Landings on other worlds are remarkably difficult to achieve.
During the last 40 years, the only objects in the Solar System
on which robotic spacecraft have soft-landed have been the Moon,
Venus, Mars and near-Earth asteroid Eros. A decade from now, it
will be the turn of ESA’s pioneering Rosetta spacecraft to land
on a comet.

Rosetta will achieve many breakthroughs during its 10 1/2 year
odyssey to Comet Wirtanen, but one of the most exciting episodes
of this ambitious mission will involve the first soft landing
on one of these cosmic icebergs. This landmark event will be
followed by the first panoramic images from a comet’s surface
and the first in situ analysis to find out what its ancient
nucleus is made of.

The historic landing on one of the most primitive objects in the
Solar System will be undertaken by a unique spacecraft which has
been built by a European consortium under the leadership of the
German Space Agency (DLR), together with ESA and institutes from
Austria, Finland, France, Hungary, Ireland, Italy, Germany and
the UK.

Engineers at the European Space Research and Technology Centre
(ESTEC) in the Netherlands have recently integrated the landing
gear on the 100 kg Rosetta Lander, clearing the way for the
installation of the box-shaped spacecraft on the exterior of
the three tonne orbiter. In the coming weeks, the two spacecraft
will complete their systems tests and the lander release
mechanism will be checked. Then it will be time to ship the
Rosetta ‘mother craft’ and its small piggyback companion to the
launch complex in Kourou, French Guiana.

So how will Rosetta’s little Lander make its mark in the annals
of space exploration? Jean-Christophe Salvignol, the Rosetta
spacecraft and Lander mechanical engineer, outlined the complex
procedure.

"Shortly after launch, the four main attachment points used to
carry the Lander during launch are released. The Lander is
then only supported by its central motor, the MSS, during all
the cruise."

"In the summer of 2012, the Lander will be released from the
Rosetta orbiter at an altitude of about one kilometre above
the nucleus. By this time, the instruments on the orbiter will
have mapped every square centimetre of the comet’s surface,
enabling scientists to select a suitable landing site.

"The MSS will then gently push the Lander away at crawling
speed, no more than 0.5 metres per second. Once the landing
gear is deployed, the spacecraft will edge towards its target,
prevented from tumbling by an internal flywheel that provides
stability as its spins. A single cold gas thruster will be
able to provide a gradual upward push to improve the accuracy
of the descent."

After a nail-biting 30 minute wait by the helpless mission team
back on Earth, sensors on board the Lander will record the
historic moment of touchdown. Since the nucleus is so small,
its gravitational pull will be extremely weak — millions of
times weaker than on Earth — causing the Lander to touch down
at no more than walking pace. Nevertheless, a damping system
in the landing gear will be available to reduce the shock of
impact and to prevent rebound. Other events will occur in quick
succession.

"Hopefully, gradient will not be a problem, since the spacecraft
is designed to stay upright on a slope of up to about 30
degrees," said Jean-Christophe. "However, the Lander carries
two harpoons. One of these will be fired at the moment of
touchdown to anchor the spacecraft to the surface and prevent
it from bouncing. Ice screws on each leg will also be rotated
to bite into the nucleus and secure the Lander in place. The
second harpoon will be held in reserve for use later in the
mission if the first one becomes loose.

"Meanwhile, the science programme will quickly get under way.
In the primary phase, which relies on battery power, the most
important scientific measurements will be completed. It will
last in the order of 60 hours. The secondary science will be
conducted using the remaining battery power and energy from the
solar cells on the exterior of the Lander. The duration of this
phase should be about 3 months. Anyway, no one knows precisely
how long it will survive. This will depend on a number of
factors: power supply, temperature or surface activity on the
comet."

What we can be sure of is that Rosetta will revolutionise our
knowledge of comets, providing new insights into the nature
and origins of these primordial objects, the building blocks
>from which the planets were born.

Rosetta will be shipped to Kourou spaceport in French Guiana
during early September. The Ariane 5 launch from Kourou is
scheduled for the night of 12-13 January 2003.

USEFUL LINKS FOR THIS STORY

* More about the Rosetta Lander
http://sci.esa.int/content/doc/e0/2272_.htm
* Rosetta Lander home page at DLR
http://www.rosetta-lander.net
* Rosetta home page
http://sci.esa.int/rosetta/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&oid=30413&
ooid=30414]
Lander Integrated on Orbiter

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&oid=30413&
ooid=30415]
Lander with Legs Deployed

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=13&cid=12&oid=30413&
ooid=30416]
Lander FM Vibration Test in Z