[meteorite-list] Dawn Mission Status - November 2005

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun Nov 20 00:28:27 2005
Message-ID: <200511200526.jAK5Qwb06519_at_zagami.jpl.nasa.gov>

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20051117/dawnstatus.html

Dawn's Early Light
Volume 4, Issue 2
November 2005
    
Dawn Mission Status
Christopher T. Russell
Dawn Principal Investigator, UCLA

In mid-October, the Dawn mission team was asked by NASA Headquarters to
cease all work except that which was critical to maintaining the
viability of the Dawn mission to launch on a delayed schedule, still
achieving all of its scientific objectives. This action was taken in
response to concerns about the availability of funding in FY2006 to
cover any problems that might arise during environmental and performance
testing, particularly with regard to several pieces of subsystem
hardware perceived to have experienced significant problems. The chief
items of concern are the Power Processing Units (PPUs) that provide the
high voltage power to the thrusters in the ion propulsion system, one of
the redundant Attitude Control Electronics (ACE) boxes, and the xenon tank.

Concern regarding the flight xenon tank arose because two qualification
tanks ruptured at lower than expected pressure during testing,
instigating a thorough review of the integrity of the flight tank. The
Dawn team chose to reduce the xenon load in the flight tank from 450 to
425 kg to increase the safety margin (reducing a generous fuel load
margin but not affecting the science return). A recommendation from the
group charged with reviewing the Dawn tank, chartered by the NASA
independent Technical Authority guidelines, is expected in early December.

The "stand down" order reduces expenditures while an independent
assessment of the project is performed by a team assembled by the
Discovery Program Office at Marshall Space Flight Center. This
Independent Assessment Team (IAT) has visited the PPU manufacturer (L3,
which took over from Boeing), and will spend time during November and
December with

Dawn is currently in the Assembly, Test and Launch Operations (ATLO)
project phase. To date, nearly all the hardware has been successfully
delivered to Orbital and integrated to the spacecraft. Delivery of the
remaining PPU and ACE are pending. The solar array arrived and a
deployment test was conducted at Orbital. Power has been applied to the
high voltage system as the first step towards testing of the ion
propulsion system, which will culminate in a hot-fire test next year.
Flight software delivery 5.1.1, which includes all functionality, has
been delivered to the flight system and to the dual testbeds.

ATLO work will proceed at a slower pace during the stand down. The goal
is to fully integrate the spacecraft and perform a Comprehensive
Performance Test (CPT), which fully exercises the spacecarft
subsystems. This will be followed by a flight system CPT, in
preparation for bakeout and instrument integration. Thermal vacuum
testing, originally scheduled for January at the Goddard Space Flight
Center, has been postponed until the project restart.

Assuming the project receives authority to proceed to launch, a
replanning effort will be undertaken beginning in February to develop a
schedule, taking into account the funding constraints. At that time,
the operations teams will be reconstituted, and the test program will
ramp up. A series of end-to-end information system flow tests and
mission scenario tests have been already planned and partially scripted.

Because of the flexibility afforded by the ion propulsion, Dawn's launch
period extends at least into late 2007. The original launch period in
June 2006 was chosen based on projected readiness to launch at that
time. The baseline trajectory includes a Mars Gravity Assist in 2009,
which fixes the timing of subsequent events. The neutral mass margin
presently increases as the launch date moves toward 2007. Work is
underway to define the sensitivity of the new trajectory to launch date
in order to facilitate the replanning effort.
Received on Sun 20 Nov 2005 12:26:58 AM PST