@Hobbes's answer calls attention to Emily Lakdawalla's book The Design and Engineering of Curiosity : How the Mars Rover Performs Its Job which led me to reading excerpts in several Planetary Society blogposts including Book Update: The Design and Engineering of Curiosity which includes the following image(s) and description.

At the bottom there is something called a secondary thwack arm.

Question: What is a secondary thwack arm, and what gets thwacked? I certainly hope it's not the bee trap! (shown in the linked original image link below)

("bonus points" for an image of the primary thwack arm.)

Original image but answer shows it as well.

Parts of the CUriosity Chirma 1-millimeter Sieve Pathway

A figure from the forthcoming book The Design and Engineering of Curiosity: How the Mars Rover Performs its Job, by Emily Lakdawalla, illustrating the parts of Curiosity's sample handling mechanism that sift and portion out powdered samples of Martian material using a sieve with 1-millimeter holes.

NASA / JPL / MSSS / Emily Lakdawalla

  • $\begingroup$ probably no need for a thwack-arm tag. $\endgroup$
    – uhoh
    Jan 12, 2019 at 23:49

1 Answer 1


The key search terms for this one are "lakdawalla thwack arm".

enter image description here

According to one of Emily Lakdawalla's blog posts, the thwack arms are used to knock material out of the sieves, much in the way you might thwack a flour sifter against a kitchen counter to clear it:

Both the 150-micrometer sieve and the 1-millimeter sieve hardware is mounted on motorized swing arms that wind up an internal spring mechanism, which releases at a certain point in the range of motion and "thwacks" (high-g impact driven by the spring) the sieves against hardstops in order to clean the sieves of material that might be blinding them. Testing has shown this to be a very effective way of cleaning the sieves inside CHIMRA in order to support the many sample processing operations we have to do.

In another post, Lakdawalla recommends that you, @uhoh, should buy the book:

But if you're the kind of person who hears the words "secondary thwack arm" and must find out what a thwack arm is and why it's called that, and also beg to know if there is a primary thwack arm, my first book will give you many happy geeky moments.

This tweet thread includes a picture of the primary thwack arm, but desktop web Twitter isn't making it easy to snatch the picture.

enter image description here

  • $\begingroup$ I added what I think is the picture you are after; please delete if I guessed wrong. I don't know any way to transmit the picture to you except by adding it to the answer. $\endgroup$ Jan 13, 2019 at 1:32
  • $\begingroup$ I was after the second, more "overviewy" pic from the same tweet, but no biggie. $\endgroup$ Jan 13, 2019 at 1:51
  • $\begingroup$ Argh, well, feel free to bin it. $\endgroup$ Jan 13, 2019 at 1:53
  • $\begingroup$ I've removed the image from the question as it's now redundant, it's great to have them all in one place here, and despite the illustration of the primary thwack arm, I'll still look for her "first book (that) will give (me) many happy geeky moments." I hereby officially award the "bonus points" as well. $\endgroup$
    – uhoh
    Jan 13, 2019 at 2:19
  • 2
    $\begingroup$ this one ? $\endgroup$
    – user20636
    Jan 13, 2019 at 10:44

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