Pathfinder landing looks tricky
Maldwyn G.T. Morris
Does nobody else think that the Pathfinder landing sequence, as
described at:
http://mpfwww.arc.nasa.gov/mpf/edl/edl1.html
is incredibly complicated - Have they tested the procedure on Earth ?
Is there no way of landing a probe on Mars without bouncing
around and hoping you land on a good spot ?
Maldwyn
Michael K. Heney
In article <33B679...@ripe.net>,
Maldwyn G.T. Morris <mal...@ripe.net> wrote:
>Does nobody else think that the Pathfinder landing sequence, as
>described at:
>
> http://mpfwww.arc.nasa.gov/mpf/edl/edl1.html
>
>is incredibly complicated - Have they tested the procedure on Earth ?
Yes, they did. Didn't yuou see the pictures of the tests on the page
you referenced?
>Is there no way of landing a probe on Mars without bouncing
>around and hoping you land on a good spot ?
Sure, there are a number of ways of landing on Mars. This particular method
was selected because it resulted in a lighter vehichle, and because it allows
for a much more lineral definition of what a "good spot" is.
With a standard landing gear, you'd need sufficient fuel to brake to
zero velocity (or awfully close to it), landing legs that could handle the
touchdown forces (plus a margin for a "hard" landing, and requires a level
area for the touchdown. If you land on a boulder, you're SOL.
The airbag method lets you minimize your landing rockets and reduces the
precision needed in your landing rocket system (which simplifies things a
lot), supports "hard landings" in an effective way (you bounce), and manages
the boulder problem by rolling off it to a flatter area, in essence settling
into a good area rather than depending on hitting one at the end of powered
descent.
It's an interesting, innovative solution to landing; look at the requirements
for a powered vertical robotic landing like Viking and tell me which is
more inherently complicated.
>
>Maldwyn
====
Mike Heney
Webmaster, Space Frontier FOundation
http://www.space-frontier.org
Frank Crary
In article <33B679...@ripe.net>,
Maldwyn G.T. Morris <mal...@ripe.net> wrote:
>Does nobody else think that the Pathfinder landing sequence, as
>described at:
> http://mpfwww.arc.nasa.gov/mpf/edl/edl1.html
>is incredibly complicated - Have they tested the procedure on Earth ?
Yes, it is complicated, but it has been tested on Earth. Well, most
of the phases of the landing have been tested. They didn't do anything
dropping a full test article from orbit... I'm a bit concerned about
the size/frequency distribution of the rocks used in tests of the
airbag landing; they photos I've seen of the test site didn't look
like the Viking landing sites, but I'm told that's an artifact of
the camera angles, and they really did test it on the expected
size/frequency distribution of rocks.
>Is there no way of landing a probe on Mars without bouncing
>around and hoping you land on a good spot ?
Well, Pathfinder doesn't really have to hope it lands in a good spot.
But, yes, there are other ways to land a probe on Mars: The two
Viking landers just used parachutes and retro-rockets. But they
probably got lucky: One landed within meters of a big rock, big
enough to have caused a fatal accident on landing, if the lander
had come down on top of it. Pathfinder is fairly immune to that
sort of problem. In any case, the technique for landing Pathfinder
on Mars wasn't intended to be simple. It was designed to be
robust (to landing on an unknown surface) and low mass. Pathfinder
was originally, well, a pathfinder mission: A technology demonstration
of the landing technique and a few other things, as the first mission
in a program to land a network of atmospheric and seismic stations.
The network got cut back, and then cut back more, until there wouldn't
have been enough stations to make a useful network. Then the network
was canceled, and the scientific mission of Pathfinder was beefed up.
But it still has many elements, including the landing technique,
which are more like a technology demonstration than a pure science
mission would be. However, the landing technique is certainly low mass,
and I hope it is sufficiently robust. We may get a good test of that:
Mars isn't being cooperative. I found out today that, after months
with very little dust activity, the latest HST images show a major
dust storm forming in the Vallis system, and expanding southeast and
downhill from there. That puts the Pathfinder landing site directly
in the path of the storm, and the timing is right to suggest Pathfinder
may end up landing in the middle of a significant dust storm.
Frank Crary
CU Boulder
Henry Spencer
>is incredibly complicated - Have they tested the procedure on Earth ?
Within limits. It's hard to do a completely realistic test in 1G and
1atm conditions.
It *is* a trifle complicated. Originally it was going to be just parachute
and airbags, but that turned out not to work quite well enough...
>Is there no way of landing a probe on Mars without bouncing
>around and hoping you land on a good spot ?
Well, you can do what the Vikings did: simply land on rocket thrust.
That gives you a more controlled landing, at the price of considerably
greater hardware complexity (especially compared to the original
few of the instruments the Apollo crews left on the Moon were almost
completely useless because the exhaust of the landing contaminated the
area quite badly).
The "hoping you land on a good spot" problem does remain, however, because
there's too long a time lag between Mars and Earth for the exact landing
spot to be selected during descent, and the combination of poor mapping
resolution and inadequate guidance precision makes it impossible to pick
the exact spot in advance.
The 1998 Mars lander will land on rockets, and that's probably what will
be done henceforth. Mars Pathfinder was originally MESUR Pathfinder, an
engineering test vehicle for MESUR, a project to deploy an array of simple
Mars surface stations. In that context, it made sense to try to develop a
simple, lightweight landing system. Unfortunately, (a) the simple landing
system didn't work out quite as well as hoped, and (b) MESUR is dead.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | he...@zoo.toronto.edu
Aaron Gilliland
Michael K. Heney wrote:
> Sure, there are a number of ways of landing on Mars. This particular method
> was selected because it resulted in a lighter vehichle, and because it allows
> for a much more lineral definition of what a "good spot" is.
>
> It's an interesting, innovative solution to landing; look at the requirements
> for a powered vertical robotic landing like Viking and tell me which is
> more inherently complicated.
Of course, we hope it doesn't bounce into a huge rift, void of
light...That'd be a nice sticky hole for NASA to climb out of :)
- Bubbis >:-)
Frank Crary
In article <5p9v84$2...@lace.colorado.edu>,
Frank Crary <fcr...@rintintin.Colorado.EDU> wrote:
>Mars isn't being cooperative. I found out today that, after months
>with very little dust activity, the latest HST images show a major
>dust storm forming in the Vallis system, and expanding southeast and
>downhill from there. That puts the Pathfinder landing site directly
>in the path of the storm, and the timing is right to suggest Pathfinder
>may end up landing in the middle of a significant dust storm.
Fortunately, the description I heard was a bit exaggerated (and I
should have said heading northeast, not southeast, towards the
landing site...) The storm is rather large, but basically a local
one. Those tend to move with the prevailing winds, not downhill,
and are unlikely to cross the equator. That means it's unlikely
the storm will reach the landing site, and unlikely to be a problem
if it does.
Frank Crary
CU Boulder
Magnus Redin
Henry Spencer <he...@zoo.toronto.edu> writes:
> Mars Pathfinder was originally MESUR Pathfinder, an engineering
> test vehicle for MESUR, a project to deploy an array of simple Mars
> surface stations. In that context, it made sense to try to develop a
> simple, lightweight landing system. Unfortunately, (a) the simple
> landing system didn't work out quite as well as hoped, and (b) MESUR
> is dead.
How do the parachute and airbag concept scale with size? It might well
be that it could function withouth retrorockets with a lighter
payload.
Regards,
--
--
Magnus Redin Lysator Academic Computer Society re...@lysator.liu.se
Mail: Magnus Redin, Rydsv=E4gen 214B, 584 32 LINK=F6PING, SWEDEN
Phone: Sweden (0)13 260046 (answering machine) and (0)13 214600