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.
