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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 1 Sep 2009 13:24:30 -0700 (PDT)
Message-ID: <200909012024.n81KOUVa024781_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_8_30_09.asp

Dawn Journal
Dr. Marc Rayman
August 30, 2009
 
Dear Indawnmitables,
 
The Dawn mission remains on course as the spacecraft continues to thrust
with its ion propulsion system, patiently, persistently, and gently
changing its orbit to keep its appointment with protoplanet Vesta in two
years. Meanwhile, closer to mission control and in stark contrast, brave
firefighters work hard to protect JPL and the nearby homes of many of
its employees and others in the community.
 
The probe has continued in "quiet cruise" since the last log. During this
month, engineers did give the robot a few extra tasks to ensure it remains
healthy, but these were routine. When each such assignment was conducted
the first time or two that Dawn was in space, they were treated as special
activities, with even greater diligence than is normally applied to the
unforgiving and complex undertaking of flying a spacecraft far from Earth.
Now however, the commands for these activities are stored onboard well ahead
of time along with the routine commands for thrusting, communicating
with Earth, and carrying out all the other functions the spacecraft
normally conducts without the mission control team devoting extra attention.
 
Included in the maintenance procedures were instructions to perform a
sequence of movements of the mechanism that points ion thruster #1, to
power off reaction wheel #2 and return #1 to service, and to operate the
gyroscopes for about 4 days. For readers who do not have their copies of
the Dawn operations manuals handy, some information about these 3 kinds
of operations was provided in a previous log.
 
Another event that is now considered routine occurred on August 15. For
the second time this year, a particle
of space radiation struck a particularly sensitive electrical component
on the spacecraft, depositing enough energy to interfere with the
operation of a circuit. When this happened in January 2008,
it caused Dawn to enter safe mode, interrupting its other activities.
Thanks to software the team transmitted to the ship later that year,
now the interplanetary explorer is immune to strikes in that formerly
vulnerable location.
 
As Dawn continues its long (in space and in time) solar system journey
to match orbits with Vesta and later with Ceres, both of which reside
farther from the Sun than the probe has yet traveled, some readers may
note a surprising trend in the statistics for the mission. The famously
unimaginative ending of each of these logs reveals that Dawn's distance
from Earth has been diminishing since November 2008. Indeed, the probe's
maximum separation from its planet of origin occurred on November 10.
Today, it is as far from Earth as it was on June 2, 2008. By January 2010,
it will be as close as it was in March 2008. Is this progress?
 
Earth and Dawn, each following its own path, are both in orbit around
the Sun. As grateful residents of the planet know, their world's orbit
doesn't change very much. The planet keeps following the same nearly
circular path around the Sun year after year after year. Today Earth is
about 1.01 astronomical units (AU) from the
Sun, and it never strays very far from its average distance of 1.00 AU.
As Dawn has traveled independently of Earth, thanks to the push from its
Delta rocket, its orbit has been farther
from the Sun than Earth's. The gradual effect of ion thrusting and the
much more abrupt boost from Mars have caused that orbit to change
considerably since then. To enter orbit around Vesta, Dawn will have to
match the giant asteroid's orbit around the Sun, ranging from 2.15 AU to
2.57 AU. Today, Dawn is 1.53 AU from the Sun and headed outward.
 
As we saw a year ago (that is, one
Earth-orbit-around-the-Sun ago), objects at different orbital distances
travel at different speeds. The probe, orbiting the Sun at a greater
range than Earth, travels more slowly, because the Sun's gravitational
attraction diminishes with distance. So as Dawn heads slowly for Vesta,
gradually spiraling away from the Sun, and Earth speeds around more
quickly in its orbit, sometimes our planet moves closer to the
spacecraft and sometimes it moves farther away.
 
In a continuing effort to offset the extraordinary cost of these logs
with the handsome revenue from subtle product placements, we can refer
to still another in the apparently endless line of Dawn clocks (many of
which have been described in recent logs and all of which are available
in the Dawn gift shop on your planet). On this clock, the minute hand is
shorter than the hour hand. The motion of the former represents Earth,
traveling closer to the Sun (at the clock's center) and more quickly.
Dawn is at the tip of the hour hand, moving more slowly in its larger
orbit. (We'll ignore for now that the hour hand should be growing in
length, as the spacecraft recedes from the Sun.) Some of the time (such
as between noon and about 12:30), the distance between the ends of the
hands increases, but then the situation reverses; the faster minute hand
begins moving closer and closer to the hour hand as the time approaches
about 1:05.
 
Earth and Dawn are exhibiting the same repetitive behavior, albeit more
complicated because of Dawn's ever-changing orbital speed and distance
from the Sun. They will continue to draw closer until January, when
Earth, coming from behind, passes Dawn and moves on ahead. The explorer
will not need to take note however, as its sights are set on the
asteroid belt.
 
So for readers tracking the distances reported in each log, don't
despair. The continuously declining separation between Earth and its
celestial envoy is a reflection of the elegant mechanics of the cosmos
and not the result of inattentive engineers setting the spacecraft on
the wrong path.
 
Next month, as Earth and the spacecraft continue their separate solar
system dances, together with the Sun they will briefly make an
attractive arrangement. On September 18, Dawn will be just as far from
the Sun as it is from Earth, at 1.56 AU from each. Earth and the Sun
will be 1.00 AU apart. The trio formed a very similar pleasing pattern
last year. A triangle such as this, with two
sides of equal length, is usually called "isosceles."
 
Although there is nothing inherently significant for the mission about
this alignment, we can use one more clock example to illustrate this
isosceles triangle. In this case, we put Dawn at the center and Earth at
the 12. (This clock may not be as useful for telling time as some of the
others that are available, but it would still make a great gift.) The
Sun would be next to the sixth little tick mark, where the minute hand
would point at about 6 minutes and 15 seconds after the hour. (Note that
this depiction of the geometry illustrates the angles and the relative
separations of Dawn, Earth, and the Sun; hence, the clock may be any
size. Several sizes are available in the gift shop and we helpfully
recommend the most expensive one.)
 
Not only is Dawn on course for the asteroid belt, on course for
returning new and exciting discoveries from its enigmatic destinations,
it is on a new and better course than it had been. According to inside
sources, the vast team of writers specifically assigned to create the
next log is already planning to explain what has changed and why. Just
as all loyal readers, your correspondent is hoping for an interesting
description of this improvement in the mission.
 
Dawn is 250 million kilometers (156 million miles) from Earth, or 620
times as far as the moon and 1.66 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take 28
minutes to make the round trip.
                
Received on Tue 01 Sep 2009 04:24:30 PM PDT