[meteorite-list] Dawn Journal - December 30, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 4 Jan 2010 17:58:07 -0800 (PST)
Message-ID: <201001050158.o051w7A4011346_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_30_09.asp

Dawn Journal
Dr. Marc Rayman
December 30, 2009

Dear Dawnters and Sons,
 
The Dawn mission continues to go smoothly, as Earth's distant envoy
carries out its interplanetary journey. Although the craft still devotes
most of its time to the slow but efficient reshaping of its orbit around
the Sun to match Vesta's, controllers gave it some extra assignments
since the last log to ensure its systems remain healthy and to prepare
for its studies of Vesta.
 
Dawn usually interrupts ion thrusting once a week for about 8 hours to
point its main antenna to Earth. On November 30, however, instead of
resuming thrusting, it dutifully followed different instructions that
were stored onboard.
 
The spacecraft began the 5 days of special activities by activating the
gamma ray and neutron detector (GRaND).
Despite its name, GRaND is not at all pretentious, but its capabilities
are quite impressive. It will reveal the atomic constituents of the
surfaces of Vesta and Ceres. GRaND's measurements of space radiation
this month showed it to be in excellent health. After a week of smooth
operation, it was deactivated on December 7.
 
The visible and infrared mapping spectrometer (VIR) and the primary
science camera also were turned on for the first time in
more than half a year. As these sensors yield complementary data,
controllers want to refine earlier measurements of exactly how their
views overlap. This will allow scientists to correlate observations from
the instruments in order to glean as much as possible about the nature
of the protoplanets the craft will orbit. Dawn rotated to point at a
star and then observed it simultaneously with VIR and the camera. By
measuring precisely where the star registers in each device, their
relative alignments can be pinned down. Upon completing the sequence of
commands to acquire the desired data, the spacecraft turned to point its
main antenna to Earth again and began transmitting the results during
the next scheduled session with the Deep Space Network a few hours later.
 
The VIR team quickly discovered that a subtle incompatibility between
certain instructions in the program for recording the signals from the
star caused its shutter to remain closed. (VIR also has a reusable
protective cover, but that operated as intended.) The unit continued to
function and stayed healthy, but it did not perform the planned
observations. The science camera imaged the target, but the purpose was
to compare where the star appeared in the 2 instruments. The VIR
commands are easily corrected, and the calibration will be executed
again early next year.
 
Earlier this year, engineers developed new software for the science
camera to improve its efficiency in mapping the distant worlds Vesta and
Ceres. The software was updated once before in space, and the process
followed this week was the same. As last year, loading software into the
primary and the backup cameras was performed as entirely separate activities;
each camera was off while the other was being upgraded. This was the only
major work this week that was not accomplished with commands that had
previously been stored on the spacecraft. After the new software was
installed, each camera was directed to carry out a set of tests, and the
results confirmed that both were operating correctly.
 
Among the other tasks this week was an annual evaluation of the backup
star tracker, a device that recognizes star patterns so the spacecraft
can calculate its orientation. To verify
that the tracker remained healthy, the unit was powered on and operated.
It correctly took pictures, identified the stars, and then determined
the direction it was pointed. The tests verified that the unit remains
in good condition and ready to be called into service in the unlikely
event a problem with the primary tracker occurs.
 
On December 4, after completing all of its scheduled activities for the
week, Dawn turned once again to point ion thruster #1 in the direction
needed for propelling itself to Vesta, and resumed emitting high-speed
xenon ions. It has continued since then with its familiar schedule of
quiet cruise.
 
As the effect of the thrust continues to build up, tomorrow Dawn will
pass another milestone. The thrusting since the beginning of the mission
will have achieved the equivalent of accelerating the spacecraft by 2.00
miles per second (3.22 kilometers per second, or 7200 miles per hour).
This is well in excess of what most spacecraft accomplish with their
propulsion systems but is less than 1/3 of the planned maneuvering for
the mission. To achieve this extraordinary velocity, Dawn has expended
less than 126 kg (278 pounds) of xenon propellant during 474 days of
powered flight. While the day-to-day change is small (as we will discuss
in greater detail in February), with 24 hours of thrusting yielding just
7.2 meters per second (16 miles per hour), the benefit of its acclaimed
patience is becoming evident.
 
As we have discussed several times (see, for example, this previous log,
Dawn's actual speed has not changed by the
values just presented. In the complex orbital dance it performs,
partnered principally by the Sun but with others joining in as well
(Mars being the most significant this year), the
more it thrusts and climbs away from the Sun, the slower it travels.
Nevertheless, the equivalent change in speed (that is, the change that
would be achieved in the absence of the complications from being in
orbit) is a handy measure of the effect of any spacecraft's maneuvering.
 
While Dawn continues pushing away from the Sun and deeper into the
asteroid belt, the distance to Earth is still declining, as it has been
since November 2008. The separation between the planet and the
probe varies just as the distance between the tips of the hour hand and
minute hand increases and decreases every hour. That suggests that it's
time once again to refer to one of the clocks available in the Dawn gift
shop on your planet. (If you didn't get around to preparing for the
recent festivities marking the universe's reaching its present age,
don't despair. Although there are only 5 trillion shopping days until
the next such gala celebration, Dawn gift shops in most galaxies are
offering attractive discounts right now.)

To picture the changing alignment, let's recall the clock described 365
days ago, with the Sun at the center. Dawn is at the tip of the minute h
and and Earth is at the tip of the shorter hour hand. One year ago today,
the celestial alignment corresponded to the position of the hands at about
6:01:45. At that time, Dawn was 2.49 astronomical units (AU) from Earth.
In the intervening year, Earth has completed 1 orbit around the Sun,
returning to where it was. Having
traveled more slowly, Dawn is in a different position now that happens
to be much closer to Earth. Today the alignment is similar to that at
6:30:00. Even though Dawn is farther from the Sun today than it was 1
year ago (as if the length of the minute hand had increased), in its
current location around the clock face, it is 0.84 AU from Earth, only
1/3 of what it was at the end of last year. The cosmic hands will
continue to move into still-closer alignment until late next month, when
the Sun, Earth, and Dawn will lie nearly along a straight line.
 
Picturing Dawn's position relative to Earth and the Sun may help some
readers gain perspective on the explorer's interplanetary journey, and
we will continue to present such illustrations (at least as long as the
increased revenue for the gift shop makes it profitable to do so).
Nevertheless, it is worth bearing in mind that from Dawn's perspective,
the location of Earth is of little importance (except when it needs to
point its antenna there). The ship travels on its own course around the
Sun, independent of the motions of the distant celestial port from which
it set sail more than 2 years ago. Dawn's sights remain firmly fixed on
the destinations ahead, where it seeks to unlock secrets about the dawn
of the solar system.
 
Dawn is 0.84 AU (125 million kilometers or 78 million miles) from Earth,
or 345 times as far as the moon and 0.85 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take 14
minutes to make the round trip.
Received on Mon 04 Jan 2010 08:58:07 PM PST