[meteorite-list] Dawn Journal - November 30, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/11/30/dawn-journal-november-30/

Dawn Journal | November 30
by Marc Rayman
November 30, 2015

Dear Superintendawnts and Assisdawnts,

An intrepid interplanetary explorer is now powering its way down through 
the gravity field of a distant alien world. Soaring on a blue-green beam 
of high-velocity xenon ions, Dawn is making excellent progress as it spirals 
closer and closer to Ceres, the first dwarf planet discovered. Meanwhile, 
scientists are progressing in analyzing the tremendous volume of pictures 
and other data the probe has already sent to Earth.

[Graphic]
Dawn's spiral descent from its third mapping orbit (HAMO), at 915 miles 
(1,470 kilometers), to its fourth (LAMO), at 240 miles (385 kilometers). 
The two mapping orbits are shown in green. The color of Dawn's trajectory 
progresses through the spectrum from blue, when it began ion-thrusting 
in HAMO, to red, when it arrives in LAMO. The red dashed sections show 
where Dawn is coasting for telecommunications. It requires 118 spiral 
revolutions around Ceres to reach the low altitude (and additional revolutions 
to prepare for and conduct the trajectory correction maneuver described 
below). Compare this to the previous spiral. (Readers with total recall 
will note that this is fewer loops than illustrated last year. The flight 
team has made several improvements in the complex design since then, shortening 
the time required and thus allowing more time for observing Ceres.) 
Image credit: NASA/JPL-Caltech

Dawn is flying down to an average altitude of about 240 miles (385 kilometers), 
where it will conduct wide-ranging investigations with its suite of scientific 
instruments. The spacecraft will be even closer to the rocky, icy ground 
than the International Space Station is to Earth's surface. The pictures 
will be four times sharper than the best it has yet taken. The view is 
going to be fabulous!

Dawn will be so near the dwarf planet that its sensors will detect only 
a small fraction of the vast territory at a time. Mission planners have 
designed the complex itinerary so that every three weeks, Dawn will fly 
over most of the terrain while on the sunlit side. (The neutron spectrometer, 
gamma ray spectrometer and gravity measurements do not depend on illumination 
from the sun, but the camera, infrared mapping spectrometer and visible 
mapping spectrometer do.)

Obtaining the planned coverage of the exotic landscapes requires a delicate 
synchrony between Ceres' and Dawn's movements. Ceres rotates on its axis 
every nine hours and four minutes (one Cerean day). Dawn will revolve 
around it in a little less than five and a half hours, traveling from 
the north pole to the south pole over the hemisphere facing the sun and 
sailing northward over the hemisphere hidden in the darkness of night. 
Orbital velocity at this altitude is around 610 mph (980 kilometers per 
hour).

Last year we had a preview of the plans for this fourth and final mapping 
orbit (sometimes also known as the low altitude mapping orbit, or LAMO), 
and we will present an updated summary next month.

The planned altitude differs from the earlier, tentative value of 230 
miles (375 kilometers) for several reasons. One is that the previous notion 
for the altitude was based on theoretical models of Ceres' gravity field. 
Navigators measured the field quite accurately in the previous mapping 
orbit (using the method outlined here), and that has allowed them to refine 
the orbital parameters to choreograph Dawn's celestial pas de deux with 
Ceres. In addition, prior to Dawn's investigations, Ceres' topography 
was a complete mystery. Hubble Space Telescope had shown the overall shape 
well enough to allow scientists to determine that Ceres qualifies as a 
dwarf planet, but the landforms were indiscernible and the range of relative 
elevations was simply unknown. Now that Dawn has mapped the topography, 
we can specify the spacecraft's average height above the ground as it 
orbits. With continuing analyses of the thousands of stereo pictures taken 
in August - October and more measurements of the gravity field in the 
final orbit, we will further refine the average altitude. Finally, we 
round the altitude numbers to the nearest multiple of five (both for miles 
and kilometers), because, as we will discuss in a subsequent Dawn Journal, 
the actual orbit will vary in altitude by much more than that. (We described 
some of the the ups and dawns of the corresponding orbit at Vesta here. 
The variations at Ceres will not be as large, but the principles are the 
same.)

[Image]
Dawn had this view of Urvara crater in mapping cycle #4 from an altitude 
of 915 miles (1,470 kilometers) during the third mapping orbit. (Urvara 
is a Vedic goddess associated with fertile lands and plants.) The crater 
is 101 miles (163 kilometers) in diameter. It displays a variety of features, 
including a particularly bright region on the peak at the center, ridges 
nearby, a network of fissures, some smooth regions and much rougher terrain. 
You can locate all the areas shown in this month's photos on the Ceres 
map presented last month. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To attain its new orbit, Dawn relies on its trusty and uniquely efficient 
ion engine, which has already allowed the spacecraft to accomplish what 
no other has even attempted in the 58-year history of space exploration. 
This is the only mission ever to orbit two extraterrestrial destinations. 
The spaceship orbited the protoplanet Vesta for 14 months in 2011-2012, 
revealing myriad fascinating details of the second most massive object 
in the main asteroid belt between Mars and Jupiter, before its March 2015 
arrival in orbit around the most massive. Ion propulsion enables Dawn 
to undertake a mission that would be impossible without it.

While the ion engine provides 10 times the efficiency of conventional 
spacecraft propulsion, the engine expends the merest whisper of xenon 
propellant, delivering a remarkably gentle thrust. As a result, Dawn achieves 
acceleration with patience, and that patience is rewarded with the capability 
to explore two of the last uncharted worlds in the inner solar system. 
This raises an obvious question: How cool is that? Fortunately, the answer 
is equally obvious: Incredibly cool!

The efficiency of the ion engine enables Dawn not only to orbit two destinations 
but also to maneuver extensively around each one, optimizing its orbits 
to reap the richest possible scientific return at Vesta and Ceres. The 
gentleness of the ion engine makes the maneuvers gradual and graceful. 
The spiral descents are an excellent illustration of that.

Dawn began its elegant downward coils on Oct. 23 upon concluding more 
than two months of intensive observations of Ceres from an altitude of 
915 miles (1,470 kilometers). At that height, Ceres' gravitational hold 
was not as firm as it will be in Dawn's lower orbit, so orbital velocity 
was slower. Circling at 400 mph (645 kilometers per hour), it took 19 
hours to complete one revolution around Ceres. It will take Dawn more 
than six weeks to travel from that orbit to its new one. (You can track 
its progress and continue to follow its activities once it reaches its 
final orbit with the frequent mission status updates.)

[Image]
Dawn took this picture of Dantu crater from an altitude of 915 miles (1,470 
kilometers) during the third mapping orbit, in mapping cycle #4. (Dantu 
is a timekeeper god who initiates the cycle of planting rites among the 
Ga people of the Accra Plains of southeastern Ghana. You can find Dantu, 
but not Ghana, on this map.) The crater is about 77 miles (125 kilometers) 
across. Note the isolated bright regions, the long fissures, and the zigzag 
structure at the center. Scientists are working to understand what these 
indicate about the geological processes on Ceres. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

On Nov. 16, at an altitude of about 450 miles (720 kilometers), Dawn circled 
at the same rate that Ceres turned. Now the spacecraft is looping around 
its home even faster than the world beneath it turns.

When ion-thrusting ends on Dec. 7, navigators will measure and analyze 
the orbital parameters to establish how close they are to the targeted 
values and whether a final adjustment is needed to fit with the intricate 
observing strategy. Several phenomena contribute to small differences 
between the planned orbit and the actual orbit. (See here and here for 
two of our attempts to elucidate this topic.) Engineers have already thoroughly 
assessed the full range of credible possibilities using sophisticated 
mathematical methods. This is a complex and challenging process, but the 
experienced team is well prepared. In case Dawn needs to execute an additional 
maneuver to bring its orbital motion into closer alignment with the plan, 
the schedule includes a window for more ion-thrusting on Dec. 12-14 (concluding 
on Dawn's 3000th day in space). In the parlance of spaceflight, this maneuver 
to adjust the orbit is a trajectory correction maneuver (TCM), and Dawn 
has experience with them.

The operations team takes advantage of every precious moment at Ceres 
they can, so while they are determining whether to perform the TCM and 
then developing the final flight plan to implement it, they will ensure 
the spacecraft continues to work productively. Dawn carries two identical 
cameras, a primary and a backup. Engineers occasionally operate the backup 
camera to verify that it remains healthy and ready to be put into service 
should the primary camera falter. On Dec. 10, the backup will execute 
a set of tests, and Dawn will transmit the results to Earth on Dec. 11. 
By then, the work on the TCM will be complete.

Although it is likely a TCM will be needed, if it turns out to be unnecessary, 
mission control has other plans for the spacecraft. In this final orbit, 
Dawn will resume using its reaction wheels to control its orientation. 
By electrically changing the speed at which these gyroscope-like devices 
rotate, the probe can control its orientation, stabilizing itself or turning. 
We have discussed their lamentable history on Dawn extensively, with two 
of the four having failed. Although such losses could have been ruinous, 
the flight team formulated and implemented very clever strategies to complete 
the mission without the wheels. Exceeding their own expectations in such 
a serious situation, Dawn is accomplishing even more observations at Ceres 
than had been planned when it was being built or when it embarked on its 
ambitious interplanetary journey in 2007.

[Image]
Dawn took this picture in its third mapping orbit at an altitude of 915 
miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit. 
The prominent triplet of overlapping craters nicely displays relative 
ages, which are apparent by which ones affect others and hence which ones 
formed later. The largest crater, Geshtin, is 48 miles (77 kilometers) 
across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian 
goddess of the vine.) A subsequent impact that excavated Datan crater, 
which is 37 miles (60 kilometers) in diameter, obliterated a large section 
of Geshtin's rim and made its own crater wall in Geshtin's interior. (Datan 
is one of the Polish gods who protect the fields but apparently not this 
crater.) Still later, Datan itself was the victim of a sizable impact 
on its rim (although not large enough to have merited an approved name 
this early in the geological studies of Ceres). 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Now the mission lifetime is limited by the small supply of conventional 
rocket propellant, expelled from reaction control system thrusters strategically 
located around the spacecraft. When that precious hydrazine is exhausted, 
the robot will no longer be able to point its solar arrays at the sun, 
its antenna at Earth, its sensors at Ceres or its ion engines in the direction 
needed to travel elsewhere, so the mission will conclude. The lower Dawn's 
orbital altitude, the faster it uses hydrazine, because it must rotate 
more quickly to keep its sensors pointed at the ground. In addition, it 
has to fight harder to resist Ceres' relentless gravitational tug on the 
very large solar arrays, creating an unwanted torque on the ship.

Among the innovative solutions to the reaction wheel problems was the 
development of a new method of orienting the spacecraft with a combination 
of only two wheels plus hydrazine. In the final orbit, this 'hybrid control"
will use hydrazine at only half the rate that would be needed without 
the wheels. Therefore, mission controllers have been preserving the units 
for this final phase of the expedition, devoting the limited remaining 
usable life to the time that they can provide the greatest benefit in 
saving hydrazine. (The accuracy with which Dawn can aim its sensors is 
essentially unaffected by which control mode is used, so hydrazine conservation 
is the dominant consideration in when to use the wheels.) Apart from a 
successful test of hybrid control two years ago and three subsequent periods 
of a few hours each for biannual operation to redistribute internal lubricants, 
the two operable wheels have been off since August 2012, when Dawn was 
climbing away from Vesta on its way out of orbit.

Controllers plan to reactivate the wheels on Dec. 15. However, in the 
unlikely case that the TCM is deemed unnecessary, they will power the 
wheels on on Dec. 11. The reaction wheels will remain in use for as long 
as both function correctly. If either one fails, which could happen immediately 
or might not happen before the hydrazine is depleted next year, it and 
the other will be powered off, and the mission will continue, relying 
exclusively on hydrazine control.

[Image]
Dawn recorded this view in its third mapping orbit at an altitude of 915 
miles (1,470 kilometers) in mapping cycle #5. The region shown is located 
between Fluusa and Toharu craters. The largest crater here is 16 miles 
(26 kilometers) across. The well defined features indicate the crater 
is relatively young, so subsequent small impacts have not degraded it 
significantly. As elsewhere on Ceres, some strikingly bright material 
is evident, particularly in the walls. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn will measure the energies and numbers of neutrons and gamma rays 
emanating from Ceres as soon as it arrives in its new orbit. With a month 
or so of these measurements, scientists will be able to determine the 
abundances of some of the elements that compose the material near the 
surface. Engineers and scientists also will collect new data on the gravity 
field at this low altitude right away, so they eventually can build up 
a profile of the dwarf planet’s interior structure. The other instruments 
(including the camera) have narrower fields of view and are more sensitive 
to small discrepancies in where they are aimed. It will take a few more 
days to incorporate the actual measured orbital parameters into the corresponding 
plans that controllers will radio to the spacecraft. Those observations 
are scheduled to begin on Dec. 18. But always squeezing as much as possible 
out of the mission, the flight team might actually begin some photography 
and infrared spectroscopy as early as Dec. 16.

Now closing in on its final orbit, the veteran space traveler soon will 
commence the last phase of its long and fruitful adventure, when it will 
provide the best views yet of Ceres. Known for more than two centuries 
as little more than a speck of light in the vast and beautiful expanse 
of the stars, the spacecraft has already transformed it into a richly 
detailed and fascinating world. Now Dawn is on the verge of revealing 
even more of Ceres' secrets, answering more questions and, as is the marvelous 
nature of science and exploration, raising new ones.

Dawn is 295 miles (470 kilometers) from Ceres. It is also 3.33 AU (309 
million miles, or 498 million kilometers) from Earth, or 1,270 times as 
far as the moon and 3.37 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 55 minutes 
to make the round trip.