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Dawn Checked Out and On Its Way to Vesta

Carol A. Raymond
Dawn Deputy Principal Investigator, JPL

Since launch on September 27th, 2007, the Dawn spacecraft operations team at JPL performed the initial checkout of the Dawn flight system, and high-duty-cycle thrusting to Vesta using the Ion Propulsion System (IPS) began on December 17th. The flight system and the operations team are both performing well, and the spacecraft has been thrusting flawlessly for the past 2.5 months. Minor issues either have been resolved, or will be fixed in a software update planned for later this year.

The science payload was turned on and several calibrations were performed during the Initial Checkout (ICO) period, which extended 60 days from launch. Subsequently, tests were performed during several planned interruptions in thrusting, so-called ?forced coast periods? during February and early April. All instruments are fully functional, and preliminary calibrations indicate that the framing cameras (FCs), Visible and Infrared Spectormeter (VIR), and Gamma Ray and Neutron Detector (GRaND) are meeting their performance goals. Both framing cameras have been calibrated using star fields and Saturn in the clear and seven color filters, while VIR has used its internal calibration target, stars, and the planet Mars to verify its performance. GRaND has done a functional checkout with the exception of annealing the instrument, and observed the quiet space background. An image of Eta Carinae and NGC 3532 (Wishing Well Cluster), taken by the primary FC is shown in Fig. 1 on the next page. The GRaND bismuth germanate (BGO) scintillator spectrum is compared to the spectrum from a similar instrument on Lunar Prospector in Fig 2. Visible and IR row spectra collected by VIR are shown in Fig. 3.

The data from these tests (Level-1a) have been delivered to the Dawn Science Center and is being evaluated by the Science team.

The Dawn FCs are built by the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany in partnership with the Germany Space Agency (DLR). GRaND was built by Los Alamos National Laboratory in New Mexico, and is operated by the Planetary Science Institute. The Instituto Nacional di Astrofisica (INAF) in Rome worked with Galileo Avionica in Florence to build the VIR instrument with support from ASI, the Italian Space Agency.

More extensive calibrations and stray light tests are planned for the Mars flyby in February, 2009 and the ensuing four-month-long optimal coast. In addition, there are bi-annual forced coast periods built into the trajectory to do calibrations thoroughout the mission.

Fig 1. Color Image taken by the primary framing camera of Eta Carinae (upper left) and the Wishing Well Cluster (NGC 3532).




Fig 2. Comparison of BGO event counts between Dawn and similar instrument on Lunar Prospector.




Fig 3. Uncalibrated Arcturus row spectra acquired by VIR in visible (top) and infrared (bottom) range. The region from 3500-5000 nm was saturated to achieve higher signal from 1000-2000 nm and perform a geometrical calibration.