In the second term of every academic year since 1986 Darwin College has organised a series of eight public lectures. Each series has been built around a single theme, approached in a multi-disciplinary way, and with each lecture prepared for a general audience by a leading authority on his or her subject. Forseeing Space Weather The concept of space weather describes the variety of changing environmental conditions within the space between the Sun and the Earth, driven by fluctuations in solar activity. On the timescale of human evolution the Sun has remained broadly unchanged. Small fluctuations in the rate of solar energy output combined with cyclic variations in the Earth’s orbit are likely to have driven long-term global climate change, but through human eyes these effects have been either gradual or utterly imperceptible. However, humankind’s scientific curiosity over the last 400 years has advanced our understanding of the Sun’s dynamic nature tremendously. Galileo’s 17th century observations of the Sun revealed sun spots, dark features on the solar disk that waxed and waned over several weeks. Encouraged by advances in atomic physics, early 20th century physicists theorised that electromagnetic emissions from the Sun were responsible for the majestic aurora borealis or “northern lights”, hinting that solar activity must be highly variable over timescales of minutes. But it was the advent of the space age in the latter half of the 20th century that revealed the true nature of the space environment surrounding our planet. Following the discovery of radiation belts surrounding the planet by the first Earth-orbiting US satellite came the first direct measurements of the solar wind – the blizzard of electrically charged particles constantly emitted by the Sun. While the Earth’s atmosphere and magnetic field shield the surface of the Earth from the biological hazards of space weather, the same is not true for some of the technologies developed in the last century. Our society’s increased exploitation of space for communications, defence and monitoring relies upon satellites that operate in the harsh radiation environment above the Earth’s projective atmosphere. Aircrews and airline passengers spend significant periods above the densest and most projective portion of the lower atmosphere and risk exposure to increased radiation doses. The ionosphere’s refractive effects on radio waves determines the viability of many communications links, but these are liable to change rapidly due solar activity while geomagnetic storms have the potential to disrupt electricity generating and distribution systems. In all of these cases, the natural processes in the space environment remain the same as ever, but our adoption and reliance on vulnerable technology forces us to prepare for the increased hazard to society due to space weather.