The MESSENGER Venus Flybys: Opportunities for Synergy with Venus Express

Abstract

The trajectory of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, launched by NASA on 3 August 2004 and destined to be the first probe to orbit Mercury, includes two flybys of Venus during the period that the ESA Venus Express mission is operational in Venus orbit. MESSENGER's first Venus flyby occurred on 24 October 2006, at a closest approach distance of 3140 km, but no scientific observations were made because Venus was at superior conjunction and no direct communication with the MESSENGER spacecraft (or with Venus Express) was possible for an extended period. All MESSENGER instruments, however, will be trained on Venus during the spacecraft's second flyby on 6 June 2007, when closest approach will be at 300 km altitude over 12°S, 107°E, in the uplands of Ovda Regio. The Mercury Dual Imaging System will image the night side in near-infrared bands, and color and higher-resolution monochrome mosaics will be made of both the approaching and departing hemispheres. The Ultraviolet and Visible Spectrometer on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument will make profiles of atmospheric species on the day and night sides as well as observations of the exospheric tail on departure. The Visible and Infrared Spectrograph on MASCS will observe the planet near closest approach to sense cloud chemical properties and near-infrared returns from the lower atmosphere and surface. The laser altimeter will serve as a passive 1064-nm radiometer and will measure the range to one or more cloud decks for several minutes near closest approach. The combined observations of Venus Express and MESSENGER will permit simultaneous and complementary observations of particular value for characterization of the particle and field environment at Venus. MESSENGER's Energetic Particle and Plasma Spectrometer (EPPS) will observe charged particle acceleration at the Venus bow shock and elsewhere. The Magnetometer will provide measurements of the interplanetary magnetic field (IMF), bow shock signatures, and pickup ion waves as a reference for EPPS and Venus Express observations. The encounter will enable two-point measurements of IMF penetration into the Venus ionosphere, primary plasma boundaries, and the near-tail region. During the MESSENGER flyby the instruments on Venus Express will be operated to maximize the synergy between the two spacecraft for this unique opportunity