From Newsgroup: sci.space.news


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.

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