The Hubble Space Telescope has given us no end of gorgeous astronomical photographs. This one is of an enormous cavity of gas carved out by the stellar wind from the nebula N44f; the image comes from the ESA/Hubble Information Centre. You’re looking at what happens when a cloud of gas is inflated by fast-moving particles from a hot young star. This stellar wind is moving at about 7 million kilometers per hour, far faster than the Sun’s (a sedate 1.5 million kilometers per hour). Keep this image in mind when thinking about new propulsion concepts like Robert Winglee’s M2P2, which uses the solar wind to push a magnetic sail. You can read about Winglee’s work at this Web site at the University of Washington. And note that when we talk about the solar wind pushing a magnetic sail, we are discussing something different than a solar sail, which gets its push solely from the momentum imparted by photons. They are two entirely different concepts.