Characterization of the Lunar Plume Excavated by the LCROSS Impactor

Astronomy and Astrophysics – Astrophysics

Scientific paper

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[6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

The impact of the 2000 kg LCROSS Centaur Earth Departure Upper Stage (EDUS) at the lunar south pole provides a unique opportunity to study plume dynamics following the impact of a body of known mass, composition, impact velocity, and entry angle. In February 2009, Universities Space Research Association (USRA) selected the Apache Point Observatory (APO) in Sunspot, NM as one of four NASA-funded ground-based LCROSS observation sites. The goal of the APO observation team is to image the LCROSS plume to measure the radius and velocity of the expanding plume boundary and determine the mass of the ejecta that is thrown up into sunlight by the EDUS impact. These mass measurements will in turn be used to calculate the expected water vapor optical depth as a function of time for a given estimate of water vapor content in the regolith. They will also allow a calibration of models used to estimate mass ejection by random meteorite impacts on the Moon, which have been observed and monitored by the Meteoroid Environment Office at NASA’s Marshall Space Flight Center. We will observe the impact on 09 October 2009 with three cameras simultaneously covering visible and near infrared wavelengths (from 0.6 to 1.7 microns). At APO, we will use the Astrophysical Research Consortium 3.5-meter telescope coupled with the Agile visible light frame transfer CCD camera, which is capable taking a continuous series of 0.3 second images with zero latency between exposures. Also at APO, we will use New Mexico State University’s 1 meter telescope with a StellaCam visible light video camera, which is capable of exposure times as short as 1/12,000 second sampled at a video rate of 1/30 second. Finally, we will image the plume in near infrared wavelengths (0.9 to 1.7 micron) with the 0.6-meter f/40 telescope at Tortugas Mountain in Las Cruces, NM using a Goodrich Sensors Unlimited InGaAs IR video camera. This camera is a similar design to that of the two near infrared cameras (NIR1 and NIR2) on the LCROSS Shepherding Spacecraft. Pixel resolutions range from 0.435 arcseconds for the Agile camera (corresponding to 0.77 km at lunar distance at the time of impact), to 0.211 arcseconds (0.36 km) for the Goodrich IR video camera. We discuss the challenges of observing an extended plume immediately adjacent to the brightly illuminated lunar limb with large aperture telescopes. We have developed an observation strategy based on extensive testing with a variety of broadband and neutral density filter combinations. As the actual surface brightness of the plume is uncertain before impact, we will employ a strategy of bracketing our exposures to cover possible surface brightness from 4.0 to 8.5 mags/square arcsecond. In the event that we are successful in observing the plume, we will present images of the expanding plume from all three cameras and a preliminary analysis of the ejecta plume size and velocity. This work is supported by USRA grant number GR0002970.

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