[meteorite-list] Dawn Journal - August 21, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/08/21/dawn-journal-august-21/

Dawn Journal 
by Dr. Marc Rayman
August 21, 2015

Dear Unhesidawntingly Enthusiastic Readers,

An ambitious explorer from Earth is gaining the best views ever of dwarf 
planet Ceres. More than two centuries after its discovery, this erstwhile 
planet is now being mapped in great detail by Dawn.

The spacecraft is engaged in some of the most intensive observations of 
its entire mission at Ceres, using its camera and other sensors to scrutinize 
the alien world with unprecedented clarity and completeness. At an average 
altitude of 915 miles (1,470 kilometers) and traveling at 400 mph (645 
kilometers per hour), Dawn completes an orbit every 19 hours. The pioneer 
will be here for more than two months before descending to its final orbit.

The complex spiral maneuver down from the second mapping orbit at 2,700 
miles (4,400 kilometers) went so well that Dawn arrived in this third 
mapping orbit on Aug. 13, which was slightly ahead of schedule. (Frequent 
progress of its descent, and reports on the ongoing work in the new orbit, 
are available here and on Twitter @NASA_Dawn.) It began this third mapping 
phase on schedule at 9:53:40 p.m. PDT on Aug. 17.
Map of Ceres with named craters

We had a detailed preview of the plans last year when Dawn was more than 
six thousand times farther from Ceres than it is today. (For reasons almost 
as old as Ceres itself, this phase is also known as the high altitude 
mapping orbit, or HAMO, although we have seen that it is the second lowest 
of the four mapping orbits.) Now let's review what will happen, including 
a change mission planners have made since then.

The precious pictures and other data have just begun to arrive on Earth, 
and it is too soon to say anything about the latest findings, but stand 
by for stunning new discoveries. Actually, you could get pictures about 
as good as Dawn's are now with a telescope 217 times the diameter of 
Hubble Space Telescope. An alternative is to build your own interplanetary 
spaceship, travel through the depths of space to the only dwarf planet 
in the inner solar system, and look out the window. Or go to the Ceres 
image gallery.

Dawn has already gained fabulous perspectives on this mysterious world 
from its first and second mapping orbits. Now at one third the altitude 
of the mapping campaign that completed in June, its view is three times 
as sharp. (Exploring the cosmos is so cool!) That also means each picture 
takes in a correspondingly smaller area, so more pictures are needed now 
to cover the entire vast and varied landscape. At this height, Dawn's 
camera sees a square about 88 miles (140 kilometers) on a side, less than 
one percent of the more than one million square miles (nearly 2.8 million 
square kilometers). The orbital parameters were chosen carefully so that 
as Ceres rotates on its axis every nine hours (one Cerean day), Dawn will 
be able to photograph nearly all of the surface in a dozen orbital loops.
his image, taken by NASA's Dawn spacecraft, shows the brightest spots 
on dwarf planet Ceres from an altitude of 2,700 miles (4,400 kilometers). 
The image, with a resolution of 1,400 feet (410 meters) per pixel, was 
taken on June 24, 2015.

When Dawn explored the giant protoplanet Vesta from comparable orbits 
(HAMO1 in 2011 and HAMO2 in 2012), it pointed its scientific instruments 
at the illuminated ground whenever it was on the dayside. Every time its 
orbit took it over the nightside, it turned to point its main antenna 
at Earth to radio its findings to NASA's Deep Space Network. As we explained 
last year, however, that is not the plan at Ceres, because of the failure 
of two of the ship's reaction wheels. (By electrically changing the 
speed at which these gyroscope-like devices rotate, Dawn can turn or stabilize 
itself in the zero-gravity conditions of spaceflight.)

We discussed in January that the flight team has excogitated innovative 
methods to accomplish and even exceed the original mission objectives 
regardless of the condition of the wheels, even the two operable ones 
(which will not be used until the final mapping orbit). Dawn no longer 
relies on reaction wheels, although when it left Earth in 2007, they were 
deemed indispensable. The spacecraft's resilience (which is a direct 
result of the team's resourcefulness) is remarkable!

One of the many ingredients in the recipe for turning the potentially 
devastating loss of the wheels into a solid plan for success has been 
to rotate the spacecraft less frequently. Therefore, sometimes Dawn will 
wait patiently for half an orbit (almost 9.5 hours) as it flies above 
ground cloaked in the deep darkness of night, its instruments pointed 
at terrain they cannot detect. Other times, it will keep its antenna fixed 
on Earth without even glancing at the sunlit scenery below, because it 
can capture the views on other revolutions. This strategy conserves hydrazine, 
the conventional rocket propellant used by the small jets of the reaction 
control system in the absence of the wheels. It takes more time, but because 
Dawn is in orbit, time is not such a limited resource. It will take 12 
passages over the illuminated hemisphere, each lasting nearly 9.5 hours, 
to bring the entirety of the landscape within view of its camera, but 
we will need a total of 14 full revolutions, or 11 days (29 Cerean days, 
for those of you using that calendar), to acquire and transmit all the 
data. The Dawn team calls this 11-day period "11 days," or sometimes 
a "cycle."

In quite a change from the days that there simply didn't seem to be 
enough hydrazine onboard to accomplish all of the mission's ambitious 
objectives, engineers and the spacecraft itself have collaborated to be 
so efficient with the precious molecules that they now have some to spare. 
Therefore, mission planners have recently decided to spend a few more 
in this mapping orbit. They have added extra turns to allow the robot 
to communicate with Earth during more of the transits over the nightside 
than they had previously budgeted. This means Dawn can send the contents 
of its computer memory to Earth more often and therefore have space to 
collect and store even more data than originally planned. An 11-day mapping 
cycle is going to be marvelously productive.
Dawn Survey Orbit Image 46

But Dawn has goals still more ambitious than taking pictures and recording 
infrared and visible spectra of the lands passing underneath it. It will 
conduct six complete mapping cycles, each one looking at a slightly different 
angle. This will effectively yield stereo views, which when combined will 
make those flat images pop into full three dimensionality.

In its first mapping cycle, which is taking place now, the explorer aims 
its instruments straight down. For the second, it will keep the camera 
pointed a little bit back and to the left, making another full map but 
with a different perspective. For the third, it will look a little back 
and to the right. The fourth map will be viewing the scenery ahead and 
to the left. The fifth map will be of the terrain immediately ahead, and 
the sixth will be farther back than the third but not as far to the right.

In addition to the stereo pictures and the many spectra (which reveal 
the nature of the minerals as well as the surface temperature), Dawn will 
use the color filters in its camera to record the sights in visible and 
infrared wavelengths.

As always, mission planners schedule more observations than are needed, 
recognizing that glitches can occur on a complex and challenging expedition 
in the forbidding depths of space. So even if some data are not collected, 
the goals can still be accomplished.

The probe also will continue to acquire spectra both of neutrons and of 
gamma rays. It is unlikely to detect more than a whisper of neutrons from 
Ceres at this height, but the radiation coming from elsewhere in space 
now will serve as a useful calibration when it measures stronger nuclear 
emanations from one quarter the altitude starting in December, allowing 
scientists to inventory Ceres' atomic constituents.

Precise measurements of Dawn's radio signal will reveal more details 
of the dwarf planet's gravitational field and hence the distribution 
of mass within. When the spacecraft is not aiming its main antenna at 
Earth, it will broadcast through one of its three auxiliary antennas, 
and the Deep Space Network will be listening (almost) continuously throughout 
the 84 orbits.
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As at Vesta, Dawn's polar orbits are oriented so that the craft always 
keeps the sun in view, never entering Ceres' shadow, even when it is 
nighttime on the ground below. But its course will take the robot out 
of sight from Earth occasionally, and the behemoth of rock and ice will 
block the radio signal. Of course, Dawn is quite accustomed to operating 
in radio silence. It follows timed instructions (called sequences) that 
cover a full mapping cycle, so it does not require constant contact. And 
in budgeting how much data Dawn can collect and transmit, mission planners 
have accounted for the amount of time Ceres will eclipse its view of Earth.

Thanks to the uniquely efficient and exceptionally gentle thrust of the 
ion engines, as well as the flexibility inherent in being in orbit, Dawn 
operations generally can be more leisurely than those with conventional 
chemical propulsion or missions that only fly past their targets rather 
than stay for as long as needed. In that spirit, controllers had allowed 
the time in the spacecraft's main computer to drift off, as there was 
no need to keep it particularly accurate. But recently the clock was off 
by so much that they decided to correct it, so before the mapping began, 
they adjusted it by a whopping 0.983 seconds, eliminating a large (but 
still tolerable) offset.

Many residents of Earth's northern hemisphere are completing their leisurely 
summer vacations. As we saw in February, Dawn has measured the orientation 
of Ceres' spin axis and found that it is tipped about four degrees (compared 
with Earth's axial tilt of 23 degrees). The sun then never moves very 
far from the dwarf planet's equator, so seasonal variations are mild. 
Nevertheless, northern hemisphere summer (southern hemisphere winter) 
began on Ceres on July 24. Because Ceres takes longer to revolve around 
the sun than Earth, seasons last much longer. The next equinox won't 
occur until Nov. 13, 2016, so there is still plenty of time to plan a 
summer vacation.

Meanwhile, Dawn is working tirelessly to reveal the nature of this complex, 
intriguing world. Now seeing the exotic sights with a shaper focus than 
ever, the probe's meticulous mapping will provide a wealth of new data 
that scientists will turn into knowledge. And everyone who has ever seen 
the night sky beckon, everyone who has heard the universe's irresistible 
invitation, and everyone who has felt the overpowering drive for a bold 
journey far from Earth shares in the experience of this remarkable interplanetary 
adventure.

Dawn is 905 miles (1,456 kilometers) from Ceres. It is also 2.06 AU (191 
million miles, or 308 million kilometers) from Earth, or 775 times as 
far as the moon and 2.03 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 34 minutes 
to make the round trip.