So how is the craft only 9,00 miles in altitude??
Stuart McDaniel
Lawndale, NC
Secr.,
Cleve. Co. Astronomical Society
IMCA #9052
Member - KCA, KBCA, CDUSA
-----Original Message-----
From: Ron Baalke
Sent: Wednesday, May 04, 2011 1:59 PM
To: Meteorite Mailing List
Subject: [meteorite-list] Dawn Journal - May 3, 2011
http://dawn.jpl.nasa.gov/mission/journal_05_03_11.asp
Dawn Journal
Dr. Marc Rayman
May 3, 2011
Dear Dawntalizingly Close Readers,
Dawn is on the threshold of a new world. After more than three and a
half years of interplanetary travel covering in excess of 2.6 billion
kilometers (1.6 billion miles), we are closing in on our first
destination. Dawn is starting its approach to Vesta.
The interplanetary cruise phase of the mission ends today and the
15-month Vesta phase begins. The first three months are the "approach
phase," during which the spacecraft maneuvers to its first science
orbit. Many of the activities during approach were discussed in detail
in March and April last year, and now we are about to see those plans
put into action.
The beginning of the phase is marked by the first images of the alien
world Dawn has been pursuing since it left Earth. Vesta will appear as
little more than a smudge, a small fuzzy blob in the science camera's
first pictures. But navigators will analyze
where it shows up against the background stars to help pin down the
location of the spacecraft relative to its target. To imagine how this
works, suppose that distant trees are visible through a window in your
house. If someone gave you a photo that had been taken through that
window, you could determine where the photographer (Dawn) had been
standing by lining up the edge of the window (Vesta) with the pattern
of
the background trees (stars). Because navigators know the exact
position
of each star, they can calculate where Dawn and Vesta are relative to
each other. This process will be repeated as the craft closes in on
Vesta, which ultimately will provide a window to the dawn of the solar
system.
Even though the mysterious orb is still too far away to reveal new
features, it will be exciting to receive these first images. For most
of
the two centuries that Vesta has been studied, it has been little more
than a pinpoint of light. Interrupting thrusting once a week this
month
to glimpse its protoplanetary destination, Dawn will watch it grow
from
about five pixels across to 12. By June, the images should be
comparable
to the tantalizing views obtained by the Hubble Space Telescope. As
the
approach phase continues and the distance diminishes, the focus will
grow still sharper and new details will appear in each subsequent set
of
pictures. During the approach phase, images will be released in
periodic
batches, with priority viewing for residents of Earth. The flow will
be
more frequent thereafter.
The visible and infrared mapping spectrometer (VIR)
will join the camera in spying Vesta on
May 10 and again later in the approach phase. At the end of June, Dawn
will watch Vesta for a full Vestian day of 5 hours, 20 minutes. When
the
camera searches for moons on July 9 and 10, it will also enjoy another
full pirouette. By the third and final time the spacecraft observes
Vesta throughout a complete rotation on its axis, during a set of
observations from July 23 to 25, Dawn will be in orbit.
On July 16, when the ship is at an altitude of around 15,500
kilometers
(9,600 miles) and propelling itself with its ion propulsion system in
the same way it has been for more than 900 days of interplanetary
travel, Vesta will gently take hold. For the first time since
September
27, 2007, when Dawn rode atop the second and third stages
of the Delta rocket for a short time in Earth orbit, it will be bound
to a planetary body.
The precise time and distance at which Vesta gains control of its
visitor depend not only on subtleties of the thrusting until then but
also on the strength of the giant asteroid's gravity. Among the many
characteristics of Vesta yet to be known well is its mass. Astronomers
have estimated it by detecting the tiny changes Vesta induces in the
orbits of other asteroids and even of Mars, but those measurements
yield
only approximate values. One of Dawn's objectives is to determine
Vesta's mass and to map its gravitational field.
The approach phase concludes when Dawn is ready to commence its survey
orbit in the second week of August. We will consider the timing of the
beginning of this next phase in a subsequent log.
While the start of the approach phase is defined by the beginning of
the
navigation imaging, other changes are being made today as well, both
in
procedures used by the operations team and in the configuration of the
spacecraft. Let's consider just one subsystem: attitude control.
(To achieve a certain mystique about their work,
engineers use the term "attitude" to describe the orientation of the
probe in the weightless conditions of spaceflight; the system also
happens to have a very enthusiastic attitude about its work.) Since
August Dawn has controlled its attitude with
its reaction control system, the small thrusters
that operate with hydrazine propellant. (When the craft is using the
ion
propulsion system, which is most of the time, the ion thruster helps
control the attitude.)
At the beginning of the approach phase, the ship returns to using
reaction wheels, gyroscope-like devices which, when electrically spun
faster or slower, rotate (or stop the rotation of) the spacecraft.
During Vesta operations, Dawn will turn much more frequently, as it
points its sensors at the alien world it is exploring, aims its main
antenna to Earth frequently to transmit its precious findings, and
follows a complex flight profile to
travel from one science orbit to another. The reaction wheels will be
used until Dawn has departed from Vesta in July 2012, providing more
accurate control of the attitude while conserving hydrazine.
To enable the explorer to point its camera and VIR even more
delicately,
the ship's gyroscopes are powered on. Not to be confused with the
reaction wheels, these devices help determine exactly what the
attitude
is so that the system can command the wheels to achieve the desired
attitude. The gyroscopes are not needed for most of Dawn's activities
during the interplanetary cruise phase of its mission, so they have
been
off for most of the mission so far.
The gyroscopes serve another purpose at Vesta, which we discussed in
more detail in January. The probe usually relies on star trackers for
sensing its attitude. Each tracker takes pictures of the stars. Its
internal computer processes the images, finding familiar patterns of
stars to determine where it is pointed, just as you might use some of
the constellations visible from your planet to orient yourself at
night.
When some component (such as the main antenna or an ion thruster)
needs
to be oriented in such a way that the star trackers happen to point at
Vesta, the gyroscopes will take over so the spacecraft doesn't lose
track of its attitude. There will be much to discover about the
enigmatic 530-kilometer-diameter (330-mile) rocky world, but its
ability
to block starlight is not in doubt.
While the science camera and VIR will be turned on and off as needed
during the Vesta phase, the gamma-ray and neutron detector (GRaND)
is being activated today and will remain on
until the departure next year. Most of that time, the majority of the
signals it detects will be from space radiation known as cosmic rays.
But the closer it gets to Vesta, the more gamma rays and neutrons it
will receive from the surface, gradually allowing scientists to
formulate a census of the atomic constituents. GRaND's greatest
ability
to sense the faint radiation will be in the low altitude mapping
orbit.
The instruments were tested during a planned coast period in March,
and each was in excellent condition. Dawn had
another scheduled hiatus in thrusting from April 11 to 19, but this
one
was not intended for calibrations or tests. Rather, controllers had
planned this for an upgrade to the software in the craft's main
computer.
When version 9.0 of the software was installed last year,
it was intended to be used at Vesta. By
coincidence, the day after they rebooted the computer to start running
with 9.0, the operations team began thinking about adding a new
capability to the software. The motivation was the development of
excessive friction in reaction wheel no. 4.
While Dawn performs perfectly well with
the other three wheels, the unavailability of one wheel meant that
there
was no longer a spare. Since then, three tests of wheel no. 4 have
shown
that it cannot be restored as a backup prior to Vesta and probably not
for the rest of the mission. Therefore, to regain the robot's
resilience
to the loss of almost any component, work began immediately at Orbital
Sciences Corporation and JPL on new software that would allow safe and
stable attitude control with only two wheels. (Of course, the
spacecraft
can function with all wheels powered off, relying on the reaction
control system, but ever-cautious engineers wanted the two-wheel
option
to reduce the hydrazine expenditure for complex Vesta and Ceres
operations.)
The installation of software on our probe flying in deep space is a
delicate task. To begin running with the new version, the computer has
to be rebooted. That same computer constantly performs such essential
functions as maintaining a steady attitude and acceptable
temperatures.
Controllers followed the same intricate procedures they used
successfully to load new software in November 2007, April 2009, and
June 2010. Preparing the spacecraft, radioing the
new software to it, rebooting the computer, and commanding the craft
back to its normal flight configuration all went exactly according to
plan. Although more than a week was allocated, it only took three
days.
Dawn is now running what the team officially designates OBC flight
software version 10.0, but what the more zany team members refer to as
10.0 or "ten oh." It may be surprising that even with the complex and
rigorous work to overcome myriad challenges of operating the first
explorer from Earth to take up residence in the main asteroid belt,
normally dispassionate engineers can display such frivolity.
Now with new software, the spacecraft is beginning the approach phase.
Its journey has been long, but the reward is almost in view. Since
leaving Earth in September 2007, Dawn has made about one and three
quarters circuits around the sun as it spirals outward.
Earth itself (along with your
correspondent and some readers) has completed more than three and a
half
orbits in that time. But on May 14, Vesta will finish its first
revolution around the sun since Dawn has been in flight; the mission
will then have been under way for exactly one Vestian year.
We have seen before that objects travel more
slowly in more distant orbits, where the force of gravity holding them
is weaker. Dawn has been climbing the solar system hill, traveling
farther and farther from the sun at the bottom. It began its journey
on
Earth, partway up the hill. Now far above Mars, the probe is closing
in
on Vesta. As the adventurer and the mysterious world each race around
the sun at nearly 21 kilometers per second (47,000 mph), Dawn is
gradually closing in for its rendezvous. Two months ago, the
spacecraft's course was bringing it toward Vesta at 0.7 kilometers per
second (1,600 mph). Today, having
completed more thrusting to bring its orbit into a closer and closer
match with Vesta's, the craft is approaching at about 0.37 kilometers
per second (830 mph). The speed will continue to diminish as Dawn
gradually reshapes its flight path to be exactly the same as Vesta's.
Soon, they will travel together around the sun.
Meanwhile, the distance between them continues to shrink. Since the
middle of March, Vesta has outshone everything in Dawn's sky save the
sun. By the middle of April, a sharp-eyed passenger would notice that
Vesta is more than a pinpoint of light like the myriad stars and
distant
planets; it would appear as a tiny disk, hinting of the exciting
adventure ahead. (The passenger also might notice that his luggage was
left back on Earth, more than 320 million kilometers or 200 million
miles away.) Now, with Dawn's interplanetary cruise ending and the
approach beginning, Vesta is coming into its sights, as the ship
prepares to sail into port after an extraordinarily long journey
across
the lonely emptiness of the vast interplanetary seas.
Dawn is 1.2 million kilometers (760,000 miles) from Vesta, or 3.2
times
the average distance between Earth and the moon. It is also 1.90 AU
(284
million kilometers or 177 million miles) from Earth, or 715 times as
far
as the moon and 1.89 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 32
minutes
to make the round trip.
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