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Discussion below this post lead me to wonder:

Question: How many solar system bodies have had their magnetic fields directly measured?

Inferences from radio or spectroscopic observations don't count for the purposes of this question, it's got to be some kind of in situ magnetometer, some magnetic field detector that has flown past or around or landed on a body, measured the magnetic field and reported the data back to Earth.

A measurement of zero, or of tiny fluctuations about zero certainly count as a measurement.

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Here's a potentially incomplete list, we have direct measurements of at least these bodies:

Planets and moons:

Asteroids:

Comets:

Other notes:

  • There wasn't a magnetometer on New Horizons so we don't have Pluto.
  • Dawn ended up not flying a magnetometer so we don't have Ceres and Vesta.
  • Neither Mariner 9 nor Mars Express had a magnetometer, and Mars Global Surveyor didn't approach very close to Deimos, so we don't have detailed readings of Deimos.

Methods and disclaimers:

I made this list by trying to first figure out which bodies we've visited at all (landed or flyby), then which spacecraft did the visit, and then whether that spacecraft had a magnetometer by looking for the actual data or an article or publication based on it. This wiki page was helpful for the asteroids and comets.

This method is unlikely to give false positives but might give false negatives if I couldn't find information on a magnetometer or a measurement. For example, it's likely Cassini flew close enough to Mimas to have done measurements, but the measurements aren't interesting enough to be reported on separately. The raw data for the entire mission is available here, but no separate data for Mimas is listed.

When a body was visited by multiple spacecraft, I haven't always listed all of them.

We have done magnetic measurements on samples brought back from comets, and while that is a fairly direct measurement, it's not in-situ, so I didn't list them here.

So to answer your question: At least 30.

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    $\begingroup$ Wow! Sadly for the Dawn spacecraft at Ceres and Vesta, A magnetometer and laser altimeter were considered for the mission, but were not ultimately flown. However, they might have made indirect measurements using charged particles from the solar wind as a probe. For Deimos, you can consider just posting a new question about it. $\endgroup$
    – uhoh
    Commented Jun 9 at 15:24
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    $\begingroup$ While searching some more I've actually become less sure whether Mariner 9 had a magnetometer at all. I've posted a question about it. $\endgroup$
    – Mqrius
    Commented Jun 9 at 15:47
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    $\begingroup$ Wow! Most of your links are not just general references that say magnetic measurements were taken, but the journal articles describing analysis of the magnetic measurements in detail! $\endgroup$ Commented 2 days ago
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    $\begingroup$ I figured that was the most reliable way to make sure that measurements were actually taken — find either journal articles using the data, or the direct data itself. I didn't manage it for each of them but it does make me confident that the list is mostly correct and fairly complete. $\endgroup$
    – Mqrius
    Commented 2 days ago

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