Six decades have passed since a University of Chicago physicist pondered why the tail of a comet, no matter the direction it traveled, always pointed away from the Sun. From this and other observations made by astronomers, Eugene Parker theorized that there must be some kind of stream of particles flowing away from the Sun. He called this the “solar wind.”
It turns out that such a stream of charged particles does exist, a moving plasma of electrons, protons, and other particles that varies in density and speed. Over time, as observations mounted, astronomers moved from a position of skepticism about Parker’s solar-wind idea to one of acceptance. Then, nearly 30 years ago, scientists launched their first mission, Ulysses, to make direct measurements
Astronomers have since come to realize the profound importance of this solar wind for our planet and the rest of the Solar System. For example, we now know that a few hundred million years after its formation, the early solar wind accelerated ions in the upper Martian atmosphere to an escape velocity, stripping the young planet of much of its atmosphere over time.