The rate of discovery of near-earth asteroids outpaces current abilities to analyze them. Knowledge of an asteroid’s physical properties is essential to deflect them. I developed open-source algorithms that combine images from robotic telescopes, open data, and school math to determine asteroids’ size, rotation period, strength, and mutual orbital period in the case of binary asteroids. I took observations of the Didymos binary asteroid, and my algorithm determined its size to be 820 metres, with a 2.26-hour rotation period and rubble-pile strength. I measured a 35-minute decrease in the mutual orbital period after impact by the 2022 NASA DART Mission. External sources validated the findings. Every citizen scientist can now be a planetary defender.