[meteorite-list] Dawn Journal - September 27, 2011

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_09_27_11.asp

Dawn Journal
Dr. Marc Rayman
September 27, 2011

Dear Dawnniversaries,

Dawn's fourth anniversary of being in space is very different from 
its previous ones. Indeed, those days all were devoted to reaching 
the distant destination the ship is now exploring.  Celebrating its 
anniversary of leaving Earth, Dawn is in orbit around a kindred 
terrestrial-type world, the ancient protoplanet Vesta.
 
The adventurer spent August on Vesta's shores and now it's ready to dive
in. Dawn devoted most of this month to working its way down from the
2,700-kilometer (1,700-mile) survey orbit to its current altitude of
about 680 kilometers (420 miles) and changing the orientation of the
orbit>. (For a more detailed
discussion of the altitude, go here <#Vesta> or continue reading
patiently for another six paragraphs.) The sensationally successful
observing campaign in survey orbit produced captivating views, revealing
a complex, fascinating landscape. Now four times closer to the surface,
the probe is nearly ready for an even more comprehensive exploration
from the high altitude mapping orbit (HAMO). The plans for HAMO have
changed very little since it was described on the third anniversary of
Dawn's launch. 

Dawn's spiral descent went extremely well. We have seen before that bodies 
travel at higher velocities in lower altitude orbits, where the force of
gravity is greater. For example, Mercury hurtles around the sun faster
than Earth in order to balance the stronger pull of gravity, and Earth's
speed is greater than that of more remote Vesta. Similarly, satellites
in close orbits around Earth, such as the International Space Station,
race around faster than the much more distant moon. When it began its
spiral on August 31, Dawn's orbital speed
high above Vesta was 76 meters per second (170 mph), and each revolution
took nearly 69 hours. Under the gentle thrust of its ion propulsion
system, the spacecraft completed 18 revolutions of Vesta, the loops
getting tighter and faster as the orbital altitude gradually decreased,
until it arrived at its new orbit on schedule on Sept. 18. In HAMO, Dawn
orbits at 135 meters per second (302 mph), circling the world beneath it
every 12.3 hours.

When Dawn's itinerary called for it to stop thrusting, it was very close
to HAMO but not quite there yet. As mission planners had recognized long
beforehand, small differences between the planned and the actual flight
profiles were inevitable. Extensive and sophisticated analysis has been 
undertaken in recent years to estimate the size of such discrepancies so 
the intricate plans for completing all the work at Vesta could account for 
the time and the work needed to deliver the robotic explorer to the 
intended destination. In order to accomplish the intensive program of 
observations with its scientific instruments, the spacecraft must follow 
an orbital path carefully matched to the sequences of
commands already developed with painstaking attention to detail. The
beauty of Dawn's artistically choreographed pas de deux with Vesta
depends on the music and the movements being well synchronized.

During its descent, Dawn paused frequently to allow controllers to
update the flight profile, accounting for some of the variances in its
course along the way. Following the completion of thrusting, navigators 
tracked the ship more extensively as it sailed around Vesta, measuring 
its orbit with great accuracy. This revealed not only the details of the 
orbital parameters (such as size, shape, and orientation) but also more 
about the character of Vesta's gravity field  than could be detected at
higher altitudes. With the new information, the team designed two short
maneuvers to adjust the orbit. The first, lasting four hours, was
executed last night, and the second, half an hour shorter, will be
completed tonight. After further measurements to verify the final orbit,
the month of HAMO observations will begin on Sept. 29.

When the main portion of the thrusting was finished on Sept. 18, there
was still more for Dawn to do than let navigators determine its exact
orbit. It trained its sensors on Vesta, acquiring more exciting and
valuable data. Although these observations are not part of the
meticulously orchestrated and systematic mapping in HAMO, they
contribute to the overall effort to squeeze as much as possible out of
the precious time at Vesta. Engineers also performed more tests with the
visible and infrared mapping spectrometer.  In addition, they acquired images
with the backup science camera, confirming that it is still fully functional 
and ready for action should its perfectly healthy twin ever become infirm.

Following instructions loaded earlier, on Sept. 21 the spacecraft
reconfigured its memory to prepare for the great volume of data it will
collect in HAMO. One of the software functions took longer than
expected, causing the main computer to reset. The robot is designed to
enter safe mode after a reboot, so it dutifully powered off nonessential
systems, turned to point at the sun (the only celestial reference easily
detectable anywhere along Dawn's long route through the solar system),
and waited for further instructions. Controllers detected the condition
late that night and quickly identified the cause. With calm
professionalism they swiftly executed all the steps necessary to return
Dawn to its normal flight configuration less than two days later, and
operations have continued smoothly.
 
As the spacecraft flies around Vesta, its altitude is constantly
changing. Indeed, it would vary even if the orbit were a perfect circle,
because Vesta is not a perfect sphere. This is similar to flying in a
plane over Earth. If the plane maintains a constant altitude above sea
level, the distance above the ground can change because the elevation of
the ground itself varies, coming closer to the aircraft on mountains and
farther in valleys. The topography on Vesta is even more pronounced,
reflecting the tortured history it has experienced during 4.5 billion
years in the rough and tumble asteroid belt.

The event that created the huge gouge centered near the south pole, now
officially known as Rheasilvia (after the vestal virgin who was the
mythical mother of Romulus and Remus, a weird story unlikely to be
clarified by Dawn's investigations), has left Vesta not only with
astounding and jumbled terrain but also with an overall shape that is
very peculiar. Indeed, although this world is smaller than Earth, it
displays some of the most extreme topography in the solar system. The
tremendous mountain in the center of Rheasilvia towers almost twice as
high above the surrounding plains as Mt. Everest does above sea level.
Despite their being widely separated, the difference in elevation
between the highest features near the equator and the lowest points deep
in craters punched into Rheasilvia is more than 60 kilometers (37 miles).

Even if we imagined Dawn as being stationary while Vesta rotated
underneath it, the altitude would change as the misshapen surface surges
and subsides. In addition, the craft's path is not a perfect circle, and
the lower the orbit, the more it will deviate, as the irregular gravity
field tugs on it with changing strength depending on where in that
complex field the spacecraft is. When Dawn pushes down to closer orbits,
we will discuss more about the actual height above the surface. In the
meantime, for simplicity, these logs will continue to present the
altitude as an average value, measured with respect to the average
distance from the center of Vesta to its rocky surface. This is
analogous to using sea level on Earth for the reference to describe
altitude for aircraft and satellites. It is on that basis that the
altitude in HAMO is given as 680 kilometers (420 miles).

On the last three September 27s, we have summarized Dawn's progress on
its journey. Now that it is at its first destination, the best measure
of its progress is the stunning images and other scientific results it
has transmitted to Earth. In addition to special announcements at press
conferences in the coming months, beautiful and intriguing views
continue to be posted here
<http://dawn.jpl.nasa.gov/multimedia/imageoftheday/archives.asp?month=2011-September>
every day.

For those who would like to track the probe's progress in the same terms
used on previous (and, we boldly predict, subsequent) anniversaries, we
present here the fourth annual summary, reusing the text from last year
with updates where appropriate. Readers who wish to cogitate about the
extraordinary nature of this deep-space expedition may find it helpful
to compare this material with the first parts of the logs from its first,
second, and third anniversaries. (On this special day, members of
the operations team will further reflect upon the mission with the help
of cupcakes decorated with the Dawn/Vesta logo.)

In its four years of interplanetary travels, the spacecraft has thrust
for a total of about 988 days, or 68% of the time (and about
0.000000020% 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 254 kilograms (561
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 6.85 kilometers per second (15,300 miles per
hour). As previous logs have described (see here <journal_8_24_08.asp>
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 barely half 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.
(For a comparison with probes that enter orbit around Mars, refer to
this earlier log <journal_12_06.asp#perspective>.)

Since launch, our readers who have remained on or near Earth have
completed four revolutions around the sun, covering about 25.1 AU
(3.76 billion kilometers or 2.34 billion
miles). Orbiting farther from the sun, and thus moving at a more
leisurely pace, Dawn has traveled 19.4 AU (2.91 billion kilometers or
1.81 billion miles). As it climbed away from the sun to match its orbit
to that of Vesta, it continued to slow down to Vesta's speed. Since
Dawn's launch, Vesta has traveled only 16.4 AU (2.46 billion kilometers
or 1.53 billion miles).

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 <#numbers>, leaving that much more
for the grateful Numerivores. In order to
make the table below comprehensible (and to fulfill our commitment of
environmental responsibility), we recycle some more text here on the
nature of orbits.

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 ventured as far out of that plane in order to achieve
orbit around another body as Dawn has.)

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.

	Minimum distance from the Sun (AU) 	Maximum distance from the Sun
(AU) 	Inclination
Earth's orbit  	0.98 	1.02 	0.0
Dawn's orbit on Sept. 27, 2007 (before launch) 	0.98 	1.02 	0.0
Dawn's orbit on Sept. 27, 2007 (after launch) 	1.00 	1.62 	0.6
Dawn's orbit on Sept. 27, 2008 	1.21 	1.68 	1.4
Dawn's orbit on Sept. 27, 2009 	1.42 	1.87 	6.2
Dawn's orbit on Sept. 27, 2010 	1.89 	2.13 	6.8
Dawn's orbit on Sept. 27, 2011 	2.15 	2.57 	7.1
Vesta's orbit 	2.15 	2.57 	7.1
Ceres's orbit 	2.54 	2.99 	10.6

 
For readers who are not overwhelmed by the number of numbers, the table
may help to demonstrate how Dawn has transformed its orbit during the
course of its mission. Note that now the spacecraft's orbit around the
sun is the same as Vesta's. Achieving that was, of course, the objective
of the long flight that started in the same solar orbit as Earth. While
simply flying by Vesta would have been far easier, matching orbits with 
it has required the extraordinary capability of the ion propulsion system. 
Without it, NASA's Discovery Program would not have been able to afford a 
mission to explore this fascinating world, and a mission to both Vesta and 
Ceres would be impossible.

Amazing and inspiring as its extraordinary trek has been, climbing the
solar system hill atop a blue-green pillar
of xenon ions, gently reshaping its orbit with the finesse of a sculptor
creating a cosmic masterpiece, traveling far from its home planet
through the forbidding and lonely depths of interplanetary space, it is
the destination, and not the journey, that provides the grand prize. For
most of the two centuries prior to Dawn's arrival, Vesta was known as
little more than a small fuzzy patch of light amidst the stars. The
sharply focused picture that we are developing now of a complex alien
world, with a dramatic history and a truly unique character, is the
great reward for the long years and the billions of kilometers (miles)
to get there. As the expedition continues, how can anyone not thrill to
the experience of Vesta simultaneously becoming both more familiar and
yet more mysterious?

Now as the operations team completes preparations for the next phase of
its scrutiny of Vesta, Dawn embarks on its fifth year of spaceflight
doing what it was designed to do. At the limits of human ingenuity,
powered by the zeal of those who seek to perceive and to understand the
beauty of the cosmos, the stalwart ship forges ahead with its
exploration of a relic from the dawn of the solar system.

Dawn is 680 kilometers (420 miles) from Vesta. It is also 1.59 AU (238
million kilometers or 148 million miles) from Earth, or 665 times as far
as the moon and 1.59 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 26 minutes
to make the round trip.