[4] For The Shinano River Plan: 11 (July 1969), one of the first projects in Mail Art by Sending Stones, Horikawa was inspired by the landing of Apollo 11 on the Moon.
Horikawa then brought the stones to the local post office, where he shipped them to eleven people, reflecting the Apollo mission number.
[6] In addition, Horikawa published an accompanying statement arguing for the need to focus on the events on Earth at the time despite the significance of the first Moon landing.
In December 1969, as part of Psychophysiology Research Institute (Seishin Seirigaku Kenkyūjo), a mail-art collective, he mailed a stone to Richard Nixon as a Christmas present.
[8] By contrasting rocks from outer space with earthbound ones, art historian Reiko Tomii has suggested that Horikawa’s work highlighted two fronts of the Cold War.