[meteorite-list] Dawn Journal - May 3, 2011

Wed May 4 20:34:21 EDT 2011

On July 16, DAWN will enter a circum-Vestan orbit at
an altitude of 9,600 miles above Vesta. Its altitude will
be lowered progressively once the orbit has been tweaked
to the precise parameters needed. Vesta is not a "sphere,"
so orbiting its center of gravity causes your distance from
its surface to vary like you were on a roller coaster. Give
it plenty of room; don't want to clip the asteroid, ya know...
It's a long way to the nearest body shop.

Sterling K. Webb
-------------------------------------------------------------------------------
----- Original Message ----- 
From: "Stuart McDaniel" <actionshooting at carolina.rr.com>
To: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>; "Meteorite Mailing List" 
<meteorite-list at meteoritecentral.com>
Sent: Wednesday, May 04, 2011 6:11 PM
Subject: Re: [meteorite-list] Dawn Journal - May 3, 2011

> 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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