[meteorite-list] Dawn Journal - March 31, 2015

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 1 Apr 2015 10:46:26 -0700 (PDT)
Message-ID: <201504011746.t31HkQIk027650_at_zagami.jpl.nasa.gov>

http://dawnblog.jpl.nasa.gov/?p=2437

Dawn Journal
by Dr. Marc Rayman
March 31, 2015
 
Dear Dawnticipating Explorers,

Now orbiting high over the night side of a dwarf planet far from Earth,
Dawn arrived at its new permanent residence on March 6. Ceres welcomed
the newcomer from Earth with a gentle but firm gravitational embrace.
The goddess of agriculture will never release her companion. Indeed, Dawn
will only get closer from now on. With the ace flying skills it has demonstrated
many times on this ambitious deep-space trek, the interplanetary spaceship
is using its ion propulsion system to maneuver into a circular orbit 8,400
miles (13,500 kilometers) above the cratered landscape of ice and rock.
Once there, it will commence its first set of intensive observations of
the alien world it has traveled for so long and so far to reach.

For now, however, Dawn is not taking pictures. Even after it entered orbit,
its momentum carried it to a higher altitude, from which it is now descending.
>From March 2 to April 9, so much of the ground beneath it is cloaked in
darkness that the spacecraft is not even peering at it. Instead, it is
steadfastly looking ahead to the rewards of the view it will have when
its long, leisurely, elliptical orbit loops far enough around to glimpse
the sunlit surface again.

Among the many sights we eagerly anticipate are those captivating bright
spots. Hinted at more than a decade ago by Hubble Space Telescope, Dawn
started to bring them into sharper focus after an extraordinary journey
of more than seven years and three billion miles (nearly five billion
kilometers). Although the spots are reflections of sunlight, they seem
almost to radiate from Ceres as cosmic beacons, drawing us forth, spellbound.
Like interplanetary lighthouses, their brilliant glow illuminates the
way for a bold ship from Earth sailing on the celestial seas to a mysterious,
uncharted port. The entrancing lights fire our imagination and remind
us of the irresistible lure of exploration and the powerful anticipation
of an adventure into the unknown.

As we describe below, Dawn's extensive photographic coverage of the sunlit
terrain in early May will include these bright spots. They will not be
in view, however, when Dawn spies the thin crescent of Ceres in its next
optical navigation session, scheduled for April 10 (as always, all dates
here are in the Pacific time zone).

As the table here shows, on April 14 (and extending into April 15), Dawn
will obtain its last navigational fix before it finishes maneuvering.
Should we look forward to catching sight of the bright spots then? In
truth, we do not yet know. The spots surely will be there, but the uncertainty
is exactly where "there" is. We still have much to learn about a dwarf
planet that, until recently, was little more than a fuzzy patch of light
among the glowing jewels of the night sky. (For example, only last month
did we determine where Ceres' north and south poles point.) Astronomers
had clocked the length of its day, the time it takes to turn once on its
axis, at a few minutes more than nine hours. But the last time the spots
were in view of Dawn?s camera was on Feb. 19. From then until April 14,
while Earth rotates more than 54 times (at 24 hours per turn), Ceres will
rotate more than 140 times, which provides plenty of time for a small
discrepancy in the exact rate to build up. To illustrate this, if our
knowledge of the length of a Cerean day were off by one minute (or less
than 0.2 percent), that would translate into more than a quarter of a
turn during this period, drastically shifting the location of the spots
from Dawn's point of view. So we are not certain exactly what range of
longitudes will be within view in the scheduled OpNav 7 window. Regardless,
the pictures will serve their intended purpose of helping navigators establish
the probe's location in relation to its gravitational captor.

Dawn's gradual, graceful arc down to its first mapping orbit will take
the craft from the night side to the day side over the north pole, and
then it will travel south. It will conclude its powered flight over the
sunlit terrain at about 60 degrees south latitude. The spacecraft will
finish reshaping its orbit on April 23, and when it stops its ion engine
on that date, it will be in its new circular orbit, designated RC3. (We
will return to the confusing names of the different orbits at Ceres below.)
Then it will coast, just as the moon coasts in orbit around Earth and
Earth coasts around the sun. It will take Dawn just over 15 days to complete
one revolution around Ceres at this height. We had a preview of RC3 last
year, and now we can take an updated look at the plans.

Dawn's final swoop down to RC3 orbit. The sun is off the figure far to
the left, and Ceres' north pole points up. The farther Dawn is to the
right side of Ceres here, the smaller a crescent it sees, because the
illumination is from the left. The white circles are at one-day intervals.
The trajectory is solid where Dawn is thrusting with its ion engine, which
is most of the time. The labels show four optical navigation sessions,
where it pauses to turn, point at Ceres, conduct the indicated observation,
turn to point its main antenna to Earth, transmit its findings, turn back
to the orientation needed for thrusting, and then restart the ion engine.
Dawn was captured into orbit on March 6. Note the periods on the right
side of the figure between OpNav 5 (on March 1) and OpNav 6 (on April
10) when Dawn pauses thrusting for telecommunications and radio navigation
but does not take pictures because it would have to point its instruments
too close to the sun. Apodemeter is the Dawn team's word for the highest
altitude in orbit, in analogy with the more common term apogee, which
applies for Earth orbits. (Demeter is the Greek counterpart of the Roman
goddess Ceres.) Dawn was at its apodemeter of 46,800 miles (75,400 kilometers)
on March 18. For more on Dawn's approach trajectory, see the overall description
and figures from other perspectives in November (including the motion
into and out of this flat depiction), further details (including the OpNavs)
in February and an animation in March. Credit: NASA/JPL

The dwarf planet is around 590 miles (950 kilometers) in diameter (like
Earth and other planets, however, it is slightly wider at the equator
than from pole to pole). At the spacecraft's orbital altitude, it will
appear to be the same size as a soccer ball seen from 10 feet (3 meters)
away. Part of the basis upon which mission planners chose this distance
for the first mapping campaign is that the visible disc of Ceres will
just fit in the camera's field of view. All the pictures taken at lower
altitudes will cover a smaller area (but will be correspondingly more
detailed). The photos from RC3 will be 3.4 times sharper than those in
RC2.

There will be work to do before photography begins however. The first
order of business after concluding ion thrusting will be for the flight
team to perform a quick navigational update (this time, using only the
radio signal) and transmit any refinements (if necessary) in Dawn?s orbital
parameters, so it always has an accurate knowledge of where it is. (These
will not be adjustments to the orbit but rather a precise mathematical
description of the orbit it achieved.) Controllers will also reconfigure
the spacecraft for its intensive observations, which will commence on
April 24 as it passes over the south pole and to the night side again.

As at Vesta, even though half of each circular orbit will be over the
night side of Ceres, the spacecraft itself will never enter the shadows.
The operations team has carefully designed the orbits so that at Dawn?s
altitude, it remains illuminated by the sun, even when the land below
is not.

It may seem surprising (or even be surprising) that Dawn will conduct
measurements when the ground directly beneath it is hidden in the deep
darkness of night. To add to the surprise, these observations were not
even envisioned when Dawn's mission was designed, and it did not perform
comparable measurements during its extensive exploration of Vesta in 2011-2012.

The measurements on the night side will serve several purposes. One of
the many sophisticated techniques scientists use to elucidate the nature
of planetary surfaces is to measure how much light they reflect at different
angles. Over the course of the next year, Dawn will acquire tens of thousands
of pictures from the day side of Ceres, when, in essence, the sun is behind
the camera. When it is over the night side in RC3, carefully designed
observations of the lit terrain (with the sun somewhat in front of the
camera, although still at a safe angle) will significantly extend the
range of angles.

In December, we described the fascinating discovery of an extremely diffuse
veil of water vapor around Ceres. How the water makes its way from the
dwarf planet high into space is not known. The Dawn team has devised a
plan to investigate this further, even though the tiny amount of vapor
was sighted long after the explorer left Earth equipped with sensors designed
to study worlds without atmospheres.

It is worth emphasizing that the water vapor is exceedingly tenuous. Indeed,
it is much less dense than Earth's atmosphere at altitudes above the International
Space Station, which orbits in what most people consider to be the vacuum
of space. Our hero will not need to deploy its umbrella. Even comets,
which are miniscule in comparison with Ceres, liberate significantly more
water.

There may not even be any water vapor at all now because Ceres is farther
from the sun than when the Herschel Space Observatory saw it, but if there
is, detecting it will be very challenging. The best method to glimpse
it is to look for its subtle effects on light passing through it. Although
Dawn cannot gaze directly at the sun, it can look above the lit horizon
from the night side, searching intently for faint signs of sunlight scattered
by sparse water molecules (or perhaps dust lofted into space with them).

For three days in RC3 after passing over the south pole, the probe will
take many pictures and visible and infrared spectra as it watches the
slowly shrinking illuminated crescent and the space over it. When the
spacecraft has flown to about 29 degrees south latitude over the night
side, it will no longer be safe to aim its sensitive instruments in that
direction, because they would be too close to the sun. With its memory
full of data, Dawn will turn to point its main antenna toward distant
Earth. It will take almost two days to radio its findings to NASA's Deep
Space Network. Meanwhile, the spacecraft will continue northward, gliding
silently high over the dark surface.

On April 28, it will rotate again to aim its sensors at Ceres and the
space above it, resuming measurements when it is about 21 degrees north
of the equator and continuing almost to the north pole on May 1. By the
time it turns once again to beam its data to Earth, it will have completed
a wealth of measurements not even considered when the mission was being
designed.

Loyal readers will recall that Dawn has lost two of its four reaction
wheels, gyroscope-like devices it uses to turn and to stabilize itself.
Although such a loss could be grave for some missions, the operations
team overcame this very serious challenge. They now have detailed plans
to accomplish all of the original Ceres objectives regardless of the condition
of the reaction wheels, even the two that have not failed (yet). It is
quite a testament to their creativity and resourcefulness that despite
the tight constraints of flying the spacecraft differently, the team has
been able to add bonus objectives to the mission.

Dawn will finish transmitting its data after its orbit takes it over the
north pole and to the day side of Ceres again. For three periods during
its gradual flight of more than a week over the illuminated landscape,
it will take pictures (in visible and near-infrared wavelengths) and spectra.
Each time, it will look down from space for a full Cerean day, watching
for more than nine hours as the dwarf planet pirouettes, as if showing
off to her new admirer. As the exotic features parade by, Dawn will faithfully
record the sites.

It is important to set the camera exposures carefully. Most of the surface
reflects nine percent of the sunlight. (For comparison, the moon reflects
12 percent on average, although as many Earthlings have noticed, there
is some variation from place to place. Mars reflects 17 percent, and Vesta
reflects 42 percent. Many photos seem to show that your correspondent's
forehead reflects about 100 percent.) But there are some small areas that
are significantly more reflective, including the two most famous bright
spots. Each spot occupies only one pixel (2.7 miles, or 4.3 kilometers
across) in the best pictures so far. If each bright area on the ground
is the size of a pixel, then they reflect around 40 percent of the light,
providing the stark contrast with the much darker surroundings. When Dawn's
pictures show more detail, it could be that they will turn out to be even
smaller and even more reflective than they have appeared so far. In RC3,
each pixel will cover 0.8 miles (1.3 kilometers). To ensure the best photographic
results, controllers are modifying the elaborate instructions for the
camera to take pictures of the entire surface with a wider range of exposures
than previously planned, providing high confidence that all dark and all
bright areas will be revealed clearly.

Dawn will observe Ceres as it flies from 45 degrees to 35 degrees north
latitude on May 3-4. Of course, the camera's view will extend well north
and south of the point immediately below it. (Imagine looking at a globe.
Even though you are directly over one point, you can see a larger area.)
The territory it will inspect will include those intriguing bright spots.
The explorer will report back to Earth on May 4-5. It will perform the
same observations between 5 degrees north and 5 degrees south on May 5-6
and transmit those findings on May 6-7. To complete its first global map,
it will make another full set of measurements for a Cerean day as it glides
between 35 degrees and 45 degrees south on May 7.

By the time it has transmitted its final measurements on May 8, the bounty
from RC3 may be more than 2,500 pictures and two million spectra. Mission
controllers recognize that glitches are always possible, especially in
such complex activities, and they take that into account in their plans.
Even if some of the scheduled pictures or spectra are not acquired, RC3
should provide an excellent new perspective on the alien world, displaying
details three times smaller than what we have discerned so far.

Dawn activated its gamma ray spectrometer and neutron spectrometer on
March 12, but it will not detect radiation from Ceres at this high altitude.
For now, it is measuring space radiation to provide context for later
measurements. Perhaps it will sense some neutrons in the third mapping
orbit this summer, but its primary work to determine the atomic constituents
of the material within about a yard (meter) of the surface will be in
the lowest altitude orbit at the end of the year.

Dawn will conduct its studies from three lower orbital altitudes after
RC3, taking advantage of the tremendous maneuverability provided by ion
propulsion to spiral from one to another. We presented previews last year
of each phase, and as each approaches, we will give still more up-to-date
details, but now that Dawn is in orbit, let's summarize them here. Of
course, with complicated operations in the forbidding depths of space,
there are always possibilities for changes, especially in the schedule.
The team has developed an intricate but robust and flexible plan to extract
as many secrets from Ceres as possible, and they will take any changes
in stride.

Each orbit is designed to provide a better view than the one before, and
Dawn will map the orb thoroughly while at each altitude. The names for
the orbits - rotation characterization 3 (RC3); survey; high altitude
mapping orbit (HAMO); and low altitude mapping orbit (LAMO) - are based
on ancient ideas, and the origins are (or should be) lost in the mists
of time. Readers should avoid trying to infer anything at all meaningful
in the designations. After some careful consideration, your correspondent
chose to use the same names the Dawn team uses rather than create more
helpful descriptors for the purposes of these blogs. That ensures consistency
with other Dawn project communications. After all, what is important is
not what the different orbits are called but rather what amazing new discoveries
each one enables.

The robotic explorer will make many kinds of measurements with its suite
of powerful instruments. As one indication of the improving view, this
table includes the resolution of the photos, and the ever finer detail
may be compared with the pictures during the approach phase. For another
perspective, we extend the soccer ball analogy above to illustrate how
large Ceres will appear to be from the spacecraft's orbital vantage point.

Mapping
Orbit Dawn code
name Tentative dates (changes are guaranteed) Altitude
in miles
(kilometers) Resolution in
feet (meters)
per pixel Resolution compared to Hubble Orbit
period Equivalent
distance of
a soccer ball
1 RC3 April 23 ?
May 9 8,400
(13,500) 4,200
(1,300) 24 15
days 10 feet
(3.0 meters)
2 Survey June 6-30 2,700
(4,400) 1,400
(410) 72 3.1
days 3.3 feet
(1.0 meters)
3 HAMO Aug 4 ?
Oct 15 900
(1,450) 450
(140) 215 19
hours 13 inches
(33 cm)
4 LAMO Dec 8 ?
end of mission 230
(375) 120
(35) 850 5.5
hours 3.3 inches
(8.5 cm)
 
As Dawn orbits Ceres, together they orbit the sun. Closer to the master
of the solar system, Earth (with its own retinue, including the moon and
many artificial satellites) travels faster in its heliocentric orbit because
of the sun?s stronger gravitational pull at its location. In December,
Earth was on the opposite side of the sun from Dawn, and now the planet?s
higher speed is causing their separation to shrink. Earth will get closer
and closer until July 22, when it will pass on the inside track, and the
distance will increase again.

In the meantime, on April 12, Dawn be equidistant from the sun and Earth.
The spacecraft will be 2.89 AU or 269 million miles (433 million kilometers)
from both. At the same time, Earth will be 1.00 AU or 93.2 million miles
(150 million kilometers) from the sun.

It will be as if Dawn is at the tip of a giant celestial arrowhead, pointing
the way to a remarkable solar system spectacle. The cosmos should take
note! Right there, a sophisticated spaceship from Earth is gracefully
descending on a blue-green beam of xenon ions. Finally, the dwarf planet
beneath it, a remote remnant from the dawn of the solar system, is lonely
no more. Almost 4.6 billion years after it formed, and 214 years after
inquisitive creatures on a distant planet first caught sight of it, a
mysterious world is still welcoming the new arrival. And as Dawn prepares
to settle into its first close orbit, ready to discover secrets Ceres
has kept for so long, everyone who shares in the thrill of this grand
and noble adventure eagerly awaits its findings. Together, we look forward
to the excitement of new knowledge, new insight and new fuel for our passionate
drive to explore the universe.

Dawn is 35,000 miles (57,000 kilometers) from Ceres, or 15 percent of
the average distance between Earth and the moon. It is also 3.04 AU (282
million miles, or 454 million kilometers) from Earth, or 1,120 times as
far as the moon and 3.04 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 Wed 01 Apr 2015 01:46:26 PM PDT