[meteorite-list] Dawn Journal - September 27, 2016

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 30 Sep 2016 15:00:04 -0700 (PDT)
Message-ID: <201609302200.u8UM04Wu006022_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
September 27, 2016

Dear Dawnniversaries,

Nine years ago today, Dawn set sail on an epic journey of discovery and
adventure. The intrepid explorer has sailed the cosmic seas and collected
treasures that far exceeded anything anticipated or even hoped for. It
began its voyage at Earth with a fiery ascent atop a Delta rocket. After
escaping from its home planet's gravitational grasp, it flew through
the solar system perched on a pillar of blue-green xenon ions that enabled
the probe to accomplish a mission that would have been impossible with
conventional propulsion. In 2009, with its sights set on more distant
lands, Dawn swept past Mars, taking some of the planet's orbital
energy for its own. By its fourth anniversary, Dawn was conducting an
extensive orbital investigation of protoplanet Vesta, the second most
massive resident of the main asteroid belt. Dawn found it to be quite
unlike typical asteroids. Rather than a big chunk of rock, Vesta is like
a small planet, and scientists recognize it as being more closely related
to the rocky planets of the inner solar system (including Earth) than
to the much smaller asteroids. Vesta's nearer brethren are the blue
and white planet where Dawn began its mission nine years ago and the red
one it flew by 17 months later. By its fifth anniversary of leaving Earth,
the interplanetary spaceship was on its way to yet another distant, alien
world. Under the careful guidance of its human colleagues, Dawn completed
its 2.5-year journey from Vesta to Ceres last year. Now a perpetual companion
of the first dwarf discovered, the veteran space traveler will spend all
future anniversaries in orbit around Ceres, even after its operational
lifetime has concluded.

By February of this year, the spacecraft had exceeded all of its original
objectives established by NASA. Doing so involved orbiting Vesta for 14
months and, at that time, Ceres for almost a year. On June 30, Dawn's
prime mission concluded, and on July 1, its "extended mission" began.

[Dawn LAMO Image 147]
This simulated view of Ahuna Mons, Ceres' highest mountain, was made
with bonus stereo pictures Dawn acquired from an altitude of 240 miles
(385 kilometers). Ahuna Mons is likely a cryovolcano ("cold volcano"),
formed by cryomagma composed of salty mud rising from underground. The
volcano is geologically young, probably between 50 and 240 million years.
(We discussed in May how ages are estimated, but the analysis for Ahuna
Mons cannot yet pin down the age more accurately.) As Ceres is nearly
4.6 billion years old, a structure that developed so recently suggests
that some of the conditions that were necessary may persist even today.
(So far, scientists have identified no other cryovolcanoes on Ceres.)
It took somewhere between a few hundred and few hundred thousand years
for the volcano to build up to its present size. The elevation of the
summit is about 13,000 feet (4,000 meters), and the mountain is 11 miles
(17 kilometers) across at the base. Note the streaks from rockfalls down
the steep slopes (about 35 degrees). This view is from the north, and
in the foreground is a crater coincidentally 11 miles (17 kilometers)
across. From the lowest point in this crater to the top of the volcano
is 24,800 feet (7,560 meters) vertically across a horizontal distance
of only nine miles (15 kilometers). With 2.7 percent of Earth's gravity,
this could be a very nice extraterrestrial hike.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

One year ago today, the ship was in its third Ceres mapping orbit, scrutinizing
the exotic landscapes 915 miles (1,470 kilometers) beneath it. Less than
four weeks later, it started powering its way down through the uncharted
depths of Ceres gravitational field to undertake the final planned observations
of its long mission.

When ion thrusting concluded on Dec. 13, 2015, Dawn was orbiting closer
to Ceres than the International Space Station is to Earth. From its vantage
point only 240 miles (385 kilometers) high, the probe used its suite of
sophisticated sensors to develop a richly detailed portrait of the only
dwarf planet in the inner solar system. Dawn's reason for venturing
to its fourth mapping orbit was to collect about 35 days of neutron spectra,
35 days of gamma-ray spectra and 20 days of gravity measurements. Given
the complexity of operating in the low, tight orbit, mission planners
expected it could take about three months to acquire these precious data
and transmit them to Earth. Operations turned out to be essentially flawless,
and by the time Dawn left that orbit on Sept. 2, it had accumulated 183
days of neutron spectra, 183 days of gamma-ray spectra and 165 days of
gravity measurements. In addition, the spacecraft amassed a sensational
bonus of 38,000 high resolution photos (including stereo and color) as
well as more than 11 million infrared spectra and 12 million spectra in
visible wavelengths. The original plan was not to take any pictures or
visible or infrared spectra at the lowest altitude.

For such an overachiever, it's fitting that now, on its ninth anniversary,
the spacecraft is engaged in activities entirely unimagined on its eighth.
With the critical loss of two of the four reaction wheels used to orient
and stabilize the ship in space, the flight team (and your correspondent)
considered it unlikely Dawn would survive long enough to celebrate a ninth
anniversary. And everyone was confident that whether it was operating
or not, it would still be in the fourth mapping orbit. There was a clear
intent never to go anywhere else. But as we explained last month, with
the extraordinary wealth of information Dawn gleaned, the team has been
developing plans for new and previously unforeseen work at higher altitudes.
Next month, we will detail the first set of new observations from an orbital
perch of about 920 miles (1,480 kilometers).

For now, Dawn is using its ion engine #2 to gradually raise its orbit.
We have seen how the spacecraft's uniquely capable propulsion system
leads to intriguing spiral trajectories. Right now, on the ninth anniversary
of the last moment Dawn's rocket stood motionless at Cape Canaveral's
Space Launch Complex 17B, Dawn is 660 miles (1,060 kilometers) above Ceres.
With its signature combination of exceptional gentleness and exceptional
efficiency, the ion engine will propel Dawn to an altitude 20 miles (35
kilometers) higher by the end of the day today. (In contrast, by the end
of the day it launched nine years ago, Dawn had gained about 175,000 miles,
or 280,000 kilometers, in altitude. The Delta rocket provided a much stronger
thrust at much lower efficiency. We will discuss this further below.)
Dawn Launch

Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station,
Sept. 27, 2007. Note the sun rising on the left edge of the picture. The
intricate sequence of activities between the time this photo was taken
and Dawn's separation from the rocket to fly on its own is described
here. Image credit: KSC/NASA

You can follow Dawn's ascent to its new orbit by flying right behind
it as it loops around Ceres or by checking the frequent mission status

Nine years after launch, as Dawn maneuvers in orbit around a distant dwarf
planet in order to conduct new observations, it is convenient to look
back over its long trek through deep space. For those who would like to
track the probe's progress in the same terms used on past anniversaries,
we present here the ninth annual summary, reusing text from previous years
with updates where appropriate. Readers who wish to reflect upon Dawn's
ambitious journey may find it helpful to compare this material with the
Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh
and eighth anniversaries.

In its nine years of interplanetary travels, the spacecraft has thrust
for a total of 2,044 days (5.6 years), or 62 percent of the time (and
0.000000041 percent of the time since the Big Bang). While for most spacecraft,
firing a thruster to change course is a special event, it is Dawn's
wont. All this thrusting has cost the craft only 890 pounds (404 kilograms)
of its supply of xenon propellant, which was 937 pounds (425 kilograms)
on Sept. 27, 2007. The spacecraft has used 68 of the 71 gallons (256 of
the 270 liters) of xenon it carried when it rode its rocket from Earth
into space.

The thrusting since then has achieved the equivalent of accelerating the
probe by 24,800 mph (39,900 kilometers per hour). As previous logs have
described (see here for one of the more extensive discussions), because
of the principles of motion for orbital flight, whether around the sun
or any other gravitating body, Dawn is not actually traveling this much
faster than when it launched. But the effective change in speed remains
a useful measure of the effect of any spacecraft's propulsive work.
Dawn has far exceeded the velocity change achieved by any other spacecraft
under its own power. (For a comparison with probes that enter orbit around
Mars, refer to this earlier log.) It is remarkable that Dawn's ion
propulsion system has provided 97 percent of the change in speed that
the entire Delta rocket provided.
Dawn LAMO Image 147

Dawn had this view on June 1, 2016, from an altitude of 240 miles (385
kilometers). It is northeast of the scene we saw earlier this year of
Kupalo Crater. Kupalo is relatively young, and the impact that formed
it ejected material that blanketed the surrounding area, muting the appearance
of the older crater shown here. There are few craters visible in this
picture because there has not been enough time since the Kupalo impact
for the steady but slow rain of interplanetary debris to excavate many
new craters. We saw some examples of this in pictures in April and discussed
it further in May. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Since launch, our readers who have remained on or near Earth have completed
nine revolutions around the sun, covering 56.6 AU (5.3 billion miles,
or 8.5 billion kilometers). Orbiting farther from the sun, and thus moving
at a more leisurely pace, Dawn has traveled 38.6 AU (3.6 billion miles,
or 5.8 billion kilometers). As it climbed away from the sun, up the solar
system hill, to match its orbit to that of Vesta, it continued to slow
down to Vesta's speed. It had to go even slower to perform its graceful
rendezvous with Ceres. In the nine years since Dawn began its voyage,
Vesta has traveled only 36.6 AU (3.4 billion miles, or 5.5 billion kilometers),
and the even more sedate Ceres has gone 34.0 AU (3.2 billion miles, or
5.1 billion kilometers). (To develop a feeling for the relative speeds,
you might reread this paragraph while paying attention to only one set
of units, whether you choose AU, miles, or kilometers. Ignore the other
two scales so you can focus on the differences in distance among Earth,
Dawn, Vesta and Ceres over the nine years. You will see that as the strength
of the sun's gravitational grip weakens at greater distance, the
corresponding orbital speed decreases.)

Another way to investigate the progress of the mission is to chart how
Dawn's orbit around the sun has changed. This discussion will culminate
with a few more numbers than we usually include, and readers who prefer
not to indulge may skip this material, leaving that much more for the
grateful Numerivores. (If you prefer not to skip it, click here.) In order
to make the table below comprehensible (and to fulfill our commitment
of environmental responsibility), we recycle some more text here on the
nature of orbits.

Orbits are ellipses (like flattened circles, or ovals in which the ends
are of equal size). So as members of the solar system family (including
Earth, Vesta, Ceres and Dawn) follow their paths around the sun, they
sometimes move closer and sometimes move farther from it.
Dawn LAMO Image 147

Dawn's interplanetary trajectory (in blue). The dates in white show
Dawn's location every Sept. 27, starting on Earth in 2007. Note that
Earth returns to the same location, taking one year to complete each revolution
around the sun. When Dawn is farther from the sun, it orbits more slowly,
so the distance from one Sept. 27 to the next is shorter. In addition
to seeing Dawn's progress on this figure on previous anniversaries
of launch, we have seen it other times as well, most recently in July.
(And, to answer an important question raised last month, this image, along
with others, also will be seen for a short time this afternoon on a yummy
chocolate cake at the Dawn flight team's novennial celebration.)
Image credit: NASA/JPL

In addition to orbits being characterized by shape, or equivalently by
the amount of flattening (that is, the deviation from being a perfect
circle), and by size, they may be described in part by how they are oriented
in space. Using the bias of terrestrial astronomers, the plane of Earth's
orbit around the sun (known as the ecliptic) is a good reference. Other
planets and interplanetary spacecraft may travel in orbits that are tipped
at some angle to that. The angle between the ecliptic and the plane of
another body's orbit around the sun is the inclination of that orbit.
Vesta and Ceres do not orbit the sun in the same plane that Earth does,
and Dawn must match its orbit to that of its targets. (The major planets
orbit closer to the ecliptic, and part of the arduousness of Dawn's
journey has been changing the inclination of its orbit, an energetically
expensive task.)

Now we can see how Dawn has done by considering the size and shape (together
expressed by the minimum and maximum distances from the sun) and inclination
of its orbit on each of its anniversaries. (Experts readily recognize
that there is more to describing an orbit than these parameters. Our policy
remains that we link to the experts' websites when their readership
extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft
had terminated ion thrusting on its anniversaries; the orbits of its destinations,
Vesta and Ceres, are included for comparison. Of course, when Dawn was
on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly
Earth's orbit. After launch, it was in its own solar orbit.

        Minimum distance
from the Sun (AU) Maximum distance
from the Sun (AU) Inclination
Earth's orbit 0.98 1.02 0.0?
Dawn's orbit on Sept. 27, 2007 (before launch) 0.98 1.02 0.0?
Dawn's orbit on Sept. 27, 2007 (after launch) 1.00 1.62 0.6?
Dawn's orbit on Sept. 27, 2008 1.21 1.68 1.4?
Dawn's orbit on Sept. 27, 2009 1.42 1.87 6.2?
Dawn's orbit on Sept. 27, 2010 1.89 2.13 6.8?
Dawn's orbit on Sept. 27, 2011 2.15 2.57 7.1?
Vesta's orbit 2.15 2.57 7.1?
Dawn's orbit on Sept. 27, 2012 2.17 2.57 7.3?
Dawn's orbit on Sept. 27, 2013 2.44 2.98 8.7?
Dawn's orbit on Sept. 27, 2014 2.46 3.02 9.8?
Dawn's orbit on Sept. 27, 2015 2.56 2.98 10.6?
Dawn's orbit on Sept. 27, 2016 2.56 2.98 10.6?
Ceres' orbit 2.56 2.98 10.6?

For readers who are not overwhelmed by the number of numbers, investing
the effort to study the table may help to demonstrate how Dawn patiently
transformed its orbit during the course of its mission. Note that five
years ago, the spacecraft's path around the sun was exactly the same
as Vesta's. Achieving that perfect match was, of course, the objective
of the long flight that started in the same solar orbit as Earth, and
that is how Dawn managed to slip into orbit around Vesta. While simply
flying by it would have been far easier, matching orbits with Vesta required
the exceptional capability of the ion propulsion system. Without that
technology, NASA's Discovery Program would not have been able to
afford a mission to explore the massive protoplanet in such detail. But
now, Dawn has gone even beyond that. Having discovered so many of Vesta's
secrets, the stalwart adventurer left it behind in 2012. No other spacecraft
has ever escaped from orbit around one distant solar system object to
travel to and orbit still another extraterrestrial destination. Dawn devoted
another 2.5 years to reshaping and tilting its orbit even more so that
now it is identical to Ceres'. Once again, that was essential to
the intricate celestial choreography in March 2015, when the behemoth
tenderly took hold of the spacecraft. They have been performing an elegant
pas de deux ever since.
Dawn LAMO Image 147

This shows where Dawn's infrared mapping spectrometer detected water
ice in Oxo Crater. The crater is 6 miles (10 kilometers) in diameter.
This view was constructed from bonus photographs Dawn took from an altitude
of 240 miles (385 kilometers). Blue, green and infrared pictures were
combined with stereo pictures to provide this perspective. (Colors are
enhanced to bring out subtle differences your eye would not otherwise
detect, and the vertical scale has been exaggerated by a factor of two.)
Compare this with the Oxo Crater photograph shown in the April Dawn Journal.
Here, we are looking from the upper left of that photo toward the lower
right. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Its ion propulsion system has allowed Dawn to do even more than orbit
two distant and fascinating bodies. At each one, the spacecraft has changed
its orbits extensively, optimizing its views to conduct detailed studies,
something it would not have been able to do with conventional propulsion.

Dawn passed a coincidental pair of milestones in its orbital mission at
Ceres last week. The dwarf planet reached out to take Earth's emissary
into a gentle but permanent gravitational embrace on March 6, 2015. Sept.
23, 2016, was 1,500 Cerean days later. (Ceres turns on its axis in 9 hours,
4 minutes, considerably faster than Earth, although not all that different
from the giant planet Jupiter, which takes 9 hours, 56 minutes). Interestingly,
on Sept. 22, Dawn completed its 1,500th orbital revolution around Ceres.

Given the equality between the number of orbits and the number of Cerean
days, you may be tempted to conclude that Dawn orbits at the same rate
that Ceres rotates. Please resist this temptation! Dawn's early orbits
took weeks to complete, and as the spacecraft maneuvered to lower altitudes,
eventually they took days and then hours. In its lowest altitude, the
spacecraft circled Ceres in only 5.4 hours. (For a reminder of the details
of the orbits, see this table and this diagram depicting preliminary orbit
sizes.) So, it truly is a coincidence that the average has worked out
so that Dawn has revolved as many times as Ceres has rotated. And now
that Dawn is raising its altitude and thus increasing the time required
to complete an orbit, such a coincidence will not occur again. Ceres is
very stubborn and will keep rotating at the same rate. Dawn, much nimbler
and more flexible, is currently in a 13-hour orbit. By the time it completes
ion thrusting next week, the orbit period will be almost 19 hours.
Dawn LAMO Image 147

This topographical map of Ceres was made from Dawn's stereo photos
taken in the third mapping orbit. (For experts, the topography is referenced
to an ellipsoid of 299.5 by 299.5 by 277.1 miles, or 482.0 by 482.0 by
446.0 kilometers.) 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 map shows all
the 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. (To avoid confusion,
note that the topographical map here has the prime meridian on the left,
but the IAU map has it in the middle.) The scales for horizontal distance
in this figure apply at the equator. Rectangular maps like this distort
distances at other latitudes. A similar version of this map is here. Image

Now in the 10th year of its deep-space expedition, Dawn is not satisfied
simply to rest on its laurels. The explorer (along with its support team
on distant Earth) is committed to remaining as prolific and profitable
at Ceres as it was during earlier years of its extraordinary and innovative
mission of discovery. The largest body between Mars and Jupiter is a relict
from the dawn of the solar system, a strange and fascinating world of
rock, ice and salt that likely has been geologically active for more than
4.5 billion years. Ceres was first glimpsed from Earth more than 200 years
ago but held her secrets close until Earth finally answered her cosmic
invitation. Now, after so very long, Ceres is whispering those wondrous
secrets to her permanent companion. Dawn is listening carefully!

Dawn is 660 miles (1,060 kilometers) from Ceres. It is also 1.99 AU (185
million miles, or 297 million kilometers) from Earth, or 760 times as
far as the moon and 1.98 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 33 minutes
to make the round trip.
Received on Fri 30 Sep 2016 06:00:04 PM PDT

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