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Mars Probe Brings the "Weather Rock" New Respect
radioweather writes "What looked to casual observers like a malfunction, a dangling wire with
something on the end, seen in the
first photo of
the meteorological mast on NASA's
Phoenix Mars Lander,
actually
turned out to be the real instrument. Surprisingly, it is much like the
novelty 'weather rock' seen as a novelty gag around the world. The instrument
called the 'Telltale'
is described as a 'passive wind indicator' and uses an extremely lightweight
Kapton tube hanging in Kevlar fiber. Images taken of the instrument will show
the deflection of the Telltale due to the Martian wind."
When you're sending something into orbit or further, payload weight is a concern. Cutting weight, moving parts, and simplifying things is generally a good idea on this sort of thing.
Remember that the surface air pressure on Mars is very small compared to on Earth. So you need a much lighter and more delicate instrument for the air to be able to move it. Anything resembling a traditional weather vane would probably not respond to the tenuous Martian breezes. Even if it worked at first, it might well get stuck after the first of those Martian dust storms blew dust into its pivot. Disclaimer: Yes I am a meteorologist. No, I have not been to Mars or worked on any instrument that went there.
Err... I'm almost ashamed to admit that I have the complete works of Simon and Garfunkel (3CD set, literal transcriptions of every LP they released, 2 per disc), and that one isn't on it.
If you believe everything you read, you'd better not read. - Japanese proverb
Design choices:
(a) tell tale: simple; robust to launching forces; reliable; lightweight; works well as long as winds are relatively constant; works poorly if winds are turbulent; gives wind speed AND direction
(b) weather vane: has moving parts including bearings that require protection from the elements; delicate structure that needs to be made robust to launching forces; heavier than tell tale; works well in any winds, although the mass of vane averages, and therefore can mask, turbulence; unless paired with anemometer (those spinning things), only gives wind direction
I'm betting that the fine engineers at NASA who are working within parameters like keeping weight down to absolute minimum, not being able to repair anything at Mars, wanting something that might potentially work for years to come, and not knowing what the range of winds they might encounter would be making a pretty good decision in selecting a tell tale style design.
But what do I know? I'm just a guy who was at Caltech for graduate school and knew people who worked at JPL.
Put my fist through my alarm clock with its ding-dong death inside my ear. - The Blackjacks.
Quite difficult, but it is being planned...
Tedious Bloggy Stuff - hooray?
The kapton tube does not swing in response to wind, it just deflects. The stronger the wind, the more it deflects. Imagine that it's a spring. Ever seen a car's radio antenna flex on the highway? Same idea.
Insects use exactly the same sort of mechanism to detect gentle air movements. This is one reason it's so hard to catch a fly with your hand when the fly has landed somewhere: the air currents generated by your comparatively large and slow-moving hand are easily detected by the fly.
Put my fist through my alarm clock with its ding-dong death inside my ear. - The Blackjacks.
I wonder why they don't use a sonic anemometer: precise, works in any conditions and at any wind speed, no mechanical parts and it can take fast readings several times per second to measure turbulence. Yes, I've operated one in Antarctica.
Non-Linux Penguins ?