[meteorite-list] Dawn Journal - November 28, 2014

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2014/11/28/dawn-journal-november-28/

Dawn Journal
by Marc Rayman
November 28, 2014

Dear Unidawntified Flying Objects,

Flying silently and smoothly through the main asteroid belt between Mars 
and Jupiter, Dawn emits a blue-green beam of high velocity xenon ions. 
On the opposite side of the sun from Earth, firing its uniquely efficient 
ion propulsion system, the distant adventurer is continuing to make good 
progress on its long trek from the giant protoplanet Vesta to dwarf planet 
Ceres.

This month, let's look ahead to some upcoming activities. You can use 
the sun in December to locate Dawn in the sky, but before we describe 
that, let's see how Dawn is looking ahead to Ceres, with plans to take 
pictures on the night of Dec. 1.

The robotic explorer's sensors are complex devices that perform many 
sensitive measurements. To ensure they yield the best possible scientific 
data, their health must be carefully monitored and maintained, and they 
must be accurately calibrated. The sophisticated instruments are activated 
and tested occasionally, and all remain in excellent condition. One final 
calibration of the science camera is needed before arrival at Ceres. To 
accomplish it, the camera needs to take pictures of a target that appears 
just a few pixels across. The endless sky that surrounds our interplanetary 
traveler is full of stars, but those beautiful pinpoints of light, while 
easily detectable, are too small for this specialized measurement. But 
there is an object that just happens to be the right size. On Dec. 1, 
Ceres will be about nine pixels in diameter, nearly perfect for this calibration.

The images will provide data on very subtle optical properties of the 
camera that scientists will use when they analyze and interpret the details 
of some of the pictures returned from orbit. At 740,000 miles (1.2 million 
kilometers), Dawn's distance to Ceres will be about three times the 
separation between Earth and the moon. Its camera, designed for mapping 
Vesta and Ceres from orbit, will not reveal anything new. It will, however, 
reveal something cool! The pictures will be the first extended view for 
the first probe to reach the first dwarf planet discovered. They will 
show the largest body between the sun and Pluto that has not yet been 
visited by a spacecraft, Dawn's destination since it climbed out of 
Vesta's gravitational grip more than two years ago.
Image of Ceres

This will not be the first time Dawn has spotted Ceres. In a different 
calibration of the camera more than four years ago, the explorer descried 
its faint destination, far away in both time and space. Back then, still 
a year before arriving at Vesta, Dawn was more than 1,300 times farther 
from Ceres than it will be for this new calibration. The giant of the 
main asteroid belt was an indistinct dot in the vast cosmic landscape. 
Now Ceres is the brightest object in Dawn's sky save the distant sun. 
When it snaps the photos, Ceres will be as bright as Venus sometimes appears 
from Earth (what astronomers would call visual magnitude -3.6).
First image of Vesta

[Image]
Dawn's first extended picture of Ceres will be only slightly larger 
than this image of Vesta taken on May 3, 2011, at the beginning of the 
Vesta approach phase. The inset shows the pixelated Vesta, extracted from 
the main picture in which the overexposed Vesta can be seen against the 
background of stars. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To conserve hydrazine, a precious resource following the loss of two reaction 
wheels, Dawn will thrust with its ion propulsion system when it performs 
this calibration, which requires long exposures. In addition to moving 
the spacecraft along in its trajectory, the ion engine stabilizes the 
ship, enabling it to point steadily in the zero-gravity of spaceflight. 
(Dawn's predecessor, Deep Space 1, used the same trick of ion thrusting 
in order to be as stable as possible for its initial photos of comet Borrelly.)

As Dawn closes in on its quarry, Ceres will grow brighter and larger. 
Last month we summarized the plan for photographing Ceres during the first 
part of the approach phase, yielding views in January comparable to the 
best we currently have (from Hubble Space Telescope) and in February significantly 
better. The principal purpose of the pictures is to help navigators steer 
the ship into this uncharted, final port following a long voyage on the 
interplanetary seas. The camera serves as the helmsman's eyes. Ceres 
has been observed with telescopes from (or near) Earth for more than two 
centuries, but it has appeared as little more than a faint, fuzzy blob 
farther away than the sun. But not for much longer!

The only spaceship ever built to orbit two extraterrestrial destinations, 
Dawn's advanced ion propulsion system enables its ambitious mission. 
Providing the merest whisper of thrust, the ion engine allows Dawn to 
maneuver in ways entirely different from conventional spacecraft. In January, 
we presented in detail Dawn's unique way of slipping into orbit. In 
September, a burst of space radiation disrupted the thrust profile. As 
we saw, the flight team responded swiftly to a very complex problem, minimizing 
the duration of the missed thrust. One part of their contingency operations 
was to design a new approach trajectory, accounting for the 95 hours that 
Dawn coasted instead of thrust. Let's take a look now at how the resulting 
trajectory differs from what we discussed at the beginning of this year.

In the original approach, Dawn would follow a simple spiral around Ceres, 
approaching from the general direction of the sun, looping over the south 
pole, going beyond to the night side, and coming back above the north 
pole before easing into the targeted orbit, known by the stirring name 
RC3, at an altitude of 8,400 miles (13,500 kilometers). Like a pilot landing 
a plane, flying this route required lining up on a particular course and 
speed well in advance. The ion thrusting this year had been setting Dawn 
up to get on that approach spiral early next year.

The change in its flight profile following the September encounter with 
a rogue cosmic ray meant the spiral path would be markedly different and 
would require significantly longer to complete. While the flight team 
certainly is patient - after all, Earth's robotic ambassador won't 
reach Ceres until 213 years after its discovery and more than seven years 
after launch - the brilliantly creative navigators devised an entirely 
new approach trajectory that would be shorter. Demonstrating the extraordinary 
flexibility of ion propulsion, the spacecraft now will take a completely 
different path but will wind up in exactly the same orbit.
Trajectory

[Graphic]
In this view, looking down on the north pole of Ceres, the sun is off 
the figure to the left and Ceres' counterclockwise orbital motion around 
the sun takes it from the bottom of the figure to the top. Dawn flies 
in from the left, travels out ahead of Ceres, and then is captured on 
the way to the apex of its orbit. The white circles are at one-day intervals, 
illustrating how Dawn slows down gradually at first. (When the circles 
are closer together, Dawn is moving more slowly.) After capture, both 
Ceres' gravity and the ion thrust slow it even more before the craft 
accelerates to the end of the approach phase. (You can think of this perspective 
as being from above. Then the next figure shows the view from the side, 
which here would mean looking toward the action from a location off the 
bottom of the graphic.) Credit: NASA/JPL

The spacecraft will allow itself to be captured by Ceres on March 6, only 
about half a day later than the trajectory it was pursuing before the 
hiatus in thrust, but the geometry both before and after will be quite 
different. Instead of flying south of Ceres, Dawn is now targeted to lead 
it, flying out ahead of it as the dwarf planet orbits the sun, and then 
the spacecraft will begin to gently curve around it. (You can see this 
in the figure to the left.) Dawn will come to 24,000 miles (38,000 kilometers) 
and then will slowly arc away. But thanks to the remarkable design of 
the thrust profile, the ion engine and the gravitational pull from the 
behemoth of rock and ice will work together. At a distance of 41,000 miles 
(61,000 kilometers), Ceres will reach out and tenderly take hold of its 
new consort, and they will be together evermore. Dawn will be in orbit, 
and Ceres will forever be accompanied by this former resident of Earth.

If the spacecraft stopped thrusting just when Ceres captured it, it would 
continue looping around the massive body in a high, elliptical orbit, 
but its mission is to scrutinize the mysterious world. Our goal is not 
to be in just any arbitrary orbit but rather in the particular orbits 
that have been chosen to provide the best scientific return for the probe's 
camera and other sensors. So it won't stop but instead will continue 
maneuvering to RC3.

Ever graceful, Dawn will gently thrust to counter its orbital momentum, 
keeping it from swinging up to the highest altitude it would otherwise 
attain. On March 18, nearly two weeks after it is captured by Ceres' 
gravity, Dawn will arc to the crest of its orbit. Like a ball thrown high 
that slows to a momentary stop before falling back, Dawn's orbital ascent 
will end at an altitude of 47,000 miles (75,000 kilometers), and Ceres' 
relentless pull (aided by the constant, gentle thrust) will win out. As 
it begins descending toward its gravitational master, it will continue 
working with Ceres. Rather than resist the fall, the spacecraft will thrust 
to accelerate itself, quickening the trip down to RC3.

There is more to the specification of the orbit than the altitude. One 
of the other attributes is the orientation of the orbit in space. (Imagine 
an orbit as a ring around Ceres, but that ring can be tipped and tilted 
in many ways.) To provide a view of the entire surface as Ceres rotates 
underneath it, Dawn needs to be in a polar orbit, flying over the north 
pole as it travels from the nightside to the dayside, moving south as 
it passes over the equator, sailing back to the unilluminated side when 
it reaches the south pole, and then heading north above terrain in the 
dark of night. To accomplish the earlier part of its new approach trajectory, 
however, Dawn will stay over lower latitudes, very high above the mysterious 
surface but not far from the equator. Therefore, as it races toward RC3, 
it will orient its ion engine not only to shorten the time to reach that 
orbital altitude but also to tip the plane of its orbit so that it encircles 
the poles (and tilts the plane to be at a particular orientation relative 
to the sun). Then, finally, as it gets closer still, it will turn to use 
that famously efficient glowing beam of xenon ions against Ceres' gravity, 
acting as a brake rather than an accelerator. By April 23, this first 
act of a beautiful new celestial ballet will conclude. Dawn will be in 
the originally intended orbit around Ceres, ready for its next act: the 
intensive observations of RC3 we described in February.
Comparison of Trajectories

[Graphic]
North is at the top of this figure and the sun is far to the left. Ceres 
orbital motion around the sun carries it straight into the figure. The 
original approach took Dawn over Ceres' south pole as it spiraled directly 
into RC3. On the new approach, it looks here as if it flies in over the 
north pole, but that is because of the flat depiction. As the previous 
figure shows, the approach takes Dawn well ahead of Ceres. The upper part 
of the green trajectory is not in the same plane as the original approach 
and RC3; rather, it is in the background, "behind" the graphic. As 
Dawn flies to the right side of the diagram, it also comes forward to 
the plane of the figure to align with the targeted RC3. As before, the 
circles, spaced at intervals of one day, indicate the spacecraft's speed; 
where they are closer together, the ship travels more slowly. (You can 
think of this perspective as being from the side and the previous figure 
as showing the view from above, off the top of this graphic.) Credit: 
NASA/JPL

Dawn's route to orbit is no more complex and elegant than what any crackerjack 
spaceship pilot would execute. However, one of the key differences between 
what our ace will perform and what often happens in science fiction movies 
is that Dawn's maneuvers will comply with the laws of physics. And if 
that's not gratifying enough, perhaps the fact that it's real makes 
it even more impressive. A spaceship sent from Earth more than seven years 
ago, propelled by electrically accelerated ions, having already maneuvered 
extensively in orbit around the giant protoplanet Vesta to reveal its 
myriad secrets, soon will bank and roll, arc and turn, ascend and descend, 
and swoop into its planned orbit.
Earth, Sun, & Dawn

[Graphic]
Illustration of the relative locations (but not sizes) of Earth, the sun, 
and Dawn in early December 2014. (Earth and the sun are at that location 
every December.) The images are superimposed on the trajectory for the 
entire mission, showing the positions of Earth, Mars, Vesta, and Ceres 
at milestones during Dawn's voyage. Credit: NASA/JPL

And all this will take place far, far from Earth. Indeed, Dawn is on a 
very different heliocentric orbit from that of the planet it left behind 
in 2007. In December, their separate paths will take them to opposite 
sides of the sun. We will not have a similar celestial arrangement until 
2016, by which time the craft will be in its lowest altitude orbit at 
Ceres. (We invite our future selves to return to the past to tell us here 
how the view is. __ )  From our terrestrial perspective this year, Dawn 
will appear to be less than one solar diameter from the sun's limb on 
Dec. 9 and 10.

As Earth, the sun, and the spacecraft come closer into alignment, radio 
signals that go back and forth must pass near the sun. The solar environment 
is fierce indeed, and it will interfere with those radio waves. While 
some signals will get through, communication will not be reliable. Therefore, 
controllers plan to send no messages to the spacecraft from Dec. 4 through 
Dec. 15; all instructions needed during that time will be stored onboard 
beforehand. Occasionally Deep Space Network antennas, pointing near the 
sun, will listen through the roaring noise for the faint whisper of the 
spacecraft, but the team will consider any communication to be a bonus.

Dawn is big for an interplanetary spacecraft (or for an otherworldly dragonfly, 
for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, 
more than 3.8 times as far as the sun, 352 million miles (567 million 
kilometers) away, humankind lacks any technology even remotely capable 
of glimpsing it. But we can bring to bear something more powerful than 
our technology: our mind's eye. From Dec. 8 to 11, if you block the 
sun's blazing light with your thumb, you will also be covering Dawn's 
location. There, in that direction, is our faraway emissary to new worlds. 
It has traveled three billion miles (4.8 billion kilometers) already on 
its extraordinary extraterrestrial expedition, and some of the most exciting 
miles are still ahead as it nears Ceres. You can see right where it is. 
It is now on the far side of the sun.

The sun!

This is the same sun that is more than 100 times the diameter of Earth 
and a third of a million times its mass. This is the same sun that has 
been the unchallenged master of our solar system for more than 4.5 billion 
years. This is the same sun that has shone down on Earth all that time 
and has been the ultimate source of so much of the heat, light and other 
energy upon which the planet's residents have been so dependent. This 
is the same sun that has so influenced human expression in art, literature, 
mythology and religion for uncounted millennia. This is the same sun that 
has motivated scientific studies for centuries. This is the same sun that 
is our signpost in the Milky Way galaxy. And humans have a spacecraft 
on the far side of it. We may be humbled by our own insignificance in 
the universe, yet we still undertake the most valiant adventures in our 
attempts to comprehend its majesty.

Dawn is 780,000 miles (1.3 million kilometers) from Ceres, or 3.3 times 
the average distance between Earth and the moon. It is also 3.77 AU (350 
million miles, or 564 million kilometers) from Earth, or 1,525 times as 
far as the moon and 3.82 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take one hour 
and three minutes to make the round trip.