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In this issue:
Dawn Mission Status
Dawn Instruments are Delivered to Orbital
Ceres Results Published in Nature
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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.
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