[meteorite-list] Dawn Journal - September 1, 2011

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_09_01_11.asp

Dawn Journal
Dr. Marc Rayman
September 1, 2011

Dear Magdawnificents,

Dawn has completed the first phase of its exploration of Vesta with
tremendous success, and the peripatetic adventurer is now in powered
flight again, on its way to a new location from which to scrutinize its
subject. Meanwhile, scientists are deeply engaged in analyzing the
magnificent views the stalwart surveyor has transmitted to Earth.

Most of August was devoted to survey orbit.  At an altitude of about 2,700
kilometers (1,700 miles), the ship sailed slowly around the world
beneath it, completing a loop every 69 hours. Vesta rotates faster,
turning once on its axis each 5 hours, 20 minutes. As we saw in the
previous log, the survey orbit phase of the
mission consisted of seven revolutions around Vesta, providing ample
opportunities to acquire the rich bounty of data that scientists yearned
for. 

As Dawn follows its course, it passes over the north pole, then heads
south on the day side of Vesta. On each orbit, it trained its sensors on
the illuminated surface and filled its memory with the spectacular
sights. On the other half of its orbit, gliding high above the dark
landscape, it radioed its findings to distant Earth.

As we discussed last year, Vesta has
seasons, just as your planet probably does. For readers on Earth, for
example, it is summer in the northern hemisphere, and a region around
the south pole is in constant darkness. On Vesta right now, the southern
hemisphere is facing the sun, so everywhere between about 52 degrees
north latitude and the north pole is in a long night. That ten percent
of the surface is presently impossible to see. Because Dawn will stay in
orbit around Vesta as together they travel around the sun, in 2012 it
will be able to see some of this hidden scenery as the seasons advance.

The campaign of acquiring data in survey orbit was very complex. On the
second, fourth, fifth, and sixth loops, the strategy included collecting
more than Dawn's memory could accommodate in the half of an orbit in
which it was over sunlit terrain. Therefore, during those orbits,
mission planners incorporated instructions to turn away from looking at
Vesta to allow the spacecraft to point its main antenna to Earth for
five to six hours. That provided time to transmit enough of its precious
findings to make room for still more during the rest of the passage over
the day side.

On the first and third revolutions, the computer in the visible and
infrared mapping spectrometer (VIR) 
encountered an unexpected condition, so it stopped collecting data. When
the spacecraft was next on the night side, controllers reconfigured the
instrument so it could resume normal operation for the subsequent lap.
Engineers and scientists from Italy who developed the complex device and
from JPL are working closely together to establish the underlying cause.
They have taken advantage of the extended periods in each orbit when the
main antenna is pointing to Earth to run diagnostic tests on the unit.
All indications are that it is healthy, and evidence points strongly to
the glitches being related to some detail of the mode in which VIR
collects and processes data. The team is confident that once they
understand the behavior, they will be able to formulate plans to operate
the spectrometer in ways that avoid triggering it.

Thanks to the strategy to perform more observations than needed,
even with the interruptions, VIR
accumulated a fantastic wealth of information. The principal scientific
objective of survey orbit was to collect 5,000 sets of spectra or
"frames." A spectrum is the intensity of light at different wavelengths,
and each frame consists of visible and infrared spectra at 256 locations
on Vesta's complex and mysterious surface. By the end of survey orbit,
Dawn had obtained well in excess of 13,000 frames, or more than three
million spectra. Acquiring more than one spectrum of the same location
is valuable, as different angles of incident or reflected sunlight allow
scientists to gain greater insight into the mineralogical composition
and properties of the material. With an initial plan of observing 52
percent of the surface with VIR from survey orbit, the team is elated
now to have spectra from about 63 percent.

The science camera has similarly
overachieved. The intent was to photograph 60 percent of Vesta, but the
entire 90 percent not in the darkness of northern winter has been
captured at least five times. With pictures taken from multiple angles,
stereo views can be constructed; and images at different times allow
features to be observed under varied lighting conditions. All of the
camera's color filters were used, providing coverage in the near
infrared and visible. Until recently, Vesta was known as little more
than a smudge of light, but now scientists have more than 2,800 photos
from Dawn's survey.

A selection of stunning scenes of the latest world to come into the
realm of humankind's knowledge is here
<http://dawn.jpl.nasa.gov/multimedia/image_of_the_day_archive.asp>. 
As scientists pore through
the treasure trove, they will continue to add their favorite views to
that site.

This mission has already revealed far more about Vesta than a flyby
mission could. While much more data will be obtained during the rest of
Dawn's residence there, the six gigabytes from VIR and the three
gigabytes from the camera so far are enough to keep researchers busy
(and extremely happy!) for a very long time as they tease out the nature
of this alien world.

Even before the outstandingly successful survey orbit had begun,
navigators were starting to plan the flight to the next science orbit.
Throughout Dawn's approach  and survey orbit, they have been refining 
measurements of Vesta's mass and therefore its gravitational
strength. The closer Dawn has come, the better they have been able to
detect variations in the gravity field that are due to the uneven
distribution of mass within the protoplanet.  With their improved charts 
of the waters around Vesta, they plotted the ship's course, and it is now 
under sail.  Thrusting with the ion propulsion system began on August 31 
at 4:05 p.m.  PDT, and this trip to the high altitude mapping orbit
will take a month.

In survey orbit, Dawn was 2,700 kilometers (1,700 miles) above Vesta.
Its next orbital target lies at an altitude of about 680 kilometers (420
miles). The separation between them may seem relatively small, but
maneuvering in orbit requires far more work than may be evident simply
from the distance. In addition, Dawn is doing even more than flying down
to a lower altitude. Each of the observation orbits at Vesta is designed
to optimize a set of scientific investigations. Scientists want to shift
the plane of Dawn's orbit in going from survey to HAMO in order to
change the illumination presented to the sensors.

To visualize the nature of the shift, picture the orbit as a ring around
the world, going over both poles and crossing the equator at right
angles. Many globes are supported within a ring like this. Now for this
explanation, we have the permission of the residents to ignore Vesta's
rotation, so the ring is like a circle of constant longitude hovering in
space. For the purpose of illustration, let's say survey orbit is at a
longitude of 15 degrees. (The distant sun would be at a longitude of 0
degrees. Shining south of the equator at a latitude of 27 degrees, the
star is more than 2.25 times farther from Vesta than it is from Earth.)
HAMO is not only four times closer to the surface than survey but also
rotated so it is at 30 degrees longitude. Of course, Vesta will continue
to turn on its axis, but with the plane of Dawn's orbit changed, the
angle of sunlight falling on the surface below will be different. (Once
again, "longitude" is used here only to illustrate the relative
orientation of the orbit planes; it is not intended to describe a
relationship to specific coordinates on the ancient, battered, rocky world.)

To travel from survey to HAMO, Dawn will have to accomplish the
equivalent of a change in speed of around 65 meters per second (145
mph). Compared to the 6.8 kilometers per second (15,200 mph) it achieved
in its trek from Earth to Vesta, this is very modest indeed.
Nevertheless, as the spacecraft spirals lower, the flight plan
is much more complex than it was during the interplanetary flight. The 
outcome will be described in the next log.

Even as Dawn ventures closer to the giant that holds it in orbit, the
splendid results of its first detailed survey of Vesta will continue to
dazzle and excite us. The images and other data beamed to Earth are
filled not only with scientific value but also with the exhilaration of
discovery and the thrill of exploration. The drama upon beholding some
of humankind's first views of an alien world is something everyone can
experience. After all, it is the collective passion for extending our
reach beyond the confines of our terrestrial neighborhood and the shared
hunger for knowledge of the cosmos that enabled an emissary from Earth
to take up residence far from home, deep in the asteroid belt. There,
our species' yearning for noble adventures is now being fulfilled.

Dawn is 2,600 kilometers (1,600 miles) from Vesta. It is also 1.35 AU
(202 million kilometers or 125 million miles) from Earth, or 555 times
as far as the moon and 1.34 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 22
minutes to make the round trip.