[meteorite-list] Dawn Journal - March 28, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 29 Mar 2010 11:27:04 -0700 (PDT)
Message-ID: <201003291827.o2TIR4jd005557_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_03_28_10.asp

Dawn Journal
Dr. Marc Rayman
March 28, 2010

Dear Dawnthorities,
 
Dawn continues patiently forging through the asteroid belt, its
permanent residence, as it climbs away from Earth and the Sun.
Having thrust with its ion propulsion system for more than 1.5
years, the spacecraft remains healthy and on target for its
rendezvous with alien worlds.
 
Our interplanetary adventurer still has a great deal of ion
thrusting to complete before it can begin its orbital exploration
of Vesta next year. Although it will suspend thrusting for a few
weeks this summer to conduct some special activities (to follow
along, be sure to renew your subscription to these logs the first
time our helpfully persistent telemarketers call), it will devote
most of the time until early August 2011 in powered flight,
continuously reshaping its orbit around the Sun.
 
In addition to keeping the ship sailing smoothly and on course,
Dawn's engineers (who reside and work on distant Earth) are
developing the detailed instructions that will guide it into orbit
around Vesta and throughout its year of operations there. This
process began last month and will continue even as the probe
begins executing the first of the commands in May 2011.
 
Mission controllers compile Dawn's instructions by assigning a
time to each individual command. Groups of these timed commands
are known as a "sequence." During the current interplanetary
cruise phase of the mission, sequences generally extend for 5
weeks, but some special activities may use sequences as short as a
few hours. Usually more than one sequence is executing at a time,
but like all the instruments in an orchestra, they are carefully
synchronized and coordinated so the overall score accomplishes the
composer's artistic intent.
 
Readers may recall that the mission is separated into phases.
Following the "launch phase" was the 80-day "checkout phase". The
current "interplanetary cruise phase," which began
on December 17, 2007, is the longest. It ends when the "Vesta
phase" begins. (Other phases may occur simultaneously with those
phases, such as the "oh man, this is so cool phase," the "what
clever name are we going to give this phase phase," and the "lunch
phase.") Because the mission at Vesta is so complex, it is further
divided into sub-phases. The Vesta sequences that are being
developed now are for the "approach phase." Approach begins in
early May 2011 and concludes 3 months later when Dawn has
maneuvered to the first orbit from which it will conduct intensive
science observations, known as survey orbit.
 
Most of the approach phase is dedicated to the final ion thrusting
required to slip into orbit around Vesta. All of Dawn's thrusting
contributes to rendezvousing with Vesta, but the terminal
thrusting will be controlled slightly differently. We will
describe the process of using ion propulsion to enter orbit around
another solar system body in an upcoming log. For now, however,
let's take a look at some of the other activities during the
approach phase. While these are being timed in the sequences down
to the second, part of the strategy for developing these sequences
is to allow the team a means to update the times as the probe
closes in on its target. The ion propulsion system provides
flexibility in the timing that is different from most missions,
and to take advantage of the benefits, the sequences must be
correspondingly flexible. All the relative timing within a
sequence will be fixed, but the time each sequence is activated
can change. So, for example, even though we may change the date
the first Vesta approach sequence begins executing by as much as a
few days, once that adjustment is made, all the events within the
sequence will shift by exactly the same interval. Some small
changes other than timing, such as details of the probe's
orientation, may be made as well to reflect the latest information
available before it is time to transmit the sequences to the
spacecraft more than a year from now.
 
The principal activity other than thrusting during approach is the
acquisition of images of Vesta with Dawn's main science camera,
primarily for navigation. From the
distant vantage point of Earth, astronomers can determine Vesta's
location with astonishing accuracy, and the Dawn navigation team
achieves extraordinary accuracy in establishing the probe's
position, but for the craft to enter orbit, still greater accuracy
is required. Therefore, Dawn will observe Vesta's location against
the background of stars, and the photographs will be analyzed by
celestial navigators to pin down the relative location of the ship
and the port of call it is approaching. To distinguish this method
from the one by which Dawn is usually navigated, making use of its
radio signal, this supplementary technique with pictures is
generally known as "optical navigation." There are 24 optical
navigation sessions during the 3-month approach phase. Many of
these will be combined with observations of Vesta designed to help
prepare for subsequent scientific measurements.
 
The positions of the spacecraft and protoplanet will be determined
well enough with the current navigation method that engineers will
know which stars will appear to be near Vesta from Dawn's
perspective. It is the analysis of /precisely/ where Vesta appears
relative to those stars that will yield the necessary navigational
refinement. When Dawn is closer to Vesta, the giant asteroid will
occupy most or all of the camera's view, and stars won't be
visible. 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.
 
For the optical navigation observations, Dawn will halt thrusting
and align itself so that Vesta and, when possible, the stars are
in view of the camera. It will spend half an hour or more taking
images and storing them for transmission at the next scheduled
communications session. The information extracted from the images
will be used to calculate where the probe is relative to its
destination. Engineers then will update the design of the
trajectory for the spacecraft to follow to reach its intended
orbit and fine-tune the ensuing thrust profile to ensure that Dawn
accomplishes the revised flight plan.
 
The first optical navigation images will be acquired when Dawn is
about 1.2 million kilometers (750 thousand miles) from Vesta, or
more than 3 times the separation between Earth and the Moon.
Dawn's camera is designed for mapping Vesta from orbit. Therefore,
instead of a high-power telescope with a narrow field of view, the
camera has a relatively low magnification but covers a broad area.
The camera achieves the equivalent of a magnification of about 3
compared to unaided human eyes. When these first optical
navigation images are taken, distant Vesta will appear to be only
about 5 pixels across. But at that stage, navigators will need to
know its location, not its appearance, so the images will be of
great value.
 
For 8 of the approach observation periods, in addition to the
camera, the visible and infrared mapping spectrometer (VIR)
will be trained on Vesta. By taking some early measurements with
the camera and VIR, scientists will have the opportunity to make
fine adjustments to the instrument parameters in the sequences for
later observations.
 
In one of the optical navigation sessions in July, the camera will
acquire many images of the space around Vesta in a search for
moons. Astronomers have looked for moons of Vesta before, and will
do so again before the explorer reaches its vicinity. Although
none has been discovered, Dawn's unique vantage point will provide
more data. The existence of moons would be of interest both for
science and for mission safety.
 
When Dawn suspends thrusting to check for moons, it also will
collect a series of images as Vesta rotates. Like Earth and all
other solar system bodies, Vesta spins. It completes one turn on
its axis (one Vestal "day") in about 5 hours, 20 minutes. These
measurements will help characterize the alien world still more to
aid in navigation and to prepare for subsequent observations with
the science instruments. The moon search will be during the second
of 3 observations of a full rotation.
 
Over the course of the 3-month approach, it will be exciting to
watch Vesta grow from little more than a tiny smudge in the first
optical navigation images until it is too large to fit in the
camera's view at the end of the phase. By early June 2011, the
images will surpass the best that can be obtained with the Hubble
Space Telescope. All succeeding observations will yield better and
better views, both rewarding us and tantalizing us as Dawn
prepares for its more intensive studies in later Vesta phases.
 
The spacecraft will glide into a very high orbit in late July and
continue thrusting, gently as always, until early August, when it
will arrive in its survey orbit at an orbit at an altitude of
about 2700 kilometers (1700 miles). The activities to be conducted
in the survey phase will be described when mission planners are
working on those sequences.
 
In the meantime, the team is running some of the approach
sequences through the Dawn spacecraft simulator at JPL down the
hall from mission control. The simulator includes some hardware
that is virtually identical to what is on the spacecraft and some
software to take the place of other hardware components. The
simulator is one of several methods used to check complex
sequences before they are approved for transmission to the
spacecraft.
 
It is both unnecessary and impossible to test all sequences. The
simulator operates in real-time, so it would take 3 months to run
all the approach sequences, and the Dawn team has too many other
tests to perform with the simulator to allow that. Because much of
the approach phase consists of ion thrusting, an activity which is
quite familiar not only to the spacecraft but also to mission
controllers (as well as regular readers of these logs), there is
no need to test the thrusting periods. Engineers review each
sequence to determine which portions would benefit from testing.
 
While the spacecraft simulator is hard at work at JPL, the actual
spacecraft continues its work elsewhere. On February 28, Dawn and
the Sun were equidistant from Earth.
Now, as the distant explorer continues to propel itself toward its
rendezvous with Vesta, it is farther from Earth than the Sun ever
is. Moreover, even as the probe and the planet follow their
separate paths around the Sun, Dawn will remain farther from Earth
than the Sun. The orbits of Mercury, Venus, Mars, and many other
members of the solar system family occasionally bring them closer
to our planet than the Sun, but Dawn has enlarged its orbit so
much that it never will return to the region of the solar system
in which it began its ambitious journey of discovery.
 
Dawn is 1.27 AU (191 million kilometers or 118 million miles) from
Earth, or 525 times as far as the Moon and 1.28 times as far as
the Sun. Radio signals, traveling at the universal limit of the
speed of light, take 21 minutes to make the round trip.
Received on Mon 29 Mar 2010 02:27:04 PM PDT