[meteorite-list] Dawn Journal - April 28, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 30 Apr 2010 16:08:30 -0700 (PDT)
Message-ID: <201004302308.o3UN8UtW018408_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
April 28, 2010

Dear Adawnherents,
Dawn remains on course and on schedule for its appointments with
Vesta and Ceres, colossal protoplanets in the main asteroid belt.
Under the gentle and continuous thrust of its ion propulsion system,
its journey through the solar system brings it ever closer to its
first target.
Last month's log included an overview of
many of the spacecraft's activities during the final 3 months
before its August 2011 arrival in the first science orbit at
Vesta. In this "approach phase," the probe will observe Vesta with
its camera and one of its spectrometers to gain a better fix on
its trajectory and to perform some preliminary characterizations
of the alien world prior to initiating its in-depth studies. The
discussion did not cover the principal activity, however, which is
one very familiar not only to the spacecraft but also to readers
of these logs. The majority of the time will be devoted to
continuing its ion-powered flight. Let's take a more careful look
at how this remarkable technology is used to deliver the
adventurer to the desired orbit around Vesta.
Thrusting is not necessary for a spacecraft to remain in orbit,
just as the Moon remains in orbit around Earth and Earth and other
planets remain in orbit around the Sun without the benefit of
propulsion. All but a very few spacecraft spend most of their time
in space coasting, following the same orbit over and over unless
redirected by a gravitational encounter with another body. With
its extraordinarily efficient ion propulsion system, Dawn's
near-continuous thrusting gradually changes its orbit. Thrusting
since December 2007 has propelled Dawn
from the orbit in which the Delta rocket deposited it after launch
to orbits of still greater distance from the Sun.
The flight profile was carefully
designed to send the craft by Mars in February 2009, so our
explorer could appropriate some of the planet's orbital energy for
the journey to the more distant asteroid belt, of which it is now
a permanent resident. In exchange for Mars raising Dawn's orbit,
Dawn lowered Mars' orbit, ensuring the solar system's energy
account remained balanced.
While spacecraft have flown past a few asteroids in the main belt
(although none as large as the behemoth Vesta nor the still more
massive dwarf planet Ceres), no probe has ever attempted to orbit
one, much less two. For that matter, this is the first mission
ever undertaken to orbit /any/ two solar system targets. Dawn's
unique assignment would be quite impossible without ion
propulsion. But with its light touch on the accelerator, taking
nearly 4 years to travel from Earth past Mars to Vesta and more
than 2.5 years from Vesta to Ceres, how will it enter orbit around
Vesta, how will it break back out of orbit, and how will it enter
orbit around Ceres?
Whether conventional spacecraft propulsion or ion propulsion is
employed, entering orbit requires accompanying the destination on
its orbit around the Sun. This intriguing challenge was addressed
in part in February 2007, as all readers with perfect memory recall.
In August 2008, we considered another aspect of
what is involved in climbing the solar system's hill, with the Sun
at the bottom, Earth partway up, and the asteroid belt even
higher. (Readers at that time in the past thoughtfully sent
greetings through time to us, which we
are now delighted to receive! On behalf of all present readers, we
return the kind gesture with our own greetings.) We saw that Dawn
needs to ascend that hill, but it is not sufficient simply to
reach the elevation of each target nor even to travel at the same
speed as each target; the explorer also needs to travel in the
same direction. Probes that leave Earth to orbit other solar
system bodies traverse outward from (or inward toward) the Sun,
but then need to turn in order to move along with the body they
will orbit.
Those of you who have traveled around the solar system before are
familiar with the routine of dropping into orbit. The spacecraft
approaches its destination at very high velocity and fires its
powerful engine for some minutes or perhaps even about an hour, by
the end of which it is traveling slowly enough that the planet's
gravity can hold it in orbit and carry it around the Sun. These
exciting events can range from around 0.6 to 1.5 kilometers per
second (1300 to 3400 miles per hour). With 10 thousand times less
thrust than a typical propulsion system on an interplanetary
spacecraft, Dawn could never
accomplish such a rapid maneuver. As it turns out, however, it
doesn't have to.
Dawn's method of getting into orbit is quite different, and the
key is expressed in an attribute of the ion propulsion system that
has been referred to 26 times (trust or verify; it's your choice)
before in these logs: it is gentle. Dawn's entire thrust profile
for its long interplanetary flight has been devoted largely to the
gradual reshaping of its orbit around the Sun so that by the time
it is in the vicinity of Vesta, its orbit will be very much like
Vesta's. Only a small change will be needed to let the giant
asteroid's gravity capture it, so even that gentle ion thrust will
be quite sufficient to let the craft slip into orbit.
To get into orbit, a spacecraft has to match speed, direction, and
location with its target. A mission with conventional propulsion
first gets to the location and then, with the planet's gravity and
its own fuel-guzzling propulsion system, very rapidly achieves the
required speed and direction. By spiraling out to the orbit of
Vesta (and later Ceres), Dawn works on its speed, direction, and
location all at the same time, so they all gradually reach the
needed values just at the right time.
To think about this facet of the difference between achieving this
goal with the different technologies, imagine you want to drive
your car along next to another traveling west at 100 kilometers
per hour (60 miles per hour). The analogy with the conventional
technology would be similar to heading north toward an
intersection where you know the other car will be. You arrive
there at the same time and execute a whiplash-inducing left turn
at the last moment using the brakes, steering wheel, accelerator,
and probably some adrenaline. When you drive an ion propelled car,
operating with 10 times the fuel efficiency, you take a different
path from the start, one more like a long, curving entrance ramp
to a highway. When you enter the ramp, you slowly (perhaps even
gently) build speed. You approach the highway gradually, and by
the time you have reached the far end of the ramp, your car is
traveling at the same speed and in the same direction as the other
car. Of course, to ensure you are there when the other car is, the
timing is entirely different from the first method, but the
sophisticated techniques of orbital navigation are up to the task.
In late July 2011, as the probe follows its approach trajectory to
Vesta, their paths will be so similar they will be moving at
nearly the same direction and speed around the Sun (about 20.5
kilometers per second or almost 46 thousand miles per hour). When
at a range of about 16 thousand kilometers (9900 miles), the
spacecraft will be traveling at less than 50 meters per second
(110 miles per hour) relative to its destination. That combination
of distance and velocity will allow Vesta to take gentle hold of
Dawn. The spacecraft will not even notice the difference, but it
will be in orbit around its first celestial target, even as it
continues ion thrusting to reach the planned orbit more than 2
weeks later.
With the gradual trajectory changes inherent in ion propulsion,
sharp changes in direction and speed are replaced by smooth,
gentle curves. Dawn is propelling itself along a spiral path
around the Sun as it journeys from Earth out to Vesta, the first
loop having been completed in June 2009.
It will arrive at Vesta before it
completes the second revolution. Then its flight profile will be
designed to spiral around Vesta as the probe and protoplanet
together orbit the Sun. Dawn's first loop around Vesta will be
about 10 days, and its second will take 4. It will stop thrusting
when it is in "survey orbit," where one revolution takes just
under 3 days. After collecting a rich bounty of pictures and other
important scientific data from this altitude of about 2700
kilometers (1700 miles), it will resume thrusting, spiraling down
to lower and lower orbits, requiring hundreds of revolutions.
Dawn's speed will increase as its orbital altitude decreases,
so the loops will progressively become shorter.
In 2012, after completing months of close-range scientific
observations, it will reverse the spirals, gradually climbing away
from the world it has been studying just as it gradually climbed
away from the Sun. Vesta's gravitational hold will weaken as Dawn
moves farther and faster, its graceful motion ultimately exceeding
the strength of the invisible tether that bound it. As gently as
it arrived, it will depart. In July of that year, it will once
again be on its own in orbit around the Sun, and navigators will
instruct it to point its ion thruster to spiral outward more in
order to undertake its pursuit of Ceres.
These spiral paths do not occur naturally. Under the predictable
and calculable effects of the gravity of the Sun and other bodies
(including Vesta or Ceres), Dawn is programmed to orient its
thruster in just the right direction at the right time to propel
itself on the desired trajectory. A great deal of work was
required before launch to devise such a plan. Changes since then
have been determined by knowledge
gained during the mission, such as an update to the prediction of
how much power the solar array will yield.
Engineers have completed work on the approach phase for now. They
have reviewed the sequences (the timed instructions the spacecraft
will follow) in detail and have tested portions of them in the
spacecraft simulator at JPL. The sequences are mature enough that
they will be ready for controllers to update and refine as
necessary next year before being radioed to the spacecraft. Now
the operations team is turning its attention to the subsequent
phase of the Vesta mission, survey orbit, where the intensive
observations will begin. We will learn more about that in the next
Dawn's controllers certainly do not focus all their efforts on
preparing for Vesta. (Your correspondent devotes some of his to
dancing, but perhaps that's a topic for a future log.) Of course,
keeping the spacecraft healthy and on course is essential as well.
In addition to commanding it to sustain the needed thrusting, with
a weekly hiatus for telecommunications, they perform routine
maintenance to ensure the ship remains in top shape. For example,
engineers recently adjusted the spacecraft's master clock. Always
in the glow of the distant Sun, and never needing to rest or take
a break from its duties, the robot has no need to switch to
daylight saving time. Nevertheless, a time change was called for
because the onboard time had gradually drifted from the correct
value. It had last been set on February 27, 2008, and has remained
sufficiently accurate for all Dawn's needs.With the gradual nature
of this mission, precise timing is generally not necessary, so
although they have closely monitored the clock, controllers have
allowed it to run without correction. When they commanded the
transition from ion thruster #1 to thruster #2 in January,
they expected the clock to change slightly, and indeed it did.
Thruster #2 uses a different power control unit from thrusters #1 and #3.
The #2 controller is mounted
closer to the electronics assembly that includes Dawn's clock, and
now that that device is powered, the heat it dissipates warms the
clock a little, so the clock rate is slightly altered. Although
much larger values could be accommodated, when the time offset had
crept up to 1.37 seconds, operators set it back to the correct
time, and they included a change to account for the warmer
environment. (Readers may wish to pause for 1.37 seconds to
contemplate the difficulties of synchronizing clocks that are
farther apart than the Sun.)
An improved version of the test to measure the overlap of the
views of the visible and infrared mapping spectrometer (VIR) and
the prime science camera was executed successfully. When the
measurement was carried out in December,
a conflict between commands in the VIR
sequence prevented the intended data from being acquired.
As if maintaining the spacecraft's health and powered flight and
developing detailed plans for Vesta weren't enough to keep Dawn's
engineers happy, they also are continuing work on a new version of
the software for the primary computer, scheduled to be transmitted
to the spacecraft in June. The mission also will mark 3 milestones
that month, and it may not be a surprise if your correspondent
marcs them in the next log.
Dawn is 1.62 AU (243 million kilometers or 151 million miles) from
Earth, or 1.61 times as far as the Moon and 650 times as far as
the Sun. Radio signals, traveling at the universal limit of the
speed of light, take 27 minutes to make the round trip.
Received on Fri 30 Apr 2010 07:08:30 PM PDT

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