[meteorite-list] Dawn Journal - July 5, 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 6 Jul 2007 09:12:48 -0700 (PDT)
Message-ID: <200707061612.JAA02313_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_7_05_07.asp

Dawn Journal
Dr. Marc Rayman
July 5, 2007

Dear Countdawns,

The countdown is underway for Dawn's liftoff on July 8 at 4:04:49 pm EDT.

Launch had been planned for July 7, but unfavorable weather at Cape
Canaveral led to the postponement today of the planned liftoff from July
7. The open and close times of Dawn's daily launch windows for the first
few days of its launch period are in a previous log and are transmitted
daily in the Telepath Report.

In the last log, we followed the plan for
what the Delta launch vehicle will do as it delivers its passenger, the
Dawn spacecraft, to space. During the 28.5 minutes of their shared
flight, the rocket is in control. The timeline is the same for a launch
on July 8 as on July 7. For later launches, the coast in Earth orbit
will be a little longer, but events before and after will not change.

Following separation, Dawn has three primary objectives: 1) get sunlight
on the arrays, 2) establish contact with mission control at JPL, and 3)
revel in the beginning of a remarkable mission of exploration. Most of
what it does to accomplish the first two steps will be standard
procedure for the spacecraft throughout the mission when it encounters a
problem and needs to enter "safe mode," in which it will await
instructions from Earth. Of course, separation from the launch vehicle
is anything but a problem. Engineers have taken advantage of their
extensive work developing the directions Dawn will follow to reach its
safe configuration by having it execute nearly the same program as soon
as it is flying independently in space. Future logs are sure to have
reason to discuss safe mode again.

As few of Dawn's components as possible are turned on during launch,
because with its large solar wings folded against its side (and a
segment of the flight in Earth's shadow), power is provided by a large
battery on the spacecraft. Conserving energy (a responsibility familiar
to readers on Earth) is vitally important. While on the rocket, Dawn's
computer and a few other devices are operating, heaters are activated as
needed, and some data are recorded, but mostly the probe simply waits
for the signal that indicates it and the third stage have parted ways.

Because the craft will be returning a tremendous bounty of rich
scientific information from distant Vesta and Ceres, its radio system is
powerful. Therefore, the transmitter remains off until the solar arrays
can provide essentially endless power.

When the third stage releases Dawn, it will leave the spacecraft
spinning slowly, with xenon propellant spinning inside in the opposite
direction. In addition, the springs that push
the spent stage and the eager spacecraft apart are likely to impart a
slightly unbalanced push, so Dawn is expected to be turning slowly
around all axes. When the computer determines that Dawn has separated,
it waits 8 minutes 20 seconds for the friction between the xenon and the
spacecraft to lower the spacecraft's spin rate enough that it can be
stabilized by the attitude control system. Known
to its friends as ACS, this system is responsible for controlling the
spacecraft's orientation.

After waiting the prescribed time, software directs ACS to begin using
its sensors to determine the direction and rate of the spin. Then ACS
will command the small rocket thrusters of the reaction control system
to fire, gradually stopping the unwanted
rotations. The process of bringing the attitude under control can take
as little as 1 minute or as long as 15 minutes, depending upon the
imbalance in the separation forces and details of the xenon behavior.

Once the spin is fully controlled, it is safe for Dawn to deploy its
large solar arrays. Each wing is divided into 5 panels, which are
stacked against each other and secured to the spacecraft by cables
during launch. To release the wings, small heaters press against the
cables, causing them to weaken and break. When they are no longer
restrained by the cables, the wings unfold under the gentle urging of
springs. With its wings folded, the spacecraft is 1.84 meters (6 feet 1
inch) wide. When they open, the two wings span 19.74 meters (64 feet 9
inches) tip to tip. The software provides 12 minutes 47 seconds to allow
the cables to release and the arrays to extend to their full reach.

Although ACS remains in control throughout the solar array deployment,
after the computer has allowed for the programmed time to elapse, it
requests ACS to perform another stabilization, now with the new, much
larger configuration of the spacecraft. ACS may report back that this
is complete in as little as 1 minute or as long as 15 minutes.

Just as when a teneral dragonfly spreads wide its new wings for the
first time, these intricately patterned marvels must be pointed at the
Sun. Up to this time, Dawn has paid attention only to itself, without
regard to the external universe. (Of course, it continues coasting away
from Earth with the energy given to it by
its recent companion, the Delta rocket.) Supported on small extensions
from each corner of the boxy body of the spacecraft are solar cells,
just like those on the arrays. But these cells are not intended to meet
Dawn's electrical needs; instead, ACS uses them to find the location of
the Sun. This is not very different from using your eyes to find the
Sun, a particularly appropriate analogy both for dragonflies and for
those readers who have eyes that allow them to see in all directions
simultaneously. Once it has established where the Sun is, it rotates
with its thrusters to point the arrays in that direction. Depending upon
the orientation the probe happens to be in prior to this activity, it
can take as little as 1 minute and as long as 18 minutes to locate the
Sun and complete the turn.

As soon as light from the solar system's master, the star at the center,
reaches the arrays, the battery begins to recharge, and all of Dawn's
electrical needs for the rest of its 8-year mission will be satisfied by
the energy the solar cells receive from the Sun.

The computer waits another 4 minutes after the arrays are fully
illuminated by the Sun to make sure all systems remain stable, and then
it activates its power-hungry radio transmitter. It should take about 4
minutes 30 seconds for the transmitter to warm up and begin sending
radio signals, reporting on the status of all systems.

The spacecraft is well prepared to resolve a wide range of problems as
it progresses through the list of tasks to complete between separating
from the Delta and powering on its radio. If it has not been delayed by
correcting any anomalies, the entire sequence could take as little as 32
minutes 37 seconds and as long as 77 minutes 37 seconds; otherwise, this
could stretch to over 3 hours. In mission control at JPL, the operations
team, taking a cue from one of the virtues
Dawn will display as it traverses the solar system, will remain patient.
Nevertheless, everyone will look forward to verifying that it is
starting its long journey in good health.

But Dawn's radio signals may not reach Earth quite yet. Without
information on where that planet is, the spacecraft cannot know where to
point its antenna. (For most of the mission, Dawn will know where it is
in relation to Earth and other solar system bodies, but at this early
stage, having just begun its flight, such information will not yet be
available onboard.)

After it has finished directing its solar arrays at the Sun, the
spacecraft begins a roll around the line between it and the Sun, turning
once per hour, perhaps appearing like an exotic and lazy windmill. Given
the direction of its departure from home, the Sun and Earth will be at
about right angles from Dawn's perspective. So as it makes its slow
spin, it uses an antenna pointed at the same right angle to the solar
arrays. The antenna sweeps out a broad beam, like a wide searchlight
sending its signal out to anyone who happens to see it.

Antennas at the Deep Space Network complex near Canberra, Australia and
at the European Space Agency's facility in Perth, Australia will be
ready to detect Dawn's transmissions and pass the data on to JPL. These
stations should be able to receive signals during about half of each
rotation of the spacecraft, or about 30 minutes every hour.

It is impossible to predict where Dawn's antenna will be pointed when it
begins transmitting, so it might be aimed at Earth immediately, or it
could take as long as 30 minutes until the spacecraft's rotation brings
it around to start the half hour of terrestrial coverage.

With all these steps, the time from liftoff to the receipt of the first
radio signal may be as little as about 1 hour 1 minute or as long as 2
hours 16 minutes even if Dawn encounters no surprises along the way, and
more than 3 hours 30 minutes if it does. If you are entering your
planet's friendly betting pool on when Dawn's data first will light up
the computers in mission control, you are advised to consider that the
likelihood that all circumstances will conspire to yield the shortest
possible time is extraordinarily low. That time is more a theoretical
minimum than a practical guide, and although mission control will be
ready, no one there will be expecting signals that early.

Once controllers see the data, they will begin evaluating the
spacecraft's condition. Over the course of the subsequent few days, they
also will review the data it stored during launch and begin configuring
it for further operations.

Meanwhile, Dawn will continue racing away from Earth. In less than 2
hours 15 minutes from liftoff, it will be more than 35,800 kilometers
(22,200 miles) high, passing the ring of satellites in geosynchronous
orbit, and thus will be more remote than the great majority of
spacecraft launched in Earth's half century of probing and utilizing
space. It will go beyond the most distant point in the moon's elliptical
orbit less than 29 hours after launch, traveling farther from home than
humans have ever ventured. Yet that is but the very beginning of Dawn's
journey.

Distant though it will be, it may be possible for terrestrial observers
with capable telescopes to glimpse the probe in the first week or two of
its travels. (Other spacecraft have been observed not long after they
left Earth. See http://www.jpl.nasa.gov/releases/98/ds1palomar.html for
what this former member of the Deep Space 1 team considers to be the
best image ever taken of that spacecraft.) It would be very faint,
perhaps no more than a speck amidst a sea of stars in the constellation
Cetus near right ascension 0 hours 52 minutes and declination -18?.
(These approximate coordinates will change by a few degrees if Dawn's
launch does not occur on July 8 at the opening of its window. For a
launch at a later time that day, the position will move to slightly
higher right ascension. The dependence upon the day in the launch period
is more complex, but in general, if the launch takes place on a later
day, the location will shift to slightly higher right ascension and
higher declination.) For anyone interested in trying to observe the
spacecraft, please visit http://ssd.jpl.nasa.gov/horizons.cgi and change
the target body to (no surprise here) "Dawn" to find its exact location.

If all goes according to plan, this will be the last log written when
Dawn is bound to Earth. We hope readers throughout the cosmos join in
wishing the explorer well as it gets underway for a journey that offers
new knowledge, excitement, the rewards - and the risks - of facing the
unknown, and the spirit of adventure that compels humankind to undertake
such bold quests.
Received on Fri 06 Jul 2007 12:12:48 PM PDT