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

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/09/27/dawn-journal-september-27-2/

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 
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 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.