[meteorite-list] Dawn Journal - February 29, 2012

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 29 Feb 2012 15:06:00 -0800 (PST)
Message-ID: <201202292306.q1TN60G1021477_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_02_29_12.asp

Dawn Journal
Dr. Marc Rayman
February 29, 2012

Dear Ups and Dawns,

Dawn is continuing its exploits at Vesta, performing detailed studies of
the colossal asteroid from its low altitude mapping orbit (LAMO). The
robotic ambassador is operating extremely well on behalf of the
creatures it represents on a distant planet. On this second intercalary
day of its ambitious adventure, the spacecraft is doing exactly what it
was designed to do: exploring a previously uncharted alien world.

Although we usually describe LAMO as being at an average altitude of 210
kilometers (130 miles), that does not mean it is at a constant
altitude. As we saw on the fourth anniversary of Dawn's departure from
Earth, there are two reasons the spacecraft's height changes.
One is that the elevation of the surface
itself changes, so if the probe flew in a perfect circle around Vesta,
its altitude would vary according to the topography. Like the planet
from which Dawn embarked upon its deep space journey in 2007 (and even
some of the residents there), Vesta is broadest near its equator, and
that is where the ground generally reaches its greatest distance from
the center. In addition, the ancient surface, battered over billions of
years in the rough and tumble of the asteroid belt, displays remarkable
variations in shape. The giant Rheasilvia basin is a scar from an
extraordinary impact that excavated a region encompassing the south pole
more than 500 kilometers (over 300 miles) in diameter. This immense
gouge has left that part of Vesta at a much lower elevation than
elsewhere. In the center of the enormous depression is the second
tallest mountain known in the solar system, soaring to well over twice
the height of Mt. Everest. The vertical range from the highest locations
near the equator to the bottoms of the deepest craters within Rheasilvia
is more than 60 kilometers (37 miles). So as Dawn loops around in just
over four hours, the surface underneath it rises and falls dramatically.

The second reason is that the orbit itself is not exactly a circle.
Let's ignore for a moment the effect of the topography and focus solely
on the shape of the craft's path around Vesta. As Vesta rotates and Dawn
revolves, the gravitational forces acting on the orbiter are always
changing because of the irregular distribution of material inside the
geologically complex protoplanet. This effect occurred at the higher
altitudes as well, but it was much less pronounced there. Now that the
adventurer is deep in the gravity field, the peaks and valleys of its
own motion are magnified.

Navigators were very careful in choosing the parameters for LAMO,
recognizing that the orbital waters were turbulent. Nevertheless, their
mapping of the gravitational currents proved quite accurate, and the
spacecraft has followed the planned course quite well. The lengthy and
relatively technical discussions in the two previous logs
described why the ship drifts off a little, but operators occasionally
nudge it back with the ion propulsion system.

Orbits usually are best described by ellipses, like flattened circles.
Now Vesta's bumpy gravity field does not allow perfectly smooth, regular
orbits at low altitude. Moreover, the variations in the strength of the
gravitational attraction transform the orbits. Sometimes, the difference
between the high point of a loop and the low point is less than 16
kilometers (10 miles). As the changing forces reshape the orbit, the
ellipse gets more exaggerated, with the low points going lower and the
high points going higher. The differences within one revolution grow to
be more than 75 kilometers (47 miles). Thanks to the ingenious design of
the orbital trajectory however, those same forces then will gradually
attenuate the profile, causing it to become more round again. This
pattern repeats every 11.5 days in LAMO. It is almost as if the orbit
breathes slowly, its envelope expanding and contracting.

This evolution of the orbit occurs above the rugged shape of Vesta
itself. These two effects have conspired so that Dawn has been less than
170 kilometers (106 miles) from the rocky surface on several occasions
when it was over equatorial regions. At its greatest altitude in LAMO,
Dawn occasionally reaches to more than 290 kilometers (180 miles). This
happens when it is deep in the southern hemisphere, soaring over the low
elevation terrain of Rheasilvia.

These changes in the distance to the ground were known before Dawn
arrived in LAMO, and they do not compromise the ongoing campaign to
learn as much as possible about this survivor from the dawn of the solar
system. As it revolves around the behemoth beneath it, the spacecraft
uses its gamma ray and neutron detector (GRaND) to record these subatomic
particles, which carry the signature of the elements within the top meter
(yard) of the surface. Navigators' extraordinarily accurate measurements of
the ship's orbital motion reveal subtleties in the gravity field and hence the
distribution of material throughout the gigantic asteroid. Controllers have
taken advantage of the low altitude and smooth operations to collect more
observations with the camera and the visible and infrared mapping spectrometer
(VIR). More than 7500 pictures have been acquired so far in LAMO, and VIR has
returned nearly one million spectra. These provide a fabulous scientific bonus,
affording scientists a much more detailed view of Vesta than had been planned
with survey and the high altitude mapping orbit (HAMO).

The acquisition of science data was interrupted on February 21 when the
main computer was temporarily overloaded with tasks. The system
correctly responded by rebooting the computer, which put the spacecraft
into safe mode. Because this occurred during a communications session,
controllers observed the event (albeit delayed by the long travel time
for radio signals to reach Earth). They quickly diagnosed the problem
and began the meticulous commanding to bring the robot back its normal
configuration. Within a few days, it had resumed its normal schedule of
observations.

In some sense, even the GRaND and gravity measurements now are a bonus.
When the detailed timeline for Dawn's residence at Vesta was formulated,
mission planners allowed 70 days in LAMO, which began on December 12 and
so would have concluded on February 20. As we saw at the end of 2011,
because the unique approach, the intensive observations in survey orbit and
HAMO, and the complex spiral flights
from each science orbit to the next have all been accomplished so well
(perhaps even unexpectedly well), the 40 days that were held in reserve
to overcome problems are now being used to prolong the studies at low
altitude. With all sensors fully operational, the robotic explorer is
making the best possible use of its precious time at Vesta, revealing
more and more exciting details of a mysterious world deep in the
asteroid belt.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.33 AU (498
million kilometers or 309 million miles) from Earth, or 1240 times as
far as the moon and 3.36 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 55 minutes
to make the round trip.
Received on Wed 29 Feb 2012 06:06:00 PM PST