[meteorite-list] Dawn Journal - July 29, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/07/29/dawn-journal-july-29/

Dawn Journal 
by Dr. Marc Rayman
July 29, 2015

Dear Descendawnts,

Flying on a blue-green ray of xenon ions, Dawn is gracefully descending 
toward dwarf planet Ceres. Even as Dawn prepares for a sumptuous new feast 
in its next mapping orbit, scientists are continuing to delight in the 
delicacies Ceres has already served. With a wonderfully rich bounty of 
pictures and other observations already secured, the explorer is now on 
its way to an even better vantage point.

Dawn was in its second mapping orbit at an altitude of 2,700 miles (4,400 
kilometers) when it took this picture of Ceres. This area shows relatively 
few craters, suggesting it is younger than some other areas on Ceres. 
Some bright spots are visible, although they are not as prominent as the 
most famous bright spots. Scientists do not yet have a clear explanation 
for them, but you can register your vote here. Click on the picture (or 
follow the link to the full image) for a better view of some interesting 
narrow, straight features in the lower left. 
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

Dawn takes great advantage of its unique ion propulsion system to maneuver 
extensively in orbit, optimizing its views of the alien world that beckoned 
for more than two centuries before a terrestrial ambassador arrived in 
March. Dawn has been in powered flight for most of its time in space, 
gently thrusting with its ion engine for 69 percent of the time since 
it embarked on its bold interplanetary adventure in 2007. Such a flight 
profile is entirely different from the great majority of space missions. 
Most spacecraft coast most of the time (just as planets do), making only 
brief maneuvers that may add up to just a few hours or even less over 
the course of a mission of many years. But most spacecraft could not accomplish 
Dawn's ambitious mission. Indeed, no other spacecraft could. The only 
ship ever to orbit two extraterrestrial destinations, Dawn accomplishes 
what would be impossible with conventional technology. With the extraordinary 
capability of ion propulsion, it is truly an interplanetary spaceship.

In addition to using its ion engine to travel to Vesta, enter into orbit 
around the protoplanet in 2011, break out of orbit in 2012, travel to 
Ceres and enter into orbit there this year, Dawn relies on the same system 
to fly to different orbits around these worlds it unveils, executing complex 
and graceful spirals around its gravitational master. After conducting 
wonderfully successful observation campaigns in its preantepenultimate 
Ceres orbit 8,400 miles (13,600 kilometers) high in April and May and 
its antepenultimate orbit at 2,700 miles (4,400 kilometers) in June, Dawn 
commenced its spiral descent to the penultimate orbit at 915 miles (1,470 
kilometers) on June 30. (We will discuss this orbital altitude in more 
detail below.) A glitch interrupted the maneuvering almost as soon as 
it began, when protective software detected a discrepancy in the probe’s 
orientation. But thanks to the exceptional flexibility built into the 
plans, the mission could easily accommodate the change in schedule that 
followed. It will have no effect on the outcome of the exploration of 
Ceres. Let's see what happened.

[Graphic]
Dawn's spiral descent from its second mapping orbit (survey), at 2,700 
miles (4,400 kilometers), to its third (HAMO), at 915 miles (1,470 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 survey orbit, to red, when it arrives in HAMO. The red dashed sections 
show where Dawn is coasting for telecommunications. Compare this to the 
previous spiral. Image credit: NASA/JPL-Caltech

Control of Dawn's orientation in the weightless conditions of spaceflight 
is the responsibility of the attitude control system. (To maintain a mystique 
about their work, engineers use the term "attitude" instead of 'orientation." 
This system also happens to have a very positive attitude about its work.) 
Dawn (and all other objects in three-dimensional space) can turn about 
three mutually perpendicular axes. The axes may be called pitch, roll 
and yaw; left/right, front/back and up/down; x, y and z; rock, paper and 
scissors; chocolate, vanilla and strawberry; Peter, Paul and Mary; etc., 
but whatever their names, attitude control has several different means 
to turn or to stabilize each axis. Earlier in its journey, the spacecraft 
depended on devices known as reaction wheels. As we have discussed in 
many Dawn Journals, that method is now used only rarely, because two of 
the four units have failed. The remaining two are being saved for the 
ultimate orbit at about 230 miles (375 kilometers), which Dawn will attain 
at the end of this year. Instead of reaction wheels, Dawn has been using 
its reaction control system, shooting puffs of hydrazine, a conventional 
rocket propellant, through small jets. (This is entirely different from 
the ion propulsion system, which expels high velocity xenon ions to change 
and control Dawn’s path through space. The reaction control system is 
used only to change and control attitude.)

Whenever Dawn is firing one of its three ion engines, its attitude control 
system uses still another method. The ship only operates one engine at 
a time, and attitude control swivels the mechanical gimbal system that 
holds that engine, thus imparting a small torque to the spacecraft, providing 
the means to control two axes (pitch and yaw, for example, or chocolate 
and strawberry). For the third axis (roll or vanilla), it still uses the 
hydrazine jets of the reaction control system.

On June 30, engine #3 came to life on schedule at 10:32:19 p.m. PDT to 
begin nearly five weeks of maneuvers. Attitude control deftly switched 
from using the reaction control system for all three axes to only one, 
and controlling the other two axes by tipping and tilting the engine with 
gimbal #3. But the control was not as effective as it should have been. 
Software monitoring the attitude recognized the condition but wisely avoided 
reacting too soon, instead giving attitude control time to try to rectify 
it. Nevertheless, the situation did not improve. Gradually the attitude 
deviated more and more from what it should have been, despite attitude 
control's efforts. Seventeen minutes after thrusting started, the error 
had grown to 10 degrees. That's comparable to how far the hour hand of 
a clock moves in 20 minutes, so Dawn was rotating only a little faster 
than an hour hand. But even that was more than the sophisticated probe 
could allow, so at 10:49:27 p.m., the main computer declared one of the 
'safe modes,' special configurations designed to protect the ship and 
the mission in uncertain, unexpected or difficult circumstances.

The spacecraft smoothly entered safe mode by turning off the ion engine, 
reconfiguring other systems, broadcasting a continuous radio signal through 
one of its antennas and then patiently awaiting further instructions. 
The radio transmission was received on a distant planet the next day. 
(It may yet be received on some other planets in the future, but we shall 
focus here on the response by Earthlings.) One of NASA's Deep Space Network 
stations in Australia picked up the signal on July 1, and the mission 
control team at JPL began investigating immediately.

Engineers assessed the health of the spacecraft and soon started returning 
it to its normal configuration. By analyzing the myriad diagnostic details 
reported by the robot over the next few days, they determined that the 
gimbal mechanism had not operated correctly, so when attitude control 
tried to change the angle of the ion engine, it did not achieve the desired 
result.

Because Dawn had already accomplished more than 96 percent of the planned 
ion-thrusting for the entire mission (nearly 5.5 years so far), the remaining 
thrusting could easily be accomplished with only one of the ion engines. 
(Note that the 96 percent here is different from the 69 percent of the 
total time since launch mentioned above, simply because Dawn has been 
scheduled not to thrust some of the time, including when it takes data 
at Vesta and Ceres.) Similarly, of the ion propulsion system's two computer 
controllers, two power units and two sets of valves and other plumbing 
for the xenon, the mission could be completed with only one of each. So 
although engineers likely could restore gimbal #3's performance, they 
chose to switch to another gimbal (and thus another engine) and move on. 
Dawn's goal is to explore a mysterious, fascinating world that used to 
be known as a planet, not to perform complex (and unnecessary) interplanetary 
gimbal repairs.

[Image]
This view of Ceres from the second mapping orbit shows some bright material 
that is not confined to "spots." The crater on the right with bright material 
is Haulani, visible on the left side of the topographical map below. Image 
credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

One of the benefits of being in orbit (besides it being an incredibly 
cool place to be) is that Dawn can linger at Ceres, studying it in great 
detail rather than being constrained by a fast flight and a quick glimpse. 
By the same principle, there was no urgency in resuming the spiral descent. 
The second mapping orbit was a perfectly fine place for the spacecraft, 
and it could circle Ceres there every 3.1 days as long as necessary. (Dawn 
consumed its hydrazine propellant at a very, very low rate while in that 
orbit, so the extra time there had a negligible cost, even as measured 
by the most precious resource.)

The operations team took the time to be cautious and to ensure that they 
understood the nature of the faulty gimbal well enough to be confident 
that the ship could continue its smooth sailing. They devised a test to 
confirm Dawn’s readiness to resume its spiral maneuvers. After swapping 
to gimbal #2 (and ipso facto engine #2), Dawn thrust from July 14 to 16 
and demonstrated the excellent performance the operations team has seen 
so often from the veteran space traveler. Having passed its test with 
flying colors (or perhaps even with orbiting colors), Dawn is now well 
on its way to its third mapping orbit.

The gradual descent from the second mapping orbit to the third will require 
25 revolutions. The maneuvers will conclude in about two weeks. (As always, 
you can follow the progress with your correspondent's frequent and succinct 
updates here.) As in each mapping orbit, following arrival, a few days 
will be required in order to prepare for a new round of intensive observations. 
That third observing campaign will begin on August 17 and last more than 
two months.

Although this is the second lowest of the mapping orbits, it is also known 
as the high altitude mapping orbit (HAMO) for mysterious historical reasons. 
We presented an overview of the HAMO plans last year. Next month, we will 
describe how the flight team has built on a number of successes since 
then to make the plans even better.

The view of the landscapes on this distant and exotic dwarf planet from 
the third mapping orbit will be fantastic. How can we be so sure? The 
view in the second mapping orbit was fantastic, and it will be three times 
sharper in the upcoming orbit. Quod erat demonstrandum! To see the sights 
at Ceres, go there or go here.

Part of the flexibility built into the plans was to measure Ceres' gravity 
field as accurately as possible in each mapping orbit and use that knowledge 
to refine the design for the subsequent orbital phase. Thanks to the extensive 
gravity measurements in the second mapping orbit in June, navigators were 
able not only to plot a spiral course but also to calculate the parameters 
for the next orbit to provide the views needed for the complex mapping 
activities.

[Image]
This map of Ceres depicts the topography ranging from 4.7 miles (7.5 kilometers) 
low in indigo to 4.7 miles (7.5 kilometers) high in white. (As a technical 
detail, the topography is shown relative to an ellipsoid of dimensions 
very close to those in the paragraph below.) The names of features have 
been approved by the International Astronomical Union following the system 
described in December. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. 

We have discussed some of the difficulty in describing the orbital altitude, 
including variations in the elevation of the terrain, just as a plane 
flying over mountains and valleys does not maintain a fixed altitude. 
As you might expect on a world battered by more than four billion years 
in the main asteroid belt and with its own internal geological forces, 
Ceres has its ups and downs. (The topographical map above displays them, 
and you can see a cool animation of Ceres showing off its topography here.) 
In addition to local topographical features, its overall shape is not 
perfectly spherical, as we discussed in May. Ongoing refinements based 
on Dawn's measurements now indicate the average diameter is 584 miles 
(940 kilometers), but the equatorial diameter is 599 miles (964 kilometers), 
whereas the polar diameter is 556 miles (894 kilometers). Moreover, the 
orbits themselves are not perfect circles, and irregularities in the gravitational 
field, caused by regions of lower and higher density inside the dwarf 
planet, tug less or more on the craft, making it move up and down somewhat. 
(By using that same principle, scientists learn about the interior structure 
of Ceres and Vesta with very accurate measurements of the subtleties in 
the spacecraft's orbital motions.) Although Dawn’s average altitude will 
be 915 miles (1,470 kilometers), its actual distance above the ground 
will vary over a range of about 25 miles (40 kilometers).

In March we summarized the four Ceres mapping orbits along with a guarantee 
that the dates would change. In addition to delivering exciting interplanetary 
adventures to thrill anyone who has ever gazed at the night sky in wonder, 
Dawn delivers on its promises. Therefore, we present the updated table 
here. With such a long and complex mission taking place in orbit around 
the largest previously uncharted world in the inner solar system, further 
changes are highly likely. (Nevertheless, we would consider the probability 
to be low that changes will occur for the phases in the past.)

Mapping
Orbit	Dawn code
name	Tentative dates (further changes are likely)	Altitude
in miles
(kilometers)	Resolution in
feet (meters)
per pixel	Resolution compared to Hubble	Orbit
period	Equivalent
distance of
a soccer ball
1	RC3	April 23 –
May 9	8,400
(13,600)	4,200
(1,300)	24	15
days	10 feet
(3.0 meters)
2 	Survey	June 6-30	2,700
(4,400)	1,400
(410)	73	3.1
days	3.3 feet
(1.0 meters)
3 	HAMO	Aug 17 –
Oct 23	915
(1,470)	450
(140)	217	19
hours	13 inches
(33 cm)
4 	LAMO	Dec 15 –
end of mission	230
(375)	120
(35)	850	5.5
hours	3.3 inches
(8.5 cm)

Click on the name of each orbit for a more detailed description. As a 
reminder, the last column illustrates how large Ceres appears to be from 
Dawn's perspective by comparing it with a view of a soccer ball. (Note 
that Ceres is not only 4.4 million times the diameter of a soccer ball 
but it is a lot more fun to play with.)

Resolute and resilient, Dawn patiently continues its graceful spirals, 
propelled not only by its ion engine but also by the passions of everyone 
who yearns for new knowledge and noble adventures. Humankind's robotic 
emissary is well on its way to providing more fascinating insights for 
everyone who longs to know the cosmos.

Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 1.95 AU 
(181 million miles, or 291 million kilometers) from Earth, or 785 times 
as far as the moon and 1.92 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 32 minutes 
to make the round trip.