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I've been searching the web for documentation of Mariner 10's trajectory, notably the perihelion radii, but so far have turned up nothing quantitative, just some rough trajectory diagrams. I've looked at publications by project folks such as Giberson, Cunningham, Dunne, and Bourke, the most trajectory-savvy of the four. In those publications and presentations there's lots of quantitative info about Mercury periapses, of course, but not about heliocentric distances.

QUESTION: What was Mariner 10's lowest perihelion radius?

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  • $\begingroup$ I think it encountered Mercury pretty close to (Mariner 10's) perihelion each time, so perhaps we could get a better rough value by just checking Mercury's altitude at each flyby $\endgroup$
    – Jack
    Commented Aug 8, 2018 at 20:17
  • $\begingroup$ @Jack Yep, that is my first estimate, but looking at en.wikipedia.org/wiki/Mariner_10#/media/… makes me think that Mariner 10's perihelion was a bit inside of Mercury's position at the time of the flybys. But maybe I'm trying to pull more accuracy out of that diagram than it warrants. $\endgroup$
    – Tom Spilker
    Commented Aug 8, 2018 at 21:01

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Rodger D. Bourke and Joseph G. Beerer, "Mariner Mission to Venus and Mercury in 1973"

http://www.gravityassist.com/IAF3-1/Ref.%203-110.pdf

page 55:

At Mercury encounter the spacecraft has nearly reached its closest approach to the Sun. Perihelion passage occurs some five days after encounter at a distance of 0.46 AU.

At first glance, this "0.46 AU" might seem like disappointingly low accuracy, but the real piece of information here is that perihelion is 5 days after encounter.

For the 176 day heliocentric orbit the spacecraft entered, 5 days offset from perihelion is equivalent to a heliocentric distance difference of 0.0041 AU.

Combined with ephemerides from JPL HORIZONS, confirming the first Mercury encounter was the closest of the three at 0.4656 AU, the perihelion of Mariner 10 must have been 0.4615 AU

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    $\begingroup$ It’s awesome when people take the time to answer a question years later, even the obscure ones. +1 $\endgroup$
    – Ludo
    Commented Jul 9, 2021 at 9:31

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