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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 25 Feb 2015 11:42:39 -0800 (PST)
Message-ID: <201502251942.t1PJgdn9016790_at_zagami.jpl.nasa.gov>

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.
Received on Wed 25 Feb 2015 02:42:39 PM PST