spare parts from the Mars Orbiter Laser
Altimeter (MOLA) project. Unlike its
predecessor, SLA-II’s receiver contained
a variable-gain amplifier, which allowed
the experiment’s ground-based operator
to avoid the detector saturation that had
happened to SLA-I occasionally. NASA
has posted data sets from both SLA
missions online at denali.gsfc.nasa.gov/
The SLA experiments showed scientists how to analyze on-orbit laser ranging measurements of the Earth’s surface
and to perform laser-pulse waveform
analysis to assess overall surface roughness and tree heights in forested regions.
Lidar altimetry around
the solar system
The unpiloted U.S. lunar spacecraft
known as Clementine, which flew in
1994, was most famous for finding
evidence of a large water ice deposit
in a crater near the moon’s south pole.
However, Clementine also carried a lidar
altimetry experiment that used the modern Nd:YAG laser technology unavailable during the Apollo era.
Clementine’s lidar transmitter was a
frequency-doubled Nd:YAG laser; the
spacecraft caught the reflected pulse
in its high-resolution camera equipped
with a silicon avalanche photodiode
detector (APD). The compact ( 2. 37 kg)
system collected data for nearly three
months at a vertical resolution of about
40 m; the spacecraft ran out of electrical
power about five months after launch.
Combined with the satellite’s multi-spectral imaging data, Clementine’s
lidar altimetry provided a detailed and
uniform topological map of the moon
between latitudes 60 degrees north and
60 degrees south.
Although lidar measured the moon
in the 1970s and the 1990s, the quest for
more precise data continues. The recent
Japanese satellite known as Kaguya or
Selenological and Engineering Explorer
(SELENE) carried a laser altimeter as
well as radar and a terrain camera.
Polar-orbiting 100 km above the lunar
surface, SELENE’s Nd:YAG laser started
gathering topographical data in late 2007.
Space lidar systems
must have 1,000 to
10,000 times better
airborne ones. Thus,
to fly in space, lidars
need more powerful
or bigger receiver
The Mars Orbiter Laser Altimeter (MOLA)
measured the Red Planet’s global dimensions. In this 90-degree 3D view, the large
blue spot is the massive Hellas impact
basin in Mars’ southern hemisphere.
The science team produced a global topographical map with a spatial resolution of
better than 0.5 degree and used the data
to model the mechanical properties of
the lunar lithosphere. The spacecraft also
found new features within craters near
the lunar polar regions.
In November 1996, NASA launched
its first spacecraft to the Red Planet in
two decades. The Mars Global Surveyor
(MGS) carried MOLA, a lidar system
that produced a precise pole-to-pole
topographic map of the martian landscape over the course of a Mars year
(687 Earth days).
Orbiting at an average altitude of
378 km, MOLA found that the northern hemisphere of Mars is flatter than
the southern hemisphere and that the
planet’s south pole is about 6 m higher in
elevation than its north pole. Despite the
“flatness,” the lidar survey revealed the
northern plains have subtle ridges that
were probably caused by tectonic activity.
The MOLA instrument also measured
seasonal variations in the carbon-dioxide
“snow” cover in the polar regions.
Part of MOLA’s laser stopped working in mid-2001, but a near-infrared
sensor on the instrument continued to
study cloud coverage on the Red Planet.
Communications between Earth and
MGS failed in November 2006 and
NASA officially ended the mission two
For NASA’s Phoenix mission, which
landed near the north polar cap of Mars
in May 2008, the Canadian Space
Agency built a meteorological station
with a lidar system looking up at the
martian atmosphere from ground level.
Last fall, the lidar detected snow falling
from martian clouds about 4 km above
the landing site, although the snow
vaporized before it hit the ground.
MESSENGER, the NASA mission
en route to the planet Mercury, is carrying a laser altimeter to gather the most
precise topography yet of this small,
rocky body, which was surveyed only
by a flyby spacecraft in 1974 and 1975.
MESSENGER will insert itself into
orbit around Mercury in March 2011.
ICESat and CALYPSO
With the ICESat satellite, launched in
2003, NASA deployed lidar technology for environmental studies of Earth,
rather than simple altimetry. ICESat (or
the Ice, Cloud and land Elevation Satellite) is part of the U.S. space agency’s
Earth Observing System mission.
Designed and built at NASA Goddard,
the Geoscience Laser Altimeter System
(GLAS) aboard ICESat measures ice-sheet topography and changes in cloud
and atmospheric properties.
Not long after reaching Earth orbit,
though, GLAS started experiencing
problems with its three lasers (three had
been considered sufficiently redundant
for a planned five-year mission). Over
the years, each of the lasers has failed
at least once, and one of the three is
now completely inoperable. As a result,
GLAS is collecting data for two brief
windows per year. The ICESat science