Why Cabeus A1?

[Jim Mosher]

Those who read the LCROSS FaceBook page may have noticed that a
presentation explaining the decision to target "Cabeus A1" has been
released at:

http://www.nasa.gov/pdf/386497main_target-selection_web2.pdf

It would appear that a major driving factor behind the choice was a
finding that ejecta from the "sweet spot" on the floor of Cabeus A1
would have to rise only 0.63 km above the surface to reach sunlight,
compared to 1.21 to 3.92 km at other potential targets (Slide 10).
Since the modeled dust density falls off very rapidly with height,
this would greatly increase the expected signal strength. The
presentation also suggests that anything more than 0.33 km above the
impact site will be visible from Earth, which implies that the lowest
and most brightly reflecting parts of the cloud will be just as
visible to amateurs and professionals on Earth as they will be to the
Shepherding Spacecraft. However, since the Moon's surface is below
the 0.33 km Earth masking level, the latter number also seems to imply
that the impact flash itself will be invisible from Earth (as it would
be at all the other impact sites considered).

I am not entirely sure I understand these figures, since the cratered
ridges on the north rim of Cabeus A1 would seem (on the basis of
observed shadow lengths) to rise at least 2 km (perhaps as much as 3
km) above the main crater floor. Indeed, it would seem considerable
relief is required to permit permanent shadows this far from the
pole. At this distance from the pole (8.5°), if dust reaches sunlight
after just 0.63 km, it would seem the "sweet spot" must be placed near
the extreme outer edge of the permanently shadowed area -- a strategy
it would seem could be used in any of the other target craters, and
that should work better at sites closer to the pole. Indeed, given the
3.5 km (diameter) uncertainty in targeting (Slide 11), and the large
distance from the pole, it would seem the impact point in Cabeus A1
would have to be dangerously close to the outer edge of the permanent
shadow. I'm not sure exactly where the permanent shadow ends, but the
sun angle over Cabeus A1 at impact time will be approximately 9.2°
above the horizontal, and finding sunlight just 0.63 km above a level
surface does not allow you to move in more than 3.9 km from the then-
current (and slightly longer than permanent) shadow tip (and you could
move in even less if the crater bowl slopes down into the shadow).
Closer to the pole (where the sun angle is lower) you could move in
much farther from the shadow tip and still find sunlight at the same
height, or lower, above the surface. But there is much about the
LCROSS presentations I don't understand.

Two other points of interest are the hint that Cabeus A1 may have been
selected despite a possible failure of the neutron detection
experiment on LRO ("LEND") to confirm the hydrogen signal purportedly
found there by Lunar Prospector (2nd paragraph of Slide 21; the actual
LEND data have been suppressed due to issues of confidentiality); and
that the decision to target Cabeus A1, which seemed to be announced
with such confidence at the September 11th press conference, could be
abandoned in favor of some other target in the Cabeus area as late as
September 25th (Slide 23).

-- Jim

[Jim Mosher]

There is a bit of confusion about when the final LCROSS target
selection will be made. Slide 23 of the previously mentioned Cabeus
A1 selection document:

http://www.nasa.gov/pdf/386497main_target-selection_web2.pdf

implies the last time a target crater other than Cabeus A1 can be
selected is at Trajectory Correction Maneuver #7 on September 25,
2009, while Slide 4 implies the point where the Shepherding Spacecraft
will impact can be modified as late as TCM 8.

Evidently TCM 7 has passed, for a Sept. 25th Facebook announcement
states: "LCROSS Lunar Impactor Mission Mission Day 99. In the wee
hours of this morning, the Mission Ops team successfully executed
Trajectory Correction Maneuver #7. This manuever fine-tuned LCROSS's
trajectory required to meet targeting requirements towards Cabeus."
But the ambiguous reference to "Cabeus" makes it is unclear if LCROSS
is now irrevocably committed to Cabeus A1, or has some flexibility
left.

-- Jim