why is the mars rover's lifespan 90 days

Ronald F. Guilmette wrote:

Question: What exactly
prompted this dramatic (and welcome) change in JPL's estimates of the
probable lifespan of the rovers?

Nothing really. The rovers were designed to last 90 Martian days ("Sols"),
but of course, they may last considerably longer if the conditions are
favorable. With the computer program problem tied down, perhaps they will
last longer, but the "90-day" figure was the lifetime that they wanted as a
minimum (its a little like a 90-day warranty). The solar panel power output
is decreasing at the same rate as the pre-flight models predicted (dust and
overall efficiency reduction with time), and the batteries will eventually
give up the ghost, so while the rover should last about three months, how much
longer than this they will remain fully functional is about anybody's guess.

With regards to obtaining juice from the solar cells as
the seasons on Mars progress, and as the average sun angle dips slowly
lower and lower towards the horizon (due to the changing seasons), I do
understand that the lower sun angles mean that the sun's rays will have
to pass through increasing amounts of atmosphere, but is that the primary
issue? Or is the primary issue that fact that the solar panels themselves
will be angled ever more steeply towards the gradually sinking average
daytime sun position in the sky?

The power generated depends on the angle between the sun and the solar panels.
The greater the angle between the sun's direction and the "normal" (a line
perpendicular to the plane of the solar arrays) the lower the power output
from the arrays will be (goes roughly as the cosine of that angle). Thus,
maximum solar power generation will be near local noon on Mars and output will
be dropping rapidly towards zero as sunset approaches. The rovers are near
the equator, so the sun will be fairly high at local noon all year round, but
in a few months, it will not be quite as high in the sky at noon as it is now.
This, combined with the dust settling on the solar arrays, will eventually
have a noticable impact in the amount of electricity the arrays can generate.

If the latter then the obvious question arises... There might be some
small amount of risk involved, but given a small hill, or a medium-sized
nicely rounded rock in the general vicinity where the rover happens to be
prospecting at that moment, couldn't ground controllers drive the rover's
front (or rear) wheels up onto the hill and maybe get it into a nice angle,
relative to the sun, where it could ``sun itself'' for awhile, rather like
some cold-blooded lizard on a river bank, charging up its body temperature?

Well, it would be difficult in Gusev, as it is pretty flat, but I would think
that the advantage gained would be fairly small.

P.S. Alright, one last question: If the ``dust accumulating on the solar
panels'' is in fact a major (or _the_ major) factor that will eventually
lead to the demise of the rovers, then why, oh why didn't JPL do the
obvious thing and attach a little automated triple-jointed extra arm to
each rover, with a little dust brush on the end of it, to wipe the dust
off the panels?

Weight and complexity are the two main reasons. The rover's lifespan might be
only 90 days or so with or without the solar panels being cleaned of dust. Do
you put on a more complex system to get rid of the dust and have to leave out
a vital instrument, only to have the rover fail on down the road for a reason
which has nothing to do with the panels? This is the logic used. It would be
better to use the available time with more instruments and payload than try to
rig something which may or may not be effective. The dust may be rather sharp
and abrasive, so trying to "dust-off" the panels might just scratch them,
making the problem worse. The next long-range rover design is scheduled to
use an RTG, so solar panels will not be needed. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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David Knisely wrote in message ...

P.S. Alright, one last question: If the ``dust accumulating on the solar
panels'' is in fact a major (or _the_ major) factor that will eventually
lead to the demise of the rovers, then why, oh why didn't JPL do the
obvious thing and attach a little automated triple-jointed extra arm to
each rover, with a little dust brush on the end of it, to wipe the dust
off the panels?

Weight and complexity are the two main reasons. The rover's lifespan might be
only 90 days or so with or without the solar panels being cleaned of dust. Do
you put on a more complex system to get rid of the dust and have to leave out
a vital instrument, only to have the rover fail on down the road for a reason
which has nothing to do with the panels? This is the logic used. It would be
better to use the available time with more instruments and payload than try to
rig something which may or may not be effective. The dust may be rather sharp
and abrasive, so trying to "dust-off" the panels might just scratch them,
making the problem worse. The next long-range rover design is scheduled to
use an RTG, so solar panels will not be needed. Clear skies to you.


Thank you for your detailed responses to my questions.

I see your point about the difficult engineering (and weight) tradeoffs.

Question: Where may one find information on the web regarding this `next
generation rover' you speak of? I've looked (not very hard) but was unable
to find anything.

(My assumption is that with the successes, so far... knock on wood... of
spirit and Opportunity, the money will be there for further follow-on
efforts, whenever JPL is ready to spend it.)

Ronalde F. Guilmette wrote:
Question: Where may one find information on the web regarding this `next
generation rover' you speak of? I've looked (not very hard) but was unable
to find anything.


Its merely a proposal, since we don't know exactly what the final version will
look like (the current MER rover missions had various forms and designations
before they finally got the current configuration). However, its project name
is the Mars Science Laboratory, and its probable mission timeframe is around
2009. It is a much larger rover which may be as big as a small minivan, and
may use a "smart lander" to put the rover down into difficult areas where
previous missions could not go. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************