[meteorite-list] Dawn Journal - January 30, 2011

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 3 Feb 2011 18:15:17 -0800 (PST)
Message-ID: <201102040215.p142FHNA025839_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_01_30_11.asp

Dawn Journal
Dr. Marc Rayman
January 30, 2011

Dear Providawntial Readers,

Dawn continues its flight through the asteroid belt, steadily heading
toward its July rendezvous with Vesta, where it will take up residence
for a year. The spacecraft has devoted most of the time since the
previous log to its familiar routine of thrusting
patiently with its ion propulsion. But both the distant craft and the
operations team each took a little time away from this month, spending
it instead in the future. Although the project has not yet perfected
time travel, it has achieved enough capability to conduct two successful
ventures into the near future.
 
On Jan. 10, Dawn performed some of the activities that it will execute
in its low altitude mapping orbit (LAMO) at Vesta. From the probe's
vantage point in LAMO, much of the sky will be
filled by the ancient protoplanet, only about 180 kilometers (110 miles)
away. Vesta will appear as large as a soccer ball from a mere 18
centimeters (7 inches). Completing one revolution every four hours in
LAMO, the spacecraft will devote most of its time to training its
scientific instruments on the rocky world beneath it, teasing out the
secrets Vesta holds about the dawn of the solar system. Other times, the
robotic explorer will loop around the body while aiming its main antenna
at distant Earth, transmitting its findings and receiving new instructions.
 
As we have seen in previous logs, to control its orientation in the
zero-gravity of spaceflight, Dawn usually relies on one or both of its
star trackers. Despite the utterly perplexing etymology, these devices
track stars in order to help the attitude control system establish the
spacecraft's orientation (or "attitude"). The tracker's camera takes
pictures of stars five times per second. Its internal computer
recognizes patterns of stars, much as you might recognize some of the
beautiful constellations visible from your planet and use them to orient
yourself at night.
 
The spacecraft's two star trackers are mounted so each will have a
different but clear view of the star-studded sky when the scientific
instruments are pointed at Vesta. But during some turns and while the
main antenna is pointed to Earth, there will be times when the nearby
world will obstruct their views. You too might have difficulty seeing
stars if a soccer ball were positioned immediately in front of your face.
 
For those times when both star trackers end up pointing at Vesta, and
thus are unable to provide the attitude control system with the
orientation, the probe relies on gyroscopes. Using these spinning
masses, attitude control can sense turns and keep track of how the
attitude changes even when the star trackers are unable to yield useful
information.
 
While Dawn is in the great void of interplanetary space, there is
nothing to interfere with the star trackers. (They are aligned so that
neither one points near the sun in any of the spacecraft's normal
orientations.) When the ship sailed by Mars for a gravitational boost on
its way to the asteroid belt, it used the gyros when the planet blocked
the view of the stars. That worked well enough for that brief event, but
engineers wanted to confirm that their strategy of swapping from star
trackers to gyros would be effective during the much longer events that
will occur in orbit around Vesta.
 
To verify their plans, the spacecraft was configured to operate as it
will in LAMO. It pointed its instruments as if Vesta were nearby and
rotated to keep them aimed at the surface, just as it will when it
circles the colossal body. (The instruments remained unpowered, because
this was a test of the attitude control system and, of course, there was
nothing for them to observe.) After a time following that pointing
profile, Dawn turned to direct its antenna to Earth and held the planet
in its radio sights. Following those activities, it conducted a nearly
identical pattern. The second time, however, the onboard sequence of
commands included a crucial difference.
Engineers had incorporated extra instructions to prevent data from the
star tracker from being used by the attitude control system when it
stopped pointing at the surface of Vesta and started its next turn. This
trick in software perfectly mimicked the effect of about 530 kilometers
(330 miles) of giant protoplanet blocking starlight. Dawn continued
through the sequence without missing a beat. It pirouetted toward Earth
and held its orientation there with its gyros, moving with grace and
accuracy just as a blindfolded dancer might display on a stage, relying
on her vestibular system to accomplish her masterful performance. After
two and a half hours, the sequence restored star tracker data.
 
For the entirety of the test, the spacecraft recorded detailed data on
its attitude and on other parameters so engineers subsequently could
assess how their plans worked. The analysis showed that the robot's
performance was even better than anticipated.
 
After the successful test, the spacecraft reconfigured for normal
interplanetary thrusting and set course again for the real Vesta.
Meanwhile, mission controllers were preparing for their own simulation
of life in the vicinity of Vesta.
 
Dawn's exploration of that rocky protoplanet will require much more than
a sophisticated probe carrying out its assignments in the forbidding
depths of the asteroid belt. The operations team will need to keep it
healthy and furnish it with up-to-date plans. In many cases, team
members will need to analyze or process data quickly and deliver their
results to the next person in line, and each step has to proceed on
schedule to keep the mission advancing smoothly and productively. In an
undertaking as complex as orbiting a remote, massive, and previously
unexplored world, surprises are sure to occur, and some of them likely
will be unwelcome. As part of formulating intricate plans for the year
Dawn will spend in orbit, the team has developed strategies to account
for the unexpected. Some of these plans were exercised this month in an
"operational readiness test," or ORT, conducted not in the harsh, alien
setting of the asteroid belt but rather in the mysterious, unique
environment of JPL.
 
The Dawn project performed ORTs in 2007 (as described here
<journal_6_07.asp#ort>, there <journal_6_10_07.asp#ort>, and elsewhere
<journal_9_03_07.asp#proficiency>) as launch grew near and twice last
year (one of which was described in July <journal_07_26_10.asp#complex>)
to prepare for Vesta. Some ORTs focus on the team's ability to conduct
the mission as planned, and in others, including the one this month, the
team faces problems. Organized and overseen by test conductors (also
known as simulation supervisors, sim sups, or more inspired names when
they dream up more creative challenges for the operations team), the
ORTs are nearly as elaborate as real operations, both in their planning
and their execution, and they are very valuable experiences for the
participants.
 
In this ORT, the Dawn team spent a week in the summer, when the
spacecraft will be in the approach phase,
only a few weeks from its first science orbit, known as "survey orbit."
Over the course of the first few days
of the exercise, they received more and more bad news. A record-breaking
outpouring of radiation from the sun had damaged some of the memory
components in Dawn's central computer and degraded its solar arrays.
Dawn's unique mission to orbit two solar system targets is enabled by
its ion propulsion system, and the near-constant thrusting depends on
the uniquely powerful solar arrays. Just as the boost in predicted power
in 2009 provided an increase in the time Dawn could spend at Vesta , a decrease
now would translate to a shorter residence there before having to depart
for Ceres. In addition, the simulated damage to the computer meant that
many of the images already acquired of Vesta for navigation
could not be recovered and transmitted,
and the plans for subsequent storage of engineering and science data
would have to be curtailed.

It also turned out that analysis of the earlier images in this rehearsal
revealed that the tilt of Vesta's rotational axis was different from
what astronomers had calculated from telescopic observations. Vesta,
just as all large bodies in the solar system, rotates in a regular
fashion around an axis. As we have seen,
Dawn will take up a polar orbit around Vesta, the perfect choice for
observing all of the illuminated surface. To do so, the orientation of
the pole in space needs to be known. In other words, navigators need to
know exactly where Vesta's axis points. To understand this, consider the
globe of Earth. It is obvious it spins on its axis, but that axis just
happens to point near the familiar star Polaris; it equally well could
point elsewhere in space (as it has in the past and will again).
Measurements from observatories, including the Hubble Space Telescope in
2007 and 2010, have been used to estimate the direction of Vesta's axis,
and Dawn will improve upon those during the approach phase so the probe can be
targeted to an orbit that takes it over the poles. In the manufactured
future of the ORT, it was revealed that the actual orientation of the
axis was farther from the prediction than many scientists had expected
it might be.
 
That was not enough for the ever-thoughtful, endlessly creative sim
sups. They also informed the team that the faux solar radiation
inflicted even greater damage on other interplanetary spacecraft, so
Dawn's scheduled use of the Deep Space Network
would have to be cut back to allow those other missions to engage in
recovery operations. To add to the difficulties, we were astonished to
be informed that one member of the team had participated in some quite
interesting activities that landed him in prison. Another, after
involvement in an incident that merited headlines in some famous
tabloids (although, curiously, not the intergalactically read Dawn
Journal), also was unavailable to solve technical problems. Other
engineers on the team had to fill in to make sure all the work was
accomplished.
 
Still more misfortunes beset the beleaguered operators (who also managed
some good laughs over the details provided by the test conductors), but
they worked through all the problems, using a combination of the plans
they had already developed and creative solutions devised during the
course of the ORT. The challenges of these simulated operations in the
future were compounded by their being faced when the team also was
continuing with real operations in the present.
 
The actual surprises at Vesta surely will be different from those in the
rehearsal (still, your correspondent is going to keep his eyes on those
two adventurous team members), but the ORT gave the team an excellent
sense of operations in difficult conditions. Although this was the last
of the ORTs, more preparatory work remains before Dawn reaches its first
intriguing destination.
 
As the probe continues its journey, less and less of what departed Earth
atop a powerful Delta rocket more than three years ago is still onboard.
With its famously frugal use of xenon propellant, it was only this month
that the fuel gauge dipped to half. Dawn's tank carried 425 kilograms (937
pounds) at launch. It took more than 2.2 years of powered flight to exhaust
half of that supply, during which the ion propulsion system imparted the
equivalent of 5.7 kilometers per second (nearly 13,000 mph), far more than
any spacecraft has been able to change its own velocity.

And yet the adventurer continues to propel itself, gradually maneuvering
so its orbit around the sun will match that of its target, an uncharted
world that beckons. Paying no attention to the blue-green wake of xenon
ions behind it, the explorer's sights remain set ahead, on a destination
growing ever closer, on the opportunity to uncloak the mysteries of
Vesta, on the new knowledge that it will gain, and on the new questions
that it will raise. It seeks not to satisfy but rather to sustain the
powerful drive for exploration that compelled curious creatures, humble
yet bold, confined to the vicinity of almost incomprehensibly distant
Earth, to reach within themselves that they might then reach out into
the cosmos.
 
Dawn is 6.1 million kilometers (3.8 million miles) from Vesta, or 16
times the average distance between Earth and the moon. It is also 2.78
AU (416 million kilometers or 259 million miles) from Earth, or 1,065
times as far as the moon and 2.83 times as far as the sun. Radio
signals, traveling at the universal limit of the speed of light, take 46
minutes to make the round trip.
Received on Thu 03 Feb 2011 09:15:17 PM PST