[meteorite-list] Dawn Journal - June 30, 2012

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_06_30_12.asp

Dawn Journal
Dr. Marc Rayman
June 30, 2012

Dear Upside Dawn Readers,

Dawn is now seeing Vesta in a new light. Once again the probe is
diligently mapping the ancient protoplanet it has been orbiting for
nearly a year. Circling the alien world about twice a day, the ardent
adventurer is observing the signatures of Vesta's tortured history,
including the scars accumulated during more than 4.5 billion years in
the main asteroid belt between Mars and Jupiter.

Having successfully completed its orbital raising maneuvers to ascend to
its second high-altitude mapping orbit (HAMO2), Dawn looks down from
about 680 kilometers (420 miles). This is the same height from which it
mapped Vesta at the end of September and October 2011. The lifeless
rocky landscape has not changed since then, but its appearance to the
spacecraft's sensors has. The first high-altitude mapping orbit (HAMO1)
was conducted shortly after southern hemisphere
summer began on Vesta, so the sun was well south of the equator. That
left the high northern latitudes in the deep darkness of winter night.
With its slower progression around the sun than Earth, seasons on Vesta
last correspondingly longer. Thanks to Dawn's capability to linger in
orbit, rather than simply conduct a brief reconnaissance as it speeds by
on its way to its next destination, the probe now can examine the
surface with different lighting.

Much of the terrain that was hidden from the sun, and thus the camera,
during HAMO1 is now illuminated. Even the scenery that was visible then
is lit from a different angle now, so new observations will reveal many
new details. In addition to the seasonal northward shift in the position
of the sun, Dawn's orbit is oriented differently in HAMO2, as described
last month, so that makes the opportunity for new insights and discoveries 
even greater.

The strategy for mapping Vesta is the same in HAMO2 now as it was in
HAMO1. Dawn's orbital path takes it nearly over the north pole. (As we
saw last month, the orbit does not go exactly over the poles
but rather reaches to 86 degrees latitude.  That slight difference is not 
important for this discussion.) During the ship's southward passage over 
the sunlit side, the camera and the visible and infrared mapping 
spectrometer (VIR) acquire their precious data. After passing (almost) 
above the south pole, Dawn sails north over the night
side. Instead of pointing its sensors at the deep black of the ground
below, the probe aims its main antenna to the extremely distant Earth
and radios its findings to the exquisitely sensitive receivers of the
Deep Space Network. The pattern repeats as the indefatigable spacecraft
completes loop after loop after loop around the gigantic asteroid every
12.3 hours.

As Dawn revolves, Vesta rotates on its axis beneath it, turning once
every 5.3 hours. Just as in HAMO1, mission planners artfully
choreographed this celestial pas de deux so that over the course of 10
orbits, lasting just over five days, the camera would be able to view
nearly all of the lit surface. A set of 10 orbits is known to Dawn team
members (and to you, loyal readers) as a mapping cycle.

Until a few months ago, HAMO2 was planned to be four cycles. Thanks to
the determination in April that Dawn could extend its residence at Vesta
and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has 
been increased to six mapping cycles (plus even a little more, as we 
shall see below), promising a yet greater scientific return.

In cycle 1, which began on June 23, the camera was pointed at the
surface directly underneath the spacecraft. The same view will be
obtained in cycle 6. In cycles 2 through 5, images are acquired at other
angles, providing different perspectives on the complex and dramatic
landscape. Scientists combine the pictures to formulate topographical
maps, revealing Vesta's full three-dimensional character from
precipitous cliffs and towering peaks of enormous mountains to gently
rolling plains and areas with mysterious ridges and grooves to vast
troughs and craters punched deep into the crust. Knowing the elevations
of the myriad features and the angles of slopes is essential to
understanding the geological processes and forces that shaped this
exotic mini-planet. In addition to the exceptional scientific value, the
stereo imagery provides realistic, exciting views for anyone who wants
to visualize this faraway world. If you have not traveled there
yourself, be sure to visit the Image of the Day
</multimedia/imageoftheday/archives.asp> regularly and the video gallery
</multimedia/videos.asp> occasionally to see what you and the rest of
humankind had been missing during the two centuries of Vesta's
appearance being only that of a faint, tiny blob in the night sky.

With 3-D movies and other familiar stereo pictures, only two angles are
needed. That's sufficient to reproduce what our two eyes would perceive,
but it does not tell the entire story. A left-right pair reveals nothing
about the up-down dimension. Scientists chose the directions to point
Dawn's camera that yield the best combinations of perspective and
illumination to construct a complete contour map.

In cycle 2, the craft soars over the sunlit side with its camera pointed
both ahead and to the left of the ground directly below. In cycle 3, the
instrument will be targeted behind and slightly to the left. Cycle 4
will observe the surface farther back and to the right. Cycle 5 will
look slightly ahead and to the right. Together these pictures will yield
a fabulous sense of the detailed shape of Vesta, and combining them with
the HAMO1 images will afford an extraordinarily comprehensive 3-D view.

The camera and VIR are mounted on the spacecraft so that they point in
the same direction. During these six cycles, the direction is determined
by what's needed for the topographic mapping, but VIR collects valuable
spectra as well wherever it is aimed. A spectrum is a measure of the
intensity of light at different wavelengths and is reminiscent of the
rainbow you see when a glass prism or droplets of water separate white
light into its constituent colors. The material on Vesta imprints its
signature on the light it reflects from the sun, so VIR's measurements
reveal the nature of the minerals. The sensor has already found that
Vesta displays a highly varied composition, attesting to its complex
geological history. VIR records light from ultraviolet through the
entire visible range and into the infrared. Indeed, the instrument
operates so far into the infrared that it can detect the meager heat
emitted from the surface, thereby also functioning as a remote
thermometer. Each VIR snapshot consists of the spectrum at 256 locations
on the surface, providing a great richness of information.

Compared to the camera, VIR trades greater spectral coverage for smaller
spatial coverage. VIR was the prime instrument in survey orbit,
where it was high enough that even with its narrow view, it could observe 
most of the surface. At the lower altitude of HAMO1 and HAMO2, VIR cannot 
map all of Vesta in a single mapping cycle or even in six cycles. (And 
even with all the bonus data it collected during months of operation in 
the low-altitude mapping orbit (LAMO), the proximity to the surface allowed it
to obtain excellent close-up views but only of small regions.) HAMO1 was
so outstandingly productive that VIR did see much of the surface, and
now the coverage is being increased significantly with HAMO2.

Because the mission has been going so well, mission planners decided to
devote some extra time in HAMO2 to additional VIR measurements. From
June 15 through 23, before the six mapping cycles commenced, VIR was the
star of the celestial show again. Every orbit was dedicated exclusively
to collecting as many spectra as could be transmitted to Earth. The
telecommunications link that stretches across the solar system is very
limited. By not splitting it between the camera's images and VIR's
spectra, controllers could maximize the latter's coverage of Vesta.

Dawn's exceedingly productive exploration may make its accomplishments
appear easy, but as with all such undertakings, the success is enabled
by a group of people applying their collective expertise, discipline,
creativity, and powerful drive to reveal the unknown. It is thanks to
their extraordinary investment of time and energy that the distant probe
is able to execute such an ambitious mission, unveiling an ancient world
that previously had only been glimpsed from afar by telescopes.

When the previous log was unleashed upon readers
of all dawnominations, Dawn was partway through its long spiral route
from LAMO to HAMO2. (You can see the weekly progress in altitude by
checking the May mission status reports) Complex and
challenging though it was, the flight went precisely as intended.
Because maneuvering the spacecraft exactly to its targeted destination
is so difficult, mission planners had scheduled a window to fine tune
the orbit on June 9 and 10 after the main phase of ion thrusting was
complete. This is very similar to the trajectory correction maneuvers
planned before the swing past Mars.
Nevertheless, upon carefully measuring the actual orbit following the
end of thrusting on June 4, navigators determined that it was so good
that no adjustments were needed.

Before the resumption of Vesta observations on June 15, engineers
reversed some reconfigurations of the spacecraft they had made for
operation at lower altitude. They also took advantage of the time to
perform a routine verification of the health of the back-up camera,
ensuring that it remained ready to take over if the primary camera
encountered problems. Both instruments are in excellent condition.

As Dawn continues tirelessly to scrutinize Vesta and report its
fascinating findings, the mission control team is putting the finishing
touches on the plans for its departure. On July 25, the ship will begin
climbing out of HAMO2, its sights set on Ceres. Just as during the
approach phase, however, it will pause
occasionally for some additional observations. As Vesta grows farther
and smaller but sunlight touches more of the high northern latitudes,
the instruments will take some parting shots. We will describe those
plans in the next log. As we shall see, even as Dawn says goodbye to its
companion of more than a year deep in the main asteroid belt, it will
continue to discover new secrets to thrill and delight all the
passionately curious and bold creatures who champion the eager explorer
on its interplanetary voyage. Through this robot, they are transported
far, far into space to behold sights and gain knowledge that otherwise
would remain forever beyond their reach.

Dawn is 680 kilometers (420 miles) from Vesta. It is also 3.17 AU (474
million kilometers or 294 million miles) from Earth, or 1305 times as
far as the moon and 3.12 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 53 minutes
to make the round trip.