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http://dawn.jpl.nasa.gov/mission/journal_07_27_16.html

Dawn Journal
Dr. Marc Rayman
July 27, 2016

Dear Exdawnsions,

Humankind dispatched Dawn on an extraordinary extraterrestrial expedition
in 2007. It visited Mars briefly in 2009 and spent 14 months orbiting protoplanet Vesta in 2011-2012, revealing fascinating details of that uncharted, alien world. After traveling for another two and a half years through the interplanetary void, the spacecraft arrived at Ceres in March 2015. It has now conducted an outstandingly successful exploration of
the only dwarf planet in the inner solar system. Dawn greatly surpassed
its objectives at both Vesta and Ceres, accomplishing well more than was envisioned when NASA decided to undertake this ambitious mission. Having realized its raison d'ntre, the official end of Dawn's prime mission was
June 30.

Following the conclusion of the prime mission, the adventurer began its "extended mission" of performing more Ceres observations without missing
a beat. We described in April some of what Dawn can do as it continues investigating many of the mysteries there. Dawn's extension allows for
even better measurements with the gamma ray and neutron detector of the nuclear radiation emanating from Ceres. This is like taking a longer exposure of the very faint nuclear glow, yielding a brighter, sharper picture that reveals more about the atomic constituents down to about a yard (meter) underground. The spacecraft is taking more stereo photos, continuing to improve the topographical map it created from four times higher. Scientists also are taking advantage of this opportunity to study more geological features with the visible and infrared mapping spectrometers, providing important insight into Ceres' mineralogical inventory.
Dawn LAMO Image 113

Dawn's trajectory through the solar system, starting when it departed
from Earth in 2007. Dawn's orbit around the sun is shown in blue. When
the spacecraft is in orbit around Vesta or Ceres, its trajectory is the
same color as their heliocentric orbits, which are bold when Dawn is accompanying
them around the sun. Dawn is carrying out its extended mission in Ceres' permanent gravitational embrace. We have seen Dawn's progress on this
figure before, most recently on March 4. Image credit: NASA/JPL-Caltech

Dawn has already made extraordinary discoveries at Ceres, some of which
we have described in recent months. But on a dwarf planet of 1.1 million square miles (2.8 million square kilometers), there is a great deal to
see. That, after all, is the benefit of being in orbit, lingering long
enough to make a richly detailed portrait of the exotic expanse. Indeed,
Ceres has 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. In such a vast territory, there are innumerable mysteries to unravel. And that is only the surface.

Dawn also is continuing its studies of the gravitational field to discover more about the interior structure of the largest body between Mars and Jupiter.


[Dawn LAMO Image 113]

Dawn captured this view inside Occator Crater on March 26 from an altitude
of 240 miles (385 kilometers). We have explained that the bright areas
are salts, which reflect much more sunlight than typical materials on
Ceres. Recent analysis of Dawn's infrared spectra shows the salt is mostly sodium carbonate. (This is the brightest region on Ceres, but you can
see another of the many reflective deposits in one of the pictures below
of two adjoining craters.) Occator Crater formed 80 million years ago.
Another part of this geologically young crater is shown immediately below. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

[Dawn LAMO Image 108]

Dawn observed the edge of Occator Crater from an altitude of 240 miles
(385 kilometers) on January 26. The crater is 57 miles (92 kilometers)
in diameter. Note the detail on the steep walls and the fractures both
inside and outside the crater that generally are parallel to the rim.
Look carefully to spot some very large boulders (as described here), especially

near the top center and left.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In the coming months we will discuss other intriguing activities and how
Dawn will make measurements never even considered before. But for now,
let's look at how this extension came about.

As readers of the Dawn Journals know (and as you will be reminded below), there has been very good reason in recent years to believe the spacecraft would not operate beyond the end of its prime mission. However, the veteran explorer is in very good health. It is one of Earth's most experienced
and capable ambassadors to the cosmos, we want to squeeze as much out
of this mission as we can. Ever resourceful, the Dawn team recognized
in March 2016 that the probe had the capability to do yet more and decided
to give NASA Headquarters a unique choice: remain at Ceres (as always expected) or go elsewhere.

It is worth pondering how extraordinary this is. Most spacecraft can only
make minor adjustments to their trajectories, so at the end of their prime missions, they generally go wherever they were already headed. If a spacecraft is in orbit around some planetary body, it remains in orbit. If a spacecraft is not in orbit, having previously flown past one or more bodies that
orbit the sun, its course is largely determined by the targeting for the
last encounter. A planet's gravity may have redirected it, but otherwise
its propulsion system has to do the work, and that usually can produce
only a tiny change in direction. If a spacecraft is not already in orbit around a planetary body, it won't be able to enter orbit.

Dawn is different. With its uniquely capable ion propulsion system, Dawn
is the only spacecraft ever to travel to a distant destination, orbit
it, later break out of orbit, then travel to another faraway destination,
and orbit it. And even while in orbit around Vesta and Ceres, Dawn maneuvered extensively, optimizing its orbits for its scientific investigations.
And yet this remarkable ship can do still more. It has the capability
to leave its second destination and continue its travels.

Dawn's brilliant and creative navigators analyzed possible missions to
more than 68,000 known objects. That alone is a nice illustration of the powerful potential.

The project team very quickly narrowed the list to the most interesting
body Dawn could reach after leaving Ceres, a large asteroid named Adeona.
That mission offered the best alternative to further studies of the dwarf planet.

[Dawn LAMO Image 114]

Dawn had this view of two adjoining craters on Ceres on March 26 from
an altitude of 240 miles (385 kilometers). Reflective material, most likely salt left after ice sublimated (as in Occator Crater, shown above), is
easily visible. Look carefully inside both craters to see many large boulders (light on the right and dark on the left). Also note what appears to be
the remnants of material that flowed to near the middle of the larger
crater. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

[Dawn LAMO Image 107]

Dawn observed this pair of craters on January 25 from an altitude of 240
miles (385 kilometers). The upper crater is the younger of the two, as indicated by its sharper features, which have not been eroded as much
by the gradual but persistent rain of interplanetary debris falling on
Ceres. (In May we discussed how scientists quantify the ages, although
the dates these craters formed have not yet been computed.) The wall where
the craters meet has partially collapsed into the older one.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

But how to decide between these two attractive possibilities? Some members
of the Dawn team preferred continuing the exploration of Ceres and others preferred going to Adeona. Similarly, some people prefer cake and some
prefer pie. (That's not a perfect example, because it's obvious cake is better, but you get the idea.)

NASA thoroughly evaluated the scientific potential and other aspects of
the options. Part of this was an assessment by an independent group of esteemed scientists. The conclusion was that either would be valuable
but that studying Ceres further was preferable.

From the perspective of your correspondent -- passionate about space exploration
since the age of four, a professional scientist (as well as a scientist
at heart), an engineer and a taxpayer -- this is a wonderful outcome.
How could one want anything other than such a well-considered decision?

But how is it even possible that the team could have offered to NASA the option of visiting Adeona for the extended mission? We have emphasized
for several years that Ceres would be Dawn's final home. If you had asked
even as recently as early this year whether the spacecraft could leave
Ceres (and many of you did), we would have responded that such a prospect
was unrealistic and inconceivable (and we did). We have described in great detail how the failure of two of Dawn's four reaction wheels was so serious that it was only with heroic effort that the distant robot was able to complete its original assignments. We have explained repeatedly that the spacecraft will soon expend the last of its hydrazine propellant, then immediately lose the ability to point its solar arrays at the sun, its
antenna at Earth, its scientific sensors at Ceres or its ion engine in
the direction needed to fly elsewhere. Why the change now, and how could
Dawn operate for a multiyear journey?

We have discussed in recent months how remarkably well the flight team
has done in conserving hydrazine, significantly exceeding any reasonable expectations and thereby extending Dawn's functional lifetime. Moreover, mission controllers know that the probe consumes less hydrazine at higher altitudes. Contrary to many people's notions, the dwarf planet's gravity
is appreciable, and operating so close to it requires a very high rate
of hydrazine consumption. Dawn is circling only 240 miles (385 kilometers) above Ceres, closer than the International Space Station is to Earth.
But during the long deep-space journey to Adeona, Dawn would use the precious propellant much more sparingly. So despite the loss of the two reaction wheels, under the expert guidance of its terrestrial colleagues, the ship could set sail once again for a new and distant land beyond the horizon.
Dawn LAMO Image 125

Dawn had this scenic view on June 13 while orbiting 240 miles (385 kilometers) above Ceres. This is one of the occasional photographs of the landscape reaching to the horizon, in this case near Kirnis Crater. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Isn't it incredibly cool that humankind has the capability to fire up
the ion engine on a distant interplanetary spaceship and pilot it out
of orbit around a dwarf planet to fly more than halfway around the sun
on a bold expedition of 900 million miles (1.5 billion kilometers) to investigate a huge asteroid? (Hint: the answer is yes.)

Exciting as such a voyage might seem, it is gratifying that a thoughtful, rationale decision was made that yields an even better outcome. Rather
than terminate the present mission after it has exceeded all of its original objectives, and rather than embark on that new mission, the best possible
use of Dawn is to do what it is doing right now: extracting secrets from
dwarf planet Ceres. And now we can look forward to more, as Dawn pursues
new objectives. As the extended mission progresses, we will describe marvelous new findings from the rich trove of data Dawn is returning, and we will
see how the team plans to take advantage of this unique opportunity to
learn more about the nature of the solar system.

If you share in the passion for exploration, if you thrill to new discoveries and new knowledge or even if you just want to see how many more silly
Dawn Journal greetings your correspondent can concoct, stay onboard as
Dawn's adventure at Ceres continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 2.69 AU (250
million miles, or 403 million kilometers) from Earth, or 1,090 times as
far as the moon and 2.65 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 45 minutes
to make the round trip.

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