The Probability of Striking a Macroscopic Interstellar Object on the Way to Proxima Centauri

This paper addresses a single quantitative question about interstellar objects and interstellar traversal: Does the space between stars contain enough large debris for a collision to be a design case rather than a curiosity? Working from published survey values for the known interstellar objects and for the local interstellar medium, the paper computes collision probabilities for hundred-metre, ten-metre and metre-scale thresholds. The headline values obtained are P100 = 1.20e-18; P10 = 3.79e-16; P1 = 1.20e-13. Each number is produced by the deposited computation script from inputs that are stated in full, so the result can be reproduced, checked and updated as survey measurements improve. The study is a computation from stated assumptions and published data; it presents no new observations. Its contribution is a transparent budget: a set of numbers, the relations that produce them, and an explicit account of how sensitively each number depends on the assumptions behind it.