[meteorite-list] Dawn's Early Light - April 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 6 Apr 2007 14:58:17 -0700 (PDT)
Message-ID: <200704062158.l36LwHu21100_at_zagami.jpl.nasa.gov>

D A W N ' S E A R L Y L I G H T April 2007
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The eleventh issue of the Dawn team newsletter, Dawn's Early Light,
has been posted on the Dawn website. Follow the links below to
view individual articles, or obtain the formatted pdf version. We look
forward to obtaining your feedback.

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Dawn Ready to Ship to Launch Site

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20070405/ship.html

Dawn Ready to Ship to Launch Site

Carol A. Raymond
Dawn Deputy Principal Investigator, JPL

The Dawn project successfully completed its Post-Environmental Test
Review (PETR) on March 19th, and its Pre-Ship Review, held March 20-21,
clearing the way for the Dawn spacecraft's trip to Cape Canaveral
for launch preparations. The PETR assessed the completeness and
validity of the thermal vacuum (TVAC) testing which was performed
Jan 23-Feb 18 at the Naval Research Laboratory in Washington, DC.
The spacecraft and instruments were put through a comprehensive
series of performance tests to validate the spacecraft thermal
model and operate the flight system (including instruments) in a
flight-like manner, culminating in a test-firing of two of the
three ion thrusters. Some rework was needed to adjust heater
circuit setpoints and correct wiring, and the High Voltage
Electronics Assembly was returned to JPL after TVAC for rework
of faults not related to the TVAC testing. The HVEA rework and
retesting has completed, and the box has been re-integrated to
the spacecraft. Following an acoustics test at NRL, the
spacecraft will be packed and shipped to the Astrotech
facility near Kennedy Space Center in Cape Canaveral.

Final assembly of the spacecraft at Astrotech involves
attaching the 19.7-meter solar array, which folds up
accordion-style against the spacecraft body for launch.
The array deploys after third-stage separation when thermal
knives cut the kevlar cords that holdback the pre-tensioned
panels. The high-gain antenna will also be re-installed and
a series of functional and scenario tests performed to verify
system functionality in preparation for launch. The Dawn
launch is scheduled for June 30th. This ten-day slip from the
original June 20th launch date was dictated by launch vehicle
readiness.

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Minor Planet Symposium to be held at Dawn Launch

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20070405/symposium.html

Christopher T. Russell
Dawn Principal Investigator, UCLA

The Dawn Science Team is planning to host a 3-day symposium
on the formation, evolution and current state of the minor
planets. The symposium will be held Thursday through Saturday,
June 28-30, in Cocoa Beach, Florida in conjunction with the
Dawn launch that is scheduled in the early evening of June 30th.
The nearby beach provides the closest public viewing area and
provides an excellent view of the launch pad area. The symposium
will feature invited and contributed talks, posters and ample
discussion time. Papers from the planetary astronomy, meteorite,
cosmochemistry, and small body communities are most welcome.

In order confirm meeting logistics, we request that you indicate
your interest in the symposium by email to swaas at igpp.ucla.edu.
Please indicate the topic of your presentation, and whether a
talk or poster is preferred; a title is not needed at this time.
If there is a major slip in the launch date, and the slip is
determined early enough, we would consider changing the date
of the symposium. The date of the symposium has been arranged
to accomodate minor slips with minimum inconvenience for
travellers. Cape Canaveral is a family friendly region with
plenty of attractions from the beach to the KSC Visitor's
Center to theme parks so you are encouraged to bring your
families to the meeting.

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Calibration of Dawn's Gamma Ray and Neutron Detector (GRaND) Instrument

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20070405/grand.html

Thomas H. Prettyman
GRaND Team Lead and Dawn Co-I, Los Alamos National Laboratory

The gamma ray and neutron detector (GRaND) is designed to measure the
chemical composition of the surfaces of Vesta and Ceres. GRaND will map
the near-surface abundance of major rock forming elements, long-lived
radioactive elements, and volatiles such as H, C, N and O which are the
major constituents of ices. A cutaway view of the instrument is shown in
Fig. 1. GRaND uses heritage technology from Lunar Prospector and 2001
Mars Odyssey, including a bismuth germanate (BGO) scintillator for high
efficiency gamma ray spectroscopy and boron loaded plastic scintillators
for fast and epithermal neutron detection. GRaND also includes new
sensor technologies to improve the accuracy of elemental abundance
measurements. These include a 16-element, CdZnTe (CZT) semiconductor
detector array for high resolution gamma ray spectroscopy, and
boron-loaded plastic/Li-loaded-glass phoswiches ("phosphor sandwiches")
to separately measure thermal, epithermal, and fast neutrons originating
from the asteroids.

GRaND has undergone extensive calibration and characterization at LANL
facilities and following integration with the spacecraft. Examples of
data products for gamma ray and neutron spectroscopy are shown in Fig 1.
The pulse height spectrum shown in Fig. 1a for a phoswich sensor, was
acquired using a laboratory neutron source with an energy distribution
similar to that of a planetary leakage spectrum. Thermal and epithermal
neutrons interacting in the Li-loaded glass produce a distinct peak
associated with the recoil energy of the reaction products for the
6Li(n,t)4He reaction. Epithermal neutrons that interact with the
B-loaded plastic produce a separate peak at 93 keVeq. The two peaks are
well separated in energy and can be used together to measure the thermal
and epithermal components of the neutron spectrum.

The BGO sensor has high efficiency for gamma ray detection and can
measure gamma rays over a wide energy range (see Fig. 1c for a spectrum
acquired for neutrons incident on an Fe slab). The CZT array has a more
restrictive energy range (0- to 3-MeV), but has somewhat higher energy
resolution than the BGO sensor (better than 3% at 662 keV). An ore
sample spectrum, showing prominent gamma rays from the decay of 214Bi
(from the 238U decay chain) is shown, for example, in Fig. 1b. The CZT
array enables improved accuracy for the analysis of the low energy
region of the spectrum, which is densely populated by gamma rays from
radioactive decay and nuclear reactions.

Dawn's operational plan provides ample integration time and coverage at
each asteroid, sufficient to globally map surface elemental composition.
The geochemical data provided by GRaND will, for example, provide strong
constraints on thermal evolution, including the role of water and other
volatiles in planetary development, context for the HED meteorites, and
the degree of volatile depletion in the source material from which the
asteroids accreted.

[Figure]
Figure 1. Cutaway view of the GRaND instrument and example data products
for: a) neutron spectroscopy (phoswich); b) gamma ray spectroscopy
(CdZnTe array); and c) gamma ray spectroscopy (BGO).

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Printable version of this newsletter (PDF format): To obtain a
formatted printable version of the newsletter, follow this link

http://www-ssc.igpp.ucla.edu/dawn/newsletter/pdf/20070405.pdf

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The Dawn mission has been selected as NASA's ninth Discovery
mission to be launched in June 2007 to orbit both Vesta and Ceres.
This list has been established to keep members of the scientific
community informed about the Dawn mission.

Dawn's Early Light is published on an occasional basis and
distributed electronically. To contribute material or query the
team, email us at dawnnews at igpp.ucla.edu.

Editor: Carol A. Raymond, Jet Propulsion Laboratory
Received on Fri 06 Apr 2007 05:58:17 PM PDT