[meteorite-list] Dawn Journal - February 29, 2016

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 4 Mar 2016 12:13:42 -0800 (PST)
Message-ID: <201603042013.u24KDgJ5004705_at_zagami.jpl.nasa.gov>

Dawn Journal
by Dr. Marc Rayman
February 29, 2016
 

Dear Indawnbitably Successful Readers,

A story of intense curiosity about the cosmos, passionate perseverance
and bold ingenuity, a story more than two centuries in the making, has
reached an extraordinary point. It begins with the discovery of dwarf
planet Ceres in 1801 (129 years before its sibling Pluto; each was designated
a planet for a time). Protoplanet Vesta was discovered in 1807. Following
200 years of telescopic observations, Dawn's daring mission was to explore
these two uncharted worlds, the largest, most massive residents of the
main asteroid belt between Mars and Jupiter. And now, as of February 2016,
the spacecraft has accomplished all of the objectives that NASA defined
for it in 2004, even before construction began (and before the very first
Dawn Journal, nearly a decade ago).

More than eight years after leaving its erstwhile planetary home behind
for an ambitious deep space adventure, Dawn has now collected all of the
data originally planned. Indeed, even prior to this third intercalary
day of its expedition, the probe had already actually sent back a great
deal more data for all investigations, significantly exceeding not only
the original goals but also new ones added after the ship had set sail
on the interplanetary seas. While scientists have a great deal of work
still ahead to translate the bounty of data into knowledge, which is the
greatest joy of science, the spacecraft can continue its work with the
satisfaction that it
has fulfilled its purpose and achieved an outstandingly successful mission.

[Dawn LAMO Image 23]
Dawn took this picture of the rim of Datan crater on Jan. 7 in its fourth
mapping orbit at 240 miles (385 kilometers). It flew over the same location
on Oct. 2, 2015, in its third mapping orbit at 915 miles (1,470 kilometers).
To see the improvement in detail, compare this with the earlier image
(presented fully in November but reproduced in part below to make comparison
easier). The bright material to the right of the crater rim here may help
you locate this area within the wider image. Full image and caption. Image
credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

[HAMO Image 57]
Dawn took this picture in its third mapping orbit at an altitude of 915
miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit.
The prominent triplet of overlapping craters nicely displays relative
ages, which are apparent by which ones affect others and hence which ones
formed later. The largest crater, Geshtin, is 48 miles (77 kilometers)
across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian
goddess of the vine.) A subsequent impact that excavated Datan crater,
which is 37 miles (60 kilometers) in diameter, obliterated a large section
of Geshtin's rim and made its own crater wall in Geshtin's interior. (Datan
is one of the Polish gods who protect the fields but apparently not this
crater.) Still later, Datan itself was the victim of a sizable impact
on its rim (although not large enough to have merited an approved name
this early in the geological studies of Ceres). Full image and caption.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is the only spacecraft ever to orbit two extraterrestrial destinations,
which would have been impossible without its advanced ion propulsion system.
It is the only spacecraft ever to orbit an object in the main asteroid
belt. It is also the only spacecraft ever to orbit massive bodies (apart
from the sun and Earth) that had not been visited first by a flyby spacecraft
to characterize the gravity and other properties. (By the way, Ceres is
one of eight solar system bodies that operating spacecraft are orbiting
now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko,
Mars and Saturn.)

Now in its fourth and final mapping orbit at Ceres, at an altitude of
240 miles (385 kilometers), Dawn is closer to the exotic terrain than
the International Space Station is to Earth. The benefit of being in orbit
is that the probe can linger rather than take only a brief look during
a fast flyby. Even though Dawn has met its full list of objectives at
Ceres, it continues to return new, valuable pictures and other measurements
to provide even greater insight into this relict from the dawn of the
solar system. For example, it is acquiring more nuclear spectra with its
gamma ray and neutron detector, sharpening its picture of some atomic
elements on Ceres. In addition, taking advantage of its unique vantage
point, Dawn is collecting more infrared spectra of locations that are
of special interest and soon will also take color photos and stereo photos
(as it did in the third mapping orbit) of selected areas.

Dawn has completed more than 600 revolutions since taking up residence
one year ago. The first few orbits took several weeks each, but as the
spacecraft descended and Ceres' gravitational embrace grew more firm,
its orbital velocity increased and the orbital period decreased. Now circling
in less than five and a half hours, Dawn has made 370 orbits since reaching
this altitude on Dec. 7.

[Dawn LAMO Image 23]
On Jan. 1, Dawn observed this scene at 78 degrees south latitude. This
deep in the southern hemisphere, the sun is low on the horizon (it is
three degrees north of the equator). The long shadows emphasize the topography
in this densely cratered (and therefore old) region. Landslides are evident
in the large crater wall on the left. Full image and caption. Image credit:
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The pace of observations here is higher than in the previous mapping orbits,
where the orbital periods were longer. The spacecraft flies over the landscape
faster now, and being closer to the ground, its instruments discern much
more detail but capture a smaller area. Mission controllers have developed
intricate plans for observing Ceres, but those plans depend on the spacecraft
being at the right place at the right time. As we will see below, however,
sometimes it may not be.

Suppose, for example, the intent is to observe a particular feature, perhaps
the bright center of Occator crater, the lonely, towering mountain Ahuna
Mons, the fractures in Dantu crater or artificial structures that definitively
prove the existence of extraterrestrial intelligence, utterly transforming
our understanding of the cosmos and shattering our naive perspectives
on life in the universe. Trajectory analysis indicates when Dawn will
fly over the designated location, and engineers will program it to take
pictures or infrared spectra at that time. They will also include some
margin, so they may program it to start 10 minutes before and end 10 minutes
after. But they can't afford to put in too much margin. Data storage on
the spacecraft is limited, so other geological features could not be observed.
Also, transmitting data to Earth requires pointing the main antenna at
that distant planet instead of pointing sensors at Ceres, so it would
be unwise to collect much more than is necessary.

Even if devoting additional time (and data) to trying to observe the desired
place were feasible, it wouldn't necessarily solve the problem. Dawn travels
in a polar orbit, which is the only way to ensure that it passes over
all latitudes. While Dawn soars from north to south over the sunlit hemisphere
making its observations, the dwarf planet itself rotates on its axis,
so the ground moves from east to west. If the spacecraft arrives at the
planned orbital location a little early or a little late, the feature
of interest may not even be beneath it but rather could be too far east
or west, out of view of the instruments. In that case, increasing the
duration of the observation period doesn't help.

All of that is why, as we saw last month, it requires more pictures to
fully map Ceres than you might expect. Many pictures may have to be taken
in order to fill in gaps, and quite a few of the pictures overlap with
others. Nevertheless, Dawn has done an excellent job. The spacecraft has
photographed 99.6 percent of the dwarf planet from this low altitude.
(If you aren't regularly visiting the image gallery, you are missing out
on some truly out-of-this-world scenes.)

[Dawn LAMO Image 33]
Dawn photographed this scene on Jan. 4 as it was looking toward the horizon
(as explained last month). Fluusa, the large crater from the center to
the upper left is 37 miles (60 kilometers) in diameter. (Fluusa was a
goddess of flowers for the Oscans of southern Italy who honored her to
make plants bloom and bear fruit.) Its degraded features and dense cratering
show it is old. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The flight team devises very detailed plans that tell the spacecraft what
to do every second, including where to point and what data to collect
with each sensor. When the observation plans are developed, they are checked
and double-checked. Then they are translated into the appropriate software
that the robotic ship will understand, and these instructions are checked
and double-checked. That is integrated with all the other software that
will be beamed to the spacecraft covering the same period of time, any
conflicts are resolved and then the final version is checked and, well,
you know.

This process is very involved, and it is usually well over a month between
the formulation and the execution of the plan. During that time, Dawn's
orbit can deviate slightly from the expert navigators' mathematical predictions,
preventing the spacecraft from flying over the desired targets. There
are several reasons the actual orbit may differ from the orbit used for
developing the plan. (We have seen related examples of this, including
as Dawn approached Mars, when it orbited Vesta and when it spiraled from
one mapping orbit to another.) Let's briefly consider two.

One reason is that we do not have perfect knowledge of the variations
in the strength of Ceres' gravitational pull from one location to another.
We have discussed before that measuring these tiny irregularities in the
gravity field provides insight into the distribution of mass within the
dwarf planet that gives rise to them. The team has mapped the hills and
valleys of the field quite well and even better than expected. Still,
the remaining small uncertainty can lead to slight differences between
what navigators calculate Dawn's motion will be and what its actual motion
will be as it is buffeted by the gravitational currents.

A second source of discrepancy is that Dawn's own activities distort its
orbit. Every time the reaction control system expels a tiny burst of hydrazine
to control the spacecraft's orientation, keeping it pointed at its target,
the force not only affects the orientation but also nudges the probe in
its orbit, slowing it down or speeding it up very slightly. It's up to
the spacecraft to decide exactly when to make these small adjustments,
and it is not possible for controllers to predict their timing. (In a
similar way, when you are driving, you occasionally move the steering
wheel to keep going the direction you want, even if is straight ahead.
It would be impossible to forecast each tiny movement, because they all
depend on what has already happened plus the exact conditions at the moment.)
The details of the reaction control system activity also depend on the
use of the novel hybrid control scheme, which the joint Orbital/JPL team
developed because of the failure of two of the spacecraft's four reaction
wheels. The effect of each small firing of hydrazine is very small, but
they can add up.

[Dawn LAMO Image 20]
Dawn had this view of two unnamed craters on Jan. 1. The craters are about
10 miles (16 kilometers) and 3 miles (5 kilometers) in diameter. The distinct
features show these are relatively young craters, not yet degraded by
subsequent impacts or geological processes intrinsic to Ceres. The lighting
in the craters shows that the sun is to the right, illuminating the left
side of the depressions and missing the right side. Click on the image
(or follow the link to the full image) and look carefully inside and around
the larger crater. There are many small features that are light on the
right and dark on the left. Therefore, they aren't depressions like these
two craters. Rather, they rise up, catching the light as it comes in from
the right, and their left sides are in shadow. These are large blocks
from the impact that excavated the crater. Each pixel in this picture
is 120 feet (35 meters). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It took about a month in this mapping orbit to discover many of the subtleties
of the gravity field and gain experience with how hybrid control affects
the orbit. But even before descending to this altitude, the operations
team understood the nature of these effects and was well prepared to deal
with them.

They devised several strategies, all of which are being used to good effect.
One of the ways to account for Dawn's actual orbit differing from its
planned orbit is simply to change the orbit. Simply? Well, not really.
It turns out to that to analyze the orbit and then maneuver to correct
it in a timely way is a surprisingly complicated process, but, come to
think of it, what isn't complicated when flying a spaceship around a distant,
alien world? Nevertheless, every three weeks, the flight team makes a
careful assessment of the orbit and determines whether a small refinement
with the ion propulsion system is in order. For technical reasons, if
maneuvers are needed, they will be executed in pairs, so mission planners
have scheduled two windows (each 12 hours long and separated by eight
days) about every 22 days.

Adjustments to resynchronize the actual orbit with the predicted orbit
that formed the basis of the exploration plan are known as "orbit maintenance
maneuvers." Succumbing to instincts developed during their long evolutionary
history, engineers refer to them by an acronym: OMM. (As the common thread
among team members is their technical training and passion for the exploration
of the cosmos, and not Buddhism, the term is spoken by naming the letters,
not pronouncing it as if it were a means of achieving inner peace. Instead,
it may be thought of as a means of achieving orbital tranquility and harmony.)

For both Vesta and Ceres, trajectory analyses long in advance determined
that OMMs would not be needed in the higher orbits, so no windows were
included in those schedules. There have been three OMM opportunities since
arriving at the lowest altitude above Ceres, but only the first was needed.
Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously
efficient ion engine. The orbit was good enough the next two times that
OMMs were deemed unnecessary. It is certain that some future OMMs will
be required. Your faithful correspondent provides frequent (and uncharacteristically
concise) reports on Dawn's day-to-day activities, including OMMs.

By the end of the Jan. 8 OMM, Dawn's ion propulsion system had accumulated
2,019 days of operation in space, more than 5.5 years. During that time,
the effective change in speed was 24,600 mph (39,600 kilometers per hour).
(We have discussed in detail that this is not Dawn's current speed but
rather the amount by which the ion engines have changed it.) This is uniquely
high for a spacecraft to accomplish with its own propulsion system and
validates our description of ion propulsion as delivering acceleration
with patience. (The previous record holder, Deep Space 1, achieved 9,600
mph, or 15,000 kilometers per hour.)

The effect of Dawn's gentle ion thrusting during its mission has been
nearly the same as that of the entire Delta II 7925H-9.5 rocket, with
its nine external rocket engines, first stage, second stage and third
stage. To get started on its interplanetary adventure, Dawn's rocket boosted
it from Cape Canaveral to out of Earth orbit with only four percent higher
velocity than Dawn subsequently added on its own with its ion engines.

As Dawn and Earth follow their own independent orbits around the sun (Dawn's
now tied permanently to its gravitational master, Ceres), next month they
will reach their greatest separation of the entire mission. On March 4
(about one Earth year after Ceres took hold of Dawn), on opposite sides
of the solar system, they will be 3.95278 AU (367.434 million miles, or
591.328 million kilometers) from each other. (For those of you with full
schedules, note that the maximum separation will be 5:40 a.m. PST.) They
won't be this far apart again until Feb. 6, 2025, long after Dawn has
ceased operating (as discussed below). The figure below depicts the arrangement
next month.

March Geometry

Earth's and Ceres' orbits will bring them to their maximum separation
on March 4. Earth's orbit is shown in green and Ceres' is in purple. Dawn's
interplanetary trajectory is in blue. Compare this figure with the ones
depicting Dawn and Earth on opposite sides of the sun in December 2014,
Dawn equidistant from Earth and the sun in April 2015, and Dawn and Earth
at their minimum separation in July 2015. Also note that Earth has completed
one full loop around the sun in the year since March 2015, when Dawn arrived
at Ceres. During the same period, Ceres, traveling in a higher heliocentric
orbit, has completed only about a fifth of a revolution. Credit: NASA/JPL-Caltech

Dawn has faced many challenges in its unique voyage in the forbidding
depths of space, but it has surmounted all of them. It has even overcome
the dire threat posed by the loss of two reaction wheels (the second failure
occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0
billion kilometers, ago). With only two operable reaction wheels (and
those no longer trustworthy), the ship's remaining lifetime is very limited.

A year ago, the team couldn't count on Dawn even having enough hydrazine
to last beyond next month. But the creative methods of conserving that
precious resource have proved to be quite efficacious, and the reliable
explorer still has enough hydrazine to continue to return bonus data for
a while longer. Now it seems highly likely that the spacecraft will keep
functioning through the scheduled end of its primary mission on June 30,
2016.

NASA may choose to continue the mission even after that. Such decisions
are difficult, as there is literally an entire universe full of interesting
subjects to study, but resources are more limited. In any case, even if
NASA extended the mission, and even if the two wheels operated without
faltering, and even if the intensive campaign of investigating Ceres executed
flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds
of glitches that can occur in such a complex undertaking, the hydrazine
would be exhausted early in 2017. Clearly an earlier termination remains
quite possible.

Regardless of when Dawn's end comes, it will not be a time for regret.
The mission has realized its raison d'e^tre and is reaping rewards even
beyond those envisioned when it was conceived. It has taken us all on
a marvelous interplanetary journey and allowed us to behold previously
unseen sights of distant lands. The conclusion of the mission will be
a time for gratitude that it was so successful. And until then, every
new picture or other measurement adds to the richly detailed portrait
of a faraway, exotic world. There is plenty more still to do before this
remarkable story draws to a close.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367
million miles, or 591 million kilometers) from Earth, or 1,475 times as
far as the moon and 3.99 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take one hour
and six minutes to make the round trip.
Received on Fri 04 Mar 2016 03:13:42 PM PST