What most people don't realise is how extremely light the Tell-tale assembly is: only 20 grams (0.71 ounces). The active part of the Tell-tale, the kapton tube, weighs only an incredible ten-thousandth of a gramme! (0.0001 gramme equal to 0.0000035 ounce). This light weight is necessary to achieve a measurable deflection in the rarified Martian air of only 1 percent Earth sea-level atmospheric pressure.
The kevlar fibers from which the kapton tube is suspended also had to be incredibly light of course, but mainly they had to have some very particular properties. They had to deflect uniformly and steadily, so the wind pressure could be properly read in a single photo. It would be no good if the kevlar fibers just allowed the kapton tube to oscillate randomly.
The famous dust "storms" on Mars only exert a wind force on the back of your hand equal to the force from moving the hand through the air in a slow sweeping motion here on Earth. This was explained by Tell-tale's Islandic team leader Haraldur Páll Gunnlaugsson at the June 4 University of Arizona press briefing
. Let's not go into what the Mars atmospheric
pressure would do to your hand, if you were to expose it on Mars!
During the first vibration tests - exposing the Tell-tale to the forces of a rocket launch - the instrument failed completely. It took an involved redesign effort to make it comply with the two highly conflicting parameters: sufficiently sensitive to measure the winds on Mars and sufficiently sturdy to survive launch and EDL. But the team succeeded, as we can now see in daily pictures from the surface of the planet!
Tell-tale is the name for a piece of string suspended from the sail or stays of a sailing boat, to indicate the wind direction and -force
. The University of Aarhus Tell-tale is a deceptively simple-looking instrument
, which however took some very clever engineering for it to work properly in the Martian environment. It has already supplied important information about the winds at the Phoenix landing site: we now know that the winds go through 180 degrees from South to North during the Martian day. Important knowledge, because it can be taken into account during digging, so that various samples won't be contaminated by airborne dust from previous digs.
The University of Aarhus
in Denmark is my alma mater, so I am very proud to see that it made the Tell-tale on Phoenix! (Aarhus is the second-largest city of Denmark with 250.000 inhabitants. The university is rather new, from the 1920's, and got its first Nobel prize (Jens Chr. Schou, Chemistry) in 1997). Until now it was mainly the Niels Bohr Institute in Copenhagen that made Danish contributions to JPL Mars expeditions, focusing on magnetism. As you may note from my user name, we have quite a tradition in that field (sic..) in Denmark.
The Tell-tale is described more in depth at the Aarhus University Mars Lab page: http://www.marslab.dk/
(sub-page for Tell-tale: http://www.marslab.dk/TelltaleProject.html