[meteorite-list] Dawn Journal - May 28, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/05/28/dawn-journal-may-28-2015/

Dawn Journal
Dr. Marc Rayman
May 28, 2015

Dear Emboldawned Readers,

A bold adventurer from Earth is gracefully soaring over an exotic world 
of rock and ice far, far away. Having already obtained a treasure trove 
from its first mapping orbit, Dawn is now seeking even greater riches 
at dwarf planet Ceres as it maneuvers to its second orbit.

The first intensive mapping campaign was extremely productive. As the 
spacecraft circled 8,400 miles (13,600 kilometers) above the alien terrain, 
one orbit around Ceres took 15 days. During its single revolution, the 
probe observed its new home on five occasions from April 24 to May 8. 
When Dawn was flying over the night side (still high enough that it was 
in sunlight even when the ground below was in darkness), it looked first 
at the illuminated crescent of the southern hemisphere and later at the 
northern hemisphere.

When Dawn traveled over the sunlit side, it watched the northern hemisphere, 
then the equatorial regions, and finally the southern hemisphere as Ceres 
rotated beneath it each time. One Cerean day, the time it takes the globe 
to turn once on its axis, is about nine hours, much shorter than the time 
needed for the spacecraft to loop around its orbit. So it was almost as 
if Dawn hovered in place, moving only slightly as it peered down, and 
its instruments could record all of the sights as they paraded by.

We described the plans in much more detail in March, and they executed 
beautifully, yielding a rich collection of photos in visible and near 
infrared wavelengths, spectra in visible and infrared, and measurements 
of the strength of Ceres' gravitational attraction and hence its mass.

To gain the same view Dawn had, simply build your own ion-propelled spaceship, 
voyage deep into the main asteroid belt between Mars and Jupiter, take 
up residence at the giant orb and look out the window. Or go to the image 
gallery here.

Either way, the sights are spectacular. And they have already gotten even 
better. As Dawn has been descending to its second mapping orbit, it paused 
ion-thrusting on May 16 and May 22 to take more pictures, helping navigators 
get a tight fix on its orbital location. We explained this technique of 
optical navigation earlier, but now it is slightly different. Dawn is 
so close to Ceres that the behemoth fills the camera's field of view. 
No longer charting Ceres' location relative to background stars, navigators 
now use distinctive features on Ceres itself. It was an indistinct, fuzzy 
little blob just a few months ago, but now the maps are becoming detailed 
and accurate. Mathematical analyses of the locations of specific landmarks 
in each picture allow navigators to determine where Dawn was when the 
picture was taken.

Let's see how this works. Suppose I gave you a picture I had taken in 
your house. (The last time I was there, I opted for the cover of darkness 
rather than a more visible demonstration of optical navigation, but we 
can still imagine.) Because you know the positions of the doors, windows, 
furniture, impact craters, paintings, etc., you could establish where 
I had been when I took the photo. Now that they have charted the positions 
of the features at Dawn's new home, navigators can do virtually the same 
thing.

In addition to aiding in celestial navigation, the photos provided still 
better views of the world Dawn traveled so long and so far to explore. 
Greater and greater detail is visible as Dawn orbits closer, and a tremendous 
variety of intriguing sights are coming into view. It may well be that 
the most interesting discoveries have not even been made yet, but for 
now, what captivates most people (and other readers as well) are the bright 
spots.

We have discussed them here and there in recent months, and their luminous 
power continues to dazzle us. What appeared initially as one fuzzy spot 
proved to be two smaller spots and now many even smaller regions as the 
focus has become sharper. Why the ground there reflects so much sunlight 
remains elusive. Dawn's finer examinations with its suite of sophisticated 
instruments in the second, third and then final mapping orbits will provide 
scientists with data they need to unravel this marvelous mystery. For 
now, the enigmatic lights present an irresistible cosmic invitation to 
go closer and to scrutinize this strange and wonderful world, and we are 
eager to accept. After all, we explore to learn, to know the unknown, 
and the uniquely powerful scientific method will reveal the nature of 
the bright areas and what they can tell us about the composition and geology 
of this complex dwarf planet.

After having been viewed as little more than a smudge in telescopes for 
more than two centuries since its discovery, Ceres now is seen as a detailed, 
three-dimensional world. As promised, measurements from Dawn have revised 
the size to be about 599 miles (963 kilometers) across at the equator. 
Like Earth and other planets, Ceres is oblate, or slightly wider at the 
equator than from pole to pole. The polar diameter is 554 miles (891 kilometers). 
These dimensions are impressively close to what astronomers had determined 
from telescopic observations and confirm Ceres to be the colossus we have 
described.

Before Dawn, scientists had estimated Ceres' mass to be 1.04 billion billion 
tons (947 billion billion kilograms). Now it is measured to be 1.03 billion 
billion tons (939 billion billion kilograms), well within the previous 
margin of error. It is an impressive demonstration of the success of science 
that astronomers had been able to determine the heft of that point of 
light so accurately. Nevertheless, even this small change of less than 
one percent is important for planning the rest of Dawn's mission as it 
orbits closer and closer, feeling the gravitational tug ever more strongly.

Let's put this change in context. Dawn has now refined the mass, making 
a proportionally small adjustment of about 0.01 billion billion tons (eight 
billion billion kilograms). Although no more than a tweak on the overall 
value, it is still significantly greater than the combined mass of all 
asteroids visited by all other spacecraft. Ceres is so immense, so massive 
that even if all those asteroids were added to it, the difference would 
hardly even have been noticeable. This serves as another reminder that 
the dwarf planet really is quite unlike the millions of small asteroids 
that constitute the main asteroid belt. This behemoth contains about 30 
percent of all the mass in that entire vast region of space. Vesta, the 
protoplanet Dawn orbited and studied in 2011-2012, is the second most 
massive resident there, holding about 8 percent of the asteroid belt's 
mass. Dawn by itself is exploring around 40 percent of the asteroid belt's 
mass!

Upon concluding its first mapping orbit, Dawn powered on its remarkable 
ion propulsion system on May 9 to fly down to a lower altitude where it 
will gain a better view. We examined the nature of the spiral paths between 
mapping orbits last year (and at Vesta in 2011-2012).

In its first mapping orbit, Dawn was 8,400 miles (13,600 kilometers) high, 
revolving once in 15.2 days at a speed of 150 mph (240 kilometers per 
hour). By the time it completes this descent, the probe will be at an 
altitude of 2,700 miles (4,400 kilometers), orbiting Ceres every 3.1 days 
at 254 mph (408 kilometers per hour). (All of the mapping orbits were 
summarized in this table.) We have discussed that lower orbits require 
greater velocity to counterbalance the stronger gravitational hold.

Dawn's uniquely capable ion propulsion system, with its extraordinary 
combination of efficiency and gentleness, propels the ship to its new 
orbital destination in just under four weeks. The descent requires five 
revolutions, each one faster than the one before. The flight profile is 
complicated, and sometimes Dawn even dips below the final, planned altitude 
and then rises to greater heights as it flies on a path that is temporarily 
elliptical. The overall trend, of course, is downward. As Dawn heads for 
its targeted circular orbit, its maneuvering is also generally reducing 
the orbit period, the time required to make one complete revolution around 
Ceres. Indeed, if Dawn stopped thrusting now, its orbit period would be 
about 83 hours, or 3.5 days.

Dawn will complete ion-thrusting on June 3, but it will not be ready to 
begin its next science observations then. Rather, as in the other new 
mapping orbits, the first order of business will be for navigators to 
measure the new orbital parameters accurately. The flight team then will 
install in Dawn's main computer the details of the orbit it achieved so 
it will always know its location.

In addition, the intensive campaign of observations is planned to begin 
when the robotic explorer travels from the night side to the day side 
over the north pole. With the three-day orbit period, that will next occur 
on June 5. Controllers will take advantage of the intervening time to 
conduct other activities, including routine maintenance of the two reaction 
wheels that remain operable, although they are powered off most of the 
time. (Two of the four failed years ago. Dawn no longer relies on these 
devices to control its orientation, and it is remarkable that the mission 
can accomplish all of its original objectives without them. But if two 
do function in the final mapping orbit later this year, they will help 
extend the spacecraft's lifetime for bonus studies.)

We have already presented the ambitious plans for this second mapping 
orbit, sometimes known as "the second mapping orbit" and sometimes more 
succinctly and confusingly as "survey orbit." As with all four of Dawn's 
mapping orbits, it is designed to take the spacecraft over the poles, 
ensuring the best possible coverage. The ship will fly from the north 
pole to the south over the side of Ceres facing the sun, and then loop 
back to the north over the side hidden in the deep dark of night. On the 
day side, Dawn will aim its camera and spectrometers at the lit ground, 
filling its memory to capacity with the readings. On the night side, it 
will point its main antenna to distant Earth in order to radio its findings 
home. At Dawn's altitude, Ceres will appear twice as wide as the camera's 
view. (As illustrated in this table, it will look about the size of a 
soccer ball seen from a yard, or a meter, away.) But as the dwarf planet 
rotates on its axis and Dawn sails around in its more leisurely orbit, 
eventually all of the landscape will come within sight of the instruments.

Only one noteworthy change has been made in the intricate plans for survey 
orbit since May 2014's shocking expose'. With the observations starting 
on June 5, the subsequent complex orbital flight to the third mapping 
orbit (also known as HAMO) would have begun on June 27. As we have seen, 
the rapidly changing orbit in the spiral descents requires a great deal 
of effort by the small operations team on a rigid schedule. The capable 
men and women flying Dawn accomplished the maneuvers flawlessly at Vesta 
and are well prepared for the challenges at Ceres. The work is very demanding, 
however, and so, just as at Vesta, the team has built into the strategy 
the capability to make adjustments to align most of the tasks with a conventional 
work schedule. The technical plans (even including the exquisitely careful 
husbanding of hydrazine following the loss of the two reaction wheels) 
fully account for such human factors. It turns out that leaving survey 
orbit three days later shifts a significant amount of the following work 
off weekends, making it more comfortable for the team members. Three days 
is one complete revolution, and always extracting as much from the mission 
as possible, they have devised another full set of observations for an 
eighth orbit. As a result, survey orbit may be even more extensive and 
productive than originally anticipated.

What awaits Dawn in the next mapping phase? The views will be three times 
as sharp as in the previous orbit, and exciting new discoveries are sure 
to come. What answers will be revealed? And what new questions (besides 
this one) will arise? We will know soon, as we all share in the thrill 
of this grand adventure. To help you keep track of Dawn's progress as 
it powers its way down and then conducts further observations, your correspondent 
writes brief (hard to believe, isn't it?) mission status updates. And 
although in space no one can hear you tweet, terrestrial followers can 
get even more frequent updates with information he provides for Twitter 
@NASA_Dawn.

Dawn is 3,400 miles (5,500 kilometers) from Ceres. It is also 2.30 AU 
(214 million miles, or 345 million kilometers) from Earth, or 855 times 
as far as the moon and 2.27 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 38 minutes 
to make the round trip.