[meteorite-list] Dawn Journal - September 27, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 28 Sep 2010 13:15:30 -0700 (PDT)
Message-ID: <201009282015.o8SKFUsa018547_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_09_27_10.asp

Dawn Journal
Dr. Marc Rayman
September 27, 2010

Dear Dawnniversaries,

On the third anniversary of traveling through the solar system on its
own since dispatching Dawn on a separate journey,
Earth continues to orbit the sun in much the same way it has been.
Meanwhile, the spacecraft is thrusting with its ion propulsion system,
making steady progress in reshaping its orbit to rendezvous with Vesta
in July 2011.

In its three years of interplanetary travels, the spacecraft has thrust
for a total of about 715 days, or 65% of the time (and about
0.000000014% of the time since the Big Bang). While for most spacecraft,
firing a thruster to change course is a special event, it is Dawn's
wont. All this thrusting has cost the craft only 189 kilograms (417
pounds) of its supply of xenon propellant, which was 425 kilograms (937
pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 5.01 kilometers per second (11,200 miles per
hour). As previous logs have described
for one of the more extensive discussions), because of the principles of
motion for orbital flight, whether around the sun or any other
gravitating body, Dawn is not actually traveling this much faster than
when it launched. But the effective change in speed remains a useful
measure of the effect of any spacecraft's propulsive work. Having
accomplished only one-third of the thrust time planned for its entire
mission, Dawn has already far exceeded the velocity change achieved by
any other spacecraft under its own power.

Since launch, our readers who have remained on or near Earth have
completed three revolutions around the sun, covering about 18.9 AU
(2.82 billion kilometers or 1.75 billion
miles). Orbiting farther from the sun, and thus moving at a more
leisurely pace, Dawn has traveled 15.1 AU (2.26 billion kilometers or
1.40 billion miles). As it climbs away from the sun to match its orbit
to that of Vesta, it will continue to slow down to Vesta's speed. Since
Dawn's launch, Vesta has traveled only 12.0 AU (1.80 billion kilometers
or 1.12 billion miles).

Readers with eidetic memory have already noticed that much of the text
in the three preceding paragraphs is taken nearly verbatim from the logs
that commemorated Dawn's first and second anniversaries of being in space,
with the principal changes being that the numbers are updated here and we
have generously expunged more (but not all!) humor each time. For those who
wish to cogitate about the extraordinary nature of this interplanetary
journey, comparing the first half of this log with those others may be
helpful. In order to make the table below comprehensible (and to fulfill
our commitment of environmental responsibility), we reuse some more of
the text here.

Another way to investigate the progress of the mission is to chart how
Dawn's orbit around the sun has changed. This discussion will culminate
with a few more numbers than we usually include, and readers who prefer
not to indulge may skip this material, leaving that much
more for the grateful Numerivores.

Orbits are ellipses (like flattened circles, or ovals in which the ends
are of equal size). So as members of the solar system family follow
their paths around the sun, they sometimes move closer and sometimes
move farther from it.

In addition to orbits being characterized by shape, or equivalently by
the amount of flattening (that is, the deviation from being a perfect
circle), and by size, they may be described in part by how they are
oriented in space. Using the bias of terrestrial astronomers, the plane
of Earth's orbit around the sun (known as the /ecliptic/) is a good
reference. Other planets and interplanetary spacecraft travel in orbits
that are tipped at some angle to that. The angle between the ecliptic
and the plane of another body's orbit around the sun is the
/inclination/ of that orbit. Vesta and Ceres do not orbit the sun in the
same plane that Earth does, and Dawn must match its orbit to that of its
targets. (The major planets orbit closer to the ecliptic, and no
spacecraft has had to venture as far out of that plane in order to
achieve orbit around another body as Dawn will.)

Now we can see how Dawn has been doing by considering the size and shape
(together expressed by the minimum and maximum distances from the sun)
and inclination of its orbit on each of its anniversaries. (Experts
readily recognize that there is more to describing an orbit than these
parameters. Our policy remains that we link to the experts' websites
when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft
had terminated thrusting on its anniversaries; the orbits of its
destinations, Vesta and Ceres, are included for comparison. Of course,
when Dawn was on the launch pad on September 27, 2007, its orbit around
the sun was exactly Earth's orbit. After launch, it was in its own solar
orbit.

[Table]

Readers may disregard the table or gaze into it for insight or
inspiration for as long as they like. The point of it, however, is to
illustrate that Dawn has come a long way since the launch pad, and while
considerably more work remains to climb to Vesta, the ascent ahead is
not as daunting as what has already been accomplished. On its next
anniversary, the probe will be in the same orbit around the sun that
Vesta is in; Dawn will be orbiting that distant world, where much of the
mission's scientific destiny lies.

During the intervening year, there is a great deal more to look forward
to than further travels through interplanetary space. In recent logs, we
have had an overview of the activities during the "approach phase,"
which begins in less then eight months, and how the ship will slip into orbit
around this mysterious protoplanet in July, becoming the first
spacecraft to circle a body in the main asteroid belt. We also
summarized the plans for the first intensive science phase, known as
"survey orbit," which will be conducted in August.

Dawn is a mapping mission. In survey orbit it will use the science
camera and the visible and infrared mapping spectrometer (VIR) to map
most of the surface. (The gamma-ray and neutron detector, GRaND,
will record radiation, but it will not be until a much lower altitude
that the full power of its measurements will be achieved.)

Dawn will not be able to observe /all/ of the surface from survey orbit,
because some of it will not be illuminated. The reason is simple: Vesta
has seasons. This is no different from Earth and most of the other
planets. Of course, the seasons don't refer to weather on this airless
world but rather to the sun's apparent motion relative to Vesta's
equator, a consequence of the tilt of Vesta's pole. Each of the seasons
lasts about 11 months. When Dawn begins surveying Vesta, it will be
summer in the southern hemisphere; the high northern latitudes will be
in the darkness of winter and hence concealed from the camera and VIR.
By the latter phases of our mission at the protoplanet, in 2012, the
seasons will have progressed, the sun will illume more of the northern
hemisphere, and the instruments will see previously invisible terrain.
We will consider the effect of the seasons and the implications for
observing Vesta in a future log. For now, we will focus on the science
phase that follows survey orbit.
 
While survey orbit affords the robotic explorer a broad overview of the
uncharted world, subsequent observations from a lower orbit will reveal
more details. This second mapping orbit is known to insiders on the Dawn
project, now including you, as the high altitude mapping orbit (HAMO).
(Most team members have avoided the disquietude of trying to understand
why that name is not applied to survey orbit, and readers are encouraged
to do the same.)

With its survey from an altitude of about 2700 kilometers (1700 miles)
complete, the ship will set sail again with the gentle touch of its ion
propulsion system, gradually spiraling down for a month until it arrives
at the HAMO altitude of about 660 kilometers (410 miles). As with survey
orbit, the final selection of the parameters of the orbit will be possible
only when Dawn is in the vicinity of Vesta and the massive asteroid's
gravity field has been measured accurately.

Dawn will follow a polar orbit again, affording it the opportunity to
pass over all latitudes, from the north pole to the equator to the south
pole and then back, as Vesta spins on its axis. Unlike the nearly three
days to complete one revolution in survey orbit, in HAMO the spacecraft
will loop around the world beneath it twice a day. Between Dawn's
orbital motion and Vesta's rotation, ten orbits will be required to
present most of the illuminated surface to the camera's eye. Although
they will take a brief break today to celebrate with some cake decorated
with an image of Dawn's launch (and with real xenon in the frosting),
mission controllers are now developing the detailed sequences of
commands for the spacecraft in six
groups, each corresponding to one of these 10-orbit/five-day cycles in
HAMO.

As before, the probe will devote most its time over the day side of
Vesta to acquiring a wealth of information with its sensors and most of
the time over the night side beaming those precious data back to eager
scientists on Earth. There is a whole new world to explore, and the
instruments will be commanded to gather more data during passage over
the lit side than the communications system can transmit to the Deep
Space Network during the half of the orbit over
the dark side. As a result, Dawn's computer memory will fill faster than
it can be emptied, and the spacecraft will leave HAMO with some of its
treasure trove still onboard. In the course of its flight to the next
science orbit (suggestively and even appropriately named the low
altitude mapping orbit) it will radio the stored data during the regular
hiatuses in thrusting to point its main antenna to Earth.

In two of the HAMO cycles, Dawn will peer straight down at the exotic
landscape below it, taking pictures with the camera and recording
spectra with VIR. By performing these
mapping observations twice, there will be a ready opportunity to see
most of the visible surface even if some observations are not completed
because of the occasional glitches that are inevitable in such a complex
undertaking in an unknown environment. The process of designing,
testing, verifying, and executing the intricate sequences for mapping
Vesta is far too complicated for the mission control team to wait until
any such minor problem occurs and then formulate plans to acquire the
missed data.

During the other four HAMO cycles, instead of pointing its instruments
at the nadir, the spacecraft will aim them off at an angle, providing a
different view in each cycle, effectively acquiring stereo imagery.
Scientists will combine the pictures taken in all the directions to
create topographic maps, revealing the heights of mountains, the depths
of craters, the slopes of plains, etc. This will be of great value in
inferring the nature of the geological processes that shaped this
protoplanet. In addition, it will yield exciting perspectives for
everyone who wants to visualize this alien world.

As this journey of exploration enters its fourth year, we continue to be
exceedingly grateful for the many readers who travel with us on this
voyage. An adventure of humankind, Dawn raises our collective sights and
our spirits as it strives on behalf of everyone who longs to see far
past mere terrestrial horizons. While physically we remain confined to
the vicinity of our planet, we do not allow that to limit our reach.
Powered by our ambition, our imagination, our curiosity, and our
determination, missions like Dawn take all of us along to real places
that lie beyond what our imaginations could construct. Although readers
throughout the cosmos have participated in this experience, it is ironic
that some very nearby have not been able to. Thanks to the generous
efforts of Pablo Guti?rrez-Marqu?s, the operations manager for Dawn's
science camera at the Max-Planck-Institut f?r Sonnensystemforschung (Max
Planck Institute for Solar System Research) in Katlenburg-Lindau,
Germany, it is with great delight that we welcome our hispanophone
friends onboard. Small as the spacecraft is, there is
plenty of room for everyone who shares in the wonderment of the cosmos,
the fulfillment of gaining new knowledge and new insights, the
exhilaration of discovery, and the thrill of exploration.

Dawn is 0.15 AU (23 million kilometers or 14 million miles) from Vesta,
its next destination. It is also 2.96 AU (443 million kilometers or 275
million miles) from Earth, or 1120 times as far as the moon and 2.95
times as far as the sun. Radio signals, traveling at the universal limit
of the speed of light, take 49 minutes to make the round trip.
                
Received on Tue 28 Sep 2010 04:15:30 PM PDT