[meteorite-list] Dawn Journal - January 29, 2015

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 2 Feb 2015 10:59:54 -0800 (PST)
Message-ID: <201502021859.t12Ixs57010297_at_zagami.jpl.nasa.gov>

http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/

Dawn Journal
by Marc Rayman
January 29, 2015
 
Dear Abundawnt Readers,

The dwarf planet Ceres is a giant mystery. Drawn on by the irresistible
lure of exploring this exotic, alien world, Dawn is closing in on it.
The probe is much closer to Ceres than the moon is to Earth.

And now it is even closer...

And now it is closer still!

What has been glimpsed as little more than a faint smudge of light amidst
the stars for more than two centuries is finally coming into focus. The
first dwarf planet discovered (129 years before Pluto), the largest body
between the sun and Pluto that a spacecraft has not yet visited, is starting
to reveal its secrets. Dawn is seeing sights never before beheld, and
all of humankind is along for the extraordinary experience.

We have had a preview of Dawn's approach phase, and in November we looked
at the acrobatics the spacecraft performs as it glides gracefully into
orbit. Now the adventurer is executing those intricate plans, and it is
flying beautifully, just the way a seasoned space traveler should.

Dawn's unique method of patiently, gradually reshaping its orbit around
the sun with its ion propulsion system is nearly at its end. Just as two
cars may drive together at high speed and thus travel at low speed relative
to each other, Dawn is now close to matching Ceres' heliocentric orbital
motion. Together, they are traveling around the sun at nearly 39,000 mph
(almost 64,000 kilometers per hour), or 10.8 miles per second (17.4 kilometers
per second). But the spaceship is closing in on the world ahead at the
quite modest relative speed of about 250 mph (400 kilometers per hour),
much less than is typical for interplanetary spaceflight.

[Image]
Dawn observed Ceres for an hour on Jan. 13, from a distance of 238,000
miles (383,000 kilometers). A little more than half of the surface was
revealed as Ceres rotated. This imaging session is known as OpNav 1. Credit:
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dawn has begun its approach imaging campaign, and the pictures are wonderfully
exciting. This month, we will take a more careful look at the plans for
photographing Ceres. Eager readers may jump directly to the summary table,
but others may want to emulate the spacecraft by taking a more leisurely
approach to it, which may aid in understanding some details.

While our faithful Dawn is the star of this bold deep-space adventure
(along with protoplanet Vesta and dwarf planet Ceres), the real talent
is behind the scenes, as is often the case with celebrities. The success
of the mission depends on the dedication and expertise of the members
of the Dawn flight team, no farther from Earth than the eighth floor of
JPL?s building 264 (although occasionally your correspondent goes on the
roof to enjoy the sights of the evening sky). They are carefully guiding
the distant spacecraft on its approach trajectory and ensuring it accomplishes
all of its tasks.

To keep Dawn on course to Ceres, navigators need a good fix on where the
probe and its target are. Both are far, far from Earth, so the job is
not easy. In addition to the extraordinarily sophisticated but standard
methods of navigating a remote interplanetary spacecraft, using the radio
signal to measure its distance and speed, Dawn's controllers use another
technique now that it is in the vicinity of its destination.

>From the vantage point of Earth, astronomers can determine distant Ceres'
location remarkably well, and Dawn's navigators achieve impressive accuracy
in establishing the craft's position. But to enter orbit, still greater
accuracy is required. Therefore, Dawn photographs Ceres against the background
of known stars, and the pictures are analyzed to pin down the location
of the ship relative to the celestial harbor it is approaching. To distinguish
this method from the one by which Dawn is usually navigated, this supplementary
technique is generally known as 'optical navigation." Unable to suppress
their geekiness (or, at least, unmotivated to do so), Dawn team members
refer to this as OpNav. There are seven dedicated OpNav imaging sessions
during the four-month approach phase, along with two other imaging sessions.
(There will also be two more OpNavs in the spiral descent from RC3 to
survey orbit.)

The positions of the spacecraft and dwarf planet are already determined
well enough with the conventional navigation methods that controllers
know which particular stars are near Ceres from Dawn's perspective. It
is the analysis of precisely where Ceres appears relative to those stars
that will yield the necessary navigational refinement. Later, when Dawn
is so close that the colossus occupies most of the camera's view, stars
will no longer be visible in the pictures. Then the optical navigation
will be based on determining the location of the spacecraft with respect
to specific surface features that have been charted in previous images.

To execute an OpNav, Dawn suspends ion thrusting and turns to point its
camera at Ceres. It usually spends one or two hours taking photos (and
bonus measurements with its visible and infrared mapping spectrometer).
Then it turns to point its main antenna to Earth and transmits its findings
across the solar system to the Deep Space Network.

[Animation]
This animation of Ceres rotating was made by combining images taken by
the Dawn spacecraft on Jan. 25 over the course of one hour in OpNav 2.
Dawn was 147,000 miles (237,000 kilometers) from Ceres.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

While it is turning once again to resume ion thrusting, navigators are
already starting to extract information from the images to calculate where
the probe is relative to its destination. Experts update the design of
the trajectory the spacecraft must follow to reach its intended orbital
position and fine-tune the corresponding ion thrust flight plan. At the
next communications session, the revised instructions are radioed back
across the solar system, and then the reliable robot carries them out.
This process is repeated throughout the approach phase.

Dawn turned to observe Vesta during that approach phase more often than
it does on approach to Ceres, and the reason is simple. It has lost two
of its four reaction wheels, devices used to help turn or stabilize the
craft in the zero-gravity, frictionless conditions of spaceflight. (In
full disclosure, the units aren't actually lost. We know precisely where
they are. But given that they stopped functioning, they might as well
be elsewhere in the universe; they don't do Dawn any good.)

Dawn's sentient colleagues at JPL, along with excellent support from Orbital
Sciences Corporation, have applied their remarkable creativity, tenacity
and technical acumen to devise a strategy that allows all the original
objectives of exploring Ceres to be met regardless of the condition of
the wheels, even the (currently) healthy ones. Your correspondent refers
to this as the "zero reaction wheel plan." One of the many methods that
contributed to this surprising resilience was a substantial reduction
in the number of turns during all remaining phases of the mission, thus
conserving the precious hydrazine propellant used by the small jets of
the reaction control system. Guided by their successful experience at
Vesta, experts determined that they could accommodate fewer OpNavs during
the approach to Ceres, thus saving turns. (We will return to the topic
of hydrazine conservation below.)

The images serve several purposes besides navigation. Of course, they
provide a tantalizing preview of the intriguing world observed from Earth
since 1801. Each picture whets our appetite! What will Ceres look like
as it comes into sharper focus? Will we see evidence of a subsurface ocean?
What unexpected shapes and structures will we find? What strange new features
will show up? Just what is that bright spot? Quite simply: we don't know.
It would be a pretty good idea to send a spacecraft there to find out!

Scientists scrutinize all the photos for moons of Ceres, and OpNavs 3
- 7 will include many extra images with exposures chosen to help reveal
moons. In addition, hundreds more pictures will be taken of the space
around Ceres in the hours before and after OpNav 3 to allow an even more
thorough search.

On two occasions during the approach, Dawn will take images and spectra
throughout a complete Ceres rotation of slightly over nine hours, or one
Cerean day. During that time, Dawn's position will not change significantly,
so it will be almost as if the spacecraft hovers in place as the dwarf
planet pirouettes beneath its watchful eye, exhibiting most of the surface.
These "rotation characterizations" (known by the stirring names RC1 and
RC2) will provide the first global perspectives.

As Dawn flies into orbit, it arcs around Ceres. In November, we described
the route into orbit in detail, and one of the figures there is reproduced
here. Dawn will slip into Ceres' gravitational embrace on the night of
March 5 (PST). But as the figure shows, its initial elliptical orbit will
carry it to higher altitudes before it swoops back down. As a result,
pictures of Ceres will grow for a while, then shrink and then grow again.

[Image]
Dawn's approach trajectory. We are looking down on the north pole of Ceres.
The sun is off the figure far to the left. The spacecraft flies in from
the left and then is captured on the way to the apex of its orbit. It
gets closer to Ceres during the first part of its approach but then recedes
for a while before coming in still closer at the end. Lighting by the
sun is not depicted here, but when Dawn is on the right side of the figure,
it only sees a crescent of Ceres, which is illuminated from the left.
(The white circles are at one-day intervals.) Credit: NASA/JPL

Because of the changing direction to Ceres, Dawn does not always see a
fully illuminated disk, just as the moon goes through its familiar phases
as its position relative to the sun changes. The hemisphere of the moon
facing the sun is bright and the other is dark. The half facing Earth
may include part of the lit side and part of the dark side. Sometimes
we see a full moon, sometimes gibbous, and sometimes a thin crescent.

The table shows what fraction of Ceres is illuminated from Dawn's perspective.
Seeing a full moon would correspond to 100 percent illumination. A half
moon would be 50 percent, and a new moon would be zero percent. In OpNav
6, when Ceres is 18 percent illuminated, it will be a delicate crescent,
like the moon about four days after it's new.

[Images]
Four views of Ceres as it rotates, as seen with Hubble Space Telescope,
were the best we had before OpNav 2. All of Dawn?s pictures from now on
will show finer detail. Credit: NASA, ESA, J. Parker (Southwest Research
Institute), P. Thomas (Cornell University), and L. McFadden (University
of Maryland, College Park)

OpNav images of a narrow crescent won't contain enough information to
warrant the expenditure of hydrazine in all that turning. Moreover, the
camera's precision optics and sensitive detector, designed for revealing
the landscapes of Vesta and Ceres, cannot tolerate looking too close to
the sun, even as far from the brilliant star as it is now. Therefore,
no pictures will be taken in March and early April when Dawn is far on
the opposite side of Ceres from the sun. By the end of April, the probe
will have descended to its first observational orbit (RC3), where it will
begin its intensive observations.

The closer Dawn is to Ceres, the larger the orb appears to its camera,
and the table includes the (approximate) diameter the full disk would
be, measured in the number of camera pixels. To display greater detail,
each pixel must occupy a smaller portion of the surface. So the "resolution"
of the picture indicates how sharp Dawn's view is.

We also describe the pictures in comparison to the best that have been
obtained with Hubble Space Telescope. In Hubble's pictures, each pixel
covered about 19 miles (30 kilometers). Now, after a journey of more than
seven years through the solar system, Dawn is finally close enough to
Ceres that its view surpasses that of the powerful telescope. By the time
Dawn is in its lowest altitude orbit at the end of this year, its pictures
will be well over 800 times better than Hubble's and more than 600 times
better than the OpNav 2 pictures from Jan. 25. This is going to be a fantastic
year of discovery!

[Table]

Some of the numbers may change slightly as Dawn?s trajectory is refined
and even as estimates of the strength of Ceres? gravitational tug improve.
(Dawn is already feeling that pull, even though it is not yet in orbit.)
Still, this should help you fill out your social calendar for the next
few months.

To get views like those Dawn has, you can build your own spaceship and
fly it deep into the heart of the main asteroid belt to this intriguing
world of rock and ice. Or you can visit our Ceres image gallery to see
pictures as soon as they are released. If you chose the first option,
use your hydrazine wisely!

As we discussed above, to explore Ceres without the use of the reaction
wheels that were essential to the original design, mission controllers
have worked very hard to conserve hydrazine. Let's see how productive
that effort has been. (You should be able to follow the story here without
careful focus on the numbers. They are here for the more technically oriented
readers, accountants and our old friends the Numerivores.)

Dawn launched in Sept. 2007 with 101 pounds (45.6 kilograms) of hydrazine.
The ship escaped from Vesta in Sept. 2012, four weeks after the second
reaction wheel failed during the climb out of Vesta's gravitational hole.
(By the way, Dawn is now more than one thousand times farther from Vesta
than it is from Ceres. It is even farther from Vesta than Earth is from
the sun!) At the beginning of the long interplanetary flight to Ceres,
it still had 71.2 pounds (32.3 kilograms) left. As it had expended less
than one-third of the original supply through the end of the Vesta expedition,
that might seem like plenty. But it was not. Without the reaction wheels,
subsequent operations would consume much more hydrazine. Indeed, engineers
determined that even if they still had the entire amount that had been
onboard at launch, it would not be enough. The Ceres objectives were at
serious risk!

The flight team undertook an aggressive campaign to conserve hydrazine.
They conceived more than 50 new candidate techniques for reducing hydrazine
consumption in the 30-month journey to Ceres and the 18 months of Ceres
operations and systematically but quickly assessed every one of them.

The team initially calculated that the long interplanetary flight between
the departure from Vesta and the beginning of the Ceres approach phase
would consume 27.6 pounds (12.5 kilograms) of hydrazine even if there
were no errors, no glitches, no problems and no changes in the plans.
Following the intensive conservation work, they determined that the spacecraft
might instead be able to complete all of its assignments for only 9.7
pounds (4.4 kilograms), an astonishing 65 percent reduction. (Keep track
of that mass through the end of the next paragraph.) That would translate
directly into more hydrazine being available for the exploration of Ceres.
They devised many new methods of conducting the mission at Ceres as well,
estimating today that it will cost less than 42.5 pounds (19.3 kilograms)
with the zero reaction wheel plan. (If the two remaining wheels operate
when called upon in the lowest orbit, they will provide a bonus reduction
in hydrazine use.)

Dawn's two years and four months of interplanetary cruise concluded on
Dec. 26, 2014, when the approach phase began. Although the team had computed
that they might squeeze the consumption down to as low as 9.7 pounds (4.4
kilograms), it's one thing to predict it and it's another to achieve it.
Changes to plans become necessary, and not every detail can be foreseen.
As recounted in October, the trip was not entirely free of problems, as
a burst of cosmic radiation interrupted the smooth operations. Now that
the cruise phase is complete, we can measure how well it really went.
Dawn used 9.7 pounds (4.4 kilograms), exactly as predicted in 2012. Isn't
flying spacecraft through the forbidding depths of the interplanetary
void amazing?

This success provides high confidence in our ability to accomplish all
of the plans at Ceres (even if the remaining reaction wheels are not operable).
Now that the explorer is so close, it is starting to reap the rewards
of the daring 3.0-billion-mile (4.9-billion-kilometer) journey to an ancient
world that has long awaited a terrestrial emissary. As Dawn continues
its approach phase, our growing anticipation will be fueled by thrilling
new pictures, each offering a new perspective on this relict from the
dawn of the solar system. Very soon, patience, diligence and unwavering
determination will be rewarded with new knowledge and new insight into
the nature of the cosmos.

Dawn is 121,000 miles (195,000 kilometers) from Ceres, or half the average
distance between Earth and the moon. It is also 3.63 AU (338 million miles,
or 544 million kilometers) from Earth, or 1,390 times as far as the moon
and 3.69 times as far as the sun today. Radio signals, traveling at the
universal limit of the speed of light, take one hour to make the round
trip.
Received on Mon 02 Feb 2015 01:59:54 PM PST