[meteorite-list] Dawn's Early Light - February 2004

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:27 2004
Message-ID: <200402251655.IAA17389_at_zagami.jpl.nasa.gov>

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20040226/

D A W N ' S E A R L Y L I G H T February 2004
Volume 3, Issue 1
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

The fourth issue of the Dawn team newsletter, Dawn's Early Light,
has been posted on the Dawn website.

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20040226/

Dawn Mission Status

Dawn is Confirmed to Proceed to Launch

Christopher T. Russell
Dawn Principal Investigator, UCLA

The Dawn team is pleased to announce that on February 6, Dawn was
confirmed and approved to move into its implementation phase. In
response to an increasing emphasis within the NASA Office of Space
Science to mitigating risks of schedule slips and growth in cost,
mass and power consumption, there has been increasing scrutiny of
all flight programs, and Dawn is no exception. To address this
changing risk posture we took steps to improve the mission's
technical margins and our financial and schedule reserves. As a
result, there have been several changes in the mission profile
since the original Concept Study Report. These have increased
the robustness of the mission but at some expense to the science
return.

The first adjustment to the mission was an increase in the solar
array output so that Dawn can always operate its ion thrusters
even at Ceres aphelion, and with adequate margin (15%) that could
allow for unexpected power requirements. Additionally, JPL design
principles now call for 20% mass margin at this phase in the
program. Originally Dawn had not used these margins because of the
high degree of experience gained in DS1 and the high heritage from
prior Orbital spacecraft. Now it has become imperative to comply
with the recommended mass margins for a program to be confirmed
by NASA. Also required were at least 25% cost reserves at
confirmation. The increased mass, power margin, and cost reserve
requirements could be accommodated either by moving the Ceres
rendezvous into an extended mission category or by taking
reductions in the science return at both target bodies. The
latter option was deemed more valuable and the mission was
replanned accordingly.

In order to fit the mission into the tighter mass and power box, a
Mars flyby was inserted into the trajectory, but this delayed
Vesta and Ceres arrival and thus increased the operations costs.
Dawn could not afford to extend its time in space and remain
within the cost box. The Dawn spacecraft provider, Orbital
Sciences Corporation made a generous contribution to the Dawn
project that allowed the project to re-balance risk across the
various elements and achieve its prime measurements within the
cost cap and with responsible technical and cost reserves.
However, sacrifices were made to achieve this scenario. First,
the stay times at Vesta and Ceres have been reduced from 11
months at each body to 7 months at Vesta and 5 months at Ceres.
This amount of time allows all imaging and mapping spectrometer
data to be obtained, and achieves the prime objectives of the
gamma ray and neutron detector (GRaND) and gravity investigations,
but the stay time does not allow the spacecraft to spiral down as
far as originally planned so that resolution of GRaND and the
gravity data are somewhat reduced. Part of the increase in the
mass margin and cost reserve has also come from the deletion of
the magnetometer and its boom. The Dawn Laser Altimeter had been
previously deleted before the Preliminary Design Review. While the
magnetometer was on the descope list in the approved mission, the
value of this instrument had been increasing as we learned more
about Vesta and Ceres. Vesta almost certainly has an iron core
and may have similar crustal magnetization as Mars, while Ceres
appears to be an ice planet with water-ice mantle, and rocky core
similar to Europa, Ganymede and Callisto. For Ceres and these
Galilean moons, thermal evolution models suggest that there may
be liquid water underneath the icy crust even today. A
magnetometer is the only certain way to detect the existence of
such a layer.

Dawn has a continuum of descope options remaining
that provide increased mass margin and still return
much of the key objectives. Mass growth beyond the
held margin can be accommodated by reducing the
stay time at the bodies to the science floor (4
months at each body), and raising the lowest
observing altitude. Conversely, if the mission
performs close to or above the nominal mission
specifications, then the spacecraft can stay longer
and go lower in its orbits about Vesta and Ceres,
increasing the science return of the mission. In other
words, the remaining descopes are entirely
recoverable if the flight system performance allows it.
Figure 1 illustrates the new mission scenario.

The Dawn Science Team is thrilled to have the opportunity to
move ahead with the mission to visit both Vesta and Ceres, and
welcomes the participation of the community which will be
solicited in future AOs.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20040226/news.html

Dawn News

Carol A. Raymond
Dawn Deputy Principal Investigator, Jet Propulsion Laboratory

Following confirmation, the team's focus has shifted to passing
the Critical Design Review (CDR) milestone. The CDR will take
place in June 2004. Leading up to this review the science team
will be holding two meetings, one on March 13-14 at the Lunar
Planetary Science Conference in Houston and the other at the
European Geosciences Union meeting on April 23-24 in Nice. These
meetings will begin in earnest the process of planning for the
science data acquisition and analysis. A Dawn Science Center (DSC)
is being established at UCLA, which will interface with the
instrument teams to plan observations, and will participate in,
and oversee, the data analysis and archiving. Mr. Steven Joy
(UCLA) has been named the manager of the DSC.

The Dawn Team welcomes two new Co-investigators to the team, Dr.
Maria Cristina de Sanctis, and Dr. Fabrizio Cappacio, from the
Istituto Nazionale di Astrofisica (INAF) in Rome. They are
working with Dr. Angioletta Coradini and her team on the
visible and infrared spectrometer (VIR).

Images of Ceres were acquired in December and January using the
HST Advanced Camera for Surveys (ACS) headed by Joel Parker at
SWRI and including Dawn team members Russell, McFadden, and Sykes.
The images are being used to derive a shape model, pole position,
and albedo and color maps. Also on the team are Eliot Young and
Alan Stern of SWRI.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

The Dawn mission has been selected as NASA's ninth Discovery
mission to be launched in May 2006 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 Wed 25 Feb 2004 11:55:24 AM PST