[meteorite-list] Dawn Journal - March 8, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 8 Mar 2009 15:34:28 -0700 (PDT)
Message-ID: <200903082234.PAA10651_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_3_08_09.asp

Dawn Journal
Dr. Marc Rayman
March 8, 2009

Dear Dawnlight Saving Times,
 
Now boosted into a new solar orbit courtesy of Mars, Dawn continues
its interplanetary journey. The spacecraft is healthy and coasting,
keeping its main antenna pointed to Earth, as it will for most of
the next 3 months. After that, it will resume its familiar routine
of devoting most of the time to gently thrusting with its ion
propulsion system, with only a short period each week for
communications.

Following the last log, as the probe succumbed to the gravitational
pull of the red planet, its trajectory gradually began to change.
Flying true to the plan, Dawn swooped close to Mars and then left
it behind on a new course, having taken advantage of Mars' gravity.

The spacecraft plunged to within 542 kilometers (337 miles) of the
planet, reaching that lowest altitude at 4:27:58 pm PST on
February 17. The last time it had been so close to another solar
system body was on September 27, 2007, as it left Earth to begin
its long journey to the asteroid belt. In the intervening time, it
traveled alone (although always accompanied by the good wishes of
space enthusiasts on its home planet and throughout the cosmos)
for 1.06 billion kilometers (661 million miles).
 
The targeting of the encounter was well within acceptable limits.
Before embarking on the mission, long before launch, engineers
chose 500 kilometers (311 miles) as a convenient initial target
altitude for planning purposes. With the extraordinary capability
of the ion propulsion system, Dawn easily could accommodate
significant deviations from the plan. To accomplish its mission,
the probe needed to fly anywhere through a window shaped liked a
croquet wicket, extending from more than 750 kilometers (466
miles) above the planet down to 300 kilometers (186 miles). (The
lower limit was chosen for safety, maintaining a comfortable
distance from the tenuous atmosphere and other threats the
spacecraft was not designed to handle.) The width at the bottom of
the wicket was almost 670 kilometers (415 miles).
 
In an effort to broaden our readership beyond only those sentient
beings in the universe who share in the passion for the
exploration of the solar system, the November and January logs
included some material for anyone passionate about archery. We
compared Dawn's flight by Mars to shooting an arrow at a target 47
kilometers (29 miles) away. The objective was to hit a small
region just outside the 30-centimeter (1-foot) red bull's-eye. For
the readers who have joined us because of that topic, we return to
it once again here. (We do not plan to expand upon the croquet
theme in future logs but hope our new readers attracted by
references to that sport will remain with us anyway.)

Our original target altitude of 500 kilometers corresponded to
about 17.2 centimeters (6 3/4 inches) from the center of the
bull's-eye. Using navigational data from January, we predicted
Dawn would fly by at 543 kilometers (337 miles) above the surface,
the arrow sinking into the target about 17.4 centimeters (less
than 6 7/8 inches) from the center, just outside the red circle.
Now we know that that prediction was in error by the equivalent of
about 40 micrometers (less than 1/600 of an inch). The arrow was
off from that expected location by about the thickness of a hair.

The location of the original target was at the 11:00 position, but
the base of the wicket-shaped window extended from where the hour
hand would be at 10:51:02 to 11:03:49. (The wicket is not as
symmetrical as official croquet rules might require.) As a
reminder, because we are concerned only about the hour hand, the
tick marks adjacent to the 11:00 position correspond to 10:48 and
11:12, so this wicket is very narrow. In January, it appeared our
arrow was headed for about 11:03:42. In fact, the actual
trajectory took the spacecraft through the window at the 11:03:40
position.

The gravity assist was extremely accurate indeed. The archers hit
their target and won the big prize: the continuation of the
mission of exploration in the asteroid belt, seeking answers to
questions about the dawn of the solar system. In addition to the
gravity assist, which was essential to the success of the mission,
the operations team had devised a plan to acquire some bonus data
to aid in the calibration of the science instruments, as described
last month.

In order to point its instruments at their calibration targets,
the probe oriented itself for a short time in such a way that
light reflected from Mars reached its "star tracker." This unit
(with a mystifying name, whose origin is lost in the dim mists
of time) tracks stars in order to help the attitude control system
establish the spacecraft's orientation (or
"attitude") in the zero-gravity of spaceflight. The tracker's
camera images stars and its internal computer recognizes patterns,
much as you might recognize some of the lovely constellations
visible from your planet and use them to orient yourself at night.
When Mars light entered the star tracker, the camera was dazzled,
temporarily unable to see the stars. If you reside on a planet
with a large moon, you may have experienced a similar phenomenon.
It is much harder to see stars when the bright moon interferes.

Engineers had anticipated this behavior. Because they knew the
star tracker likely would be unable to provide useful data to the
attitude control system for a while, gyroscopes had been powered
on well beforehand. Using these spinning masses, attitude control
can sense turns and keep track of how the attitude changes even
when the star tracker is not yielding accurate information.

As another preventive measure, commands stored on board
temporarily precluded protective software, known as fault
protection, from seeing any alerts indicating that the star
tracker was not able to produce valid data. Because they expected
the data to be invalid, engineers did not want fault protection to
respond under the mistaken impression that the tracker was unhealthy.

As Dawn approached Mars, with instruments powered and beginning
their calibrations, the light reached the star tracker, but it
performed better than expected. After reaching its minimum
distance, as the spacecraft rotated during its ascent, the star
tracker's line of sight moved closer still to Mars. Almost 2
minutes after the closest approach, the device finally was
overwhelmed with light and reported that it could not track stars,
declaring itself to be nonoperational and causing a software flag
to be hoisted to alert interested parties. As planned, attitude
control relied on gyros and fault protection remained blind to the
alert.

About 11 minutes later, as the spacecraft's attitude continued to
change, the star tracker's view moved far enough from Mars that
the unit once again could discern stars. When it recognized
patterns, it reported its data to attitude control, which readily
used them. All was well, and the tracker had functioned better
than anticipated, identifying stars with Mars closer to its line
of sight than anticipated.

Another 7 minutes after that, as Dawn's momentum continued to
carry it away from Mars, the stored command to restore fault
protection's ability to see any star tracker problems was
executed. The star tracker was indeed working well, but the
compulsive reader will note that the chronology above does not
include lowering the virtual flag that was raised when the tracker
announced it had stopped tracking stars. A software bug, hardy
enough to elude the operations team and survive the rigors of the
deep-space environment, prevented the flag from coming back down
when the tracker resumed normal operation. Although attitude
control was making good use of the data, when fault protection saw
the flag, it fulfilled its function; treating the tracker as if it
were unhealthy, the protective software deactivated the unit.

Fault protection's next step was to try to use the backup star
tracker. Another bug, identified last year while the spacecraft
was in flight, deprived the second tracker of the time necessary
to complete its activation routine. (That bug has been fixed in a
new version of the software scheduled to be transmitted to the
spacecraft before ion thrusting resumes in June.) With no star
tracker available, fault protection correctly followed the plan
its designers had given it: it interrupted the calibrations,
powered off the instruments, and put the spacecraft into "safe
mode", awaiting instructions from Earth on what to do next.

The operations team, following events on distant Earth (delayed by
the more than 19 minutes it took radio signals to cross the
separation), observed the signature of safe mode. There was no
urgency in responding, however, because the instrument
calibrations could not be restarted, and the crucial gravity
assist was unaffected by the spacecraft's activities. Mars would
sling Dawn in the intended direction without regard for the
probe's actions.

The cause of the "safing" was quickly determined, and the team
returned the spacecraft to its normal operational configuration
within about 2 days. In addition, all the bonus calibration data
that the instruments had transferred to the spacecraft's main
computer before they were deactivated were transmitted to Earth.

The gamma ray and neutron detector (GRaND)
obtained excellent measurements of both gamma rays and neutrons
from Mars. The instrument has been operated a number of times in
flight to measure high-energy radiation that pervades space as it
strikes the spacecraft, but, unlike the other science sensors
onboard, it can detect bodies only when it is very close to them.
Dawn's other instruments have observed distant planets and stars
several times already, but that is not possible for GRaND, even
with its suite of sophisticated detectors. Mars is the only
occasion in the mission for it to observe a specific,
well-characterized object. It was powered on in January
in anticipation of this opportunity.

GRaND obtained a thorough set of data as Dawn traveled down to its
lowest point, despite being over the night side of the planet part
of that time, because it does not require illumination by the Sun
to "see" its subject. It acquired additional valuable data as the
spacecraft receded from Mars. This was GRaND's only measurement of
a planetary body; and, apart from being quite brief, it was
performed in much the same way it will be when Dawn orbits each of
its protoplanetary destinations. It was especially good fortune
that space radiation levels were relatively low during the
encounter with Mars, making the radiation escaping from the
atmosphere and surface particularly clear for GRaND. Scientists
will compare GRaND's data with measurements of gamma rays and
neutrons by the Mars Odyssey spacecraft (which has been studying
the planet for more than 7 years), helping them prepare for
interpreting the unique observations it will make of Vesta and
Ceres to reveal many of the atomic constituents of those protoplanets.
 
The visible and infrared spectrometer's data were not stored in
the spacecraft's main computer memory before the safing because
the camera's data had priority. One of the images the camera
acquired is shown here
<http://www.nasa.gov/mission_pages/dawn/multimedia/mars-20090220.html>.
 
With the benefit of an excellent gravity assist, the Dawn project
is gratified to have Mars behind and Vesta now so clearly ahead.
Having obtained even more than was needed from the red planet,
Dawn is ever more eager to press on to its destinations in the
asteroid belt.
 
Dawn is 4.1 million kilometers (2.5 million miles) from Mars. It
is 336 million kilometers (209 million miles) from Earth, or 915
times as far as the moon and 2.26 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light,
take 37 minutes to make the round trip.
 
P.S. While the spacecraft is not concerned with changes between
standard time and daylight saving time, its human colleagues are.
Your correspondent, however, subscribes neither to the
spacecraft's indifference nor to the more traditional rigid
adherence. He does follow the convention of advancing clocks, as
we do today, but rather than setting his clocks back with everyone
else late in the year, he saves that extra hour. Every 24 years,
that should give him one extra day.
Received on Sun 08 Mar 2009 06:34:28 PM PDT