[meteorite-list] Dawn Journal - December 31, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 1 Jan 2014 16:13:06 -0800 (PST)
Message-ID: <201401020013.s020D6e5019708_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_12_31_13.asp

Dawn Journal
Dr. Marc Rayman
December 31, 2013

Dear Clairvoydawnts,

Now more than halfway through its journey from protoplanet Vesta to dwarf
planet Ceres, Dawn is continuing to use its advanced ion propulsion system
to reshape its orbit around the sun. Now that the ship is closer to the
uncharted shores ahead than the lands it unveiled astern, we will begin
looking at the plans for exploring another alien world. In seven logs
from now through August, we will discuss how the veteran adventurer will
accomplish its exciting mission at Ceres. By the time it arrives early
in 2015 at the largest object between Mars and Jupiter, readers will be
ready to share not only in the drama of discovery but also in the thrill
of an ambitious undertaking far, far from Earth.

Mission planners separate this deep-space expedition into phases. Following
the "launch phase" was the 80-day "checkout phase." The "interplanetary
cruise phase" is the longest. It began on December 17, 2007, and continued
to the "Vesta phase," which extended from May 3, 2011, to Sept. 4, 2012.
We are back in the interplanetary cruise phase again and will be until
the "Ceres phase" begins in 2015. (Other phases may occur simultaneously
with those phases, such as the "oh man, this is so cool phase," the "we
should devise a clever name for this phase phase," and the "lunch phase.")
Because the tasks at Vesta and Ceres are so complex and diverse, they
are further divided into sub-phases. The phases at Ceres will be very
similar to those at Vesta, even though the two bodies are entirely different.
Dawn thrusting with its ion propulsion system as it approaches Ceres

In this log, we will describe the Ceres "approach phase." The objectives
of approach are to get the explorer into orbit and to attain a preliminary
look at the mysterious orb, both to satisfy our eagerness for a glimpse
of a new and exotic world and to obtain data that will be helpful in refining
details of the subsequent in-depth investigations. The phase will start
in January 2015 when Dawn is about 400,000 miles (640,000 kilometers)
from Ceres. It will conclude in April when the spacecraft has completed
the ion thrusting necessary to maneuver into the first orbit from which
it will conduct intensive observations, at an altitude of about 8,400
miles (13,500 kilometers). For a reason to be revealed below, that orbit
is known by the catchy cognomen RC3.

(Previews for the Vesta approach phase were presented in March 2010 and
May 2011, and the accounts of its actual execution are in logs from June,
July, and August 2011. Future space historians should note that the differing
phase boundaries at Vesta are no more than a matter of semantics. At Vesta,
RC3 was described as being part of the approach phase. For Ceres, RC3
is its own distinct phase. The reasons for the difference in terminology
are not only unimportant, they aren't even interesting.)

The tremendous maneuverability provided by Dawn's uniquely capable ion
propulsion system means that the exact dates for events in the approach
phase likely will change between now and then. So for those of you in
2015 following a link back to this log to see what the approach plan has
been, we offer both the reminder that the estimated dates here might shift
by a week or so and a welcome as you visit us here in the past. We look
forward to meeting you (or even being you) when we arrive in the future.

Most of the approach phase will be devoted to ion thrusting, making the
final adjustments to Dawn's orbit around the sun so that Ceres's gravity
will gently take hold of the emissary from distant Earth. Next month we
will explain more about the unusual nature of the gradual entry into orbit,
which will occur on about March 25, 2015.

Starting in early February 2015, Dawn will suspend thrusting occasionally
to point its camera at Ceres. The first time will be on Feb. 2, when they
are 260,000 miles (420,000 kilometers) apart. To the camera's eye, designed
principally for mapping from a close orbit and not for long-range observations,
Ceres will appear quite small, only about 24 pixels across. But these
pictures of a fuzzy little patch will be invaluable for our celestial
navigators. Such "optical navigation" images will show the location of
Ceres with respect to background stars, thereby helping to pin down where
it and the approaching robot are relative to each other. This provides
a powerful enhancement to the navigation, which generally relies on radio
signals exchanged between Dawn and Earth. Each of the 10 times Dawn observes
Ceres during the approach phase will help navigators refine the probe's
course, so they can update the ion thrust profile to pilot the ship smoothly
to its intended orbit.

Whenever the spacecraft stops to acquire images with the camera, it also
will train the visible and infrared mapping spectrometer on Ceres. These
early measurements will be helpful for finalizing the instrument parameters
to be used for the extensive observations at closer range in subsequent
mission phases.

Dawn obtained images more often during the Vesta approach phase than it
will 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 hominin colleagues
at JPL, along with excellent support from Orbital Sciences Corporation,
have applied their remarkable creativity, tenacity, and technical acumen
to devise a plan that should allow all the original objectives of exploring
Ceres to be met regardless of the health of the wheels. 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.

When Dawn next peers at Ceres, nine days after the first time, it will
be around 180,000 miles (290,000 kilometers) away, and the pictures will
be marginally better than the sharpest views ever captured by the Hubble
Space Telescope. By the third optical navigation session, on Feb. 21,
Ceres will show noticeably more detail.

At the end of February, Dawn will take images and spectra throughout a
complete Ceres rotation of just over nine hours, or one Cerean day. During
that period, while about 100,000 miles (160,000 kilometers) distant, 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. Dawn will see most of the surface with a resolution twice
as good as what has been achieved with Hubble. (At that point in the curving
approach trajectory, the probe will be south of Ceres' equator, so it
will not be able to see the high northern latitudes.) This first "rotation
characterization," or RC1, not only provides the first (near-complete)
look at the surface, but it may also suggest to insightful readers what
will occur during the RC3 orbit phase.

There will be six more imaging sessions before the end of the approach
phase, with Ceres growing larger in the camera's view each time. When
the second complete rotation characterization, RC2, is conducted on March
16, the resolution will be four times better than Hubble's pictures. The
last photos, to be collected on March 24, will reveal features seven times
smaller than could be discerned with the powerful space observatory.

The approach imaging sessions will be used to accomplish even more than
navigating, providing initial characterizations of the mysterious world,
and whetting our appetites for more. Six of the opportunities also will
include searches for moons of Ceres. Astronomers have not found moons
of this dwarf planet in previous attempts, but Dawn's unique vantage point
would allow it to discover smaller ones than would have been detectable
in previous attempts.

When the approach phase ends, Dawn will be circling its new home, held
in orbit by the massive body's gravitational grip and ready to begin more
detailed studies. By then, however, the pictures and other data it will
have returned will already have taught Earthlings a great deal about that
enigmatic place. Ceres has been observed from Earth for more than two
centuries, having first been spotted on January 1, 1801, but it has never
appeared as much more than an indistinct blob amidst the stars. Soon a
probe dispatched by the insatiably curious creatures on that faraway planet
will take up residence there to uncover some of the secrets it has held
since the dawn of the solar system. We don't have long to wait!

Dawn is 20 million miles (32 million kilometers) from Vesta and 19 million
miles (31 million kilometers) from Ceres. It is also 2.42 AU (225 million
miles, or 362 million kilometers) from Earth, or 1,015 times as far as
the moon and 2.46 times as far as the sun today. Radio signals, traveling
at the universal limit of the speed of light, take 40 minutes to make
the round trip.
Received on Wed 01 Jan 2014 07:13:06 PM PST