LCROSS Crater Candidates Announced

Mission controllers and scientists for NASA's newest lunar explorers are breathing easier these days. The Lunar Reconnaissance Orbiter (LRO) is settling into its polar orbit nicely, readying its instruments to begin a complete survey of the Moon.

So is the companion Lunar Crater Observation and Sensing Satellite — whose acronym, LCROSS, is so much easier to remember. LCROSS skirted 1,988 miles (3,183 km) from the Moon on June 23rd, and in doing so got a gravity assist that put the craft in a looping 37-day orbit. Impress your friends by remembering this mouthful: Lunar Gravity Assist Lunar Return Orbit (LGALRO).

In any case, late last week project officials released the "short list" of candidate sites where, on October 9th at 11:30 Universal Time, LCROSS and the Centaur rocket that helped boost it will slam into the lunar landscape. These spots are lying in permanent shadow near the Moon's south pole. In the table below, Sun mask is each spot's depth (in kilometers) below the surrounding sight lines to the Sun.

LCROSS Candidate Impact Sites
Designation Crater name Sun mask Earth mask Latitude Longitude
SP_A Faustini 2.3 0.9 –87.2° 89°E
SP_B Shoemaker 3.5 –0.6 –88.5° 50°E
SP_C Cabeus 3.0 8.5 –85.0° 35.5°W
SP_CB Cabeus B 0.9 –0.5 –81.7° 54.5°W
SP_CC (none) 2.5 0.1 –83.5° 16°W
SP_D Haworth 2.7 0.8 –87.4° 5°W
SP_F (none) 1.8 0.5 –82.3° 12°E
SP_G (none) 2.4 0.4 –84.3° 1°E

Likewise, Earth mask denotes how far each spot lies below a direct sight line from Earth. On the Big Day, the lunar south pole will be tipped about 2½° toward Earth, a favorable libration that allows terrestrial observers to peek directly at the targets in Shoemaker and Cabeus B. Project officials won't select the final target until 30 days before impact. That will permit LRO to photograph the candidate sites thoroughly.

LCROSS targets
Labels identify possible impact sites near the Moon's south pole for the LCROSS spacecraft and its Centaur rocket. Colors on the map show topographic highs and lows, while gray indicates locations in permanent shadow. Click on the image for a larger view.
Brian Day, who's coordinating amateur observations for the LCROSS project at NASA's Ames Research Center, told me that ground-based observers probably won't see the momentary flash created when the 5,200-pound (2,360-kg) Centaur slams into the surface. However, seconds afterward a rising plume of debris might become as bright as a 5th-magnitude star along the limb. He thinks the plume will spread into a thin band a few arcseconds tall and perhaps 25 arcseconds wide before it fades from view.

Although many professional telescopes will be turned Moonward that night, Day hopes amateurs will join the hunt. Chances are you'll need an aperture of at least 10 or 12 inches to see anything visually. Sky & Telescope is preparing a complete guide for watching the LCROSS impacts, but for now you can check out the project's website.

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