[meteorite-list] Dawn Journal - February 25, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/02/25/dawn-journal-february-25/

Dawn Journal 
by Dr. Marc Rayman
February 25, 2016
 
Dear Fine and Dawndy Readers,

The Dawn spacecraft is performing flawlessly as it conducts the first 
exploration of the first dwarf planet. Each new picture of Ceres reveals 
exciting and surprising new details about a fascinating and enigmatic 
orb that has been glimpsed only as a smudge of light for more than two 
centuries. And yet as that fuzzy little blob comes into sharper focus, 
it seems to grow only more perplexing.

Dawn is showing us exotic scenery on a world that dates back to the dawn 
of the solar system, more than 4.5 billion years ago. Craters large and 
small remind us that Ceres lives in the rough and tumble environment of 
the main asteroid belt between Mars and Jupiter, and collectively they 
will help scientists develop a deeper understanding of the history and 
nature not only of Ceres itself but also of the solar system.

[Image]
Dawn observed Ceres for three hours, or one third of a Cerean day, on 
Feb. 3-4. The spacecraft was 91,000 miles (146,000 kilometers) from the 
dwarf planet in this imaging session, known as OpNav 3. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even as we discover more about Ceres, some mysteries only deepen. It certainly 
does not require sophisticated scientific insight to be captivated by 
the bright spots. What are they? At this point, the clearest answer is 
that the answer is unknown. One of the great rewards of exploring the 
cosmos is uncovering new questions, and this one captures the imagination 
of everyone who gazes at the pictures sent back from deep space.

Other intriguing features newly visible on the unfamiliar landscape further 
assure us that there will be much more to see and to learn - and probably 
much more to puzzle over - when Dawn flies in closer and acquires new 
photographs and myriad other measurements. Over the course of this year, 
as the spacecraft spirals to lower and lower orbits, the view will continue 
to improve. In the lowest orbit, the pictures will display detail well 
over one hundred times finer than the RC2 pictures returned a few days 
ago (and shown below). Right now, however, Dawn is not getting closer 
to Ceres. On course and on schedule for entering orbit on March 6, Earth's 
robotic ambassador is slowly separating from its destination.

"Slowly" is the key. Dawn is in the vicinity of Ceres and is not leaving. 
The adventurer has traveled more than 900 million miles (1.5 billion kilometers) 
since departing from Vesta in 2012, devoting most of the time to using 
its advanced ion propulsion system to reshape its orbit around the sun 
to match Ceres' orbit. Now that their paths are so similar, the spacecraft 
is receding from the massive behemoth at the leisurely pace of about 35 
mph (55 kilometers per hour), even as they race around the sun together 
at 38,700 mph (62,300 kilometers per hour). The probe is expertly flying 
an intricate course that would be the envy of any hotshot spaceship pilot. 
To reach its first observational orbit - a circular path from pole to 
pole and back at an altitude of 8,400 miles (13,500 kilometers) - Dawn 
is now taking advantage not only of ion propulsion but also the gravity 
of Ceres.

On Feb. 23, the spacecraft was at its closest to Ceres yet, only 24,000 
miles (less than 39,000 kilometers), or one-tenth of the separation between 
Earth and the moon. Momentum will carry it farther away for a while, so 
as it performs the complex cosmic choreography, Dawn will not come this 
close to its permanent partner again for six weeks. Well before then, 
it will be taken firmly and forever into Ceres' gentle gravitational hold.

The photographs Dawn takes during this approach phase serve several purposes. 
Besides fueling the fires of curiosity that burn within everyone who looks 
to the night sky in wonder or who longs to share in the discoveries of 
celestial secrets, the images are vital to engineers and scientists as 
they prepare for the next phase of exploration.

[Images]
Dawn acquired these two pictures of Ceres on Feb. 12 at a distance of 
52,000 miles (83,000 kilometers) during the first "rotation characterization," 
or RC1. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

[Images] 
Dawn acquired these two pictures of Ceres on Feb. 19 at a distance of 
28,000 miles (46,000 kilometers) in RC2. Dawn's trajectory took it north 
between RC1 and RC2, so the terrain within view of its camera is farther 
north here than in RC1. The angle of the sunlight is different as well. 
Nevertheless, each of these two perspectives is close in longitude to 
the two above, so some features apparent here are also visible in the 
RC1 photos. The careful observer will note that these pictures are very 
cool, especially when compared with earlier ones from Dawn and the best 
from Hubble Space Telescope, as shown in last month's Dawn Journal. Credit: 
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The primary purpose of the pictures is for "optical navigation" (OpNav), 
to ensure the ship accurately sails to its planned orbital port. Dawn 
is the first spacecraft to fly into orbit around a massive solar system 
world that had not previously been visited by a spacecraft. Just as when 
it reached its first deep-space target, the fascinating protoplanet Vesta, 
mission controllers have to discover the nature of the destination as 
they proceed. They bootstrap their way in, measuring many characteristics 
with increasing accuracy as they go, including its location, its mass 
and the direction of its rotation axis.

Let's consider this last parameter. Think of a spinning ball. (If the 
ball is large enough, you could call it a planet.) It turns around an 
axis, and the two ends of the axis are the north and south poles. The 
precise direction of the axis is important for our mission because in 
each of the four observation orbits (previews of which were presented 
in February, May, June and August), the spacecraft needs to fly over the 
poles. Polar orbits ensure that as Dawn loops around, and Ceres rotates 
beneath it every nine hours, the explorer eventually will have the opportunity 
to see the entire surface. Therefore, the team needs to establish the 
location of the rotation axis to navigate to the desired orbit.

We can imagine extending the rotation axis far outside the ball, even 
all the way to the stars. Current residents of Earth, for example, know 
that their planet's north pole happens to point very close to a star appropriately 
named Polaris (or the North Star), part of an asterism known as the Little 
Dipper in the constellation Ursa Minor (the Little Bear). The south pole, 
of course, points in exactly the opposite direction, to the constellation 
Octans (the Octant), but is not aligned with any salient star.

With their measurements of how Ceres rotates, the team is zeroing in on 
the orientation of its poles. We now know that residents of (and, for 
that mater, visitors to) the northern hemisphere there would see the pole 
pointing toward an unremarkable region of the sky in Draco (the Dragon). 
Those in the southern hemisphere would note the pole pointing toward a 
similarly unimpressive part of Volans (the Flying Fish). (How appropriate 
it is that that pole is directed toward a constellation with that name 
will be known only after scientists advance their understanding of the 
possibility of a subsurface ocean at Ceres.)

The orientation of Ceres' axis proves convenient for Dawn's exploration. 
Earthlings are familiar with the consequences of their planet's axis being 
tilted by about 23 degrees. Seasons are caused by the annual motion of 
the sun between 23 degrees north latitude and 23 degrees south. A large 
area around each pole remains in the dark during winter. Vesta's axis 
is tipped 27 degrees, and when Dawn arrived, the high northern latitudes 
were not illuminated by the sun. The probe took advantage of its extraordinary 
maneuverability to fly to a special mapping orbit late in its residence 
there, after the sun had shifted north. That will not be necessary at 
Ceres. That world's axis is tipped at a much smaller angle, so throughout 
a Cerean year (lasting 4.6 Earth years), the sun stays between 4 degrees 
north latitude and 4 degrees south. Seasons are much less dramatic. Among 
Dawn's many objectives is to photograph Ceres. Because the sun is always 
near the equator, the illumination near the poles will change little. 
It is near the beginning of southern hemisphere winter on Ceres now, but 
the region around the south pole hidden in hibernal darkness is tiny. 
Except for possible shadowing by local variations in topography (as in 
deep craters), well over 99 percent of the dwarf planet's terrain will 
be exposed to sunlight each day.

Guiding Dawn from afar, the operations team incorporates the new information 
about Ceres into occasional updates to the flight plan, providing the 
spacecraft with new instructions on the exact direction and throttle level 
to use for the ion engine. As they do so, subtle aspects of the trajectory 
change. Last month we described the details of the plan for observing 
Ceres throughout the four-month approach phase and predicted that some 
of the numbers could change slightly. So, careful readers, for your convenience, 
here is the table from January, now with minor updates.


Beginning of activity in Pacific Time zone	Distance from Dawn to Ceres 
in miles (kilometers)	Ceres diameter in pixels	Resolution in miles (kilometers) 
per pixel	Resolution compared to Hubble	Illuminated portion of disk	Activity
Dec 1, 2014	740,000 (1.2 million)	9	70 (112)	0.25	94%	Camera calibration
Jan 13, 2015	238,000 (383,000)	27	22 (36)	0.83	95%	OpNav 1
Jan 25	147,000 (237,000)	43	14 (22) 1.3	96%	OpNav 2
Feb 3	91,000 (146,000)	70	8.5 (14) 2.2	97%	OpNav 3
Feb 12	52,000 (83,000)	122	4.9 (7.8) 3.8	98%	RC1
Feb 19	28,000 (46,000)	222	2.7 (4.3)	7.0	87%	RC2
Feb 25	25,000 (40,000)	255	2.3 (3.7)	8.0	44%	OpNav 4
Mar 1	30,000 (49,000)	207	2.9 (4.6)	6.5	23%	OpNav 5
Apr 10	21,000 (33,000)	306	1.9 (3.1) 9.6	17%	OpNav 6 
Apr 14	14,000 (22,000)	453	1.3 (2.1) 14	49%	OpNav 7
 

In addition to changes based on discoveries about the nature of Ceres, 
some changes are dictated by more mundane considerations (to the extent 
that there is anything mundane about flying a spacecraft in the vicinity 
of an alien world more than a thousand times farther from Earth than the 
moon). For example, to accommodate changes in the schedule for the use 
of the Deep Space Network, some of the imaging sessions shifted by a few 
hours, which can make small changes in the corresponding views of Ceres.

The only important difference between the table as presented in January 
and this month, however, is not to be found in the numbers. It is that 
OpNav 3, RC1 and RC2 are now in the past, each having been completed perfectly.

As always, if you prefer to save yourself the time and effort of the multi-billion-mile 
(multi-billion-kilometer) interplanetary journey to Ceres, you can simply 
go here to see the latest views from Dawn. (The Dawn project is eager 
to share pictures promptly with the public. The science team has the responsibility 
of analyzing and interpreting the images for scientific publication. The 
need for accuracy and scientific review of the data slows the interpretation 
and release of the pictures. But just as with all of the marvelous findings 
from Vesta, everything from Ceres will be available as soon as practicable.)

In November we delved into some of the details of Dawn's graceful approach 
to Ceres, and last month we considered how the trajectory affected the 
scene presented to Dawn's camera. Now that we have updated the table, 
we can enhance a figure from both months that showed the craft’s path 
as it banks into orbit and maneuvers to its first observational orbit. 
(As a reminder, the diagram illustrates only two of the three dimensions 
of the ship's complicated route. Another diagram in November showed another 
perspective, and we will include a different view next month.)

[Graphic]
Section of Dawn's approach trajectory. We are looking down on the north 
pole of Ceres. (Readers who reside in the constellation Draco will readily 
recognize this perspective). The sun is off the figure far to the left. 
The spacecraft flies in from the left and then is captured (enters orbit) 
on the way to the apex of its orbit. It gets closer to Ceres during the 
first part of its approach but then recedes for a while before coming 
in still closer at the end. When Dawn is on the right side of the figure, 
it sees only a crescent of Ceres, because the illumination is from the 
left. The trajectory is solid where Dawn is thrusting with its ion engine, 
which is most of the time. The labels show where it pauses to turn, point 
at Ceres, conduct the  indicated observation, turn to point its main antenna 
to Earth, transmit its precious findings, turn back to the orientation 
needed for thrusting, and then restart the ion engine. Because RC1 and 
RC2 observations extend for a full Cerean day of more than nine hours, 
those periods are longer, both to collect data and to radio the results 
to Earth. Note that there are four periods on the right side of the figure 
between capture and OpNav 6 when Dawn pauses thrusting for telecommunications 
and radio navigation but does not take pictures, as explained here. Credit: 
NASA/JPL

We can zoom out to see where the earlier OpNavs were.

[Graphic]
All of Dawn's observations during the approach phase. Note how much shorter 
this caption is than the one above, despite the similarity of the figures. 
Credit: NASA/JPL

As the table and figures indicate, in OpNav 6, when Ceres and the sun 
are in the same general direction from Dawn's vantage point, only a small 
portion of the illuminated terrain will be visible. The left side of Ceres 
will be in daylight, and most of the hemisphere facing the spacecraft 
will be in the darkness of night. To get an idea of what the shape of 
the crescent will be, terrestrial readers can use the moon on March 16. 
It will be up much of the day, setting in the middle of the afternoon, 
and it will be comparable to the crescent Dawn will observe on April 10. 
(Of course, the exact shape will depend on your observing location and 
what time you look, but this serves as a rough preview.) Fortunately, 
our spacecraft does not have to contend with bad weather, but you might, 
so we have generously scheduled a backup opportunity for you. The moon 
will be new on March 20, and the crescent on March 23 will be similar 
to what it was on March 16. It will rise in the mid morning and be up 
until well after the sun sets.

Photographing Ceres as it arcs into orbit atop a blue-green beam of xenon 
ions, setting the stage for more than a year of detailed investigations 
with its suite of sophisticated sensors, Dawn is sailing into the history 
books. No spacecraft has reached a dwarf planet before. No spacecraft 
has orbited two extraterrestrial destinations before. This amazing mission 
is powered by the insatiable curiosity and extraordinary ingenuity of 
creatures on a planet far, far away. And it carries all of them along 
with it on an ambitious journey that grows only more exciting as it continues. 
Humankind is about to witness scenes never before seen and perhaps never 
even imagined. Dawn is taking all of us on a daring adventure to a remote 
and unknown part of the cosmos. Prepare to be awed.

Dawn is 24,600 miles (39,600 kilometers) from Ceres, or 10 percent of 
the average distance between Earth and the moon. It is also 3.42 AU (318 
million miles, or 512 million kilometers) from Earth, or 1,330 times as 
far as the moon and 3.46 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 57 minutes 
to make the round trip.