Alice—an Ultraviolet Imaging Spectrometer for the Rosetta Orbiter

We describe the design concept and scientific objectives of ALICE: a lightweight (2.2 kg), low-power (2.9 W), and low-cost UV imaging spectrometer for the ESA Rosetta Orbiter. Ultraviolet spectroscopy is a powerful tool for studying astrophysical objects, and has been applied with great success to the study of comets. ALICE is designed to obtain far-UV (FUV) spectra of the Rosetta comet nucleus and coma in the 700-2050 Å bandpass; it will achieve spectral resolutions between 9.8 and 12.5 Å across the bandpass for extended sources that fill its 0.1 x 6.0 deg. field-of-view. It employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a concave holographic reflection grating. The imaging microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and a 2-D wedge-and-strip readout array. ALICE will deepen the Rosetta Orbiter remote sensing investigation through its ability to detect and measure (1) noble gases; (2) atomic abundances in the coma; (3) major ion abundances in the tail; and (4) production rates, variability, and structure of H2O and CO/CO2 molecules that generate cometary activity. In addition, ALICE will allow an investigation of the FUV properties of the nucleus and its solid grains, and can provide unique information during asteroid flybys and at en-route planetary encounters, most notably, Mars.