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Wikipedia's SOLRAD_1; Scientific results says:

The satellite communicated results in real-time, which meant that data could only be received when there was a tracking station within range – either one of Vanguard's Minitrack stations or a few other isolated receivers.[10]:64 Thus, just one to ten minutes per orbit, some 1.2% of the satellite's active time, returned solar observations. The magnetic deflectors proved effective, allowing SOLRAD/GRAB 1 to become the first satellite to successfully observe solar X-rays. However, they also interacted with the Earth's magnetic field, causing the satellite to precess (wobble around its axis like a spinning top) so that its sensors were in shadow half of the time the satellite was in sunlight.

The linked sources there are not readily accessible by internet.

Quetions:

  1. Why exactly did SOLRAD 1 have a magnet? What was being deflected and how?
  2. Why weren't its interactions with Earth's magnetic field anticipated? This seems like a no-brainer; any engineering or physics student could have predicted that a magnet in a magnetic field in a near-vacuum would respond to the $\mathbf{\tau} = \mathbf{m} \times \mathbf{B}$ torque produced, and that there would be dynamical repercussions.

NRL solar radiation (SOLRAD) I satellite schematic, Date 1 January 1961

above: Source

NRL solar radiation (SOLRAD) I satellite schematic, Date 1 January 1961

below: Source

GRAB satellite model on display at the National Cryptologic Museum, Annapolis Junction, Maryland, United States

GRAB satellite model on display at the National Cryptologic Museum, Annapolis Junction, Maryland, United States

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    $\begingroup$ A comment, not a for-sure answer: the magnet was to deflect charged particles away from the detector so that all charge collected was from x-rays, not solar wind? $\endgroup$
    – Jon Custer
    Commented Aug 11, 2021 at 16:15
  • $\begingroup$ @JonCuster I think that sounds right. Consider adding a tentative answer? $\endgroup$
    – uhoh
    Commented Nov 12, 2021 at 0:48

1 Answer 1

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X-ray detectors are sensitive to charged particles. There are plenty of those in Earth's radiation belts. X-ray missions sometimes use "magnetic brooms" to reduce the charged particle background. The SOLRAD magnet appears to have been a crude early example.

As for the attitude effect, I suspect that for this early, hastily assembled mission nobody had thought of it. More modern magnetic brooms use magnet arrangements with zero dipole moment, so they don't produce torque. And, of course, we use magnetic torquers to control angular momentum of LEO spacecraft without expending fuel (although using precession to control attitude seems to give some aerospace engineers headaches).

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    $\begingroup$ it feels really common for historical questions to ask something like "why didn't people think about this" and the answer is so frequently that thinking of things is the hard part. there's lots of things to think about $\endgroup$
    – Erin Anne
    Commented Jan 21 at 1:50
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    $\begingroup$ @uhoh Yes, but they hadn't really thought out the cumulative effects of tiny torques in systems lacking external friction. Explorer 1's transition from spinning around its long axis to rotating end over end was a surprise, too. When you're making a physical model, you must judge when to stop estimating small effects and just get on with the job. There's no rigorous way to prove that there isn't some important effect you've left out. Experience is essential, and nobody had it in those days. Even with experience, sometimes you misestimate an effect, and thus neglect it when you shouldn't. $\endgroup$
    – John Doty
    Commented Jan 21 at 22:53
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    $\begingroup$ @uhoh They knew how to do the problems, but it didn't occur to them which problems to do. The engineers were experienced with spin-stabilized rockets, where magnetic effects are negligible. $\endgroup$
    – John Doty
    Commented Jan 21 at 23:12
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    $\begingroup$ @uhoh How would they know they needed to do that 5 minute calculation? Nobody had ever had to worry about this effect in ballistics, which was the background here. What's obvious in retrospect isn't a good way to get into history. $\endgroup$
    – John Doty
    Commented Jan 22 at 0:37
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    $\begingroup$ @uhoh To give you an idea of how disconnected this was, even a couple of decades later, for SAS-3 mass properties were in slugs and feet, angular rates were RPM, and magnetics used EMU. It wasn't so easy to use textbook physics through all this fog. $\endgroup$
    – John Doty
    Commented Jan 22 at 1:16

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