[meteorite-list] Dawn Journal - September 27, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 29 Sep 2008 10:49:53 -0700 (PDT)
Message-ID: <200809291749.KAA14143_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
September 27, 2008

Dear Dawnniversaries,

On the first anniversary of its departure from Earth,
Dawn continues with what it has been doing
for most of its time in space: with the greatest patience it is
gently reshaping its orbit around the Sun with its ion propulsion

In its first year of travels, the spacecraft has thrust for a
total of about 253 days, or 69% of the time. Dawn has been in
powered flight for 85% of the time since the beginning of its
interplanetary cruise phase in December
2007 and about 0.000000005% of the time since the Big Bang. While
for most spacecraft, firing a thruster to change course is a
special event, it is Dawn's wont. All this thrusting has cost the
craft only 67 kilograms (148 pounds) of its supply of xenon
propellant, which was 425 kilograms (937 pounds) 1 year ago.

The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 1.68 kilometers per second (3760 miles
per hour). As the preceding log described,
because of the principles of motion for orbital flight, whether
around the Sun or any other gravitating body, Dawn is not actually
traveling this much faster than when it launched. But the
effective change in speed remains a useful measure of the effect
of any spacecraft's propulsive work. Having accomplished only
one-eighth of the thrust time planned for its entire mission, Dawn
has already exceeded the velocity change required by many
spacecraft. (For a comparison with probes that enter orbit around
Mars, visit the red planet yourself or refer to a previous log.)

Since launch, our readers who have remained on or near Earth have
completed exactly 1 revolution around the Sun. (This log,
including the date it is filed, disregards that 2008 is a leap
year and that Earth actually takes almost 365.25 days to complete
one orbit. Oops -- it isn't being disregarded; in fact, it's right
there in the previous sentence, and the longer this parenthetical
text goes on, the more attention is being drawn to it. As it makes
no significant difference, we request readers do a better job of
ignoring it than the writer is doing. Please return to the flow of
the log.) Orbiting farther from the Sun than Earth, and moving at
a more leisurely pace, Dawn has not traveled even two-thirds of
the way around the Sun. Of course, unlike Earth, when it has
completed 1 full circuit (in 2009), it will not be at the same
place it started. Earth's orbit is quite repetitive, but the
combined effects of the powerful rocket launch, the extensive ion
thrusting, and the gravitational deflection from Mars next
February will cause the spacecraft to be farther from the Sun at
the end of its first revolution than it was at the beginning.

As readers who have followed the Dawn mission during 2008 know,
the spacecraft occasionally engages in activities other than
routine thrusting as its adventure progresses. On August 26,
mission controllers commanded the primary and backup cameras
to execute their calibration routines.
This not only served to confirm that both units remain healthy,
but it also let engineers verify one of the new features in the
software radioed to each camera in April that was not tested at
that time.

On September 22, an updated version of a method to establish how
much power Dawn's extraordinary solar arrays can generate was
tested successfully. The first test
was conducted in July, and it yielded only some of the desired
information. The revised procedure was very similar to the earlier
one, principally differing in the timing of some instructions and
values of parameters based on the analysis of that initial run.
Because the entire activity, even including the 41-minute
round-trip travel time for radio signals, required less than 3
hours in the middle of the afternoon, among the most significant
changes that ever-observant mission controllers detected was that
no meals were incorporated into the carefully engineered plan.

As in July, the test included rotating both solar array wings 45
degrees, so they did not point directly at the Sun, thus reducing
how much light they received and converted to electrical power.
The test was carried out during the spacecraft's routine weekly
interruption in thrusting to point its main antenna to Earth, but
the ion propulsion system was commanded into service when it
otherwise would have been idle. Its role then was not to provide
propulsion (although it did so); rather, it participated because
it is the greatest consumer of power onboard. Dawn's enormous
solar arrays, even turned partially away from the Sun and more
than 1.66 times farther from the radiant orb than Earth is, were
able to provide the 2.5 kilowatts requested by the ion drive at
full power. Later in the mission, after all the data have been
analyzed thoroughly, the next step in the solar array calibration
will be to command the arrays to rotate farther, where they are
not expected to be able to deliver all the power requested.

As Dawn begins its second year (as measured back on Earth) of
interplanetary flight, the probe steadfastly continues its long
journey in the quiet solitude of space, quite isolated from events
on or near the distant planet that used to be its home. While no
spacecraft has left the vicinity of the Earth-moon system in the
year since Dawn's departure, much has happened there, even as the
explorer has remained focused on accomplishing its voyage in deep
space. From the first circulation of protons at the Large Hadron
Collider 100 meters (330 feet) underground, to the beginning of
the Fermi Gamma-ray Space Telescope mission 550 kilometers (340
miles) overhead, to the arrival of SELENE (Kaguya) and Chang'e 1
at the moon, humankind's thrilling work to understand nature has
continued. Apparently there have been some other kinds of news as
well, from shocking revelations about celebrities, to competitions
among athletes and among politicians, to still more shocking
revelations about celebrities, but such information is harder to
find, given the news media's nearly exclusive focus on myriad
science topics. (News coverage may be different on your planet.)

Dawn is 374 million kilometers (232 million miles) from Earth, or
980 times as far as the moon and 2.49 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of
light, take 42 minutes to make the round trip.
Received on Mon 29 Sep 2008 01:49:53 PM PDT

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