Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission. Ralph: A visible/infrared imager for the New Horizons Pluto/Kuiper Belt Mission Dennis Reuter1, Alan Stern2, James Baer3, Lisa Hardaway3, Donald Jennings1, Stuart McMuldroch4, Jeffrey Moore5, Cathy Olkin2, Robert Parizek3, Derek Sabatke3, John Scherrer6, John Stone6, Jeffrey Van Cleve3 and Leslie Young2 1NASA/GSFC, Code 693, Greenbelt, MD 20771 2Dept. of Space Studies, SWRI, 1050 Walnut St., Suite 400, Boulder CO, 80302 3BATC, 1600 Commerce St, Boulder, CO 80301 4SSG Precision Optronics, 65 Jonspin Rd., Wilmington MA, 01887 5NASA/Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000 6SWRI, 6220 Culebra Rd., San Antonio TX, 78228 ABSTRACT The instrument named Ralph is a visible/NIR imager and IR hyperspectral imager that would fly as one of the core instruments on New Horizons, NASA’s mission to the Pluto/Charon system and the Kuiper Belt. It is a compact, power efficient, and robust instrument with excellent imaging characteristics and sensitivity, and is well suited to this long- duration flyby reconnaissance mission. 1. INTRODUCTION Ralph is a visible and IR remote sensing instrument scheduled to fly on the New Horizons Mission to the Pluto/Charon system and the Kuiper Belt. The instrument’s primary purpose is to measure surface characteristics, including geological processes, geomorphology, photometric properties, and surface composition. Surface temperature would be inferred from the spectral band shapes and positions of well-established, thermally-diagnostic reflectance spectral features in ices. In addition, Ralph would be used to measure haze optical depths (if present) and to search for small satellites. This paper describes the Ralph instrument and specifies its characteristics. Ralph consists of a telescope that feeds various focal planes, and its associated electronics. More specifically, the Ralph telescope feeds two sets of focal planes; the Multi-spectral Visible Imaging Camera (MVIC) a visible, near-IR imager and the Linear Etalon Imaging Spectral Array (LEISA), a short-wave IR spectral imager. The optical system uses a 75 mm aperture, F/8.7, three mirror anastigmat which provides a 5.7°x1.0° field of view. The telescope is constructed entirely from grain aligned 6061-T6 aluminum, including the diamond turned mirrors. This insures that the optical performance of the system is insensitive to temperature and dissipates thermal gradients. A dichroic beamsplitter transmits the IR at wavelengths longer than 1.1 µm to LEISA and reflects the shorter wavelengths to MVIC. Cooling of the detectors is provided by a two stage, externally-mounted passive radiator. MVIC uses two large format (5000x32 pixel) CCD arrays to provide panchromatic (400 to 1000 nm) hemispheric maps of Pluto at a double sampled spatial resolution of 1 km by 1 km or better. Four additional 5000x32 CCDs provide hemispheric maps in blue, red, Near IR and narrow band methane color channels. These arrays operate in time delay integration (TDI) mode to increase sensitivity. In addition to the TDI arrays, MVIC has a 5000x128 element frame transfer array, which would be used primarily for optical navigation of the spacecraft. LEISA is a wedged filter infra-red spectral imager that operates in a push-broom mode. It images a scene through a wedged filter (linear variable filter, LVF1) placed about 100 µm above a two-dimensional Mercury Cadmium Telluride (HgCdTe) 256x256 pixel detector array (a PICNIC array). An image is essentially formed on both the wedged filter and the array simultaneously (less than 5% broadening for an F/8.7 beam). The filter is made in two segments. The first covers from 1.25 to 2.5 microns at a spectral resolving power (λ/∆λ) ~250. This section of LEISA would be used to 5906B-51 V. 3 (p.1 of 11) / Color: No / Format: Letter / Date: 8/12/2005 9:35:05 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission. obtain surface composition maps with a spatial resolution of 10 km or better. The second segment covers 2.1 to 2.25 microns with a spectral resolving power of ~570. It would be used to obtain both compositional information and surface temperature maps using the spectral shape of solid N2 near its 35° K phase transition and other similar features (e.g., due to H2O-ice). Ralph is a joint effort of the Southwest Research Institute (SWRI, San Antonio, TX and Boulder, CO which is the home institution of Alan Stern, the Ralph Principal Investigator), Ball Aerospace and Technologies Corp. (BATC, Boulder, CO) and NASA’s Goddard Space Flight Center (GSFC, Greenbelt, MD). 2. NEW HORIZONS MISSION OVERVIEW New Horizons is planned to be NASA’s first mission to Pluto, the last unexplored planet in the Solar System, and Charon, its satellite. It also hopes to visit at least one moderately large object in the Kuiper Belt, an extended disk containing numerous primordial bodies whose planetary evolution was arrested early in Solar System formation. Kuiper Belt Objects (KBOs) are the “fossils” of planetary evolution and the Kuiper Belt is the prime “archeological” site in the Solar System. Pluto is also among the largest known KBOs. Because Charon is approximately half of Pluto’s size, the center of mass of the Pluto-Charon system lies between the two objects, making Pluto-Charon a truly binary system. The first exploration of the Pluto-Charon system and the Kuiper Belt would be both scientifically and publicly exciting. It would provide invaluable insights into the origin of the outer solar system and the ancient outer solar nebula, the origin and evolution of planet–satellite systems presumably formed by giant impacts, and the comparative geology, geochemistry, tidal evolution, atmospheres, and volatile transport mechanics of icy worlds. The baseline mission, slated for launch in January, 2006 would reach Pluto in 2015 and would also feature an active Jupiter-system flyby with closest approach about 14 months after launch. New Horizons is a Principal Investigator led mission (the PI is Alan Stern of the Southwest Research Institute of Boulder, CO and San Antonio, TX) managed by the Johns Hopkins Applied Physics Laboratory in Laurel, MD. It is the first mission of NASA’s New Frontiers program. New Horizons carries a suite of sophisticated remote sensing and in situ instruments that would provide a unique and important data set during the Pluto-Charon and KBO flybys. The instrument complement includes Ralph, Alice (an ultra violet imaging spectrometer), the Radio Science Experiment (REX), the Solar Wind Analyzer for Pluto (SWAP), the Pluto Energetic Particle Spectrometer Investigation (PEPSSI) and the Long Range Reconnaissance Imager (LORRI). This set of instruments was chosen to fulfill science requirements based upon prioritized Group 1, Group 2 and Group 3 science objectives as defined in 1996 by the Pluto Science Definition Team and listed in NASA AO 01-OSS-01 for the then proposed Pluto Kuiper Belt (PKB) mission. Group 1 objectives are required measurements while Groups 2 and 3, although highly desirable, are not mandatory requirements. Ralph, Alice and REX form the core set of instruments whose operation is necessary for mission success. The Group 1 objectives can be accomplished using only this core payload. The instrument set also provides a high degree of functional redundancy for the various measurements. Table 1 lists the science objectives that New Horizons would address. Table 1 also describes the observational parameters used to address the objectives and the primary and secondary instruments for each observation. At Pluto- Charon, the New Horizons baseline mission would fulfill all Group 1 and 2 and three of the four Group 3 measurement objectives. One group 3 objective, the magnetic field search at Pluto and Charon, would not be directly addressed. Table 1: Summary of New Horizons science objectives, the measurements that would accomplish them and the instruments used to obtain the measurements. Science Objective Primary Measurement Primary Instruments (Secondary Instruments) Group 1 Objectives 1) Characterize the global geology Map viewable disk at 1 km per line Ralph/MVIC and morphology of Pluto and pair in panchromatic band (LORRI) Charon 2) Map surface composition of IR Spectral Surface map at 5-10 Km Ralph/LEISA Pluto and Charon spatial resolution (Ralph/MVIC) 3) Characterize neutral atmosphere UV Spectral mapping; earth radio Alice, REX of Pluto and its escape rate occultation (Ralph/LEISA) 5906B-51 V. 3 (p.2 of 11) / Color: No / Format: Letter / Date: 8/12/2005 9:35:05 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission. Group 2 Objectives 1) Characterize the time variability Maps in UV, visible and IR spectral Ralph/MVIC, Ralph/LEISA, Alice of Pluto's surface and atmosphere