[meteorite-list] Dawn Journal - May 27, 2010

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 3 Jun 2010 09:35:58 -0700 (PDT)
Message-ID: <201006031635.o53GZwfY021422_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_05_27_10.asp

Dawn Journal
Dr. Marc Rayman
May 27, 2010

Dear Multitudawnous Readers,
 
After more than 2.5 years of spaceflight, and more than 6 months
in the asteroid belt, Dawn's interplanetary journey continues
smoothly. The mission remains on course and schedule for this
expedition to the dawn of the solar system.
 
Our Dawn is not the first spacecraft to use this name, although it
is traveling farther from home than any other Dawn. This month 2
more craft traveled into space carrying that appellation, at least
when translated into English. The Japanese Aerospace Exploration
Agency sent Akatsuki to Earth's neighbor Venus, and Russia's
Rassvet module was attached to the International Space Station in
Earth orbit. The solar system is vast, however, and there is
plenty of room for all such spacecraft. We send our best wishes
for success to these other Dawns as they embark on their missions.
 
While our Dawn patiently and reliably thrusts with its ion
propulsion system, gradually reshaping its path around the Sun to
match orbits with the protoplanet Vesta, the human members of the
team are very busy on distant Earth. Among their many activities
is developing the sequences the robotic explorer will use when it
begins studying that mysterious, alien world next year. We have seen
recently what will occur during the "approach phase" and how Dawn will
slip into orbit around Vesta. Now let's have a preview of what the ship
will do once it has reached the first science orbit, known as "survey
orbit." Engineers are developing those sequences now, for execution in
August 2011.
 
In survey orbit, the probe will be about 2700 kilometers (1700
miles) above the surface. During the approach phase, navigators
will measure the strength of Vesta's gravitational tug on the
spacecraft so they can compute the giant asteroid's mass with much
greater accuracy than astronomers have yet been able to determine
it. (The mass is calculated now using observations of how Vesta
perturbs the orbits of other asteroids and even of Mars.) That
knowledge will allow them to refine the survey orbit altitude, and
they may target it to be somewhat higher or lower, depending on
whether Vesta is more massive or less massive than the current
calculations show. The sequences for acquiring science data are
being designed to accommodate a reasonable range of masses.
 
Dawn will be in a near-polar orbit. Its trajectory will take it
over the north pole (which will be in darkness, because it will be
northern hemisphere winter at that time), then over the terminator
(the boundary between the illuminated and unilluminated sides),
down over the equator, over the south pole, and then across the
terminator again to pass over Vesta's night side. Such an orbit
allows the spacecraft to have a view of virtually every part of
the lit surface at some time. Each revolution in survey orbit will
take 2.5 to 3 days to complete. While this may seem like a
leisurely pace, the spacecraft will be busy the entire time.
 
When on the day side of Vesta, Dawn will conduct an intensive
campaign of observations. Vesta rotates on its axis in about 5
hours, 20 minutes (one Vestian "day"), which is faster than Dawn
will be advancing in its orbit. So from the spacecraft's
perspective, as it progresses slowly from north to south, the
globe beneath it will complete several turns on its axis. That
affords excellent opportunities for mapping the body.
 
During most of approach, Vesta will be so far away that it will
fit comfortably in the fields of view of the science camera
and the visible and infrared mapping spectrometer. Before Dawn reaches
survey orbit, however, it will be too close to capture all of the
expansive surface with its sensors in one glance. On each
revolution, the sequences will command the spacecraft to point the
instruments through profiles that will allow them to observe as
much of the surface as possible.
 
The primary objective of survey orbit is to get a broad overview
of Vesta with color pictures and with ultraviolet, visible, and
infrared spectra. The camera will obtain views with 250 meters
(820 feet) per pixel, about 150 times sharper than the best images
from the Hubble Space Telescope. The mapping spectrometer will
reveal much of the surface at better than 700 meters (2300 feet)
per pixel. While subsequent science orbits will yield more detail,
these first, new perspectives of this ancient world will represent
an exciting step in the exploration of the solar system.
 
Throughout the year at Vesta, gamma-ray spectra and neutron
spectra will be recorded with GRaND, and
ultrasensitive measurements of the spacecraft's motion using the
radio signal will reveal ever greater details of the protoplanet's
gravity field and hence its internal structure. Although such
information will be acquired in survey orbit, these investigations
will benefit most from the lower altitude orbits.
 
Survey orbit is planned to last for 6 revolutions, or about 17
days. For most of the time it is on the day side, Dawn will fill
its memory buffers with images and spectra. For most of the other
half of each orbit, as it travels over the night side, the
spacecraft will transmit those precious data through its main
antenna to eager scientists and all others curious about the
cosmos who reside on Earth. (Even when the surface below the
spacecraft is in darkness, Dawn itself will be high enough that it
will remain in sunlight, so its solar arrays will continue to
provide electrical power.) There is so much to see at Vesta, and
the instruments generate so much data, that a simple strategy of
filling the memory on the day side and emptying it on the night
side would be too limiting. Therefore, in the middle of its
second, fourth, and fifth passes over the sunlit side, Dawn will
halt its acquisition of data to spend a few hours radioing some of
its findings to Earth, making more room for subsequent measurements.
 
Because the program of activities during the residence at Vesta is
so full, and it all has to be planned in detail long before Dawn
arrives, the project needs plans that are resilient to the
inevitable problems, both large and small, that arise in such
complex and challenging endeavors. While every observation in
survey orbit is of interest, many more are scheduled than are
necessary to fulfill the scientific objectives. Therefore, even if
some are missed because of glitches in systems on the spacecraft
or on Earth, as long as others are acquired, the mission will
proceed. With the extremely rich set of measurements planned,
there is no intention of repeating some that are lost.
 
After it has completed its survey of Vesta, Dawn will resume
thrusting, spiraling down to its next science orbit for an even
closer view. We will learn more about that in a subsequent log.
 
Meanwhile, as the craft continues to propel itself toward its
destination, traveling farther and longer than ever, it will pass
3 milestones on its journey next month. Look for a NASA news
release soon on a record it will set as it keeps thrusting with
its ion propulsion system. We will describe that in the next log.
 
On June 23, Dawn will have been in flight for 1000 days. No doubt
readers will enjoy taking a minute (at least, for those who read
61,000 words per minute) to reread all the logs since launch
to recall some of what has occurred so far during the
mission. While much has already been accomplished, the great
rewards lie ahead, as Dawn pushes deeper into the asteroid belt,
where it will explore faraway new worlds.
 
On June 3, Dawn will be exactly twice as far from Earth as Earth
is from the Sun. Of course, the distance between the planet and
the star does not matter for the spacecraft; it is on its own
independent journey through the solar system. Nevertheless, such
an occasion may provide some terrestrial readers with another
opportunity to reflect upon the nature of such a journey. Dawn's
trek is not simply that of a robot in space. Although in a narrow
sense the ship is sailing the cosmic seas on its own, there is
much more to the voyage than that. Such a mission represents a
journey by a remarkable species that does not allow its physical
confinement to the vicinity of its home planet to keep it from
reaching ever farther in its pursuit of knowledge and its quest
for grand and noble adventures.
 
Dawn is 1.96 AU (293 million kilometers or 182 million miles) from
Earth, or 760 times as far as the Moon and 1.93 times as far as
the Sun. Radio signals, traveling at the universal limit of the
speed of light, take 33 minutes to make the round trip.
 
Received on Thu 03 Jun 2010 12:35:58 PM PDT