15 Apr 2004
(Source: Jet Propulsion Laboratory)
D A W N ' S E A R L Y L I G H T - April 2004
The fifth issue of the Dawn team newsletter, Dawn's Early Light, has been posted on the Dawn website. Follow the links below to view individual articles, or obtain the pdf version. We look forward to obtaining your feedback.
Dawn Mission Status
Dawn proceeds towards critical design review
Christopher T. Russell
Dawn Principal Investigator, UCLA
Dawn has now entered Phase C/D, the implementation phase, and is moving forward quickly toward launch. Dawn has made excellent progress, as described below, as has the Discovery program itself.
As previously reported, the Discovery Program Office has moved to JPL where the office has the technical support it needs to evaluate and assist the missions in development. At NASA Headquarters, Orlando Figueroa has appointed a deputy, Andy Dantzler, who will oversee the Discovery Program. These changes will ensure that Dawn and all the Discovery missions receive the support and advice that is needed to achieve success.
Great progress has been made with the Dawn spacecraft, the payload and the ground system. All spacecraft subcontractors are on board and hardware is beginning to appear. Subsystem CDRs are taking place weekly. The GRaND CDR was the first payload CDR to take place on March 30 and 31 with VIR following on April 22 and 23. The Framing Camera CDR takes place on May 18 and 19.
The technical reserves of the project are appropriate to the mission phase and green is becoming Dawn's official color. On April 13, an independent power and mass margin review was successfully conducted, confirming the project's assessment of health. Schedule reserve continues to be tight; however, an aggressive schedule is key to executing a successful low-cost mission. Recent events within the Discovery program have increased the level of oversight and the effort required to support this oversight, directly impacting schedule. As we begin to build and integrate the spacecraft and payload, the schedule is and will remain a top priority.
Dawn's Visible and Infrared Mapping Sectrometer (VIR)
Dawn's Visible and Infrared Mapping Spectrometer (VIR)
VIR Team Lead, Istituto Nazionale di Astrofisica (INAF), Rome
Hubble Space Telescope observations of Vesta's surface made ten years ago revealed a large southern polar crater and geological diversity, with regions that can be characterized spectroscopically as basalts. Figure 1 shows the surface composition map of Vesta produced from separate images in blue (439 nm), orange (673 nm), red (953 nm), and near-infrared (1024 nm) light.
The map shows that all of Vesta's surface is igneous, indicating that either the entire surface was once melted, or lava flowing from its interior completely covered its surface. A firm identification of surface geology on Vesta requires medium-high resolution spectra. The Dawn Visible-IR Mapping Spectrometer (VIR) addresses this need with a capability to acquire high spectral and spatial resolution data. Spectral coverage is important for both Vesta and Ceres, as diagnostic minerals show absorption bands in the visible and near IR regions. For this reason we developed a single spectrometer able to cover both visible and IR spectral ranges.
The Visible and Infrared sensors, that are the heart of the VIR imaging spectrometer, are housed in the same optical subsystem. The instrument is derived from the VIRTIS experiment that is presently flying on the Rosetta mission. Figure 2 shows the result of in-flight calibration of VIRTIS, compared with the ground-based measurements. The difference at the long wavelengths is due to the higher temperature of the VIRTIS box in-flight.
The optics module OM (Figure 3) contains the optical system, scan mirror, the entrance and sunshield, cover, shutter, cryocooler, in-flight calibration units (lamps), radiators, focal plane arrays (FPAs), and the proximity electronics (PEM). The OM architecture has been maintained as similar as possible to the VIRTIS for Rosetta; only the spacecraft-instrument interfaces have been modified. The optical concept is inherited from the visible channel of the Cassini Visible Infrared Mapping Spectrometer (VIMS-V) developed at Galileo Avionica. This concept matches a Shafer telescope to an Offner grating spectrometer to disperse a line image across two FPAs. The Shafer telescope and Offner spectrometer are aligned separately, then mounted and co-aligned.
The Shafer telescope combines an inverted Burch telescope and an Offner relay (M4/6 and M5). The Offner relay takes the curved, anastigmatic VIR virtual image of the inverted telescope and makes it flat and real without losing the anastigmatic quality. Coma optical aberration is eliminated by putting the aperture stop on M5 near the center of curvature of the primary mirror and thus making the telescope monocentric. The result is a telescope system that relies only on spherical mirrors yet remains diffraction limited over an appreciable portion of the spectrum and all vertical field (slit direction). The Shafer telescope is matched to the Offner grating spectrometer because both are telecentric; the entrance pupil is positioned in the front focal length (FFL) of the optical system at 750 mm in front of the primary mirror (M1). Because the pupil optics conjugate is on the grating, the same spectral beam splitting is performed for each FOV angle. The grating profiles are holographically recorded into a photoresist and then etched with an ion beam. Higher groove density in the central 30% of the conjugate pupil area generates the higher spectral resolution required in the "visible" channel, extending from the ultra-violet to the near infrared. The smaller pupil area allows the visible channel to operate partially coherently and achieve a smaller point spread function.
A laminar grating is used for the visible channel's pupil zone to increase the grating efficiency spectrum and compensate for low solar energy and low CCD quantum efficiency in the ultra-violet and near infrared regions. This improves the instrument's dynamic range by increasing the S/N at the extreme wavelengths and preventing saturation in the central wavelengths. For the infrared zones, a blazed groove profile is used that results in a peak efficiency at 5 ?m to compensate for the low signal levels expected at this wavelength. These features, combined with custom designed FPAs, result in an instrument able to collect spectra of very large dynamic range with high spatial resolution that satisfies Dawn's requirements.
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 email@example.com.
Editor: Carol A. Raymond, Jet Propulsion Laboratory