The SMART-1 way - giving the Moon some great new looks

The SMART-1 way - giving the Moon some great new looks
European Space Agency
12 July 2006

During its 15-month science mission at the Moon, ESA’s SMART-1 returned
up to 1000 images per week, whose analysis is still keeping the
scientists busy. They show the Moon’s surface in unprecedented detail
(with the kind of information that until now, scientists could only
dream about) and it is going to get better.

When you are studying the Moon there is no such thing as
‘point-and-shoot’. The scientists and mission planners on ESA’s SMART-1
lunar explorer had to make sure that when it aimed its camera at a
particular lunar feature, there was enough sunlight to illuminate the
target. That’s no easy task when the spacecraft’s five-hour orbit
carried it around the Moon from mid-day to midnight in just a couple of
hours.

Not just that, but the SMART-1 team also had to ensure that the
spacecraft surveyed as much of the Moon as possible during the mission.
“To decipher the formation and evolution of the Moon, and the processes
that shape its landscapes, we needed both global coverage and dedicated
observations of specific targets,” says Bernard Foing, SMART-1 Project
Scientist.

To accomplish this heavy load, the mission used a number of innovative
observing modes. In all, four styles of observation were used: nadir
observations, targeted observations, moon-spot pointing and push-broom
observations.

Looking ‘nadir’

During SMART-1’s first six months, the nominal mission, the emphasis
was
placed on surveying. Previously, the best digital Moon maps have been
from the US Clementine mission, revealing colour details of 200 metres
across. At its best, the SMART-1 survey maps reveal features just 40
metres across.

“SMART-1 has produced images for some very detailed maps of the Moon,”
says David Frew, SMART-1’s Science Operations Manager, from ESA’s
European Space Research and Technology Centre (ESTEC) in The
Netherlands.

To achieve the survey, the spacecraft simply pointed its cameras
straight down and continuously recorded what passed beneath the
spacecraft. Known as nadir observations, this was not as simple as it
sounds because of the Sun’s heat on the spacecraft.

Looking ‘nadir’

To keep SMART-1 cool, the spacecraft has a number of ‘radiators’ on the
side panels. On the same panels are the star trackers. These tiny
cameras watch the stars, so that SMART-1’s orientation and movement can
be computed.

If sunlight falls onto these sides, the radiators would not dissipate
the excess heat efficiently and the star trackers would stop working if
direct sunlight heated them beyond a certain point.

So the spacecraft had to twist during its orbit to keep the sunlight
off
the side panels. This twisting motion turned the cameras and so the
Science Team had to carefully calculate the image times to ensure that
the camera recorded every part of the lunar surface.

During the survey mode, the other science instruments were also
recording data. For example, the X-ray instrument, D-CIXS collected
nearly 10 full maps of the lunar surface. These data will be combined
into a single definitive map of the Moon’s surface composition. It
should help determine whether the Moon formed from the debris of a
gigantic collision between the Earth and another planet-sized body
during the early history of the Solar System.

Lunar targeting and moon-spot pointing

Towards the end of the six-month nominal mission, the team began to
experiment with targeted observations. This involved tilting the
spacecraft so that it captured a lunar feature even though the
spacecraft did not pass directly over the top of it. This was useful
for
making the most of the infrared spectrometer, SIR, because it has a
small field of view just less than one kilometre across. It allowed SIR
to measure mineralogical changes across the central peaks of lunar
craters.

The targeted observations became increasingly important throughout the
extended mission. The survey mode continued when no special targets
where available. During the northern winter phase of the extended
mission, SMART-1’s orbit carried it towards the dawn-dusk line and so
targets directly below the spacecraft were poorly lit. This meant
tilting the spacecraft towards the sunlit lunar regions. “The
illumination drives the way we observe with SMART-1,” says Frew.

Targets were selected from requests made by the instrument science
teams. Frew and SMART-1 science operations colleagues used computer
simulations of SMART-1’s orbit to determine whether the requested
observations would be possible. Once they had identified a suitable
opportunity to gather the data, the instructions were programmed into
SMART-1.

Two types of targeted observation were possible. The simplest were when
SMART-1 tilted slightly, so that the instruments would track over the
feature. Second were the moon-spot pointings. These were used to keep
looking at a specific feature as SMART-1 flew past it. For the
moon-spot
pointings, SMART-1 had to turn to keep the target in sight. Such
observations enable scientists to understand the topography and surface
roughness of lunar features because they see them from different
angles.

Push-broom

The Science Operations team also used push-broom observations. This
technique allowed colour images of the Moon to be made. SMART-1’s
camera, AMIE, had been constructed so its light-collecting detector was
split into four regions. One was clear and the other three had filters
to cut out all but certain wavelengths of red and infrared light. In
the
push-broom mode, the camera would take a continuous series of images
with a carefully selected exposure time so that the motion of the
spacecraft resulted in the surface features falling into each filter
one
after the other.

This required keeping the camera pointed exactly in the direction of
the
spacecraft’s travel. Unless the team were careful, sunlight would hit
the side panels containing the radiators and star trackers. The team
identified a number of times when, although sunlight would strike the
panels, the heating this would cause was within tolerable limits.

“The push-broom observations were entirely successful,” says Miguel
Almeida, a Science Operations Engineer at ESTEC. Now scientists will be
able to create contextual maps of surface minerals and, for instance,
search for glassy areas on the lunar surface, betraying meteorite
strikes that have melted small areas.

Scientist are now preparing for the final phase of the mission, a
series
of increasingly low altitude orbits that will allow scientists to take
their closest look of the entire mission. In particular, they want to
study the south pole to see if there are any possible landing sites for
future missions. “When you get close, areas that look smooth are
actually very rough,” says Almeida.

“SMART-1 will help to pinpoint interesting and safe terrains for future
exploration,” says Foing, “We are using our data and operational
experience to contribute to the next generation of international lunar
orbiters and landers”.

Eventually, the decreasing orbit will run SMART-1 into the Moon, ending
the mission in a small impact that will leave behind just a small
crater
of a few metres size: a little tribute to a highly successful mission.

For more information:

Bernard Foing, ESA SMART-1 Project Scientist
Email: bernard.foing @ esa.int

David Frew, ESA SMART-1 Science Operations Manager
Email: david.frew @ esa.int