of Ceres has all 114 feature names approved so far by the International 
Astronomical Union (IAU). (We described the naming convention here.) As 
more features are named, this official list and map are kept up to date. 
We saw an earlier version of this map before Dawn had flown to its lowest 
orbit and obtained its sharpest pictures. The dwarf planet is 1.1 million 
square miles (2.8 million square kilometers). That's about 36 percent 
of the land area of the contiguous United States, or the combined land 
areas of France, Germany, Italy, Norway, Spain, Sweden and the United 
Kingdom. The scales for horizontal distance in this figure apply at the 
equator. Rectangular maps like this distort distances at other latitudes. 
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

>From the XMO3 vantage point, with sunlight coming from the side, Ceres 
is gibbous and looks closer to a half moon than full. The new objective 
is to peer at Ceres when the sun is directly behind Dawn. This would be 
the same as looking at a full moon. (In the figure above, it would be 
like photographing Ceres from somewhere on the dashed line that points 
to the distant sun.)

While Dawn obtained pictures from near the line to the sun in its first 
Ceres orbit, there is a special importance to being even closer to that 
line. Let's see why that alignment is valuable.

Most materials reflect light differently at different angles. You can 
investigate this yourself (and it's probably easier to do at home 
than it is in orbit around a remote dwarf planet). To make it simpler, 
take some object that is relatively uniform (but with a matte finish, 
not a mirror-like finish) and vary the angles at which light hits it and 
from which you look at it. You may see that it appears dimmer or brighter 
as the angles change. It turns out that this effect may be used to help 
infer the nature of the reflecting material. (For the purposes of this 
exercise, if you can hold the angle of the object relative to your gaze 
fixed, and vary only the angle of the illumination, that's best. 
But don't worry about the details. Conducting this experiment represents 
only a small part of your final grade.)

Now when scientists carefully measure the reflected light under controlled 
conditions, they find that the intensity changes quite gradually over 
a wide range of angles. In other words, the apparent brightness of an 
object does not vary dramatically as the geometry changes. However, when 
the source of the illumination gets very close to being directly behind 
the observer, the reflection may become quite a bit stronger. (If you 
test this, of course, you have to ensure your shadow doesn't interfere 
with the observation. Vampires don't worry about this, and we'll 
explain below why Dawn needn't either.)

If you (or a helpful scientist friend of yours) measure how bright a partial 
moon is and then use that information to calculate how bright the full 
moon will be, you will wind up with an answer that's too small. The 
full moon is significantly brighter than would be expected based on how 
lunar soil reflects light at other angles. (Of course, you will have to 
account for the fact that there is more illuminated area on a full moon, 
but this curious optical behavior is different. Here we are describing 
how the brightness of any given patch of ground changes.)

A full moon occurs when the moon and sun are in opposite directions from 
Earth's perspective. That alignment is known as opposition. That 
is, an astronomical body (like the moon or a planet) is in opposition 
when the observer (you) is right in between it and the source of illumination 
(the sun), so all three are on a straight line. And because the brightness 
takes such a steep and unexpected jump there, this phenomenon is known 
as the opposition surge.
Dawn LAMO Image 188

Dawn observed this scene inside Yalode Crater on Oct. 13, 2015, from its 
third mapping orbit at an altitude of 915 miles (1,470 kilometers). At 
162 miles (260 kilometers) in diameter, Yalode is the second largest crater 
on Ceres. (Scientists expected to see much larger craters than Ceres displays.) 
The two largest craters within Yalode are visible in this picture. Lono 
Crater, at top right, is 12 miles (20 kilometers) in diameter. (Lono is 
a Hawaiian god of agriculture, rain and other roles.) Below Lono is the 
11-mile (17-kilometer) Besua Crater. (Besua is one of at least half a 
dozen Egyptian grain gods.) Note several chains of craters as well as 
fractures on the left and lower right. We saw a much more fractured area 
of Yalode, now named Nar Sulcus, here. (Nar is from a modern pomegranate 
feast in part of Azerbaijan. A sulcus is a set of parallel furrows or 
ridges.) You can locate this scene in the eastern part of Yalode on the 
map above near 45°S, 300°E. The photo below shows a more detailed view. 
You can see all of Yalode starting at 2:32 in the animation introduced 
here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The observed magnitude of the opposition surge can reveal some of the 
nature of the illuminated object on much, much finer scales than are visible 
in photos. Knowing the degree to which the reflection strengthens at very 
small angles allows scientists to ascertain (or, at least, constrain) 
the texture of materials on planetary surfaces even at the microscopic 
level. If they are fortunate enough to have measurements of the reflectivity 
at different angles for a region on an airless solar system body (atmospheres 
complicate it too much), they compare them with laboratory measurements 
on candidate materials to determine which ones give the best match for 
the properties.

Dawn has already measured the light reflected over a wide range of angles, 
as is clear from the figure above showing the orbits. But the strongest 
discrimination among different textures relies on measuring the opposition 
surge. That is Dawn's next objective, a bonus in the bonus extended 
mission.

You can see from the diagram that measuring the opposition surge will 
require a very large change in the plane of Dawn's orbit. Shifting 
the plane of a spacecraft's orbit can be energetically very, very 
expensive. (We will discuss this more next month.) Fortunately, the combination 
of the unique capabilities provided by the ion propulsion system and the 
ever-creative team makes it affordable.

[LAMO Image 195]
Dawn had this view on June 7, 2016, from its fourth mapping orbit. Taken 
at an altitude of 240 miles (385 kilometers), this picture shows greater 
detail in a smaller area than the picture above. Part of Lono Crater is 
at the bottom. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Powered by an insatiable appetite for new knowledge, Dawn will begin ion 
thrusting on Feb. 23. After very complex maneuvers, it will be rewarded 
at the end of April with a view of a full Ceres from an altitude of around 
12,400 miles (20,000 kilometers), about the height of GPS satellites above 
Earth. (That will be about 50 percent higher than the first science orbit, 
which is labeled as line 1 in the figure.) There are many daunting challenges 
in reaching XMO4 and measuring the opposition surge. Even though it is 
a recently added bonus, and the success of the extended mission does not 
depend on it, mission planners have already designed a backup opportunity 
in case the first attempt does not yield the desired data. The second 
window is late in June, allowing the spacecraft time to transmit its findings 
to Earth before the extended mission concludes at the end of that month.

[Occator Crater Image]
Occator Crater is shown in this mosaic of photos Dawn took at its lowest 
altitude of 240 miles (385 kilometers). The central bright area, Cerealia 
Facula, is the prime target in the planned opposition surge measurements. 
Dawn's infrared spectra show that this reflective material is principally 
sodium carbonate, a kind of salt. We described more about this mosaic 
here. For other views of Occator and its mesmerizing reflective regions, 
follow the links in the paragraph below. Full image and caption. Image 
credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For technical reasons, the measurements need to be made from a high altitude, 
and throughout the complex maneuvering to get there, Dawn will remain 
high enough to monitor cosmic rays. Ceres will appear to be around five 
times the width of the full moon we see from Earth. It will be about 500 
pixels in diameter in Dawn's camera, and more than 180,000 pixels 
will show light reflected from the ground. Of greatest scientific interest 
in the photographs will be just a handful of pixels that show the famous 
bright material in Occator Crater, known as Cerealia Facula and clearly 
visible in the picture above. Scientists will observe how those pixels 
surge in brightness over a narrow range of angles as Dawn's XMO4 
orbital motion takes it into opposition, exactly between Occator and the 
sun. Of course, the pictures also will provide information on how the 
widespread dark material covering most of the ground everywhere else on 
Ceres changes in brightness (or, if you prefer, in dimness). But the big 
reward here would be insight into the details of Cerealia Facula. Comparing 
the opposition surges with laboratory measurements may reveal characteristics 
that cannot be discerned any other way save direct sampling, which is 
far beyond Dawn's capability (and authority). For example, scientists 
may be able to estimate the size of the salt crystals that make up the 
bright material, and that would help establish their geological history, 
including whether they formed underground or on the surface. We will discuss 
this more in March.

Most of the data on opposition surges on solar system objects use terrestrial 
observations, with astronomers waiting until Earth and the target happen 
to move into the necessary alignment with the sun. In those cases, the 
surge is averaged over the entire body, because the target is usually 
too far away to discern any details. Therefore, it is very difficult to 
learn about specific features when observing from near Earth. Few spacecraft 
have actively maneuvered to acquire such data because, as we alluded to 
above and will see next month, it is too difficult, especially at a massive 
body like Ceres. The recognition that Dawn might be able to complete this 
challenging measurement for a region of particular interest represents 
an important possibility for the mission to discover more about this intriguing 
dwarf planet's geology.

Meeting the scientific goal will require a careful and quantitative analysis 
of the pixels, but the images of a fully illuminated Ceres will be visually 
appealing as well. Nevertheless, you are cautioned to avoid developing 
a mistaken notion about the view. (For that matter, you are cautioned 
to avoid developing mistaken notions about anything.) You might think 
(and some readers wondered about this in a different phase of the mission) 
that with Dawn being between the sun and Ceres (and not being a vampire), 
the spacecraft's shadow might be visible in the pictures. It would 
look really cool if it were (although it also would interfere with the 
measurement of the opposition surge by introducing another factor into 
how the brightness changes). There will be no shadow. The spacecraft will 
simply be too high. Imagine you'e standing in Occator Crater, either 
wearing your spacesuit while engaged in a thrilling exploration of a mysterious 
and captivating extraterrestrial site or perhaps instead while you're 
indoors enjoying some of the colony's specially salted Cerean savory 
snacks, famous throughout the solar system. In any case, the distant sun 
you see would be a little more than one-third the size that it looks from 
Earth, comparable to a soccer ball at 213 feet (65 meters). Dawn would 
be 12,400 miles (20,000 kilometers) overhead. Although it's one of 
the largest interplanetary spacecraft ever to take flight, with a wingspan 
of 65 feet (20 meters), it would be much too small for you to see at all 
without a telescope and would block an undetectably small amount of sunlight. 
It would appear smaller than a soccer ball seen from 135 miles (220 kilometers). 
Therefore, no shadow will be cast, the measurement will not be compromised 
by the spacecraft blocking some of the light reaching the ground and the 
pictures will not display any evidence of the photographer.

[Dawn XMO2 Image 26]
Dawn took this picture on Oct. 21, 2016, in its fifth observation orbit, 
at an altitude of 920 miles (1,480 kilometers). The two largest craters 
here display very different kinds of topography on their floors. The larger, 
Jarimba, is 43 miles (69 kilometers) across. (Jarimba is a god of fruit 
and flowers among the Aboriginal Aranda of central Australia.) Above Jarimba 
is part of Kondos Crater, which is 27 miles (44 kilometers) in diameter. 
(Kondos is a pre-Christian Finnish god of sowing and young wheat.) This 
scene is centered near 21°S, 27°E on the map above. Full image and caption. 
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even as the team was formulating plans for this ambitious new campaign, 
they successfully dealt with a glitch on the spacecraft this month. When 
a routine communications session with the Deep Space Network began on 
Jan. 17, controllers discovered that Dawn had previously entered its safe 
mode, a standard response the craft uses when it encounters conditions 
its programming and logic cannot accommodate. The main computer issues 
instructions to reconfigure systems, broadcasts a special radio signal 
through one of the antennas and then patiently awaits help from humans 
on a faraway planet (or anyone else who happens to lend assistance). The 
team soon determined what had occurred. Since it left Earth, Dawn has 
performed calculations five times per second about its location and speed 
in the solar system, whether in orbit around the sun, Vesta or Ceres. 
(Perhaps you do the same on your deep-space voyages.) However, it ran 
into difficulty in those calculations on Jan. 14 for the first time in 
more than nine years of interplanetary travel. To ensure the problematic 
calculations did not cause the ship to take any unsafe actions, it put 
itself into safe mode. Engineers have confirmed that the problem was in 
software, not hardware and not even a cosmic ray strike, which has occasionally 
triggered safe mode, most recently in September 2014.

Mission controllers guided the spacecraft out of safe mode within two 
days and finished returning all systems to their standard configurations 
shortly thereafter. Dawn was shipshape the subsequent week and resumed 
its scientific duties. When it activated safe mode, the computer correctly 
powered off the gamma ray and neutron detector, which had been measuring 
the cosmic rays, as we described above. The time that the instrument was 
off will be inconsequential, however, because there is more than enough 
time in the extended mission to acquire all the desired measurements.

The extended mission has already proven to be extremely productive, yielding 
a great deal of new data on this ancient world. But there is still more 
to look forward to as the veteran explorer prepares for a new and adventurous 
phase of its extraordinary extraterrestrial expedition.

Dawn is 5,650 miles (9,100 kilometers) from Ceres. It is also 2.87 AU 
(266 million miles, or 429 million kilometers) from Earth, or 1,135 times 
as far as the moon and 2.91 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 48 minutes 
to make the round trip.