drag against a plasma wind from a nearby star that accelerates a spacecraft away from the star. Many designs, analyses, simulations and experiments focus on this use case. The solar wind is a time varying stream of plasma that flows outwards from the Sun. Near the Earth's orbit at 1 Astronomical Unit (AU) the plasma flows at velocity ranging from 250 to 750 km/s (155–404 mi/s) with a density ranging between 3 and 10 electrons, protons, and alpha particles per cm3 along with a few heavier ions per cubic centimeter. Assuming that 8% of the solar wind is helium and the remainder hydrogen, the average solar wind plasma mass density at 1 AU is (kg/m3). At 1 AU most magnetic sail research assumes 6 protons per cm3 corresponding to a density of 10−20 and a mean wind velocity =500 km/s. On average, the plasma density decreases with the square of the distance from the Sun while the velocity is nearly constant, see Figure 4.2. The average mass density as a function of distance Astronomical Units (AU) from the Sun is: with the plasma velocity falling off very slowly. The effective solar wind seen by a spacecraft traveling at velocity (positive meaning acceleration away from the star and negative meaning deceleration toward a star) is . Deceleration in interstellar medium A spacecraft accelerated to very high velocities by other means, such as a fusion rocket or laser pushed lightsail, can decelerate even from relativistic velocitieswithout requiring the use of onboard propellant by using a magnetic sail to create drag against the interstellar medium plasma environment. For example. long duration missions, such as missions aimed to offer terrestrial life alternative evolutionary pathways, e.g. as envisioned by the Genesis project, could brake passively using magnetic sails on approach to a distant star. The Sun is the center of the heliosphere region that extends radially outwards to a termination shock at 75–90 AU, a heliosheath at 80 to 100 AU and then a theoretical heliopause at 120 AU. Beyond this is a relatively low density region called the Local Bubble which contains local interstellar cloud (which contains the Solar System) and a neighboring G-Cloud complex which contains Alpha Centauri. Less is known about the ISM than within the heliosphere, but measurements by Voyager 1 and Voyager 2 have provided important data and indirect observations have also provided information. Estimates of the number of particles