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Mission Details
The NASA Science Advisory Committee defined
the scientific goals as imaging of the entire lunar surface and imaging of the asteroid
1620 Geographos during a close flyby. SDIO's primary objective would be accomplished
by adding scientific tests and demonstrations while achieving the scientific goals.
Significant time constraints were imposed by the close flyby of Geographos. The asteroid
would be closest to Earth in late August 1994 and then would move rapidly out of
range. Figure 1 shows a timeline of the flown mission.
The Clementine spacecraft was launched into a low-Earth orbit by a Titan IIG
on January 25, 1994, only 22 months after formal acceptance of the program by NRL.
On February 3, a solid-fueled rocket integral to the spacecraft was fired to achieve
a trajectory to the Moon. In contrast to the Apollo missions, which had large
fuel reserves to support a direct trajectory to the Moon with arrival in approximately
4 days, Clementine used fuel-efficient phasing loops to get to the vicinity
of the Moon and fired its liquid-fueled propulsion system to enter lunar orbit on
February 19. On February 21, the spacecraft was maneuvered into the 5-hour period,
polar orbit that would be used to take images of the entire lunar surface. After
several days of testing to ensure proper exposure settings for the cameras and to
perfect operations, systematic imaging started on February 26 and continued until
April 22. Several additional days in lunar orbit were used to make special observations,
and the spacecraft propelled itself from lunar orbit on May 3. The plan was to again
use phasing loops around Earth and then leave the Earth-Moon system on May 27, with
an assist from lunar gravity. However an onboard malfunction on May 7 caused the
attitude control fuel to be used completely and left the spacecraft in a spin, which
ultimately prevented generation of adequate electrical energy to operate the spacecraft.
The flyby of the asteroid 1620 Geographos was no longer possible, and the mission
was effectively ended, although attempts to regain some control over the spacecraft
continued for several weeks.
Definition of the desired lunar orbit to achieve
complete imaging of the lunar surface was difficult because of several competing
factors. The orbit had to be a polar one in order to provide the opportunity to image
the entire surface. The NASA Science Advisory Committee desired that the angle between
the plane of the orbit and the vector from the spacecraft to the Sun be no greater
than approximately 30 degrees throughout the systematic imaging phase. (This angle
changes approximately 1 degree per day because of the motion of the Earth-Moon around
the Sun.) If the cameras were too close to the lunar surface during imaging, the
surface included in each image would be inadequate to ensure overlap between adjacent
imaging strips. However, imaging from longer distances reduced resolution. Also the
laser ranger used as an altimeter had a maximum range of only 640 km because of a
design feature that could not be changed quickly enough to support the January 1994
launch. With these altitude constraints, the imaging portion of each orbit would
require approximately 2 hours. During the 2 hours of imaging, between 5000 and 6000
images would be collected and stored aboard the spacecraft. The data transmission
rate was constrained by hardware available in time for the launch. By using data
compression acceptable to the NASA Science Team, it would take slightly over 2 hours
to transmit the data collected on one imaging run. As the Moon moved around the Earth
each month, communications blockages lasting a little over an hour would occur. Thus
the lunar orbit used for systematic imaging had to have a period of at least 5 hours
to allow 2 hours for data collection, 2 hours for data transmission, and 1 hour of
communications blockage. The orbital period had to be in synchronization with the
rotation of the Moon to ensure that the paths traced on the lunar surface by the
fields of view of the cameras over a sequence of orbits would result in adequate
overlap between adjacent images.
The lunar orbit used for systematic imaging
had a 5-hour period with a periselene altitude (orbital point closest to the lunar
surface) maintained between 400 and 450 km. The aposelene altitude (orbital point
farthest from the lunar surface) was allowed to vary to keep the period constant
as the periselene was adjusted to keep it within its acceptable limits. Typically
the aposelene altitude was near 2950 km. During the first 28 days of systematic imaging,
as the Moon rotated once beneath the spacecraft's orbit, the south-to-north paths
traced on the lunar surface by the cameras' fields of view had gaps between them.
These gaps were narrower than the cameras' fields of view and were covered by the
imaging paths of the second 28 days of systematic imaging. A significant orbital
adjustment had to be made at the end of the first 28 days of systematic imaging to
ensure that the imaging paths of the second 28 days filled in the gaps left during
the first 28 days. It was recognized that, since the imaging paths converged in the
polar regions, the polar regions would be completely imaged after the first 28 days
of systematic imaging. However placing the periselene in the Southern Hemisphere
during the first 28-day imaging period would allow imaging of the south pole from
a lower altitude, thus with better resolution than if the periselene were positioned
over the lunar equator for the entire imaging cycle. When the orbital adjustment
was made after the first 28 days of systematic imaging to ensure that the gaps between
imaging paths would be filled during the second 28 days, the periselene could also
be rotated into the Northern Hemisphere to allow higher-resolution imaging of the
north pole during the second 28 days of systematic imaging. This procedure was followed,
and both poles were imaged completely from an altitude of 800 km instead of the 1200-km
altitude that would have resulted if the periselene were held over the equator. The
lunar orbit was selected so that the angle between its plane and the vector from
the spacecraft to the Sun was approximately 30 degrees and decreasing at the start
of systematic imaging on February 26. By the end of systematic imaging on April 22,
the vector had passed through the orbital plane, and the angle between it and the
orbital plane was again approximately 30 degrees but increasing.
NCST trajectory experts had no experience
with translunar trajectories, lunar orbits, and heliocentric trajectories. Therefore
the Flight Dynamics Facility at NASA's Goddard Space Flight Center was given the
primary responsibility for trajectories and orbits until departure from the Earth-Moon
system, which was to occur on May 27. After May 27, the Jet Propulsion Laboratory
(JPL) was to take primary responsibility for the heliocentric trajectory to 1620
Geographos. NCST trajectory experts duplicated the calculations of these external
groups as a check on their results and as a learning experience for NCST.
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