[meteorite-list] Landing on Powder or Ice? (Rosetta)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:56 2004
Message-ID: <200403301737.JAA16475_at_zagami.jpl.nasa.gov>

http://www.esa.int/SPECIALS/Rosetta/SEM2BB57ESD_0.html

Landing on powder or ice?
European Space Agency
30 March 2004

Rosetta's lander Philae will do something never before attempted:
land on a comet. But how will it do this, when the kind of surface
it will land on is unknown?

With the surface composition and condition largely a mystery,
engineers found themselves with an extraordinary challenge; they
had to design something that would land equally well on either
solid ice or powder snow, or any state in between.
 
In the tiny gravitational field of a comet, landing on
hard icy surface might cause Philae to bounce off
again. Alternatively, hitting a soft snowy one could
result in it sinking. To cope with either possibility,
Philae will touch as softly as possible. In fact,
engineers have likened it more to docking in space.

Landing on a comet is nothing like landing on a large
planet, you do not have to fight against the pull of the
planet's gravity, and there is no atmosphere.

The final touching velocity will be about one metre per
second. That is near a walking pace. However, as anyone
who has walked into a wall by mistake will tell you, it is
still fast enough to do some damage. So, two other
strategies have been implemented.
 
Firstly, to guard against bouncing off, Philae will fire
harpoons upon contact to secure itself to the comet.

Secondly, to prevent Philae from disappearing into a
snowy surface, the landing gear is equipped with
large pads to spread its weight across a broad area -
which is how snowshoes work on Earth, allowing us
to walk on powdery falls of snow.

When necessity forced Rosetta's target comet to be
changed in Spring 2003 from Comet Wirtanen to
Comet 67P/Churyumov-Gerasimenko, the landing
team re-analysed Philae's ability to cope. Because Comet
Churyumov-Gerasimenko is larger than Wirtanen, three
times the radius, it will have a larger gravitational
field with which to pull down Philae.
 
In testing it was discovered that the landing gear is
capable of withstanding a landing of 1.5 metres per
second - this was better than originally assumed.

In addition, Rosetta will gently push out the lander
from a low altitude, to lessen its fall. In the
re-analysis, one small worry was that Philae might
just topple, if it landed on a slope at high speed.
So the lander team developed a special device called a
"tilt limiter", and attached it to the lander before
lift-off, to prevent this happening.

In fact, the unknown nature of the landing environment
only serves to highlight why the Rosetta mission is
vital in the first place. Astronomers and planetary
scientists need to learn more about these dirty
snowballs that orbit the Sun.
Received on Tue 30 Mar 2004 12:37:50 PM PST