[meteorite-list] Dawn Journal - November 26, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 1 Dec 2008 16:35:53 -0800 (PST)
Message-ID: <200812020035.QAA06094_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_11_26_08.asp

Dawn Journal
Dr. Marc Rayman
November 26, 2008

Dear Indawnviduals,
 
The Dawn spacecraft is healthy and on course for its flyby of Mars
early next year. The planet's gravity will help boost the probe on
its way to rendezvous with Vesta. While the spacecraft has its
sights set on the asteroid belt (via Mars), its path is now
bringing it closer to Earth. Meanwhile, from Earth's perspective,
Dawn appears to be approaching a blindingly close encounter with
the Sun. With so much happening in the solar system, all readers,
whether local or not, are invited to turn their attention here.
 
In the last log, we saw that Dawn was
nearing the end of an extended period of thrusting with is ion
propulsion system that began on December 17, 2007. When it left
Earth on September 27, 2007, the Delta II rocket deposited the
spacecraft into a carefully chosen orbit around the Sun. By
October 31, 2008, the spacecraft had completed the thrusting it
needed to change that orbit so it would encounter Mars at just the
right time, location, and angle to sling it on its way to Vesta.
During this interplanetary cruise phase, Dawn thrust for 270 days,
or 85% of the time. Expending less than 72 kilograms (158 pounds)
of xenon propellant, the spacecraft changed its speed by about 1.81
kilometers per second (4050 miles per hour).
 
Although controlling an interplanetary probe across hundreds of
millions of kilometers (or miles) of deep space and guiding it
accurately enough to reach its remote destination seems as if it
should be a very simple task, readers may be surprised to know
that it is not. Let's consider just one aspect of the problem.
 
Suppose you want to shoot an arrow at a target. Unlike typical
archers, you are so far from the target that you can only barely
see it. In that case, aiming for the bull's-eye is essentially out
of the question. Adding to the problem may be a variable breeze
that could nudge the arrow off course. Shooting sufficiently
accurately to get the arrow even to the vicinity of the target
would be challenging enough; hitting the precise point you want on
the target is just too difficult.
 
For readers who are principally interested in archery, this
concludes our in-depth analysis of the sport.
 
Now let's consider how to change the situation to make it more
similar to an interplanetary mission. If the arrow had a tiny
radio locator mounted on it, you would be able monitor its
progress as it flew closer to the target. This would be like
watching it on a radar screen. You might see your arrow miss the
target entirely or, if you had made a particularly good shot, hit
somewhere on it. Now if you could occasionally send a signal to
the arrow, perhaps to change the angles of the feathers, you might
not be able to alter its course drastically, but you could change
it a little. So if your initial shot had been good enough, you
could guide the arrow to the desired destination. (To buy your
radio controlled archery set, visit the Dawn gift shop on your
planet. The set may be found between the display case with xenon
ion beam jewelry and the shelves and shelves and shelves and
shelves of really cool new Dawn Journal reader action figures --
be sure to buy the one that looks just like you!)
 
Shooting the arrow is akin to launching a spacecraft, and its
flight to the target represents the interplanetary journey,
although operating a spacecraft involves far greater precision
(and fun!). Our knowledge of where the spacecraft is and where it
is heading is amazingly, fantastically, incredibly accurate, but
it is not perfect. This point is essential. Keeping most
spacecraft on course is a matter of frequently recalculating the
position, speed, and direction of travel and then occasionally
fine-tuning the trajectory through burns of the propulsion system.
 
Dawn's near-constant use of its advanced ion propulsion system for
most of 2008 changes the story, but only a little. The thrust plan
was calculated before launch and then updated once our arrow was
free of the bow. Throughout the interplanetary cruise phase, a new
thrust plan was transmitted to the spacecraft about every 5 weeks,
each time with slight updates to account for the latest
calculations of Dawn's orbit around the Sun. With this method, the
small adjustments to the trajectory have been incorporated into
the large, preplanned changes.
 
The mission control team requires about 5 weeks to design,
develop, check, double-check, transmit, and activate a 5-week set
of commands. By the time the spacecraft is executing the final
part of those instructions, it is following a flight plan that is
based on information from 10 weeks earlier. During most of the
mission, when there are months or even years of thrusting ahead of
it, subsequent opportunities to adjust the trajectory are
plentiful. In contrast, for the last period of preplanned
thrusting before Mars, controllers modified their normal process
for formulating the commands, making a fast update for the final
few days of thrusting. By including
the latest navigational data in the computations for the direction
and duration of the concluding segment of powered flight, the
mission control team put Dawn on a more accurate course for Mars
than it otherwise would have been.
 
Even with this strategy, navigators recognized long ago that
subsequent adjustments would be required. The plan for approaching
Mars has always included windows for trajectory correction
maneuvers (which engineers are physiologically incapable of
calling anything other than TCMs). Dawn's first TCM occurred on
November 20.
 
As navigators refined their trajectory calculations after
thrusting finished on October 31, they determined that the
spacecraft was quite close to the aim point they wanted, but still
not exactly on target. In fact, rather than being on a course to
sail a few hundred kilometers above Mars, the probe's path would
have taken it to the surface of the planet. Despite the power of
the ion propulsion system, Dawn does not have the capability to
bore through the rocky planet and continue on its way to Vesta.
 
Such a situation is not surprising. Suppose in the archery, the
bull's-eye were 30 centimeters (1 foot) in diameter, but we
preferred to hit a point 2.2 centimeters (7/8 inch) outside the
bull's-eye, near the 11:00 position (corresponding to where we
want Dawn to fly past Mars). As our arrow approached the target,
it might turn out that it was going to miss the target entirely,
it might be headed for some other point on the target, and it just
might be that it was headed for the bull's-eye itself. Dawn's case
was this last one, so TCM1 put it on track for the destination we
desired.
 
Amazing sports analogies for the fantastic accuracy of
interplanetary navigation usually fail to account for TCMs, as
most arrows, balls, and other projectiles do not include active
control after they are on their way. Your correspondent has
presented his own simile
<http://nmp.jpl.nasa.gov/ds1/arch/mrlogQ.html> for the astonishing
accuracy with which a spacecraft can reach a faraway destination,
but most such analogies neglect TCMs, without which deep-space
missions could not be accomplished. (Note that the accuracy is
impressive with or without TCMs. We shall extend our archery
example in a future log, making it more quantitative. It will be
important, however, to keep in mind that the ion propulsion system
provides so much maneuvering flexibility that Dawn does not need
to achieve the degree of accuracy in its gravity assist that a
mission using conventional chemical propulsion might.)
 
For reasons we will not divulge, Dawn's first TCM has been
designated TCM1. On November 20, just as it had for all of its
previous thrusting, the spacecraft pointed a thruster (TCM1 used
thruster #1) in the required direction and resumed emitting the
familiar blue-green beam of xenon ions to alter course. While
typical thrusting during the mission has lasted for almost 7 days
at a time (followed by a hiatus of 7 to 8 hours), in this case
only a short burn was necessary. Propelling itself from about 4:31
pm to 6:42 pm PST was just enough to fine-tune its course and
change its speed by a bit more than 60 centimeters per second (1.3
miles per hour). This adjustment was modest indeed, as at that
time Dawn was traveling around the Sun at more than 22.5
kilometers per second (50,400 miles per hour). Dawn and Mars,
following their separate orbits that will (almost!) intersect on
February 17, 2009, were moving relative to each other at 3.17
kilometers per second (7100 miles per hour).
 
Dawn's second TCM window (inexplicably named TCM2) is in January.
Traveling two-thirds of the way from here to Mars, the
navigational accuracy then will be still better, with smaller
deviations from the planned target point being detectable, so
another refinement in the trajectory then is likely. In the
meantime, Dawn will follow its orbital path with its ion thrusters
idle.
 
As Dawn travels through space on its own, its path has been
essentially independent of Earth's. We saw in a previous log
that the weaker grasp exerted by the
Sun at Dawn's greater distance means that it travels more slowly
around the solar system. While Earth has completed more than 1
full revolution (each revolution requiring 1 year) since launch,
Dawn has not yet rounded the Sun once. After receding from the Sun
until early August, the spacecraft began falling back, albeit only
temporarily.
 
The probe attained its maximum distance from Earth on November 10.
(For anyone who was on Earth on that date and plans to use this
information in an alibi, it may be helpful to know that the
greatest range was reached at about 3:07 am PST.) The spacecraft
was more than 384 million kilometers (239 million miles) from its
one-time home. Although it will make substantial progress on its
journey in the meantime, Dawn's distance to Earth will continue to
decrease until January 2010, when it will be less than one-third
of what it is today. In the summer of that year, however, as Earth
maintains its repetitive annual orbital motion and the explorer
climbs away from the Sun, it will surpass this month's distance to
Earth. (Readers are encouraged to memorize the contents of this
log for reference in 2010 in case we fail to include a link to
this paragraph.)
 
The complex choreography of the solar system's grand orbital dance
rarely calls for a circular orbit; rather, the dancers follow
ellipses (ovals in which the ends are of equal size) around the
Sun. Thanks to the details of the shapes of their orbits, the
greatest separation between Earth and Dawn did not occur when they
were precisely on opposite sides of the Sun, although the
alignment was close to that.
 
On December 12, their dance steps will take them to points almost
exactly on opposite sides of the Sun. For observers on Earth, this
is known as solar conjunction, because the spacecraft and the Sun
will appear to be in the same location. (Similarly, from Dawn's
point of view, Earth and the Sun will be almost coincident.) In
reality, of course, Dawn will be much farther away than Earth's
star. It will be 147 million kilometers (91.5 million miles) from
Earth to the Sun but 379 million kilometers (236 million miles)
from the planet to its cosmic envoy.
 
Its apparent proximity to the Sun presents a helpful opportunity
for terrestrial readers to locate Dawn in the sky. On December 9 -
15, the spacecraft will be less than 1 degree from the Sun,
progressing from east to west and passing just 1/3 degree south of
that brilliant celestial landmark on December 12. (As Dawn does
not orbit in the same plane as Earth, it will not pass directly
behind the Sun.) The Sun itself is 1/2 degree across, so this is
close indeed; the spacecraft will sneak in to less than 1 solar
diameter from the disk. To demonstrate how small the separation
is, if you blocked the Sun with your thumb at arm's length during
this week around conjunction (and you are exhorted to do so), you
also would cover Dawn.
 
For those interested observers who lack the requisite superhuman
visual acuity to discern the remote spacecraft amidst the dazzling
light of the Sun, conjunction still may provide a convenient
occasion to reflect upon this most recent of humankind's missions
far into the solar system. This small probe is the product of
creatures fortunate enough to be able to combine their powerful
curiosity about the workings of the cosmos with their impressive
abilities to explore, investigate, and ultimately understand.
While its builders remain in the vicinity of the planet upon which
they evolved, their robotic ambassador now is passing on the far
side of the extraordinarily distant Sun. This is the same Sun that
has been the unchallenged master of our solar system for 4.5
billion years. This is the same Sun that has shone down on Earth
throughout that time and has been the ultimate source of so much
of the heat, light, and other energy upon which the planet's
inhabitants have been so dependent. This is the same Sun that has
so influenced human expression in art, literature, and religion
for uncounted millennia. This is the same Sun that has motivated
scientific studies for centuries. This is the same Sun that acts
as our signpost in the Milky Way galaxy. And humans have a
spacecraft on the far side of it. We may be humbled by our own
insignificance in the universe, yet we still undertake the most
valiant adventures in our attempts to comprehend its majesty.
 
Solar conjunction means even more to Dawn mission controllers than
the opportunity to meditate upon what magnificent feats our
species can achieve. As Earth, the Sun, and the spacecraft come
closer into alignment, radio signals that go back and forth must
pass near the Sun. The solar environment is fierce indeed, and it
causes interference in those radio waves. While some signals will
get through, communications will be less reliable. Therefore,
controllers plan to send no messages to the spacecraft from
December 5 through December 18; all instructions needed during
that time will be stored onboard beforehand. Deep Space Network
antennas, pointing near the Sun, will listen through the roaring
noise for the faint whisper of the spacecraft, but the team will
consider any signals to be a bonus.
 
There is plenty of other work to do while waiting to resume
communications after conjunction. In addition to preparing for the
visit to Mars, engineers will continue to interpret the results of
election day. On November 4, the Dawn team voted unanimously for
more power. They commanded the spacecraft to execute a set of
steps to yield data that will reveal the full potential of the
enormous solar arrays to generate electrical power. The method was
tested first on July 21, and then refined for a test on September 22.
For this month's measurement, the commands were identical to those
used for the second test with one exception that had been planned
from the beginning: the solar arrays were rotated to point 60
degrees away from the Sun instead of 45 degrees. The solar arrays
are so powerful that when they are pointed directly at the Sun,
the spacecraft could not draw enough power to measure their full
capability.
 
The data collected show the electrical behavior of the arrays as
the ion propulsion system was commanded through its start-up,
drawing more and more power. Unlike the two tests, this
calibration was designed so that with the arrays pointed so far
from the Sun, they would not be able to provide as much power as
was requested. Engineers wanted to find the point at which the
arrays would no longer be able to satisfy the demands. They were
not disappointed; power climbed up and up until no more was
available. The prospect of having a spacecraft not be able to meet
its own power demands may seem risky, but the procedure was
carefully designed, analyzed, and simulated, and it executed
perfectly. When the ion propulsion system asked for more power
than the arrays could deliver, in the language of the trade, the
solar arrays "collapsed." Now to some (including even some
engineers unfamiliar with the terminology), this suggests
something not entirely desirable, such as 2 bent and twisted
wings, each with 5 warped panels, and 11,480 shattered solar
cells, the fragments sparkling in the sunlight as they tumbled and
floated away from the powerless probe. In this case though,
"collapse" is an electrical, not a mechanical, phenomenon and
hence would be somewhat less visually spectacular and quite
reversible -- a key attribute for a mission with well over 6 years
of space exploration ahead of it. Once all the data are analyzed,
controllers will have a better prediction for how much power the
arrays will be able to generate for the rest of the voyage.
 
Dawn is 20 million kilometers (12 million miles) from Mars. It is
383 million kilometers (238 million miles) from Earth, or 950
times as far as the moon and 2.59 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light,
take 43 minutes to make the round trip.
 
Received on Mon 01 Dec 2008 07:35:53 PM PST