  Over the course of its motion through the Galaxy, our solar system has
encountered many interstellar environments of varying characteristics.
Interstellar medium (ISM) density variations spanning seven orders of magnitude
are commonly seen throughout the general Galactic environment, and a
sufficiently dense cloud within this range has the potential to dramatically
alter the structure of the heliosphere. We present observations of the ISM
environments the Sun has most recently encountered based on high-resolution
optical spectra toward nearby stars in the direction of the historical solar
trajectory. The data were obtained with the highest-resolution spectrographs
available, including the Tull Spectrograph on the Harlan J. Smith Telescope at
McDonald Observatory and the Ultra-High-Resolution Facility on the
Anglo-Australian Telescope at the Anglo-Australian Observatory. Observations
were made of interstellar Na I and Ca II doublet absorption toward 43 bright
stars within about 500 pc. No absorption is seen out to a distance of 120 pc
(consistent with the outer boundary of the Local Bubble), but a complex
collection of absorbers is seen in stars beyond 120 pc. While common absorbers
are consistently seen in most sight lines, significant spatial variation is
also detected, even between closely spaced sight lines. This pervasive evidence
of small-scale structure not only speaks to the complexity of the morphology or
physical properties of the gas in the ISM, but also emphasizes that dramatic
structural changes to the heliosphere are common and it is important to
understand the implications of such changes, such as the modulation in the
cosmic ray flux, on planets.
