[meteorite-list] Dawn Journal - December 30, 2011

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 4 Jan 2012 16:45:28 -0800 (PST)
Message-ID: <201201050045.q050jSXs015530_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_30_11.asp

Dawn Journal
Dr. Marc Rayman
December 30, 2011

Dear Indawnmitables,

Dawn concludes 2011 more than 40 thousand times nearer to Vesta than it
began the year. Now at its lowest altitude of the mission, the bold
adventurer is conducting its most detailed exploration of this alien
world and continuing to make thrilling new discoveries.

Circling the protoplanet 210 kilometers (130 miles) beneath it every 4
hours, 21 minutes on average, Dawn is closer to the surface than the
vast majority of Earth-orbiting satellites are to that planet. There are
two primary scientific objectives of this low altitude mapping orbit
(LAMO). With its gamma ray and neutron detector (GRaND), the probe is
measuring the faint emanations of these subatomic particles from Vesta.
Some are the by-products of the bombardment by cosmic rays, radiation
that pervades space, and others are emitted through the decay of
radioactive elements. Vesta does not glow brightly when observed in
nuclear particles, so GRaND needs to measure the radiation for weeks at
this low altitude. This is analogous to using a long exposure with a
camera to photograph a dimly lit subject. If GRaND only detected the
radiation, it would be as if it took a black and white picture, but this
sophisticated instrument does more. It measures the energy of each particle,
just as a camera can measure the color of light. The energies reveal the
identities of the elements that constitute the uppermost meter (yard) of
the surface. Dawn devotes most of its time now flying over Vesta to
collecting the glimmer of radiation. It requires a long time, but this
spacecraft has demonstrated tremendous patience in its use
of the gentle but efficient ion propulsion system that made the mission
possible, so it can be patient in making these measurements.

The second motivation for diving down so low is to be close enough that
Vesta's interior variations in density affect the spacecraft's orbit
discernibly. We have seen before that the distribution of mass inside
the protoplanet reveals itself through the changing strength of its
gravitational tug on Dawn. Exquisitely sensitive measurements of the
ship's course can be translated into a three-dimensional map of the mass.
In the plans discussed for LAMO one year ago, the delicate tracking of the
spacecraft required pointing the main antenna to Earth. That provides a
radio signal strong enough to achieve the required accuracy. Since then,
navigators have determined that the radio signal received from one of
the craft's auxiliary antennas, although far weaker, is sufficient. The
main antenna broadcasts a tight beam, whereas the others emit over a
much larger angle, exchanging signal strength for flexibility in pointing.

This allows an extremely valuable improvement. The spacecraft cannot aim
GRaND at the surface and the main antenna at Earth concurrently, because
both are mounted rigidly, just as you cannot simultaneously point the
front of your car north and the back east. Therefore, in the original
plan, gravity measurements and GRaND measurements were mutually
exclusive. Now, as Dawn turns throughout its orbit to keep Vesta in
GRaND's sights, it can transmit a weak radio signal that is just
perceptible at Earth. This enables an even greater science return for
the time in LAMO.

Unlike the science camera and the visible and infrared mapping
spectrometer (VIR), GRaND and gravity observations do not depend on
the sun's illumination of the surface. Even as it orbits over a dark,
cold, silent landscape, Dawn is fully capable of continuing to build its
maps of elements and the interior structure.

The signal from the auxiliary antenna is just sufficient for the
measurement of the spacecraft's motion, but it is not strong enough to
carry data as well. So the spacecraft is still programmed to point its
main antenna to Earth three times each week, allowing the precious GRaND
observations that have been stored in computer memory to be transmitted.
As always, the myriad measurements of temperatures, voltages, currents,
pressures, and other parameters that engineers use to ensure the health
of the ship are returned during these communications sessions as well.

Although the pictures of Vesta from survey orbit and the high altitude
mapping orbit (HAMO) have exceeded scientists' expectations, not only in
quality and quantity but also in the truly fascinating content, as
enthusiastic explorers, the Dawn team could not pass up the opportunity
for more. When GRaND is pointed at the surface, the camera is too, and
already well over one thousand images have been returned, revealing detail
three times finer than the spectacular images from HAMO. For readers who
cannot go to Vesta on their own, go here </multimedia/imageoftheday/archives.asp>
for a selection of the best views, each showing surprising and captivating
alien landscapes.

In addition to the bonus photography, beginning in January VIR will take
observations. Although the instrument has already acquired nearly seven
million spectra in the higher orbits, this new vantage point will allow
sharper resolution, just as it does for the camera.

The ultra-long-distance communication between Dawn and Earth requires
extraordinary technology on both ends. Even with all the sophistication,
the amount of information that can be transmitted in a given time
remains very limited. The remote spacecraft sends data at speeds
significantly lower than a typical home Internet connection. Engineers
use that precious communications link very carefully, judiciously
selecting what information to instruct the probe to return. Because of
the high priority given to GRaND, which needs to be pointed at the
surface as long as possible, much of the limited time spent with the
main antenna aimed at Earth is devoted to transmitting that instrument's
findings (and the measurements of spacecraft subsystems). This restricts
how much data from the camera and VIR can be communicated.

In the next log, we will see another limitation on the number of camera
images and VIR spectra in LAMO. It is a consequence of another aspect of
the complex operations in this low orbit around a massive body, and that
is the small but real differences between the predicted orbit and the
actual orbit. We will cover the first part of the explanation here.

Navigators use their best knowledge of the many forces acting on Dawn to
chart an orbital course for it. The forces can be traced to three
principal sources: gravity, light, and Dawn itself. We have discussed
all of these before in detail (see, for example, this explication of the
last two), but let's review them here.
This is an involved story, so readers are advised to be in a comfortable
orbit while following it. You can safely skip the next four paragraphs
and no one ever need know.

Vesta has a complicated gravity field, and that leads to a complicated
orbit. The spacecraft does not follow a
perfectly circular, repetitive path because the gravitational pull on it
changes according to where it is as the colossus beneath it rotates and
it loops around. The map of the gravity field has been improving
throughout Dawn's residence there, but its completion awaits the LAMO
gravity measurements. In the meantime, unknown details of the variation
of mass lead to small divergences in the orbit. All the other bodies in
the solar system exert gravitational pulls on the spacecraft as well
(just as they do on you), but those are more easily accounted for. The
distances from Dawn are so great that the variations in their gravity
fields don't matter. So although the effects of the faraway objects need
to be accounted for, they do not contribute much to the discrepancies.

Dawn depends on sunlight for its power, using its large solar arrays to
make electricity to run all systems. The sun also propels the
spacecraft, because in the frictionless conditions of spaceflight, the
ship recoils slightly in response to the miniscule but persistent
pressure of the light. The force depends on
whether the light is absorbed (whereupon it is converted to electrical
power by the arrays or to heat by whatever component it illuminates) or
reflected. If it is reflected, the angle makes a difference, so smooth
shiny surfaces that act like mirrors cause different effects from the
materials that present a matte finish or are curved or angled. As the
spacecraft rotates to keep GRaND pointed at the ground below, different
parts of the ship are presented to the sun, so the force from the light
changes, and the orbit is constantly subjected to a variable disturbance.

Dawn itself adds to the complexity of its orbital path.
The spacecraft carries reaction
wheels, which are spun to help it control its orientation. These devices
gradually spin faster, so every few days they need to be slowed down.
That is accomplished by firing the small reaction control system
thrusters during windows specified by mission
controllers. In addition to the thrusters providing the needed torque on
the craft to reduce the wheels' speeds, they impart a force that changes
the orbit slightly.

The physical principles underlying all these phenomena that perturb
Dawn's orbit are understood with exceptional clarity. Although the
values of the myriad parameters involved are ascertained quite
accurately, they are not known perfectly. As a result, navigators'
prediction of the ship's course includes some degree of uncertainty.
Even their ability to determine the present orbit is subject to a
variety of small errors typical in sensitive physical measurements.

For all of these reasons, the craft's actual orbit departs slightly from
the plan, and the deviations tend to grow, albeit gradually. As
designers expected, in survey orbit and HAMO, the differences were small
enough that they did not affect the complex operations plans. Analysis
well before Dawn arrived at Vesta predicted that the discrepancies in
LAMO would be large enough that occasional adjustments of the orbit
would be necessary. Therefore, mission controllers scheduled a window
every week (on Saturdays, as it turned out) to use the ion propulsion
system to fine-tune the spacecraft's trajectory, bringing it back to the
intended orbit. These are known as "orbit maintenance maneuvers," and
succumbing to instincts developed during their long evolutionary
history, engineers refer to them by an acronym: OMM. (As the common
thread among team members is their technical training and passion for
the exploration of the cosmos, and not Buddhism, the term is spoken by
naming the letters, not pronouncing it as a means of achieving inner
peace. Instead, it may be thought of as a means of achieving orbital
tranquility and harmony.)

The LAMO phase began on December 12, and OMMs were performed on December
17 and 24. In contrast to the long periods of thrusting required with
ion propulsion for other parts of the mission, the corrections needed
were so small that each OMM needed less than 15 minutes. The whisper-like
thrust changed the spacecraft's speed by less than five centimeters per
second (one-tenth of a mph). But that was enough to nudge Dawn back to
the planned orbit.

The ship was so close to the designated course that the OMMs for
December 31 and even January 7 have already been canceled. Not executing
the OMMs allows the probe to spend more time collecting neutrons and
gamma rays from Vesta. The operations team productively uses the time
saved in designing, checking, and transmitting the OMM commands to do
other work to ensure LAMO proceeds smoothly and productively.

In the last log we discussed the complicated and
dynamic spiral descent from HAMO to LAMO, which was still in progress.
The flight required not only reducing the altitude from 680 kilometers
(420 miles) to 210 kilometers (130 miles) but also twisting the plane of
Dawn's orbit around Vesta. As with all orbiting bodies, whether around
Vesta, Earth, or the sun, the lower the orbital altitude, the shorter
the orbital period. Vesta's gravitational
grip strengthened as Dawn closed in, forcing the spacecraft to make
faster loops around it. This meant that as the probe performed the
intricate choreography to align its ion thruster with the changing
direction needed to alter its orbit, it had to pirouette faster.

When engineers command Dawn to rotate, they usually instruct it to use
the same stately speed as the minute hand on a clock.
The spacecraft may have to move a little
faster however, as it pivots to keep its solar arrays pointed at the sun
while accomplishing the required turn. Sometimes it knows that at the
end of a turn, it will have to initiate another turn. For example, it
may rotate to the orientation required to begin a session of ion
thrusting. But while it is thrusting and curving around its orbit, it
generally needs to steer the thruster to execute the maneuver. As a
result, the robot may choose to turn at a slightly different rate from
what its human team members command in order to make a smooth transition
from the first turn to the second.

On Dec. 3, when preparing for one of the final thrust segments required
to reach LAMO, the combination of all these factors caused the
spacecraft to rotate faster than usual. That led to a temporary
discrepancy between where it was pointed and where it expected to be
pointed during the turn. When protective software detected the
inconsistency, it interrupted the ongoing activities and put the
spacecraft into safe mode.

When the safe mode signal was received by the Deep Space Network, the
operations team responded with its usual calm and skill. They quickly
determined that Dawn was fully healthy, diagnosed the cause of the
safing, and began guiding the spacecraft back to its normal operational
configuration. In addition, they devised a new flight profile that would
compensate for the thrusting that was not completed. The team also
determined how to prevent the same problem from recurring for subsequent
maneuvers. While doing all this work, they were putting the finishing
touches on the first LAMO science observation sequences.
Controllers managed to complete
everything flawlessly and even kept the mission on schedule, allowing
LAMO to commence on Dec. 12.

The general plan for Dawn's three-month approach <journal_05_03_11.asp>
plus one year in orbit around Vesta was described in logs in 2010
</mission/journal.asp>. The time was apportioned among the different
science phases and the transfers between science orbits to ensure a
comprehensive and balanced exploration of this mysterious and
fascinating world. Fully appreciating that in such an exceedingly
ambitious undertaking, some unexpected problems are inevitable, mission
planners worked hard to devise an itinerary that left 40 days
uncommitted. Their strategy was that as they recovered from anomalies,
they would draw from that time and still not have to compromise any of
their carefully designed activities. They also planned that any unspent
margin would be used to extend LAMO.

To the great delight (and, to be honest, surprise) of all, not one day
of the 40-day reserve has been needed. Although there have indeed been
unanticipated difficulties, from the beginning of approach on May 3 to
this point, the team has been able to resolve all of them without having
to withdraw from that account. This is remarkable considering that Dawn
is the first visitor from Earth to Vesta, with its many unknown physical
properties. This expedition is the first ever in which humankind has
sent a spacecraft to orbit such a massive body without first conducting
a reconnaissance with a flyby spacecraft. Dawn has maintained a rapid
pace of scrutinizing its enigmatic destination. Performing all of this
so successfully without needing to use even a little of the spare time
they provided for themselves was considered quite unlikely. And yet the
entire 40 days remain available.

More ambitious operations lie ahead, with the rest of LAMO, the spiral
ascent to HAMO2 <journal_02_27_11.asp#completed>, HAMO2 itself, and the
escape in July to begin the long interplanetary cruise to reach Ceres on
schedule in February 2015. We will see in 2012 that each of these phases
includes new challenges, and it is certain new problems will arise.
Nevertheless, all 40 days are being used to extend LAMO. Therefore, the
indomitable explorer will remain at this low altitude through the end of
March, continuing to tease out secrets about the dawn of the solar
system and revealing more startling and thrilling discoveries on behalf
of everyone on distant Earth who yearns to reach out into the vastness
of space.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 2.79 AU (418
million kilometers or 260 million miles) from Earth, or 1045 times as
far as the moon and 2.84 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 46 minutes
to make the round trip.
Received on Wed 04 Jan 2012 07:45:28 PM PST