SHOEMAKER CRATER GOING WHERE WE CAN SEE

1
SHOEMAKER CRATER GOING WHERE WE CAN SEE

• Carlton Allen
• NASA JSC

2
Recommended impact site for LCROSS SHOEMAKER
CRATER This 51-km diameter crater centered at
88.1 S, 45 E is in permanent shadow Half of
its floor can be imaged by Earth-base
radar This degree of target knowledge will
strongly constrain impact models and
significantly increase the confidence of data
interpretation.
3
Lunar Orbiter IV and Clementine images of
Shoemaker show that the rim is partially
illuminated by sunlight, but the crater floor
remains dark.
Clementine mosaic
4
Radar interferometry digital elevation model
(DEM) 150 m spatial resolution and 50 m height
resolution
Radar DEM
5
Radar DEM, combined with ray tracing, highlights
five craters in which a portion or all of each
floor is in permanent shadow.
Radar DEM
6
Intact rim with numerous 1 - 10 km superimposed
craters ejecta blanked is totally subdued More
degraded than Shackleton, but less degraded than
other craters in the region
Radar DEM
7
Crater floor diameter is approximately 20
km Floor is flat and partially smooth, with
scattered impact craters Crater is simple in
plan, with no evidence of a central peak
Radar DEM
8
Thermal and epithermal neutron counts measured by
Lunar Prospector suggest concentrations of
near-surface ice.
9
Radar beams with wavelengths of 2.5, 12.6, and 70
cm have returned echoes that are typical of
highland terrain, without the strong echoes
attributable to ice.
10
In 1999 Lunar Prospector was deliberately crashed
into Shoemaker crater. Earth-based sensors
detected no ejecta plume.
LCROSS will impact the Moon with 200 x more
kinetic energy, and the impact will be observed
by from lunar orbit as well as from Earth.
11

• Many targets meet the LCROSS criteria
• nearside
• permanently shadowed
• poleward of 70 degrees latitude
• large enough to contain a 10 km landing ellipse
• The actual material to be impacted in most of
  these candidates is unknown.

12
Shoemaker crater is unique among possible targets
half of the floor has been imaged by radar.
The physical properties of the floor material
can be modeled. This target is known to be
flat, providing simple geometry for understanding
impact dynamics and the ejecta plume.
13
Importance of imaging the target area in a
spacecraft impact experiment was demonstrated by
the Deep Impact mission to Comet Temple 1.
Close-up images of the impact point, revealing
layered target material, strongly constrained the
comet model.
14
Shoemaker is large enough to cover a substantial
portion of a Lunar Prospector neutron detector
pixel, and so contributes directly to that ice
detection. Half of the floor is within sight
of Earth-based radar and was probed as part of
the null ice detection by that
technique Shoemaker crater provides a unique
opportunity to test the seemingly-contradictory
neutron and radar results concerning near-surface
ice.
15
LCROSS impact into Shoemaker will directly test
these two results at the same location, and
should significantly improve our understanding of
lunar ice.