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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 31 Jul 2015 15:19:34 -0700 (PDT)
Message-ID: <201507312219.t6VMJYrc003632_at_zagami.jpl.nasa.gov>

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.
Received on Fri 31 Jul 2015 06:19:34 PM PDT