[meteorite-list] Dawn Journal - June 27, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 30 Jun 2010 13:47:19 -0700 (PDT)
Message-ID: <201006302047.o5UKlJx4026248_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_06_27_10.asp

Dawn Journal
Dr. Marc Rayman
June 27, 2010

Dear Daw9.0s,

A new version of the Dawn spacecraft is continuing the ambitious
journey through the asteroid belt to uncharted distant worlds. Now
holding a new solar system record, the probe is thrusting with its
ion propulsion system, patiently and gently changing its orbit
around the sun to match that of the immense protoplanet Vesta (and
subsequently dwarf planet Ceres).

Even as Dawn continues pushing deeper into space, another spacecraft
that used ion propulsion to conduct an exciting mission at a near-Earth
asteroid has concluded. After traveling to and studying the diminutive
Itokawa, Japan's Hayabusa spacecraft returned to Earth on June 13. This
was long one of your correspondent's favorite missions, and he has
joined many, many other enthusiasts in congratulating the team
responsible for this impressive achievement.
 
When Dawn reaches each of its destinations, it will have a very full
program of activities to acquire pictures and other scientific
information. Brief overviews of some of its plans for Vesta were
described in recent logs, and more will be presented
later. To accomplish its mission of exploration, the spacecraft needs
some enhancements to the capabilities it has been using for its travel
through deep space to reach its targets. Those new capabilities are now
onboard.

For the third time since it left Earth in September 2007, the spacecraft
has received an upgrade of the software that runs in its primary
computer. With a sense of grandeur and drama befitting this unique
adventure, ever-poetical engineers fulfilled their dream of more than a
year by denominating it /OBC flight software version 9.0/. Revealing
their surprisingly cute and playful nature, however, most Dawn team
members prefer the hypocorism "9.0" (or "nine oh").

Engineers at JPL and Orbital Sciences Corporation began work on 9.0
almost immediately after 8.0 was installed on the spacecraft in April
2009. They continued with the careful and deliberate process of
modifying the software until January, when the extensive test program
commenced. It was crucial to verify not only that the new functions
would work correctly but also that no unintended differences were
introduced and that the existing capabilities were not compromised.

The latest software has 23 sets of changes from the previous version.
Some of these are new methods of controlling the way the spacecraft will
point its sensors at Vesta and Ceres in order to optimize the
acquisition of data. Other modifications, based on experience gained in
the ongoing operation of the spacecraft, improve its ability to handle
certain potential anomalies on its own. In addition, just as 7.0 and 8.0
did, 9.0 corrects some bugs.
 
While it may seem quite elementary to load software into a computer that
is in control of a spacecraft more than twice as far from Earth as the
sun, it actually turns out to be somewhat complex and delicate. Even in
"quiet cruise", the computer is responsible
for a great deal of activity onboard. The ion propulsion system was
inactive, which is typical when the main antenna is pointed to Earth,
but otherwise the computer was busy keeping all systems operating.
 
To install 9.0, controllers used exactly the same processes they
followed for 8.0 in April 2009. It went quite smoothly again this time,
right down to the on-time delivery of pizza to mission control during
the first day of returning the spacecraft to its normal configuration
after rebooting the computer. We know almost all readers accepted the
advice offered last year to retain a copy of the log that presented the
details of the 8.0 installation, but we happily include a link here
<http://dawn.jpl.nasa.gov/mission/.journal_4_26_09.asp>
for the convenience of the sole reader who did not
and wishes to recall what is involved. (For all other readers,
congratulations on the handsome profit you have realized on your
investment in that previous log.)

As last year, controllers had run a few tests to verify the
integrity of some critical components during the normal weekly
communications sessions in the weeks leading up to the loading of
the new software. On June 15, the spacecraft stopped thrusting on
schedule, turned to point its main antenna to Earth, and kept it
there rather than returning to the thrust direction a few hours
later. That allowed operators to perform the rest of these
detailed checks. After confirming that both the primary and backup
computers were fully healthy, they transmitted the files
containing the new software.

On June 16, with all stations in mission control at JPL reporting
all subsystems were healthy and stable, and all systems at the
Deep Space Network performing equally well, the command to reset
the computer was radioed to the distant ship. The computer
dutifully rebooted for the first time since the installation of
8.0 and began running with version 9.0. Whenever the computer
reboots, it puts the craft into safe mode. The team verified that
the new software was running smoothly and then initiated the
process of guiding the spacecraft out of safe mode and back to its
normal interplanetary cruise configuration. The schedule had
allowed until June 24, but by June 18, the robotic explorer was
fully prepared to resume its normal duties.

Because the software upgrade went so well, the Dawn project has decided
to present this exciting offer: we will install a functional copy of 9.0
on your computer or smartphone at no charge. Simply place your device
in the asteroid belt, send us the coordinates, and we'll do the rest.
 
On June 17, after the majority of reconfigurations had been completed
and while all members of the team but the insomniacs and the spacecraft
itself were slumbering, protective software that is always running
onboard detected an increase in the internal friction in reaction wheel
no. 4. Reaction wheels are devices used to control a ship's orientation
in the zero-gravity of spaceflight. By electrically controlling the
speed of these spinning units, the spacecraft can hold steady or rotate
as needed. Dawn is outfitted with four reaction wheels, although it only
uses three during normal operations. As we have seen before,
operators let each wheel have its turn at
being off for a part of the mission. The software that detected the
friction in no. 4 responded correctly by powering that unit off. If only
three wheels had been in use, it would have activated the unused wheel;
but it was unnecessary to do so this time because, by coincidence, all
wheels were operating, as is normal when the spacecraft enters safe
mode. The team had been planning to turn reaction wheel no. 1 off later
on June 17 as part of the reconfiguration. Instead, after taking some
time to reassess the spacecraft's condition, they simply left wheel no.
4 off and continued with their plans, now using wheels 1, 2 and 3
instead of 2, 3 and 4.
 
Dawn resumed ion thrusting on schedule on June 24. As it continues
propelling itself to Vesta, it does so with the recognition that it has
accomplished a greater propulsive change in velocity than any other
craft ever to leave Earth. Some spacecraft have experienced larger
velocity changes through gravitational interactions with planets, but
thanks to the extensive use of its extremely efficient ion propulsion
system, Dawn surpassed the record for the greatest change in velocity
under a ship's own power on June 5.

The previous record holder, Deep Space 1, was the first
interplanetary mission to use ion propulsion. In its 11-month
primary mission of testing advanced technologies (including ion
propulsion), its two-year extended mission devoted to the
exploration of a comet, and its final three-month hyperextended
mission of additional technology testing, DS1 accumulated so much
thrust time that it achieved an effective change in speed of 4.3
kilometers per second (9,600 mph). (As we have seen in several
earlier discussions, such as here
<http://dawn.jpl.nasa.gov/mission/journal_8_24_08.asp#correspondent>,
this "effective change in speed" is not the speed at which the craft
travels. It is a very
commonly used way to express the effectiveness of a spacecraft's
propulsion system that avoids the confounding effects of orbital
mechanics.)

Having thrust now for 635 days, or 63 percent of its time in
space, Dawn has attained a change of more than 4.4 kilometers per
second (9,800 mph), and it has much, much more powered flight ahead.

The record itself and even the total velocity change, while
perhaps fun, really are not important, however. They are
convenient measures of the progress this ship is making on its
ambitious expedition, one that would not have been possible
without ion propulsion and other innovations. The exploration of
the cosmos is not a competition; it is a shared undertaking of all
humankind. Each mission, each record, each accomplishment, each
discovery builds on the successes (and even the failures) of those
that preceded it and helps pave the way for those that will
follow. Together they all contribute to the advancement of our
understanding of the universe and our humble place within it.

Dawn is 0.32 AU (48 million kilometers or 30 million miles) from Vesta,
its next destination. It is also 2.29 AU (342 million kilometers or 213
million miles) from Earth, or 855 times as far as the moon and 2.25
times as far as the sun. Radio signals, traveling at the universal limit
of the speed of light, take 38 minutes to make the round trip.
Received on Wed 30 Jun 2010 04:47:19 PM PDT