DATA_SET_DESCRIPTION |
Data Set Overview
=================
The NASA LCROSS (Lunar CRater Observation and Sensing Satellite) mission
was designed to search for evidence of water ice in continuously
shadowed crater floors at the Moon's north or south pole. LCROSS was a
piggyback experiment to the Lunar Reconnaissance Orbiter (LRO), which
was launched in June 2009. The LCROSS mission involved crashing the LRO
upper stage into a shaded polar region of the Moon, throwing up lunar
regolith high enough to be illuminated by the Sun and observed from
Earth (and simultaneously observed from a 'chase satellite' following
several minutes behind the booster).
The main scientific goal of the LCROSS mission was to find
spectrographic evidence of water in the illuminated plume. Estimates of
the plume brightness by NASA/Ames Research Center (ARC) indicate that
large telescopes such as Keck would probably be needed to detect water
lines in the spectra with sufficient signal-to-noise. This conclusion is
based on the estimated optical depth of the ejected water, which is
related to the total regolith mass ejected and the expanding radius of
the plume. Photometric observations of the evolving plume with smaller
telescopes were sought to establish the plume radius as a function of
time and provide an estimate of the ejecta mass, which is related to the
surface brightness of the plume. These measurements could in turn be
used to calculate the water vapor optical depth as a function of time
for a given estimate of water vapor content in the regolith. Simulations
conducted at NASA/ARC indicated that the plume will rise to a height of
35 km above the lunar surface, which corresponds to an angular distance
of 18 arcseconds as seen from Earth.
We observed the Moon from the Apache Point Observatory (APO), which is
located in the Sacramento Mountains of southern New Mexico, in an effort
to characterize the LCROSS plume. Our observations addressed LCROSS
Science Goal #4: Characterize the lunar regolith within a permanently
shadowed crater on the Moon. Specifically, we proposed to set
constraints on the ejecta mass by observing the time evolution of the
expanding plume.
Scientific Background
========================
The NASA LCROSS (Lunar CRater Observation and Sensing Satellite) mission
was designed to search for evidence of water ice in continuously
shadowed crater floors at the Moon's north or south pole. The neutron
spectrometer on the Lunar Prospector orbiter detected the signature of
hydrogen concentrations in shaded areas of the moon's north and south
poles (Feldman et al. 2001). The leading theory is that this hydrogen is
locked in hydrous minerals or water ice. In September 2009, results
from the Cassini (Clark et al. 2009), Deep Impact (Sunshine et al.
2009), and Chandrayaan-1 (Pieters et al. 2009) spacecraft all indicated
the presence of surficial lunar water and hydroxyl.
The LCROSS mission was designed to crash the LRO upper stage into a
shaded polar region of the Moon, throwing up lunar regolith high enough
to be illuminated by the Sun and observed from Earth and the shepherding
spacecraft, which impacted the Moon's surface four minutes later. The
LRO/LCROSS spacecraft was launched on June 18, 2009, and crashed into
the Cabeus crater near the Moon's south pole on October 9, 2009. The upper
stage impacted the moon at 11:31:19.51 UTC at -84.68 deg latitude,
-48.69 deg longitude, Mean Earth frame, and the shepherding spacecraft
impacted the surface at 11:35:34 UTC.
Ground-based observatories from Texas westward to Hawaii were enlisted
to observe the impact, which occurred during nighttime hours in those
locations.
Observations
=================
The APO 3.5-m team acquired data on October 9, 2009, for approximately
five hours prior to impact time and 1.5 hours following impact.
Observing conditions were clear.
Instrument
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The Agile imager was used to acquire all APO 3.5-m observations. A
combination of a V filter and an ND 2.5 filter was used to reduce the
brightness of the Moon to avoid saturation of the detector. The
instrument was used in 2 x 2 binned mode with the medium gain setting
and the fast readout time. During observations of the impact, the Agile
dark slide was inserted partway into the optical path. This resulted in
an obscuration of part of the illuminated disk of the Moon as seen in
the Agile field of view, which helped to reduce the amount of scattered
light in the images.
Observing Strategy
------------------
Standard stars and other calibration data were acquired along with the
images of the Moon. These calibration files include the following:
1) bias frames
2) a dome flat (to illustrate the location of the dark slide)
3) sky flats
4) dark frames
5) reference objects: 68 Psc, 1 Aur, Hipparcos 2942, Hipparcos 24813,
Mars, and Uranus
For most of the data acquisition, images were obtained using TUI, the
telescope and instrument user interface software. However, the
observations of the Moon obtained close to the time of impact were
obtained by typing a data acquisition command into a command line. This
was done to avoid a latency that occurred when writing numerous header
cards with information obtained from the telescope hub computer.
Consequently, the FITS headers for the impact sequence Moon images do
not contain some information that is written to the FITS headers created
when acquiring data through the user interface.
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