# Why is there a large wooden ball on Mariner 3's magnetometer?

Wood is good for certain niche space exploration applications, but the one below is not for spaceflight. According to the documentation for the archived photo:

Several spacecraft were built for the Mariner Mars 1964 mission. The ones that were actually launched were referred to as Mariner C-2 and Mariner C-3 until they were renamed Mariner 3 and Mariner 4, respectively. There was also a Proof Test Model (PTM, or Mariner C-1) and a Structural Test Model (STM). This photo shows Mariner C-2 configured for system tests in May 1964. It appears to be in the Spacecraft Assembly Facility, with the observation area at the top of the photo.

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Documentation found in the Archives does not identify the purpose of the sphere covering the magnetometer during this test.

1. What is it for?

2. Why is it thick, heavy wood?

I can speculate - non magnetic materials near magnetometers, but a plexiglass or fiberglass box could have been used, both of those were already around at the time. For that matter, what about aluminum?

1. Is it really hanging off of Mariner 3's magnetometer boom?

This wooden shell looks so heavy (it's not Balsa this time)!

above: Detail cropped from Photograph Number 293-6619Ac. Image credit: NASA/JPL-Caltech.

above: Photograph Number 293-6619Ac. Image credit: NASA/JPL-Caltech.

• Is it even part of the spacecraft? Maybe part of the test rig? – Anthony X Feb 12 '17 at 18:25
• @AnthonyX yep, you are right.There's no suggestions that it is a part of the spacecraft. It just happens to be "on the spacecraft" in the photo, and I you don't see very many 1 meter wooden spheres in archival photos. The answer really surprised me! – uhoh Feb 13 '17 at 1:10
• And now I want to know what happened to this sphere, would do wonders as an art installation in a house...hanging from the ceiling or something. – NZKshatriya Feb 14 '17 at 5:05
• @NZKshatriya time to start checking the auctions? – uhoh Feb 14 '17 at 5:10
• If the wooden sphere has no cracks and the coils are intact, the sphere could be used for magnetometer tests today. – Uwe Feb 14 '17 at 8:10

I suspect the wooden sphere is a three-dimensional Helmholtz coil. There are circular groves visible filled with silicone or something similar. The groves contain the Helmholtz coils, one pair for each direction. An electrical current flowing through the coils generates the magnetic field. The Helmholtz pair of coils minimizes the nonuniformity of the field at the center of the coils. It is used as a magnetic field simulator for the magnetometer to do tests on earth.

This configuration of Helmholtz coils may be used to compensate the local earth magnetic field and to generate a magnetic field of variable direction and intensity for a thorough test of the magnetometer to measure sensitivity and linearity in all three dimensions.

The sphere is made from wood to avoid any metal and unwanted magnetic fields. Wood does not conduct electrical currents, eddy currents are not possible. It is used for preflight tests only, the weight is no problem. If they used plexiglass or fiberglass for that, they would need a model made from wood to shape the fiberglass. It was easier to use the wood sphere itself as only one piece was necessary. They did not use aluminium because any metal should be avoided. The manufacture of the sphere was easier using wood than aluminium and the better stability of aluminium was not needed there. Using thick and heavy wood provided the necessary stability of the sphere to avoid any deformation during the tests and also during the manufacture of the sphere.

• This has got to be the precisely the right answer - thank you! – uhoh Feb 13 '17 at 0:48
• @uhoh: Did you wrote an Email to the JPL archivist Julie Cooper to tell her the purpose of the wooden sphere? – Uwe Feb 15 '17 at 9:15
• I think it's a great idea, I'm kinda shy so I am hoping someone else has done it already. If you can, please do so! I think you can help them with correct details much better than I. – uhoh Feb 15 '17 at 9:22

• Aluminium is a bad material to wind coils around (like conductors in general) because any change in the current will for a while be “shadowed” by eddy currents. This can to some degree be mitigated by laminating only small sections, but this isn't as easy with aluminium as it is with wood. And even with constant magnetic fields aluminium is not great: it is actually not completely nonmagnetic, but exhibits paramagnetism. Ok, but no material is – the absolute susceptibility of Al actually seems to be roughly the same as common organic substances, about 10-8 kg.
• Fibreglass could be used, but likely wasn't as cheap to make as wood. Particularly the grooves for the Helmholtz wires could probably be much easier milled into wood than in the refractory glass fibres.
• Homogeneous plastics like plexiglass can be very cheaply mass-produced, but this requires molds etc.. Not practical for something you only need once! Today this would be a bit different thanks to 3D printers.
• Thanks! Sometimes wood is simply the best overall solution to getting the job done. One could just make six coils and try to hold them in position (rather than a spherical shell), but that could end up a real trouble spot if they shift a bit. The wood seems to solve many problems at the same time. – uhoh Feb 13 '17 at 2:21
• Plus, varnished wood laminate is so pretty. – Wayne Conrad Feb 13 '17 at 3:04
• A very good explanation of the problems of aluminium, eddy currents and paramagnetism. – Uwe Feb 13 '17 at 7:59
• Wood is various forms is still a useful material for jigs/test rigs in small-scale manufacturing, despite advances in plastic and related materials. It's easy to work to a good level of precision, quite stable, and doesn't mar aluminium surfaces. This was built in the 60s and there's no reason to assume wood would be less of an option then. A fibreglass construction would probably need a wooden former - so skip the fibreglass and go straight for wood. – Chris H Feb 13 '17 at 15:33
• The wooden sphere allows a complete symmetric design for all the three axes. All coil diameters and distances of coil pairs are the same. This would not be possible if using a design with six independent coil bobbins. If the wooden sphere is precisely manufactured, the magnetic field properties are equal for all dimensions. The design with three pairs of holes in the sphere allows the test using a one dimensional laboratory magnetometer to compare all three coil pairs under the same conditions. – Uwe Feb 13 '17 at 20:11

I just ran across the wooden sphere (either Mariner 3 or 4) in in historical footage!

In the JLP video 1965: Discovery at Mars there is an excellent video The Changing Face of Mars with introductory remarks by its producer/director/writer, Blaine Baggett, Director, Office of Communication and Education, JPL, about Mariner 3 and 4 missions to Mars embedded within his Von Karman lecture.

You can see it after 00:38:40:

I just ran across another example of a triplet of Helmholtz coils in a video about the MAVEN magnetometers.

The NASA.gov mission page Measuring Mars: The MAVEN Magnetometer links to the YouTube video MAVEN Magnetometer. In it you can see the same geometry as in the other answers here, three pairs of Helmholtz coils. But this time they are large enough to form a large space that you could bring a big section of a spacecraft into and put on a table, rather than the wooden sphere only big enough for a magnetometer housing that would need to be suspended around it.

Note also there is still a wooden bobbin for a single pair of coils in the first image.

You can also see that each of the three pairs has a smaller "satellite pair" at much larger distances; there are a total of twelve coils shown. I've asked a separate question about these: How are these “supplementary” or “satellite” Helmholtz coils used?.

T

• There is an improvement of a Helmholtz coil using 3 instead of 2 coils, the Maxwell coil. But a 3 dimensional Maxwell coil would be 9 coils together, not 12. May be the constant gradient magnetic field is further improved by using 4 coils per axis. – Uwe Oct 28 '18 at 9:20
• A Helmholtz coil pair is defined by their radius equal to their distance. So the outer pair of coils should have a larger radius than the inner coils but not a smaller. – Uwe Oct 28 '18 at 11:09
• I finally found it. This arrangement of four coils is called a Braunbek coil. There is another 4 coils arrangement with equal diameters called the Barker coil. See 1, 2, 3, 4, 5. The wikipedia link with Braunbek and Barker is available in german only. – Uwe Oct 28 '18 at 18:33
• I wrote an answer to the question on physics stackexchange. – Uwe Oct 28 '18 at 20:24
• @Uwe excellent! For physics SE I'd recommend you add a bit of physics history at the beginning: "While three orthogonally intersecting long solenoids can in principle produce a uniform field over a volume this is impractical to implement. In addition to the familliar three coil-pair solution of Helmholtz, solutions using coil-trios and coil-quartets have also been published which offer increasingly better uniformity over larger internal volumes i.stack.imgur.com/owgmH.png From Wikipedia's Helmholtz-Spule (Helmholtz Coil, in German)" – uhoh Oct 29 '18 at 0:36