[meteorite-list] Dawn Journal - December 30, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 31 Dec 2008 12:44:52 -0800 (PST)
Message-ID: <200812312044.MAA05222_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_30_08.asp

Dawn Journal
Dr. Marc Rayman
December 30, 2008

Dear Dawncember30ths,
 
Having fulfilled all of its assignments for 2008, the Dawn
spacecraft has been unusually quiescent recently. While its
operators on faraway Earth have no shortage of work, the probe
patiently coasts in its orbit around the Sun, awaiting a brief
encounter with Mars on February 17, which will steer it into a new
orbit.
 
On October 31, Dawn completed nearly all the ion thrusting that had
been planned for 2008. On November 20, mission controllers directed
the spacecraft to execute a short maneuver to fine-tune its
trajectory. Its only activity since then has been the routine
maintenance of the gimbal system used to point ion thruster #1. On
December 3, it moved the mechanism through a range of angles to
help redistribute lubricant, following the same commands that were
used 2 months earlier.
 
As viewed from Earth, Dawn passed through solar conjunction
this month, appearing to be very
close to the Sun. To visualize the geometry, suppose the Sun were
at the center of a clock, with Earth at the end of the hour hand
and the spacecraft at the tip of the minute hand. With the
relative distances at the time of conjunction, the minute hand would
be almost 1.6 times the length of the hour hand - an elegant design
indeed. (This analogy applies only for the separation as viewed from
Earth under limited circumstances. As explained in an earlier log,
while Dawn is indeed farther from the Sun
than Earth is, the planet travels more quickly around its orbit
than the spacecraft does. This would be more akin to a clock on
which the hour hand is longer than the minute hand; such
timepieces are back-ordered at Dawn souvenir shops.)
 
When Earth, the Sun, and the spacecraft are on a straight line,
such as at 6:00, the Sun and spacecraft would appear to overlap
from the perspective of an observer on Earth, near the bottom of
the clock. As we noted last month, Dawn would not pass directly
behind the Sun, because it does not orbit in the same plane as
Earth. Therefore, the precisely linear arrangement of hands at
exactly 6:00:00 never occurs. Pushing the clock analogy beyond its
limits of usefulness, the minute hand would be bent toward the
clock face, so it does not circle in quite the same plane as the
hour hand. We shall ignore that enhancement for now but return to
this point below. In the meantime, let's consider the arrangements
that have occurred.
 
On December 12, when the angle between the Sun and the spacecraft
was at its minimum, it would be analogous to the alignment of the
hands about 10 seconds from the hour, or the arrangement at
6:00:10. (Remember, this clock only has hour and minute hands;
your correspondent types too slowly to be able to construct a
useful analogy with a clock that includes a second hand.) When
most modern interplanetary craft are within about 2 degrees of the
Sun, normal communications may be less reliable. This limitation,
which lasted about 2 weeks for Dawn, would correspond to half a
minute on either side of 6:00, or between about 5:59:30 and 6:00:30.
 
Despite the powerful interference caused by radio signals passing
through the distorting environment of the Sun on their way from
the spacecraft to Earth, enough of the transmissions made it
through for engineers to confirm that the spacecraft remained
healthy throughout the conjunction period. Dawn was programmed to
modify its radio transmissions to account for the angle between it
and the Sun. Operators chose to accept a reduced return of
information from the ship's systems in exchange for boosting the
quality of the signals used for navigation because of the upcoming
flight by Mars. Some usable navigation data were obtained every
day, but, as expected, most of the data, particularly during the 4
days when the spacecraft was nearest the Sun, were too degraded to
be useful in refining the parameters of Dawn's orbit.
 
Now, as Earth and the spacecraft have progressed in their separate
travels around the Sun (making an angle today equivalent to about
6:01:45 on our Dawn clock), the radio waves traverse a less
tortuous path, so the signal quality has improved. After
collecting and analyzing more navigational data, engineers will
determine what refinement is needed to the trajectory to guarantee
Dawn encounters Mars in just the right way to provide the needed
gravitational deflection. Following the same procedure applied to
the design of Dawn's first trajectory correction maneuver (TCM),
the team will begin designing the
second TCM early next month for the spacecraft to perform on
January 15. In fact, the creative process has already begun; the
maneuver has been given the imaginative appellation TCM2. Using
those 4 characters (and perhaps a few others as well), the next
log will report on the maneuver and provide some details on the
nature of Dawn's gravitational interaction with Mars and how it
affects the trajectory.
 
The only reason for Dawn to travel to the vicinity of Mars is for
the help to reach its targets in the asteroid belt. Nevertheless,
as the probe races by, the team will take advantage of the
opportunity to accomplish some bonus goals. Some of the plans will
be covered in an upcoming log.
 
In the meantime, as the thrill of conjunction begins to fade, our
vast staff has yet to sort through all the data on how many
terrestrial readers used this convenient alignment to guide their
mental eyes toward the
spacecraft. The Dawn project sincerely hopes all observers reaped
the maximum possible inspiration and joy from solar conjunction,
as the mission will not offer another like it. Our destinations,
Vesta and Ceres, do not orbit the Sun in the same plane that Earth
does, and Dawn must match its orbit to that of its targets. (The
major planets orbit closer to the plane of Earth's orbit, and no
spacecraft has ventured as far out of that plane to orbit another
body as Dawn will.) While the probe is already in a slightly
different plane from Earth's orbit now, the gravity of Mars and
subsequent ion thrusting will propel it to still a greater angle.
As a result, when Dawn and Earth find themselves on opposite sides
of the Sun in the future, the alignment will not be as close as it
was this month. Dawn's next apparent encounter with the Sun will
be in November 2010, but it will appear to pass far enough north
of the Sun that communications should not be significantly
compromised. Following that, there will be 3 more times before the
primary mission ends in 2015 that Earth and the spacecraft will be
on opposite sides of the Sun, but in each case Dawn's path through
Earth's skies will take it farther north or south of the brilliant
landmark than in the 2008 conjunction. Nevertheless, each will be
close enough that it may provide a visual reference once again to
stir meditation upon the magnificence of a journey far away in the
depths of space.
 
Dawn is 11 million kilometers (7 million miles) from Mars. It is
372 million kilometers (231 million miles) from Earth, or 930
times as far as the moon and 2.53 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light,
take 41 minutes to make the round trip.
Received on Wed 31 Dec 2008 03:44:52 PM PST


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