Data for Scientific Research
Although the bravely stated secondary goal
of the mission was to image the entire lunar surface in 11 narrow pass bands of the
UV/Vis and NIR
cameras, there was a tacit recognition that this goal would not be fully achieved.
If we succeeded in imaging 75% of the lunar surface, it would be a major accomplishment.
NASA's DSN ground stations in Madrid, Canberra,
and Goldstone, California, shared the communications tasks with our NRL site in Pomonkey,
Maryland. As a matter of policy, DSN would only commit to 90% of the support requested
of them because of the large number of space programs using DSN. However 1 week into
the 8-week systematic imaging phase of the mission, it was clear that imaging of
the entire lunar surface was indeed possible. Although a few images had already been
missed, operational procedures and opportunities to regain the missed images were
quickly identified. Therefore the Clementine operations team became determined
not to miss a single image, and their determination inspired DSN as well. The obvious
delight of the NASA Science Team with the quality and quantity of the lunar images
also fueled our determination.
The Clementine spacecraft delivered
approximately 1.8 million digital images,
most of which were obtained during the period of systematic imaging from February
26 until April 22. Every 5 hours, between 5000 and 6000 images were transmitted.
As each image arrived in the Clementine operations center in Alexandria, Virginia,
it was processed and was available for use by operations personnel and the NASA Science
Team within 15 minutes. As the images became available, sensor experts from NRL
and LLNL, representatives of the NASA Science
Team, and other operations team personnel would use them to quickly verify the proper
operation and targeting of the sensors. Thus adjustments identified from the images
of one orbit could often be transmitted to the spacecraft before collection of images
began for the next orbit. Selected images were frequently made available over the
Internet electronic mail system; this created great interest in the Clementine
mission.
 Figure 1 shows a refined version
of a mosaic of UV/Vis camera images covering
the Moon's south polar region, from approximately 70 degrees S latitude to the pole.
The south pole would be completely imaged with the best resolution for this mission
during the first month of systematic imaging because the periselene was in the Southern
Hemisphere. At the arrival of the first polar images, members of the NASA Science
Team from the U.S. Geological Survey (USGS) started
assembling a mosaic of UV/Vis images of the south polar region as a check on the
quality of the data. As new images were transmitted, the Science Team would fit them
into the mosaic and carefully examine the overlap with adjacent images and picture
quality. This effort provided early identification of a minor spacecraft timing anomaly,
which caused some misalignment of image strips from adjacent orbits. The early identification
resulted in a quick correction so that analysis of the data from subsequent orbits
was made easier. As the mosaic took shape, it appeared to reveal a region surrounding
the pole that received no sunlight even though the Moon completely rotated once.
Although it will require extensive analysis to determine whether sunlight is always
excluded from this region, the possibility of an extremely cold, perpetually shadowed
region collecting and freezing water molecules for eons invited an attempt to find
ice in the shadowed area. In consultation with representatives of JPL
and by using a technique based on characteristic changes in the polarization signature
of radio frequency (RF) radiation with reflection from ice or rock, the spacecraft's
high gain antenna was aimed at various south and north polar regions, including the
shadowed area so that the RF signal would bounce from the selected lunar surface
area to the ground stations on Earth. Since this is an experiment that was not foreseen
prior to launch, no processing software had been prepared. Thus after the data were
collected, software for an extremely intricate data processing sequence had to be
prepared. Final analysis of this data is not yet complete. Initial lower-order analyses
reveal no ice, but there is hope that ice might still be revealed in the later stages
of the analysis.
Figure 1 is also significant for
what it promises from the Clementine images. Between 1500 and 1600 images
from a single pass band of the UV/Vis camera were used to create the mosaic. With
an altitude near 800 km at the poles, the resolution (based on the field of view
of a single pixel) is 200 m for the UV/Vis camera and 300 m for the NIR camera. At
the 400-km minimum periselene altitude, resolution was 100 m for the UV/Vis camera
and 150 m for the NIR camera. Thus the Clementine images cover the entire
lunar surface with a resolution between 100 and 200 m for the five pass bands of
the UV/Vis camera and a resolution between 150 and 300 m for the six pass bands of
the NIR camera. Images of equal or higher resolution were available for most of the
lunar surface before Clementine from Lunar Orbiter and Apollo.
However, these earlier images were almost entirely in the form of nondigital hard
copies. The digital format of the Clementine images will allow not only easier
storage and distribution but also digital processing to enhance selected information
and more easily align adjacent images. Clementine does provide significantly
higher resolution than previously available for the higher latitude regions, especially
on the Moon's far side. Presently available charts from USGS and National
Geographic show significant areas poleward from 70 degrees S latitude between
90 degrees and 120 degrees W longitude labeled "unsatisfactory photography."
The Clementine images will certainly remove this luna incognita. Mosaics,
with resolution equal to or better than in Fig. 1, can be made by using Clementine
data for every region of the lunar surface. The overlap of image paths in both polar
regions is such that mosaics like Fig. 1 can also be made by using images from the
HiRes camera with 30-m resolution. Since the HiRes mosaics will include over 70,000
images, they will not be available soon.
The use of the same cameras to image the entire
lunar surface adds information. Many discrete areas of the Moon have been imaged
with better resolution than Clementine provides. Surface features within these
regions can be precisely located relative to each other. However precise location
of features within one region relative to features in another region separated by
an expanse of lower-resolution images is very difficult, especially if different
sensors were used for the two regions. The Clementine data provide the first
opportunity not only to achieve precise relative locations for all resolvable features
on the lunar surface but also to accurately define an absolute reference grid for
the entire lunar surface.
The 11 narrow pass band filters of the UV/Vis
and NIR cameras were selected by the NASA Science Advisory Committee to provide information
useful to identify the distribution of minerals over the lunar surface. Relative
responses among the various filters can identify regions dominated by minerals rich
in iron, magnesium, titanium, or other elements. Lunar samples provide a calibration
of the mineralogy at the Apollo landing sites. During spacecraft integration
at NRL, Apollo samples were used to provide calibration information for the
Clementine sensors. Over 100,000 UV/Vis images were used in three pass bands
to cover the entire lunar surface. The average response for each image was calculated
from the response of each of the 100,000-plus pixels of each image. The red images
centered at 750 nm were used as the reference and combined with the relative response
of the bands centered at 415 and 950 nm. Use of the individual pixels of the images
will provide much higher resolution. Lunar geologists can use the results of these
multispectral displays to estimate the ages of different regions, trace ejecta paths
from craters, and recognize when material from below the surface has been uncovered.
Doppler-based range and velocity measurements
were made on the spacecraft while in lunar orbit so the location of the spacecraft
could be precisely known for later interpretation of the images and to obtain measurements
to improve the model of the Moon's gravitational potential. Since the lunar potential
model was quite accurate before the Clementine flight, improvements to the
model are expected to be small. However the accurate measurements of the location
of the spacecraft throughout the mission, coupled with the use of the laser ranger
to provide altitude measurements, will greatly extend the knowledge of lunar topology.
Since the laser ranger had a maximum range of 640 km, good altimetry data were obtained
70 degrees to 10 degrees S latitude during the first 28 days of systematic imaging
and from 10 degrees to 70 degrees N latitude during the second 28 days. Additional
data of lesser quality were obtained between 70 degrees and 75 degrees in both polar
regions and within 10 degrees of the equator. Although the laser ranger pulsed at
1 Hz, only about 25% of the pulses provided an interpretable return. Therefore the
Clementine altimetry data provides strips of measured points at intervals
averaging 10 km running from 70 degrees S to 70 N latitude. The strips are approximately
75 km apart at the equator and closer at higher latitudes. The laser ranger can resolve
altitudes to 40 m. The Doppler measurements and lunar potential model can locate
the spacecraft to 100 m relative to the center of mass of the Moon. Thus a coarse
surface profile for latitudes below 75 degrees can be generated from the Clementine
data. Superficial, initial analysis of these data has already verified the existence
of suspected old basins, revealed the presence of at least one unsuspected old basin,
and shown the huge south pole Aitken Basin on the far side to be larger and far deeper
than previously recognized. Altimetry measurements near the lunar equator were also
made by Apollo flights. Attempts will ultimately be made to combine the Clementine
and Apollo altimetry into a single grid. In an attempt to provide some surface-profile
information for latitudes greater than 75 degrees, dual images of both polar regions
were made to provide stereoscopic coverage.
The Clementine operations team could
not resist taking images of Earth from the spacecraft's unusual vantage point. Figure 2 is a splendid mosaic of Earth
assembled by the USGS from multispectral HiRes camera images taken from lunar orbit.
New pictures of Earth arriving in the control center always created a stir. Although
everyone clearly understood the physics involved, photographs showing our round Earth
with no visible support were awesome.
All data from the Clementine mission
are being processed for submission to NASA's Planetary
Data System (PDS) for archiving in the standard PDS format. This task is being
closely monitored by the NASA Science Team. The image data are being combined with
pertinent ancillary information from the spacecraft, such as sensor temperatures
and spacecraft orientation, which will be needed for proper analysis of the data.
High-precision orbit information supplied by NASA's Goddard Space Flight Center is
being used to locate the spacecraft for each image. Calibration information for each
sensor will be included with the archived data set. Data archived in the PDS are
easily available to all potential users at modest cost, especially including universities
and other research institutions, government agencies, and the general public. It
is expected that the Clementine data will keep lunar scientists occupied for
at least 10 years.
From "Clementine-A Mission to the Moon (and Beyond)" by
Donald M. Horan and Paul A. Regeon, 1995.
|
Figure 1 shows a refined version
of a mosaic of UV/Vis camera images covering
the Moon's south polar region, from approximately 70 degrees S latitude to the pole.
The south pole would be completely imaged with the best resolution for this mission
during the first month of systematic imaging because the periselene was in the Southern
Hemisphere. At the arrival of the first polar images, members of the NASA Science
Team from the U.S. Geological Survey (USGS) started
assembling a mosaic of UV/Vis images of the south polar region as a check on the
quality of the data. As new images were transmitted, the Science Team would fit them
into the mosaic and carefully examine the overlap with adjacent images and picture
quality. This effort provided early identification of a minor spacecraft timing anomaly,
which caused some misalignment of image strips from adjacent orbits. The early identification
resulted in a quick correction so that analysis of the data from subsequent orbits
was made easier. As the mosaic took shape, it appeared to reveal a region surrounding
the pole that received no sunlight even though the Moon completely rotated once.
Although it will require extensive analysis to determine whether sunlight is always
excluded from this region, the possibility of an extremely cold, perpetually shadowed
region collecting and freezing water molecules for eons invited an attempt to find
ice in the shadowed area. In consultation with representatives of JPL
and by using a technique based on characteristic changes in the polarization signature
of radio frequency (RF) radiation with reflection from ice or rock, the spacecraft's
high gain antenna was aimed at various south and north polar regions, including the
shadowed area so that the RF signal would bounce from the selected lunar surface
area to the ground stations on Earth. Since this is an experiment that was not foreseen
prior to launch, no processing software had been prepared. Thus after the data were
collected, software for an extremely intricate data processing sequence had to be
prepared. Final analysis of this data is not yet complete. Initial lower-order analyses
reveal no ice, but there is hope that ice might still be revealed in the later stages
of the analysis.
is a splendid mosaic of Earth
assembled by the USGS from multispectral HiRes camera images taken from lunar orbit.
New pictures of Earth arriving in the control center always created a stir. Although
everyone clearly understood the physics involved, photographs showing our round Earth
with no visible support were awesome.