[meteorite-list] Dawn Journal - September 17, 2006

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Sep 20 13:32:09 2006
Message-ID: <200609201724.KAA06633_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_09_06.asp

Dawn Journal
Dr. Marc D. Rayman
September 17, 2006

Dear Dawntellectuals,

There is only about three quarters of a revolution remaining around the
Sun before Dawn leaves Earth to travel on its own to distant worlds.
Meanwhile, the project team continues to prepare the spacecraft for its
mission. This work has proceeded smoothly despite the chaos of planets
apparently coming and going from our solar system.

As readers in other solar systems have no doubt followed with some
detached amusement, the definition of "planet" was in the news in this
solar system this summer. While much of the focus was on whether Pluto
should be considered a planet, Dawn's second destination, Ceres, also
was subjected to this linguistic turmoil. The International Astronomical
Union (IAU) adopted a definition of "dwarf planet" that includes Ceres,
Pluto, Eris, and perhaps more bodies yet to be characterized
sufficiently or even discovered. Ceres is the largest member of the
asteroid belt, residing between Mars and Jupiter; the other dwarf
planets are part of the Kuiper belt, spending most or all of their time
beyond the most distant planet, Neptune.

Resolution 5A passed by the 26th General Assembly of the IAU describes
the attributes a body must have to qualify as a dwarf planet. Like a
planet, it must orbit the Sun and have sufficient mass for its own
gravity to make it nearly spherical. (Vesta, the first stop on Dawn's
interplanetary itinerary, might satisfy the definition of dwarf planet,
but not enough is known yet about its gravity and shape.) Unlike planets
however, dwarf planets are characterized by not having cleared away
other objects from their part of the solar system through the effects of
their gravity. This bars any resident of the asteroid belt or the Kuiper
belt from membership in the planet club. (Another criterion, that the
body not be a satellite, excludes some of the moons of planets from
being designated as planets themselves.)

The definition is not widely accepted by the community of planetary
scientists, and it remains to be seen how the definition might be
changed. Ceres and Vesta were considered planets for half a century
following their discoveries in 1801 and 1807 respectively. All
scientific evidence indicates that with all the names humans have
applied to them, including planets, asteroids, minor planets,
protoplanets, and dwarf planets, they have steadfastly remained above
the controversy, leading their stately lives without apparent interest.

The Dawn team has never wavered about what to call these bodies; with
the utmost clarity and consistency, they have always been known as
"Ceres" and "Vesta." Team members continue to look forward to the wealth
of information the spacecraft will return from its orbits around these
fascinating places. In continuing to prepare for that, engineers are
completing another set of the comprehensive performance tests (as
explained in previous logs) to verify that the subsystems on the
spacecraft can fulfill the required functions.

Loyal readers will come to be familiar with Dawn's subsystems as we take
it through the rest of its prelaunch preparations and we join it, in
spirit if not in person, on its cosmic travels. [Editor's note: "Loyal
readers" is redundant; our recent surveys show 100% of readers in the
targeted galaxies are loyal.] As we shall see over the coming years,
there is nothing like guiding a spacecraft through the forbidding depths
of space to understand how it really works. But now let us have a very
very brief introduction to the engineering subsystems that allow Dawn to
conduct its mission. In a future log, we will describe the scientific
instruments, which will help reveal the natures of Ceres and Vesta.

The command and data handling subsystem includes the main computers that
operate the probe along with most of the other electronics. As with most
Dawn subsystems, the design includes primary and backup components so
that even if a failure occurs far from Earth, the spacecraft can
continue to fulfill its scientific mission. This subsystem keeps the
spacecraft functioning smoothly as it operates on its own in space.
Running in its three primary computers is the master software for the
spacecraft, consisting of more than 400,000 lines of C and assembly
code. In addition to its own orchestrations of spacecraft activities, it
processes commands sent by the mission operations team and issues them
when required to other subsystems. It stores the scientific data
acquired by the instruments and collects information on the performance
of the spacecraft, all to be reported back to Earth. Some engineers
would consider this to be the most important subsystem on the spacecraft.

The electrical power subsystem (OK, I know you're ahead of me on this
one) provides the power needed by all electrical components onboard. Its
solar arrays convert light from the Sun into electricity, and the
subsystem delivers high voltage to the ion propulsion subsystem and
lower voltage to all the other subsystems. Because Dawn will need high
electrical power for its ion propulsion subsystem even when far from the
Sun, the solar arrays are very large for a planetary spacecraft. Each of
the two solar array wings is almost 8.3 meters (more than 27 feet) long,
and when they are extended shortly after launch, the overall craft will
be about 19.7 meters (nearly 65 feet) from wing tip to wing tip. This
subsystem includes a powerful battery whose primary purpose is to allow
Dawn to operate while on the rocket and during the time immediately
after separation when it needs to perform a number of critical functions
to deploy its arrays and point them at the Sun. The arrays will generate
more than 10 kilowatts at Earth's distance from the Sun (enough to power
10 average households in the US). This is far more power than Dawn can
use, but when it has receded to 3 times Earth's distance from the Sun,
every watt it can yield will be of great value to the spacecraft, with
its power-hungry ion propulsion subsystem. Some engineers would consider
this to be the most important subsystem on the spacecraft.

The attitude control subsystem (despite the name, this subsystem is as
delightful to work with and is as enthusiastic about the mission as all
other subsystems) is responsible for controlling the orientation (which
engineers refer to as "attitude") of the craft in the zero-gravity of
spaceflight. This subsystem can orient the probe so that it points an
ion thruster in the direction required to reach its cosmic destinations,
directs an antenna to distant Earth, or aims the camera or other
instruments so they may observe their targets. It also will keep the
solar arrays pointed at the Sun. To determine its attitude, Dawn uses
"star trackers" (again, two are onboard, although only one is needed),
cameras that recognize star patterns and thereby reveal the direction
they are pointed. (For readers who accompanied Deep Space 1 on its
voyage, it was the failure of the sole star tracker during the extended
mission that led to the need to conduct the spectacular rescue of the
spacecraft. That is described in the logs of 2000, available at
http://nmp.jpl.nasa.gov/ds1/archives.html and in late night reruns on
most planets not in synchronous rotation around their stars.) The
subsystem also carries gyroscopes to improve the accuracy of the
pointing. For emergency use, Sun sensors can help the spacecraft
establish its approximate attitude when a star tracker is temporarily
off-line. Devices known as reaction wheels are electrically spun faster
or slower to rotate the spacecraft. Some engineers would consider this
to be the most important subsystem on the spacecraft.

For technical reasons, the reaction wheels are not sufficient for all
the pointing control Dawn will need during its long mission, so another
means is required. In addition to the reaction wheels, which are
considered part of the attitude control subsystem, there are two other
subsystems that attitude control uses to achieve the orientations it
needs. The reaction control subsystem includes 12 small thrusters that
use a conventional rocket propellant known as hydrazine; you may not be
surprised to know that only 6 thrusters are needed, so even if an entire
group of 6 failed, the mission would not be lost. Each brief pulse of a
thruster causes the spacecraft to change its speed or direction of
rotation. This subsystem will be loaded with about 45 kilograms (100
pounds) of hydrazine, although it likely will use much less than that
during the mission. Some engineers would consider this to be the most
important subsystem on the spacecraft.

Most interplanetary spacecraft use hydrazine-based propulsion not only
to turn but also to change their trajectories through space. Dawn is
able to undertake its detailed exploration of the most massive bodies in
the asteroid belt because it uses a more capable form of propulsion. The
ion propulsion subsystem accomplishes this by ionizing xenon gas; that
is, it gives it a small positive electrical charge by removing a
negatively charged electron from each neutral xenon atom. Once the xenon
is ionized, the subsystem can electrically accelerate the ions and emit
them at very high speed from any 1 of the 3 ion thrusters. The action of
each xenon ion as it is shot from a thruster at up to about 35
kilometers per second (78,000 miles per hour) causes a reaction that
pushes the spacecraft in the other direction. Dawn will launch with 425
kilograms (937 pounds) of xenon -- more than enough to allow it to
travel to and orbit its targets while setting some remarkable records to
be described in future logs. Because ion propulsion is so different from
conventional propulsion systems, it leads to many differences in the way
we design and conduct the mission, and later logs will describe this in
more detail (once our attorneys prove their case that the copyright
infringement claims by the self-proclaimed Ionic Potentate of Xenon are
invalid). In addition to its role in propelling Dawn to Vesta and Ceres,
in some cases the ion propulsion subsystem (instead of the reaction
wheels or the reaction control subsystem) is used by attitude control to
help control the direction the spacecraft points. While this subsystem
obviously is important, some engineers would consider the next one to be
the most important on the spacecraft.

The thermal control subsystem keeps all of Dawn's subsystems operating
within their required temperature ranges as the craft travels from Earth
past Mars to Vesta and then continues on to Ceres, reaching 3 times
Earth's distance from the Sun. The temperatures of delicate electronics,
precisely aligned structural elements, sensitive mechanical devices and
materials, lubricants, adhesives, hydrazine, xenon, and more all must be
controlled. This subsystem must ensure that units stay cool even when
they experience direct exposure to the searing Sun while being warmed
still more by their own electrical activity and stay warm even when they
face the paralyzing cold of darkest space. Louvers on some parts of the
spacecraft open or close in response to temperature to let heat radiate
away or be trapped on the spacecraft as necessary. Some of the
spacecraft panels are embedded with tubes of ammonia to help distribute
the heat more uniformly, carrying excess heat from electrically powered
devices to others that are powered off or otherwise in need of
additional heat. The subsystem also includes more than 140 heaters and
is one of the largest consumers of electrical power on the spacecraft.
While this subsystem obviously is important, some engineers would
consider the ion propulsion subsystem to be the most important on the
spacecraft.

The telecommunications subsystem allows Dawn to exchange information
with Earth, even at enormous distances. The spacecraft's main antenna is
1.52 meter (5 feet) in diameter, and 3 smaller antennas allow
communications when it is not possible or not convenient to point the
large dish at Earth. Dawn will communicate with mission controllers
through the 34-meter (112-foot) or 70-meter (230-foot) antennas of
NASA's Deep Space Network (DSN) in California, Spain, and eastern
Australia. While Dawn is returning scientific data from Ceres at maximum
range, the 100-watt radio signal it transmits, after traversing the vast
distance to Earth, will be less than one tenth of one millionth of one
billionth of a watt when it is received by a 34-m antenna. If this
energy were collected for the age of the universe, it would be enough to
illuminate a refrigerator light bulb for 1 second, yet it is sufficient
to carry all the images and other rich scientific data to Earth. Dawn's
receiver, always alert for faint whispers from home, can make sense of a
signal weaker than one billionth of one billionth of a watt. Some
engineers would consider this to be -- well, you get the message.

After this brief overview of the subsystems, it would be easy to lose
sight of what some engineers would consider to be more important than
any subsystem: the system. All subsystems have to work together for the
spacecraft work. Besides the instruments, some essential parts of that
spacecraft are missed in this description of active subsystems, such as
the structure upon which everything is built. In addition, to connect
the many elements of the subsystems to each other, Dawn includes 9000
wires with a total length of about 25 kilometers (15 miles). The cables
and their connectors account for more than 83 kilograms (183 pounds) of
the mass that will travel to Vesta and Ceres. When fully assembled and
loaded with its propellants, Dawn will be somewhat more than 1200
kilograms (2650 pounds).

Some engineers would consider there to be a larger system, still more
important than the entirety of the spacecraft, that is needed to make
Dawn a success. Indeed, the full system is not only what flies in space;
the complete Dawn system has many elements that remain on Earth,
including networks of computers, extensive software, antennas,
transmitters, receivers, and a team of dedicated and inquisitive people
who recognize their good fortune to participate in this grand adventure.

Now strange as it may seem, there seems to be some evidence that 2 of
our readers, despite being loyal, have not yet submitted their names to
be carried on the spacecraft. The end of the last log described our
plans to include the names of all members of what really is the largest
and most important system: the people whose spirits are carried aloft by
humankind's efforts to know the cosmos. Don't be the last one to add
your name to the spacecraft at:
http://www.dawn-mission.org/DawnCommunity/Sendname2asteroid/nameEntry.asp.
Received on Wed 20 Sep 2006 01:24:05 PM PDT