From reading a book on NASA's Voyager mission, I learned that before the launch of these spacecraft, the expectation was that the heliopause was at around Jupiter or Saturn's orbit which is about 5 to 10 AU. The book says that as the spacecraft continued to move away from the sun, the space physicists kept increasing their estimate of the position of the heliopause.
What book on Voyager were you referencing?
In any case, they started developing the Voyager spacecraft in the 1960's. The first measurement that a solar wind even existed wasn't reported until 1960-1961 [K. Gringauz using the Lunik 2 spacecraft]. They were able to determine a flux of particles (i.e., number per area per time), but did not determine speed or number density.
Biermann's hypothesis of the existence of the solar wind was between 1951-1957, thus, not much earlier. Though he did approximate the solar wind speed pretty closely with $\sim$500-1000 km/s, which would now be considered fast solar wind.
Mariner 2 was the first spacecraft to show that the solar wind was continuously emitted by the sun (observations and papers between 1962-1967). This is also the period when they first started to get semi-reliable estimates of the bulk flow speed, number density, temperature (i.e., Avg. kinetic energy in the bulk flow rest frame), etc. of the solar wind.
Once these parameters were found, and assuming that dynamic pressure $\propto$ $r^{-2}$ (assume V $\sim$ constant and adiabatic expansion, then n $\propto$ $r^{-2}$), the heliopause can be estimated using the dynamic pressure at 1 AU, $P_{1AU}$, combined with estimates of the interstellar pressure, $P_{I}$, to approximate a standoff distance, $R_{S}$.
If we use the following typical values $n \sim \ 5 \ cm^{-3}$ and $V \sim 400 \ km/s$, and $P_{I} \sim 10^{-13} \ Pa$, then $R_{S} \sim 100 \ AU$. So MHD is not required. One can simply use hydrodynamics to get a rough estimate. It is likely that the early estimates of $P_{I}$ were much higher or $P_{1AU}$ much lower. This would explain why early estimates of $R_{S}$ may have been grossly inaccurate.
That's a huge underestimate of the sun's output or a huge overestimate of what goes on in interstellar space.
Remember, people weren't even comfortable with whether a solar wind existed until the 1950's. Paul Kellogg predicted the existence of the Earth's bow shock in 1962, which was controversial at the time due to the extremely low collision rates of tenuous plasmas and uncertainties about relevant speeds to use for Mach numbers.
The book doesn't explain why it is that early estimates were wrong and I didn't see an explanation. Perhaps someone knows and will give a nice intuitive explanation for the estimates.
It was only very recently that we had any measurements of the sun's atmosphere (technically, everything within the heliosphere is considered within the sun's atmosphere). At the time when Voyager was first starting to be discussed and designed, all of this information was less than a decade old. It wasn't even until the 1950's that we had a definitive estimate of the sun and solar system age [e.g., Burbidge et al., 1957]. So the most likely reason for the "poor" estimates were due to insufficient information/measurements.
References
- Biermann, L. "Kometenschweife und solare Korpuskularstrahlung," Zeitschrift f$\ddot{u}$r Astrophysik 29, pp. 274, 1951.
- Burbidge, E.M., et al. "Synthesis of the Elements in Stars," Rev. Mod. Phys. 29(4), pp. 547, 1957.
- Gringauz, K.I., et al. "Results of Observations of Charged Particles Observed Out to R = 100,000 km, with the Aid of Charged-Particle Traps on Soviet Space Rockets," Astronomicheskii Zhurnal 37, pp. 716, 1960.
- Kellogg, P.J. "Flow of Plasma around the Earth," J. Geophys. Res. 67, pp. 3805, 1962.
- Lang, K.R. The Sun From Space, Astronomy and Astrophysics Library (Springer, Verlag Berlin, Germany), 2000.
- Neugebauer, M. and C.W. Snyder "Solar Plasma Experiment," Science 138, pp. 1095-1097, 1962.
