mars rovers life expectancy
 

I read an article that said the life expectancy of the mars rovers was 90
days. Isn't it too short for $800 million machine? What limits it to 90
days?

mars rovers life expectancy
 

pesso wrote:

I read an article that said the life expectancy of the mars rovers was 90
days. Isn't it too short for $800 million machine? What limits it to 90
days?

Considering what they've found already, I'd say we got a very good deal
indeed by any standards.
The cost of the whole program was only about one third more than a
Shuttle flight, and individual Shuttle flights tend to do far less of
interest than what the rovers are doing.
The three limiting factors on the mission duration a
1.) The need to get adequate solar power to operate; the Martian winter
is coming on, and the sun angle is getting lower in the sky, so that the
rover's top-mounted solar arrays aren't generating as much electricity
as they were when they first landed. There is probably some small loss
of solar panel efficiency due to their exposure to solar wind related
radiation on the surface, which slowly but surely degrades solar cells.
2.) Dust build-up on the top of the solar arrays causing less sunlight
to reach them and generate electric power.
3.) The thermal stress on the rover's electronics caused by the
day/night temperature swings, which will eventually cause the wiring
connections to begin to fail.

All that having been said though, NASA is hoping that the rovers
operating life goes beyond the 90 days- maybe by quite a bit- the
planned missions for the Voyager spacecraft were for them to get to
Jupiter and Saturn; the missions would be considered successful if those
two planets were reached. But both kept right on working, so Voyager 2
was able to reach Uranus and Neptune as a bonus mission, as NASA had hoped.

Pat

mars rovers life expectancy
 

In article ,
says...

snip

1.) The need to get adequate solar power to operate; the Martian winter
is coming on, and the sun angle is getting lower in the sky, so that the
rover's top-mounted solar arrays aren't generating as much electricity
as they were when they first landed. There is probably some small loss
of solar panel efficiency due to their exposure to solar wind related
radiation on the surface, which slowly but surely degrades solar cells.

Just one minor nit, Pat. The seasonal change is, indeed, responsible
for a lowering in the amount of sunlight getting to the solar cells, but
it doesn't have a lot to do with how high the sun gets in the sky. The
height of the sun at zenith isn't changing a whole lot yet -- remember,
both rovers are located near the equator.

What's making a really significant difference is that, as Mars heads
into southern summer and northern winter, it's also moving away from
periapsis and towards apoapsis. Since Mars' orbital eccentricity is
currently *much* greater than Earth's, that means that the change in the
solar energy reaching the ground on Mars between periapsis and apoapsis
is *far* greater. I think I read that, at apoapsis, Mars gets something
like 50% less solar energy than it does at periapsis (though that's from
memory, I could have that figure wrong.) It's a *very* significant
difference in insolation, though.

So -- the height in the sky the sun reaches at zenith isn't nearly as
much of a limiting factor in solar-cell use on the Martian surface as is
the movement of the planet from periapsis to apoapsis.

Doug

mars rovers life expectancy
 

In article ,
says...

snip

What's making a really significant difference is that, as Mars heads
into southern summer and northern winter, it's also moving away from
periapsis and towards apoapsis.

Ooops! I meant northern summer and southern winter. At the current
time, the southern hemisphere is what gets the shorter, warmer summers
due to their coincidence with periapsis.

Doug

mars rovers life expectancy
 

Doug... wrote:


Just one minor nit, Pat. The seasonal change is, indeed, responsible
for a lowering in the amount of sunlight getting to the solar cells, but
it doesn't have a lot to do with how high the sun gets in the sky. The
height of the sun at zenith isn't changing a whole lot yet -- remember,
both rovers are located near the equator.

What's making a really significant difference is that, as Mars heads
into southern summer and northern winter, it's also moving away from
periapsis and towards apoapsis. Since Mars' orbital eccentricity is
currently *much* greater than Earth's, that means that the change in the
solar energy reaching the ground on Mars between periapsis and apoapsis
is *far* greater.


Works fine as to total hours of sunlight per season, but not as fine as
regards the Sun in position in the sky for any rover that has solar
arrays facing straight up toward the zenith of the heavens; only at the
periods balanced between the two seasons of the two Martian hemispheres
does the light fall upon the solar arrays at a near perpendicular angle.
Equinoxes good.
Movement toward the solstices bad.

Pat

mars rovers life expectancy
 

Doug... wrote:

Ooops! I meant northern summer and southern winter. At the current
time, the southern hemisphere is what gets the shorter, warmer summers
due to their coincidence with periapsis.

Both rovers are effectively at the Martian equator- "Summer" and
"Winter" to them are but a matter of details and a few hundred miles of
latitude.

pat

mars rovers life expectancy
 

Doug... wrote:

What's making a really significant difference is that... it's also moving away from
periapsis and towards apoapsis. Since Mars' orbital eccentricity is
currently *much* greater than Earth's, that means that the change in the
solar energy reaching the ground on Mars between periapsis and apoapsis
is *far* greater.

Doug,

With 1.52 semimajor axis and .093 eccentricity,
I'm getting 1.37 AU perihelion and 1.66 AU apohelion.

1.37/1.66 = .83

..83^2 = .69

I believe you are correct that Mars' greater eccentricity
makes a substantial difference between periapsis and apoapsis
insolation.

--
Hop David
http://clowder.net/hop/index.html

mars rovers life expectancy
 

In article ,
says...
Doug... wrote:

What's making a really significant difference is that... it's also moving away from
periapsis and towards apoapsis. Since Mars' orbital eccentricity is
currently *much* greater than Earth's, that means that the change in the
solar energy reaching the ground on Mars between periapsis and apoapsis
is *far* greater.

Doug,

With 1.52 semimajor axis and .093 eccentricity,
I'm getting 1.37 AU perihelion and 1.66 AU apohelion.

1.37/1.66 = .83

.83^2 = .69

I believe you are correct that Mars' greater eccentricity
makes a substantial difference between periapsis and apoapsis
insolation.

According to a very well-done book I've been poring over, "Mars, The
Mystery Unfolds" by Peter Cattermole, the difference between perihelion
and apohelion insolation is on the order of 40%. That's QUITE a
difference.

Doug

mars rovers life expectancy
 

Doug... wrote:
In article ,
says...

Doug... wrote:


What's making a really significant difference is that... it's also moving away from
periapsis and towards apoapsis. Since Mars' orbital eccentricity is
currently *much* greater than Earth's, that means that the change in the
solar energy reaching the ground on Mars between periapsis and apoapsis
is *far* greater.

Doug,

With 1.52 semimajor axis and .093 eccentricity,
I'm getting 1.37 AU perihelion and 1.66 AU apohelion.

1.37/1.66 = .83

.83^2 = .69

I believe you are correct that Mars' greater eccentricity
makes a substantial difference between periapsis and apoapsis
insolation.


According to a very well-done book I've been poring over, "Mars, The
Mystery Unfolds" by Peter Cattermole, the difference between perihelion
and apohelion insolation is on the order of 40%. That's QUITE a
difference.

Doug



1/.69 is about 1.45 so my figures can land in the same ball park.

The percentage varies depending on whether you're considering

(apohelion insolation/perihelion insolation)
or
(perihelion insolation/apohelion insolation)

Hop David
http://clowder.net/hop/index.html