SMART-1 Maneuvers Prepare for Mission End

http://www.esa.int/esaCP/SEMQFHL8IOE_index_0.html

SMART-1 manoeuvres prepare for mission end
European Space Agency
23 June 2006

After sixteen months orbiting the Moon, ESA's lunar mission is
preparing
for the end of its scientific exploration. On 19 June, SMART-1 mission
controllers initiated a 17-day series of manoeuvres aimed at
positioning
the spacecraft to enhance science data return as the mission winds
down.

SMART-1, Europe's successful first Moon mission, is scheduled to end on
3 September 2006, impacting on the Moon's surface in a disposal plan
similar to that of many earlier lunar missions and almost three years
to
the day after its 2003 launch.

The recently started manoeuvre campaign aims to avoid having the
spacecraft intersect with the Moon at a disadvantageous time from the
scientific point of view, as it would have naturally about 17 August if
left alone. Instead, this 'extension' to mission operations will
provide
new opportunities for low-altitude scientific observations and give
optimum science returns during and after the spacecraft's controlled
impact on the Moon.

In preparation for mission end, spacecraft controllers at ESOC, ESA's
Spacecraft Operations Centre in Darmstadt, Germany, have started a
series of thruster firings to give a 'delta-velocity,' or change in
velocity, of approximately 12 metres per second. This will raise the
orbit perilune (point of closest passage over the Moon) by about 90
kilometres, and will shift the impact to 3 September.

"The shift in date, time and location for Moon intersection is also
optimised to favour scientific observations from Earth," said Gerhard
Schwehm, ESA's SMART-1 Mission Manager. "Projections based on the
current orbit indicated that the spacecraft, if left as is, would
impact
the Moon on the far side, away from ground contact and visibility. The
new location is on the Moon's near-side, at mid-southern latitudes."

For the manoeuvre campaign, the use of the electric propulsion system
(the ion engine) had to be ruled out since all Xenon propellant
reserves
were exhausted during the mission. The mission control team have
instead
developed an imaginative approach.

Innovative manoeuvre strategy

"The manoeuvre strategy consists of a series of reaction-wheel
off-loadings combined with about three hours of intermittent thrust
centred at apolune (point of furthest distance from the Moon) during
the
next 74 orbits," said Octavio Camino, Spacecraft Operations Manager at
ESOC.

The off-loading consists of braking a set of spinning wheels inside the
spacecraft, which has the effect of transferring angular momentum from
the wheels to the spacecraft and hence changing its velocity.

"We use asymmetric firing of the attitude thrusters to produce a small
velocity variation aligned with the flight direction. This will change
the orbit by an accumulative effect," added Camino.

"After these manoeuvres, science activities will resume until the
impact, with short interruptions for two trim manoeuvres to adjust the
impact time, one around the end of July and one at the beginning of
September," he concluded.

This manoeuvre campaign and the following trim manoeuvres will make it
possible to predict the exact time and location for the SMART-1 impact
with more accuracy.

Note to editors

SMART-1 is the first in a series of 'Small Missions for Advanced
Research and Technology' in which elements of the platform and
miniaturised payload technology have been conceived as a demonstration
for future scientific missions and an early opportunity for science.
SMART-1 used an innovative ion-propulsion system powered by a small
quantity of onboard Xenon and solar energy to generate electricity used
to ionise the fuel to travel to the Moon.

After a 27 September 2003 launch, SMART-1 spiralled out over a 14-month
period until being captured by the Moon on 15 November 2004, thus
successfully achieving the primary objective of demonstrating solar
electric propulsion during interplanetary travel. In addition to
helping
prove new technology from the perspective of satellite design, the
mission has also provided an opportunity to develop new ways of
conducting ground control operations based on both increased satellite
autonomy and improved tools for ground automation.

The wealth of scientific data from SMART-1 are still being processed
and
analysed. Thanks to SMART-1, scientists all over Europe and around the
world will have access to the best-resolution surface images ever taken
from lunar orbit, as well as a better knowledge of the Moon's minerals.
For the first time from orbit, SMART-1 detected Calcium and Magnesium
using an X-ray instrument. It measured compositional changes from the
central peaks of craters, volcanic plains and giant impact basins. The
camera studied impact craters, volcanic features and lava tubes, and
monitored the polar regions.