surfa ce that is not no rmal to the appro ach ve ctor because they are free to ro tate +/-16 degrees in th e  pitch axis. Th is allows the wing s to prop erly seat themselve s agai nst a ro ck surface so that equal force i s  applie d to each of the  wings’ knurled balls. T he knurled b alls can accomm odate approach vector e rrors o f  +/- 15 d egree s in a yaw axi s as well because they pro trude d ownward fro m the frame of ea ch wing. If the   rock surfa ce plan e is at an a ngle equal to or gre ater th an +/-16 d egree s in eithe r the pitch or yaw  directions, hard stops will prevent the butterfly wings from pivoting  further, preventing the switches from  closing. Typically, the IDD will p resent the RAT norm al to th e local surface plane of th e rock at an  angl e  that is within 5 to 10 degre es.  

Figure 1.  Ro ck Abr asion Tool -- Butte rfly Isometric Vie w 

The switche s were of parti cular con cern due to t he im posed requi rements. Th ey need to b e small eno ugh  to fit within the very limited  space allotted, they cann ot have any hystere sis or dead ban d characteri stics  and they ha ve to be dust proof. Additionally, the sw itches have to act as hard stop s capabl e of  withsta nding loads as high as 1 25 N acting at the kn urled balls which become s about 22 2 N at the g old  conta cts. All  of these require ment s lead to d esigning a nd ma nufactu ring custom m ade swit ches as  comm ercial fl ight micro-switche s could only sati sfy the du st proof  requi rement. The conta ct material was  made f rom 18-karat full h ard yell ow g old, whi ch is also st rong enou gh to withstand  the relatively high  comp ression load s. The gold conta cts are insulated  from th e structure of th e RAT via Ve spel bu shings.  The b ackside  of ea ch switch half is ba ck filled with epoxy at the point whe re each wi re protrude s. This  offers strain relief for the wire in a ddition to adde d bond stren gth for the gol d and vespel  bushi ng. Th e  exposed gold contact  heads are envel oped and sealed using a nickel bello ws with cu stom  end plate s that  are secured to ea ch side of the wi ngs. The breather vent ha d to b e installe d to all ow trapp ed air to e scape  becau se assembly of the gold co ntact s (Figure 2, label 1) took pl ace in Earth atmosp here. The vent wa s  made u sing a stainle ss steel set screw that wa s counter bo red and filled with a  316 SS po wder that was  heated a nd compresse d. This b ecame  a 2-mi cron filter, allowi ng gasse s to ex change a nd equali ze the   pressure on both sid es of the bellows yet prevent dus t from cont aminating the  gold co ntact s.  Had the  breath er vent not be en installed, th e spring  constant of the b utterfly switch  system would  have b een  greate r than t he 5 N calibration thre shold. 

Mounted within the bottom  of the revolv e hou sing are f our small magnet s (Figure 3, la bel 5 ) with va rying   degree s of stren gth. Th ese ma gnets, provided b y scienti sts from the Un iversity of Copen hage n,  Denm ark, will  be used to characterize the mag netic properties of the cutting s produced by the grin ding  operation s. For more info rmation, se e [Madsen et al., 2003]. The RAT mag nets are de signed to capture  magneti c particle s origi nating from the g rindin g of Martian su rface rocks. A high -resolution st ereo camera  called Panca m, develope d at Cornell University, w ill image the magneti c particl es captu red by these  magnet s. A major scien ce goal  for th e Magn etic Propertie s Experiment (MP E) on th e rovers is to  identify  the mine ral(s) responsible for the m agnetism of M artian rocks, soi l and dust. In addition  an effort will b e  280