[meteorite-list] Dawn Journal - September 27, 2015

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 29 Sep 2015 15:04:30 -0700 (PDT)
Message-ID: <201509292204.t8TM4U0k017444_at_zagami.jpl.nasa.gov>


Dawn Journal
by Dr. Marc Rayman
September 27, 2015

Dear Dawnniversaries,

Eight years ago today, Dawn was gravitationally bound to a planet. It
was conceived and built there by creatures curious and bold, with an insatiable
yearning to reach out and know the cosmos. Under their guidance, it left
Earth behind as its Delta rocket dispatched it on an ambitious mission
to explore two of the last uncharted worlds in the inner solar system.
As Earth continued circling the sun once a year, now having completed
eight revolutions since its celestial ambassador departed, Dawn has accomplished
a remarkable interplanetary journey. The adventurer spent most of its
anniversaries powering its way through the solar system, using its advanced
and uniquely capable ion propulsion system to reshape its orbit around
the sun. On its way to the main asteroid belt, it sailed past Mars, taking
some of the that red planet's orbital energy to boost its own solar orbit.
On its fourth anniversary, the probe was locked in orbit around the giant
protoplanet Vesta, the second most massive object between Mars and Jupiter.
Dawn's pictures and other data showed it to be a complex, fascinating
world, more closely related to the terrestrial planets (including one
on which it began its mission and another from which it stole some energy)
than to the much smaller asteroids.

Today, on the eighth anniversary of venturing into the cosmos, Dawn is
once again doing what it does best. In the permanent gravitational embrace
of dwarf planet Ceres, orbiting at an altitude of 915 miles (1,470 kilometers),
Dawn is using its suite of sophisticated sensors to scrutinize this mysterious,
alien orb. Ceres was the first dwarf planet ever sighted (and was called
a planet for more than a generation after its discovery), but it had to
wait more than two centuries before Earth accepted its celestial invitation.
The only spacecraft ever to orbit two extraterrestrial destinations, this
interplanetary spaceship arrived at Ceres in March to take up residence.

Although this is the final anniversary during its scheduled primary mission,
Dawn will remain in orbit around its new home far, far into the future.
Later this year it will spiral down to its fourth and final orbital altitude
at about 230 miles (375 kilometers). Once there, it will record spectra
of neutrons, gamma rays, and visible and infrared light, measure the distribution
of mass inside Ceres, and take pictures. Then when it exhausts its supply
of hydrazine next year, as it surely will, the mission will end. We have
discussed before that despite the failure of two reaction wheels, devices
previously considered indispensable for the expedition, the hardy ship
has excellent prospects now for fulfilling and even exceeding its many
goals in exploring Ceres.

Last month we described the plans for Dawn's penultimate mapping phase
at the dwarf planet, and it is going very well. The probe is already more
than halfway through this third orbital phase at Ceres, which is divided
into six mapping cycles. Each 11-day cycle requires a dozen flights over
the illuminated hemisphere to allow the camera to map the entire surface.
Each map is made by looking at a different angle. Taken together then,
they provide stereo views, so scientists gain perspectives that allow
them to construct topographical maps. The camera's internal computer detected
an unexpected condition in the third cycle of this phase, and that caused
the loss of some of the pictures. But experienced mission planners had
designed all of the major mapping phases (summarized here) with more observations
than are needed to meet their objectives, so the deletion of those images
was not significant. At this moment, the spacecraft is nearing the end
of its fourth mapping cycle, making its tenth flight over the side of
Ceres lit by the sun.

You can follow Dawn's progress by using your own interplanetary spaceship
to snoop into its activities in orbit around the distant world, by tapping
into the radio signals beamed back and forth across the solar system between
Dawn and the giant antennas of NASA's Deep Space Network, or by checking
the frequent mission status reports.

You also can see the marvelous sights by visiting the Ceres image gallery.
Among the most captivating is Occator crater (see the picture below).
As the spacecraft has produced ever finer pictures this year, starting
with its distant observations in January, the light reflecting from the
interior of this crater has dazzled us. The latest pictures show 260 times
as much detail. Dawn has transformed what was so recently just a bright
spot into a complex and beautiful gleaming landscape. Last month we asked
what these mesmerizing features would reveal when photographed from this
the present altitude, and now we know.

Scientists are continuing to analyze Dawn's pictures and other data not
only from Occator but all of Ceres to learn more about the nature of this
exotic relict from the dawn of the solar system. Many deep questions are
unanswered and remain mystifying, but of one point there can be no doubt:
the scenery is beautiful. Even now, the photos speak for themselves, displaying
wondrous sights on a world shaped both by its own complex internal geological
processes as well as by external forces from more than 4.5 billion years
in the rough and tumble main asteroid belt.

Because the pictures speak for themselves, your correspondent will speak
for the mission. So now, as every Sep. 27, let's take a broader look at
Dawn's deep-space trek. For those who would like to track the probe's
progress in the same terms used on past anniversaries, we present here
the eighth 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 logs from its first,
second, third, fourth, fifth, sixth and seventh anniversaries.

In its eight years of interplanetary travels, the spacecraft has thrust
for a total of 1,976 days, or 68 percent of the time (and about 0.000000039
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 873 pounds (396 kilograms) of its
supply of xenon propellant, which was 937 pounds (425 kilograms) on Sep.
27, 2007. The spacecraft has used 66 of the 71 gallons (252 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 24,400 mph (39,200 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. Having
accomplished 98 percent of the thrust time planned for its entire mission,
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.) The principal ion thrusting that remains
is to maneuver from the present orbit to the final one from late October
to mid-December.

Since launch, our readers who have remained on or near Earth have completed
eight revolutions around the sun, covering 50.3 AU (4.7 billion miles,
or 7.5 billion kilometers). Orbiting farther from the sun, and thus moving
at a more leisurely pace, Dawn has traveled 35.0 AU (3.3 billion miles,
or 5.2 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 eight years since Dawn began its voyage,
Vesta has traveled only 32.7 AU (3.0 billion miles, or 4.9 billion kilometers),
and the even more sedate Ceres has gone 26.8 AU (2.5 billion miles, or
4.0 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 eight 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.

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

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 Sep. 27, 2007, its orbit around the sun was exactly
Earth's orbit. After launch, it was in its own solar orbit.

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 Sep. 27, 2007 (before launch) 0.98 1.02 0.0?
Dawn's orbit on Sep. 27, 2007 (after launch) 1.00 1.62 0.6?
Dawn's orbit on Sep. 27, 2008 1.21 1.68 1.4?
Dawn's orbit on Sep. 27, 2009 1.42 1.87 6.2?
Dawn's orbit on Sep. 27, 2010 1.89 2.13 6.8?
Dawn's orbit on Sep. 27, 2011 2.15 2.57 7.1?
Vesta's orbit 2.15 2.57 7.1?
Dawn's orbit on Sep. 27, 2012 2.17 2.57 7.3?
Dawn's orbit on Sep. 27, 2013 2.44 2.98 8.7?
Dawn's orbit on Sep. 27, 2014 2.46 3.02 9.8?
Dawn's orbit on Sep. 27, 2015 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 has patiently
transformed its orbit during the course of its mission. Note that four
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, when the behemoth reached out with its
gravity and tenderly took hold of the spacecraft. They have been performing
an elegant pas de deux ever since.

Dawn takes great advantage of being able to orbit its two targets by performing
extensive measurements that would not be feasible with a fleeting visit
at high speed. As its detailed inspection of a strange and distant world
continues, we can look forward to more intriguing perspectives and exciting
insights into our solar system. On its eighth anniversary of setting sail
on the cosmic seas for an extraordinary voyage, the faithful ship is steadily
accumulating great treasures.

Dawn is 915 miles (1,470 kilometers) from Ceres. It is also 2.45 AU (228
million miles, or 367 million kilometers) from Earth, or 1,025 times as
far as the moon and 2.45 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 41 minutes
to make the round trip.
Received on Tue 29 Sep 2015 06:04:30 PM PDT

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