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 LCROSS - Lunar CRater Observation and Sensing Spacecraft
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Lunar CRater Observation and Sensing Satellite (LCROSS)

Impactor Targeting

LCROSS  Orbit From Ecliptic North showing two large LGARO loops beyond Moon's orbit
Figure 1
Figures 1 and 2 depict the Lunar Gravity-Assist, Lunar Return Orbit (LGALRO) trajectory that was used to target the LCROSS Centaur upper stage and Shepherding Spacecraft into the crater Cabeus near the lunar South Pole. The 113-day (5 day Earth-lunar transit plus 108-day Earth orbit) Lunar Gravity-Assist, Lunar Return Orbit (LGALRO) was chosen to allow LRO time to complete its two-month commissioning phase and conduct approximately a month of science data collection of polar crater measurements.
LCROSS Orbit Side View showing steep plane change for LGALRO orbit
Figure 2
The Centaur impact was timed to allow LRO and key ground observatories to observe the ejecta plume generated by a high-velocity and steep incident angle impact (Figure 3). Dual viewing of the Centaur impact by the LCROSS Shepherding Spacecraft and LRO provided additional perspective and redundant data collection. The Shepherding Spacecraft and Centaur separated October 9, 2009, at 01:50 UTC. The Shepherding Spacecraft then performed a braking maneuver burn lasting for four minutes and five seconds. This braking maneuver slowed the Shepherding Spacecraft to follow behind the Centaur by four minutes. The Centaur impacted the Moon at on October 9, 2009 at 11:31:20 UTC. Instrument observations and real-time transmission from the Shepherding Spacecraft continued until the time of its impact at 11:35:36.
LCROSS Impact Trajectory showing high-angle impact path
Figure 3

Diviner image of lunar South Pole showing Cabeus

The crater Cabeus waas chosen as the target for the LCROSS impact. The crater has a diameter of 98 km and is located approximately 100 km from the lunar South Pole. The choice of Cabeus was based on continued evaluation of all available data and consultation/input from members of the LCROSS Science Team and the scientific community, including impact experts, ground and space based observers, and observations from Lunar Reconnaissance Orbiter (LRO), Lunar Prospector (LP), Chandrayaan-1 and JAXA's Kaguya spacecraft. This decision was prompted by the best understanding of hydrogen concentrations in the Cabeus region, including cross-correlation between the latest LRO results and LP data sets.

The general consensus of lunar experts led by the LCROSS science team was that Cabeus showed, with the greatest level of certainty, the highest hydrogen concentrations at the south pole. Further consideration of the most current terrain models provided by JAXA's Kaguya spacecraft and the LRO Lunar Orbiter Laser Altimeter (LOLA) was important in the decision process.The models show a small valley in an otherwise tall Cabeus perimeter ridge, which would allow for sunlight to illuminate the ejecta cloud on Oct. 9, and much sooner than previously estimated for Cabeus. The LCROSS team concluded that Cabeus provided the best chance for meeting its mission goals.

LCROSS NIR image of Centaur crater

Just prior to impact, the Shepherding Spacecraft was able to capture near-infrared images of the Centaur impact crater. Subsequent analysis revealed that the Centaur impacted within 83±66 m (1sigma) of the planned target.

Technical Information
Overview | Mission Rationale | Spacecraft and System Description | Instrumentation | Water Detection | Targeting

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Editor: Brian Day
NASA Official: Daniel Andrews
Last Updated: October 2010