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Posted by baalke on September 21, 2007, 4:34 pm
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http://dawn.jpl.nasa.gov/mission/journal_9_21_07.asp
Dawn Journal
Dr. Marc Rayman
September 21, 2007
Dear Countdawns,
The countdown is underway for Dawn's liftoff on September 26 at
7:25:00
am EDT.
This is the second time our hero has been within a few days of launch,
and with a full 20-day launch period still ahead of it, confidence is
high the mission will get underway soon. Now the Dawn project is
ready
with a new flight profile to allow the probe to leave Earth months
later than planned and yet still keep its interplanetary appointments
on schedule.
Because this new flight plan begins with a different launch, we
present
here an update to the July 5 log, accounting for
the changes. Dawn's intention was to launch in June or July, and the
postponement was because of circumstances beyond our control;
nevertheless, we understand the difficulties this can cause our
readers.
Therefore, for those readers who have the July 5 log tattooed on
either
themselves or a relative, we have arranged with our favorite fine
tattoo
and taxidermy emporium for a discount on this log. (Certain
restrictions
may apply; this offer void in galaxies with less than the cosmic
abundance of deuterium or tattoo ink.) If weather or other minor
glitches
delay the launch by a few days, we will not publish another update.
In the last log, we began on the launch
pad with the entire Delta II 7925H-9.5 rocket, including its
passenger.
Together, they are 285,581 kilograms (629,592 pounds), and we followed
the plan for the delivery of the 1218-kilogram (2685-pound) Dawn to
space. For a launch prior to October 10 (a date chosen based on the
sophisticated mathematics of interplanetary trajectory design, not it
being your correspondent's birthday), the rocket and spacecraft will
spend about 62 minutes flying together. For launches on October 10 or
11, one phase of the launch, the coast in Earth orbit,
will be 2 minutes 40 seconds longer.
Should launch need to occur on October 12 - 15, the coast will be 4
minutes 16 seconds longer than for launches in the beginning of the
launch period. In all cases however, the relative timing of other
events
during the flight of the Delta rocket will remain as described in the
previous log.
During their shared flight, the rocket is in control. Following
separation from its conveyance to space, Dawn has three primary
objectives: 1) get sunlight on its solar 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 also 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,
detaching 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.
The Delta does not provide electrical power to the spacecraft (even
though it rides in the first-class section), so Dawn carries a large
battery. While on the rocket, as few of the probe's components as
possible are turned on. Its computer and a few other devices are
operating, heaters are activated as needed, and some data are
recorded,
but mostly the craft simply waits for the signal that indicates it and
the third stage have parted ways. Conserving energy (a responsibility
familiar to readers on Earth) is vitally important.
Now one might be tempted to conclude that with the longer time from
liftoff to separation for an autumn launch than a summer launch,
Dawn's
power reserves will be more critical. Readers are urged to avoid this
temptation with their utmost resolve! When the two solar arrays are
folded, the outermost panel on each side is oriented so the solar
cells
point out. The arrays are so powerful
that even with only 1 of the 10 panels exposed to the Sun, enough
electricity is generated to satisfy all of Dawn's needs (except
thrusting with the ion propulsion system) when at Earth's distance
from
that brilliant orb. While the battery will have been partially drained
during the last few minutes on the launch pad and during ascent, the
intermittent exposure to the Sun in the course of the "barbecue roll"
during the quiet coast in Earth orbit
will provide sufficient power for all systems that are running and
still
have enough extra to recharge the battery. By the time the barbecue
ends
and the second stage begins preparing for its second burn, Dawn's
battery should be fully charged.
Because the craft will be returning a tremendous bounty of rich
scientific information from distant Vesta and Ceres, its radio system
is
powerful. It does not have a mode in which it can transmit at low
power,
so 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. After 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
commands 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 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 a short
extension from each corner of the boxy body of the spacecraft is a
pair
of 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 again, 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 well 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 are 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 (DSN) complex in Goldstone,
California will be ready to detect Dawn's transmissions and pass the
data on to JPL. Had the launch occurred in the summer, Dawn would have
begun transmitting its signals in view of DSN and European Space
Agency
antennas in Australia. Now, following its longer travel time from
Florida, the coast in orbit will carry it farther east, so Goldstone
has
the privilege of being the first to communicate with the spacecraft.
The DSN station 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 35 minutes or as long as
2
hours 50 minutes even if Dawn encounters no surprises along the way,
and
more than 4 hours 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. One of them will try to find the time to
write another of these logs as well.
Meanwhile, Dawn will continue racing away from Earth. In less than 2
hours 45 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 leaving the launch pad, as it travels
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 imaged 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
portrait ever made of that craft.) It would be very faint, perhaps no
more than a speck amidst a sea of distant stars between the
constellations Auriga and Gemini near right ascension 6 hours 20
minutes
and declination +28.5=B0. [Note to self: before this is posted, remember
to insert a wonderfully clever remark here that connects Dawn to a
charioteer and the twins, the figures represented by these
constellations.] These approximate coordinates will change if Dawn's
launch does not occur on September 26 at the opening of the 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 a little more complex. Throughout
the launch period, the farthest from this location would occur for a
liftoff at the end of the launch window on October 15. That would
shift
the coordinates to approximately 7 hours 28 minutes and +26=B0, within
Gemini. For anyone interested in trying to observe the spacecraft,
please visit JPL's HORIZONS system
<http://ssd.jpl.nasa.gov/horizons.cgi> and change the target body to
(no
surprise here) "Dawn" to find its exact location.
Even before the navigation team gets a good fix on Dawn after launch
and
enters the trajectory data into HORIZONS, observers in Hawaii may get
a
view of Dawn's early light. With a launch on September 26 at the
opening
of the launch window, the spacecraft will exit the shadow of Earth 1
hour 19 minutes after liftoff (2:44 am Hawaii-Aleutian Standard Time,
or
HST). At that time, the spacecraft will not yet have deployed its
solar
arrays, so it may not be very bright, but its relatively small size at
that time should be somewhat compensated for by its relative proximity
to Earth. It will be about 68=B0 above the horizon when it comes into
the
sunlight, and will pass directly overhead 13 minutes later.
Dawnophiles in Hawaii, Alaska, and the Pleiades may be treated to a
particularly attractive alignment shortly after that. As viewed from
the
first two of those locations, Dawn will appear to pass less than 1.5=B0
north of the center of that familiar star cluster at about 3:14 am HST
when it is less than 18,000 kilometers (about 11,000 miles) from the
surface. (Note: "it" refers to Dawn. The Pleiades, in contrast, will
be
more than 430 light years from Earth, or more than 200 billion times
farther than Dawn.) By then it likely will have opened its solar
arrays,
presenting a much larger target for the Sun to illuminate.
Observers are advised however that, depending upon the spacecraft's
progress in the many steps described above, the arrays may already be
pointed straight at the Sun by the time it transits the Pleiades, so
the
reflection would not be directed toward Earth. The Dawn project is
confident no one's eyes will be damaged from direct exposure to this
view; indeed, the spacecraft may be quite dim. It is possible however
that before the spacecraft has completed aiming its panels at the Sun,
terrestrial spectators could see a brief bright reflection or "flare,"
a
phenomenon familiar to amateur satellite observers. We will not know
until we receive reports from witnesses.
If liftoff is delayed to later in the launch window, the views
described
here will occur later by less than the change in launch time. More
details on where to look are posted here
<http://dawn.jpl.nasa.gov/feature_stories/charts.asp>, and the Dawn
navigation and outreach teams will be standing by to update the
information as soon as possible after liftoff. We will do our best to
give some fortunate observers the opportunity to see Dawn as it
recedes
into the depths of space. If you obtain any images, we will be
interested in seeing them and would appreciate your sending them to
the
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.
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