[meteorite-list] Dawn Journal - November 29, 2011

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 1 Dec 2011 15:34:56 -0800 (PST)
Message-ID: <201112012334.pB1NYuU8020946_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_11_29_11.asp

Dawn Journal
Dr. Marc Rayman
November 29, 2011

Dear Dawnward Spirals,

Continuing its ambitious campaign of exploration deep in the asteroid
belt, Dawn has spent most of the past month spiraling ever closer to Vesta.
Fresh from the phenomenal success of mapping the alien world in detail in
October, the spacecraft and its human team members are
engaged in one of the most complicated parts of the mission. The reward
will be the capability to scrutinize this fascinating protoplanet further.

Thanks to the extraordinary performance of its ion propulsion system,
Dawn can maneuver to different orbits that are best suited for
conducting each of its scientific observations. The probe is now headed
for its low altitude mapping orbit (LAMO),
where the focus of its investigations will be on making a census of the
atomic constituents with its gamma ray and neutron sensors
and on mapping the gravity field in
order to determine the interior structure of this protoplanet.

As secondary objectives, Dawn will acquire more images with its camera
and more spectra with its visible and infrared mapping spectrometer. As
we will see in a future log, these measurements will receive a smaller
share of the resources than the high priority studies. The spectacular
pictures obtained already will keep scientists happy for years, and you
can continue to share in the experience of marveling at the astonishing
discoveries by seeing some of the best views here,
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/archives.asp?month=2011-December
including scenes captured during the spiral to LAMO.

Planning the low altitude mapping orbit around massive Vesta, with its
complicated gravity field, required a great deal of sophisticated
analysis. Before Dawn arrived, mission designers studied a range of
possible gravitational characteristics and honed the methods they would
use for plotting the actual orbit once the details of the protoplanet's
properties were ascertained. In the meantime, the team used a tentative
orbit at an altitude over the equator of 180 kilometers (110 miles).
As explained in a previous log, the altitude varies both because the orbit
is not perfectly circular and because Vesta displays such exceptional
topography. The highest elevations turn out to be at the equator, and
the average altitude of that orbit would be 200 kilometers (125 miles).
 
Now that navigators have measured Vesta's gravity, they have the
knowledge to refine the design for LAMO, and they decided to raise it by
10 kilometers (6 miles). The target then is an average altitude of 210
kilometers (130 miles). But there is more to the specification of the
orbit than simply its height. To meet all of the scientific objectives,
the orientation of this orbit needs to be different from the orientation
of the previous orbits, the high altitude mapping orbit (HAMO)
and survey orbit. To picture the different orbits, let's recall our
discussion of how the orbit shifted from survey to HAMO. Readers
unconcerned about the details of the geometry may safely (and wisely) skip
this explanation.

Think of Dawn's orbit as a ring encircling Vesta, going over both poles
and crossing the equator at a right angle. Globes of Earth often are
supported by a ring like that, and it may be helpful to have a
terrestrial globe in mind, or even in sight, as you ponder the celestial
arrangement. For the purpose of this illustration, you may rest assured
that no inhabitants (permanent or temporary) of Vesta will object if we
pretend that the world does not rotate, so a ring that is aligned with a
longitude line represents one of Dawn's orbits. To start, let's say
survey orbit hovers over the 15 degree west longitude line (and, to make
a complete circle, goes over the 165 degree east longitude line as
well). HAMO would be shifted to 30 degrees west (and 150 degrees east on
the other side of the globe). Dawn is now on its way to LAMO, which will
be at about 46 degrees west (and 134 degrees east). The complex scheme
for moving from HAMO to LAMO then involves not only lowering the
altitude but also rotating the plane of the orbit by 16 degrees. We will
delve into why this value was chosen after the spacecraft has arrived in
LAMO.

These differing orientations are a crucial element of the strategy for
gathering such a wealth of scientifically valuable data on Vesta.
The ion propulsion system allows the probe to fly from one orbit to
another without the tremendous penalty of carrying a massive supply of
propellant. In general, it requires a great deal of maneuvering to
change the plane of a spacecraft's orbit. Indeed, one of the reasons
traveling from Earth to Vesta (and later Ceres) requires ion propulsion
is the challenge of tilting the orbit around the sun. (A more extensive
discussion of this and a table showing Dawn's progress in altering the
angle of its solar orbit to match Vesta's were presented on the fourth
anniversary of launch.

As long as we have used the globe to illustrate the orientation of the
orbits, we can enhance the picture for those readers who want to sharpen
their mental images of the geometry. Suppose Vesta, which Dawn has
transformed from a smudge of light into a richly detailed world, is 30
centimeters (1 foot) in diameter. Even in this miniaturized universe,
the sun is 204 kilometers (129 miles) away. (Space is big!) To get the
alignments right, we will place the sun over (albeit very, very far
over) the prime meridian, the 0 degree longitude line on our stationary
Vesta. Setting the longitude of the sun is important here because Dawn's
orbits were chosen on the basis of their angles relative to the sun. Now
we recall that Vesta, like Earth, has seasons because its axis is tipped.
It is southern hemisphere summer there, so the sun is not over the equator;
rather, it is at about 25 degrees south latitude. (On Sept. 1,
when we first used the analogy of the globe, the sun was at 27 degrees
south latitude. Since then, it has moved a little bit north because of
the progression of seasons.) Although Earth's location doesn't matter
for this discussion, we can accurately position it 204 kilometers (129
miles) away, high above a point at 26 degrees south latitude and 26
degrees west longitude.

Survey orbit in this Vesta-centric universe is a hoop a little more than
1.5 meters (5 feet) above the 15 degree west longitude line (and, again,
the 165 degree east longitude line on the night side). It was from that
vantage point that the first thorough mapping was conducted in August.
The ring representing HAMO is only about 38 centimeters (15 inches) over
the 30 degree west (and 150 degree east) longitude line. The lower
altitude of HAMO afforded much better views of the great variety of
surface features than the reconnaissance from survey orbit. In addition,
because the orbit was shifted farther from the sun, the angle of light
on the landscape beneath the spacecraft was different, aiding in
formulating a more accurate portrait of the terrain. Dawn is now nearing
LAMO, less than 12 centimeters (only 4.7 inches) above the 46 degree
west (and 134 degree east) longitude line.

Although the ion propulsion system accomplishes the majority of the
orbit change, Dawn's navigators are employing another novel propulsion
system as well: they are enlisting Vesta itself. Some of the ion
thrusting was designed in part to put the spacecraft in certain
locations from which Vesta would twist its orbit toward the target LAMO
angle. As Dawn rotates and the world underneath it revolves (unlike the
static picture we used to visualize the orientation of the orbits), the
spacecraft feels a changing pull. There is always a tug downward, but
because of Vesta's heterogeneous interior structure, the product of its
complex geologic history, sometimes
there is also a slight force to one side or another. With their
knowledge of the gravity field, the team plotted a course that took
advantage of these variations to get a free ride. This is akin to
experienced sailors not only relying on their ships' engines but also
following routes that use known currents to let nature do some of the
work. Of course, sailors benefit from knowledge of currents measured by
those who plied the waters before them. Dawn is the first, venturing
boldly into mysterious seas never visited before. But the measurements
of the gravity field in HAMO, even though it was at a higher altitude,
gave navigators enough information about what lay ahead on the horizon
that our vessel could safely and productively ride the gravitational
currents. The flight plan from HAMO to LAMO then is a complex affair of
carefully timed thrusting and equally carefully timed coasting. Under
ion thrust, the spacecraft flies to a certain location in a certain
orbit at a certain time, waits a certain interval as Vesta propels it to
the next waypoint, and then it resumes thrusting.

This itinerary was worked out in exquisite detail when Dawn was still in
HAMO, but it is impossible to follow the mathematically perfect path.
Although navigators measured the gravity field, they were not able to
detect all of its convolutions, so the ship is subjected to slightly
different currents from those expected. Occasional firings of the
spacecraft's small jets and tiny discrepancies between the planned
strength of the ion thrust and the actual value contribute more to the
deviation of the trajectory. Mission planners studied these and other
effects thoroughly and have been well prepared to account for all of them.

Every few days during the spiral transit from HAMO to LAMO, the
spacecraft points its main antenna to Earth so navigators can get an
accurate fix on it. The time for radio signals (traveling, as all
readers can attest, at the universal limit of the speed of light) to
make the round trip allows them to measure the probe's distance. The
slight change in the frequency of the signal, known as the Doppler shift,
reveals how fast the craft is moving. Combining these with their best
mathematical description of the gravity field and other data, as well
as the plan that is onboard for upcoming
thrusting and coasting, they determine Dawn's orbit and calculate where
it will be at a certain time in the future. For most of the way between
HAMO and LAMO, that time is three days, which is just long enough for
the entire operations team to perform an intricate set of carefully
choreographed steps.

Following the computation of where the spacecraft will be, trajectory
designers develop an update to the ideal reference they formulated while
Dawn was still in HAMO, now accounting for the observed discrepancies.
They devise a new profile of thrusting and coasting. Next, others on the
operations team translate that into the sequence of timed commands that
the robotic probe will execute in order to accomplish the maneuvers.
(Some of the principal challenges faced in guiding the ship through
these ever tightening spiral loops were described in February.
As we saw then, the spacecraft even
simulated a portion of this complex flight; now it is really doing it,
and it is doing it extremely well.)

While all of that is taking place at JPL, Dawn is continuing to fly
around Vesta, carrying out the previous set of instructions it received
to thrust and coast. Right on schedule, the team completes the work,
including all the diligent checks to make sure every detail is right, in
time to transmit the new sequence to the adventurer when the one already
onboard calls for it to turn its main antenna to Earth once again.
Almost as soon as the process is finished, it starts again, with a
measurement of Dawn's latest orbit.

In some of the more dynamic parts of the transfer from HAMO to LAMO,
three days is too long to let the spacecraft fly on its own without an
update to the plan, so the team had to do all that work in two days.
Thanks to the meticulous planning, preparation, and rehearsing, this
complex work has been conducted with the smooth professionalism that has
characterized the entire mission. As a result, Dawn remains on course
and on schedule to begin its scientific observations in LAMO on December
12.

Very far from home, the spacecraft is making excellent progress in its
expedition at a fascinating world that, until a few months ago, had
never seen a probe from Earth. This unique mission to a place unlike any
that has been visited before has already produced a tremendous bounty of
fabulous pictures and other important scientific data. Soon it will be
ready to undertake a new phase of exploration, revealing ever more about
this behemoth of the asteroid belt. Both patient and ambitious, Dawn
plumbs the depths of alien waters, stirring the imagination and
nourishing the spirit and the mind of everyone on distant Earth who ever
gazes at the sky in wonder or who longs to know the cosmos.

Dawn is 240 kilometers (150 miles) from Vesta. It is also 2.41 AU (361
million kilometers or 224 million miles) from Earth, or 935 times as far
as the moon and 2.45 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 40 minutes
to make the round trip.
Received on Thu 01 Dec 2011 06:34:56 PM PST