[meteorite-list] Dawn Journal - January 27, 2012

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 27 Jan 2012 12:31:55 -0800 (PST)
Message-ID: <201201272031.q0RKVtVs006814_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_01_27_12.asp

Dawn Journal
Dr. Marc Rayman
January 27, 2012

Dear Asdawnished Readers,

Dawn is scrutinizing Vesta from its low-altitude mapping orbit (LAMO),
circling the rocky world five and a half times a day. The spacecraft is
healthy and continuing its intensive campaign to reveal the astonishing
nature of this body in the mysterious depths of the main asteroid belt.

Since the last log, the robotic explorer has devoted most of its time
to its two primary scientific objectives in this phase of the mission.
With its gamma ray and neutron detector (GRaND), it has been patiently
measuring Vesta's very faint nuclear emanations. These signals reveal
the atomic constituents of the material near the surface. Dawn also
broadcasts a radio beacon with which navigators on distant
Earth can track its orbital motion with exquisite accuracy. That allows
them to measure Vesta's gravity field and thereby infer the interior
structure of this complex world. In addition to these top priorities,
the spacecraft is using its camera and its visible and infrared mapping
spectrometer (VIR) to obtain more
detailed views than they could in the higher orbits.

As we have delved into these activities in detail in past logs, let's
consider here some more aspects of controlling this extremely remote
probe as it peers down at the exotic colossus 210 kilometers (130 miles)
beneath it.

Well, the first aspect that is worth noting is that it is incredibly
cool! Continuing to bring this fascinating extraterrestrial orb into
sharper focus is thrilling, and everyone who is moved by humankind's
bold efforts to reach into the cosmos shares in the experience. As a
reminder, you can see the extraordinary sights Dawn has by going here
</multimedia/imageoftheday/archives.asp> for a new image every weekday,
each revealing another intriguing aspect of the diverse landscape.

The data sent back are providing exciting and important new insights
into Vesta, and those findings will continue to be announced in press
releases. Therefore, we will turn our attention to a second aspect of
operating in LAMO. Last month, we saw that various forces contribute to
Dawn moving slightly off its planned orbital path. (That material may be worth
reviewing, either to enhance appreciation of what follows or as an
efficacious soporific, should the need for one ever arise.) Now let's
investigate some of the consequences. This will involve a few more
technical points than most logs, but each will be explained, and
together they will help illustrate one of the multitudinous complexities
that must be overcome to make such a grand adventure successful.

Far away, traveling through the vast expanse of (mostly) empty space,
Dawn only knows where it is because of information the mission control
team installs in it. This is typical for interplanetary spacecraft.
Earth-orbiting satellites may be able to use the Global Positioning
System (GPS) constellation or other means to find their own location,
but only a few spacecraft that have gone far from Earth have the means
to independently establish their own location. This should not be
confused with a spacecraft's ability to determine its own orientation,
which Dawn does with its star trackers, gyros, and sun sensors.
In the same way, if you were in a dark and
unidentified place on your planet, you could determine the direction you
were looking by recognizing patterns of stars, but that would not help
you ascertain your position.

Throughout the mission, controllers regularly transmit to the spacecraft
a mathematical description of its location in the solar system at any
instant over a given period of time. They also provide it with the
information needed to calculate where Earth is. That's how it is able to
point its main antenna in the correct direction when it needs to do so.
During the Vesta phase of the mission, the probe is given the additional
means it needs to determine its location relative to Vesta. All the
information sent to the spacecraft is based on navigators' best
prediction of where the spacecraft will be in the future. Dawn remains
unaware of any deviations from its expected course, so it always behaves
as if it were exactly where it would be if its motion matched the team's
projections perfectly, without the discrepancies that are sure to occur.
For the majority of the mission, both in interplanetary cruise and at
higher altitude orbits at Vesta, the effects of being slightly off the
predicted trajectory are insignificant. In LAMO, they are not.

For Dawn to aim its scientific sensors at Vesta, controllers instruct it
to point straight "down." Again, it knows how to compute where "down" is
because of the information it was given by navigators. Any disparity
between where the craft was predicted to be and where it really is along
its orbit causes it to point in a slightly different direction, not
quite truly straight down. This does not compromise the observations; it
could tolerate larger pointing errors and still capture the desired
targets in the field of view of the instruments.

Dawn is a very large spacecraft. Indeed, the wingspan from one solar
array tip to the other is 19.7 meters (nearly 65 feet). When it was
launched in 2007, this was the greatest span of any probe NASA had ever
dispatched on an interplanetary journey. The large area of solar cells
is needed to capture faint sunlight in the asteroid belt to meet all of
the electrical power needs. Each solar array wing is the width of a
single's tennis court, and the whole spacecraft would reach from a
pitcher's mound to home plate on a professional baseball field, although
Dawn is engaged in activities considerably more inspiring and rewarding
than competitive sports.

Now consider that when Dawn is looking precisely down, directly toward
the center of Vesta, its wings are level. If it is pointed off even a
little, then one of those long extensions is slightly closer to the
massive body it is circling and one is slightly farther away. Because
gravity diminishes with increasing distance, the one that is closer is
subject to a very slightly stronger pull than the farther other. If
unchecked, that lower side would gently be pulled down even more, thus
increasing the difference in gravitational attraction between the two
wings still more. Eventually, this would cause Dawn to be oriented so
that one wing points straight down toward the ancient surface below and
the other points straight up, back into the depths of space. Because
this phenomenon depends on the change in gravity from the lower point to
the higher one, it is known as "gravity gradient." Some satellites that
orbit Earth are designed to take advantage of the gravity gradient to
align their long axis with the planet below, but Dawn (and most other
spacecraft) need greater flexibility in where they point.

Rather than accepting the passive method of orienting provided by the
gravity gradient, Dawn uses its reaction wheels to train its science
instruments on Vesta. By electrically changing the rate at which these
devices spin, the ship can control its orientation in the zero-gravity,
frictionless conditions of spaceflight. When a small deviation from the
perfect orbit causes it to tip its wings a little when pointing to where
it calculates "down" to be, the spacecraft's reaction wheels work to
prevent it from succumbing to the gravity gradient, countering the
tendency of the wings to deviate still more from being level. As a
consequence, the ship remains stable and the wheels gradually spin
faster and faster as it conducts its observations.

To reduce the wheels' speeds, mission planners schedule a period almost
every day in LAMO during which the spacecraft fires its reaction control
system thrusters, a function known as "desaturating the wheels." Indeed, the
principal reason Dawn is outfitted with these small thrusters and a
modest supply of conventional rocket propellant known as hydrazine is to
manage the speed of the wheels.

The thruster firings not only provide the torque needed to reduce the
rotation rate of the wheels, but they also have the incidental effect of
propelling the spacecraft slightly. The push is small, changing the
orbital speed by no more than about one centimeter per second (around
one fiftieth of a mph, or about 120 feet per hour). But that causes Dawn
to deviate from its planned orbit, and the accumulated force from all
the firings is the largest source of trajectory discrepancies in LAMO.

To summarize so far, once Dawn has any variance at all between the
predicted orbital motion that mission controllers have radioed to it and
its actual path, its long wings will be tipped a little while it
observes Vesta. In opposing the resultant gravity gradient effect, the
reaction wheels will accelerate. When the reaction control system
thrusters fire to decelerate the wheels, they will nudge Dawn still more
off course, and the cycle will continue.

Of course, engineers have devised strategies to accommodate this
contribution (and others) to deviations from the plan. In LAMO, they
frequently measure the ship's trajectory and revise their estimates of
the future course. They transmit to the spacecraft a new prediction for
the orbit twice a week, so the main computer usually has a very good
estimate of where it is relative to Vesta and hence how to orient itself
so that its long solar arrays remain level as it acquires its fabulous
pictures and other scientific information. With the updated knowledge of
its position, Dawn can aim its sensors accurately and keep the thruster
firings from being excessive, even when it is not following its orbit
perfectly. This solution works well, but let's continue delving into the
consequences of the orbital perturbations.

While the operations team has the capability to provide the ship
regularly with a good description of where it will be, it is much more
difficult to make such frequent adjustments to its detailed itinerary.
The schedule of its myriad activities has to be planned longer in
advance. The sequences of commands, which are timed to the second, are
very complicated to develop and verify, and the operations team does not
have the resources to refine the timing as often as they can send
updates on the craft's predicted location.

Engineers took many factors into account in selecting the orbits Dawn
uses for its science observations. We saw in November
that the orbits are characterized not only
by the altitude but also by the orientation of the orbital plane. A
subsequent log will explain the choices for the planes more fully, but
for now, what matters is that, among other considerations, the orbits
were designed to ensure Dawn remains in constant sunlight. It always has
the sun in sight, never entering Vesta's shadow. Keeping Earth in view
at all times was not part of the design, and on every one of the more
than 600 revolutions around the gigantic rocky body since August 28 (the
seventh circuit in survey orbit), the spacecraft
has been temporarily behind Vesta from the geocentric point of view. In
its present orbit, these occultations last for about half an hour in
every 4.3-hour loop.

When Dawn is observing Vesta, that doesn't matter. When it is using its
ion propulsion system to transfer from one orbit to another, it also
doesn't matter. It does matter, however, when it is in contact with
Earth, because Vesta blocks the radio signal. Controllers give the
spacecraft a detailed schedule of which data to transmit and when,
making the best possible use of the precious communications link that
stretches across the solar system. The timed plan tells it not to send
high priority data during the radio blackout, but the timing of the
occultations can shift a little as the orbit departs from the plan.

The strategy to deal with the slight deviations in the timing of the
interruption in the radio link principally involves including some
padding in the plan. The schedule for the transmission of the highest
priority data places it well away from the expected gap, so no important
losses occur if Dawn is a little ahead in its orbit or a little behind
(causing the gap to occur a little earlier or a little later).

But what is there to do during and near the time the craft is predicted
to be blocked by Vesta while conducting a communications session? Dawn
rotates too slowly to make it worth turning to point its sensors at the
surface just for these periods. Of course, it could simply transmit
nothing at all. Instead, the team has it transmit data that otherwise
would be lost. There is never enough time to send to Earth all the
information the probe generates and collects. So most of the time it is
behind Vesta, it broadcasts many of the measurements of its own
subsystems that cannot be stored and sent later. And during the periods
immediately before and after the expected occultation, when there is a
chance that the signal will reach Earth, it sends bonus pictures and VIR
spectra. If the deviations from the planned orbit are small, then the
antenna will have an unobstructed view of Earth, and these data will
make it home. And if the spacecraft enters the blackout period late (or
early), then it will exit late (or early) as well, so the bonus results
sent before (or after) the occultation will be received. But in the rest
of the cases, well, Dawn will transmit those bits right back where they
came from, sending the photos and spectra into the vast rocky surface
between the spacecraft and Earth.

Last month we described one of the limitations in how much bonus
information could be obtained from LAMO. Now we have another. In summary,
because the probe can acquire more images and other data than it is
possible to return, it radios some of them during times that it is
possible they will make it to Earth. Because of realistic causes of
variation from its predicted orbital path, however, some of these
measurements will be transmitted when, from Dawn's perspective, Vesta
blocks Earth, thus preventing the broadcast signals from getting through.
The GRaND observations (as well as essential telemetry on the health of
the ship) are scheduled to be sent during times that, even with the
reasonable range of orbit discrepancies, the communications link will
not be obstructed. In this way, mission planners return as much data as
possible, taking maximum advantage of the time Dawn points its main
antenna to Earth. Having a sophisticated robot in orbit around the
second most massive resident of the asteroid belt presents truly unique
opportunities for the exploration of the solar system, and the team has
devised every strategy they could to use the time as productively as possible.

The spacecraft aims GRaND at Vesta most of the time in order to develop
a good picture of the weak nuclear glow. Controllers schedule three
periods per week, each about eight hours, in which it directs its
antenna to Earth. The orbit predictions have been extremely good,
matching the actual motion quite well. Moreover, some time is allocated
to return the camera and VIR data apart from the times that Vesta might
be in the way. As a result, the team has been rewarded with more than
3200 photos from LAMO so far. Every one is bonus, and every one is neat!

After well over four years of travel in deep space and already half a
year in orbit around Vesta, engineers recently encountered a bug lurking
in the spacecraft's software. As with most bugs, this one had waited
silently until just the right circumstances occurred to provoke it. The
combination of conditions was achieved late in the day on January 13,
and the bug caused the main computer to reboot. Dawn correctly responded
by going into safe mode. The mission control team observed this the next
day, and promptly began investigating the reason. They soon determined
the nature of the bug (as well as ways to ensure it would never be
activated again) and restored the spacecraft to its usual operating
configuration for LAMO. Even with the slow communications in safe mode,
the long time for radio signals to travel between Earth and Dawn, and
the frequent interruptions by the regular occultations by Vesta, they
had fully restored all systems by January 19. It took a few more days to
configure GRaND, but it, along with the other instruments, is now back
to its intensive inspection of Vesta.

We saw last month that the mission has been
progressing so well that the time originally allocated to deal with
anomalies had not been needed, so it is being applied to extend the
duration of LAMO. This allows even more scientific observations to be
conducted in this lowest altitude. Far from the planet it left in 2007,
in a region of the solar system in which no other spacecraft has ever
taken up residence, Dawn will continue its exploration of Vesta,
alternating between examining the alien world below and transmitting its
discoveries to Earth. Meanwhile, everyone who ponders what undiscovered
lands lie beyond our sight, everyone who hungers for exciting challenges
and noble adventures, and everyone who values turning the unknown into
the known profits from the great treasures this stalwart cosmic
ambassador sends to its erstwhile home, a faraway place it will never
visit again.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.08 AU (461
million kilometers or 286 million miles) from Earth, or 1155 times as
far as the moon and 3.13 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take 51 minutes
to make the round trip.
Received on Fri 27 Jan 2012 03:31:55 PM PST