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Mars Probe Brings the "Weather Rock" New Respect

Posted by timothy on from the if-tail-is-wet dept.

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."

9 of 144 comments (clear)

  1. Why not a weather vane? by Robert1 · · Score: 1, Interesting

    Why didn't they put in a lightweight weather vane with a small fan? That way they could tell direction and velocity. With the rock you can basically gauge the initial wind, but once it starts swinging in the opposite direction its practically impossible to tell if the deflection is from an opposing wind or merely the pendulum swinging. It becomes even more useless if there are sudden changes in wind speed/direction since it will just seemingly bounce round in random directions none of which are reflective of the actual wind direction or velocity.

    I don't see how this is any better than a weather vane, cheaper, or smaller. It sure is way more useless.

    1. Re:Why not a weather vane? by TheRedSeven · · Score: 3, Interesting
      From TFA:

      The Telltale consists of a gallows that is mounted on the top of the Meteorological Mast of the Lander. The active element of the instrument is an extremely lightweight Kapton tube hanging in Kevlar fibres. Images taken of the instrument will show the deflection of the Telltale due to the wind. A mirror is mounted below the active part to enable better direction information. (My emphasis)
      The wonderful addition of a high-tech "mirror" allows multiple perspectives that would allow you to detect direction. Looks like some overpaid engineer already thought of this.

      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.
    2. Re:Why not a weather vane? by hcpxvi · · Score: 5, Interesting

      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.

    3. Re:Why not a weather vane? by pz · · Score: 2, Interesting

      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.
    4. Re:Why not a weather vane? by pz · · Score: 3, Interesting

      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.
    5. Re:Why not a weather vane? by WebCowboy · · Score: 2, Interesting

      Why didn't they put in a lightweight weather vane with a small fan? Actually a vane and fan is inferior for this particular purpose. This "rock on a string" can be used to determine both direction and intensity of winds just as well, plus it is lighter and mechanically less complex than a vane and fan--important in the very cold and dusty environment on Mars. The rotating parts would more easily wear, freeze or seize up more easily in such an environment.

      With the rock you can basically gauge the initial wind, but once it starts swinging If a wind is sustained the rock would not swing. Furthermore sensors and computer analysis of the motion can provide more information than you would think. Without restricted degrees of movement this instrument can determine wind directions on different planes (vs just one with a weather vane) and the swinging can be used to figure out the size and direction of gusts, and could even detect "eddies" or small tornadoes, updrafts, etc. Can't do that as well with a weather vane.
    6. Re:Why not a weather vane? by dargaud · · Score: 2, Interesting

      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.

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  2. Re:Is there life on mars.... by KillerBob · · Score: 2, Interesting

    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
  3. Re:off topic - sending a rock by Ford+Prefect · · Score: 4, Interesting

    Would it be difficult to design a small launcher that could contain scooped rocks, dirt, etc from Mars and be launched towards Earth?

    Quite difficult, but it is being planned...

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