If we can use solar photons to drive a sail, and perhaps use their momentum to stabilize a threatened observatory like Kepler, what about that other great push from the Sun, the solar wind? Unlike the stream of massless photons that exert a minute but cumulative push on a surface like a sail, the solar wind is a stream of charged particles moving at speeds of 500 kilometers per second and more, a flow that has captured the interest of those hoping to create a magnetic sail to ride it. A ‘magsail’ interacts with the solar wind’s plasma. The sailing metaphor remains, but solar sails and magsails get their push from fundamentally different processes.
Create a magnetic field around your spacecraft and interesting things begin to happen. Those electrons and positively charged ions flowing from the Sun experience a force as they move through the field, one that varies depending on the direction the particles are moving with respect to the field. The magsail is then subjected to an opposing force, producing acceleration. The magsail concept envisions large superconducting wire loops that produce a strong magnetic field when current flows through them, taking advantage of the solar wind’s ‘push.’
A magsail sounds like a natural way to get to the outer Solar System or beyond, but the solar wind introduces problems that compromise it. One is that it’s a variable wind indeed, weakening and regaining strength, and although I cited 500 kilometers per second in the introductory paragraph, the solar wind can vary anywhere from 350 to 800 kilometers per second. An inconstant wind raises questions of spacecraft control, an issue Gregory Matloff, Les Johnson and Giovanni Vulpetti are careful to note in their 2008 title Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2008). Here’s the relevant passage:
While technically interesting and somewhat elegant, magsails have significant disadvantages when compared to solar sails. First of all, we don’t (yet) have the materials required to build them. Second, the solar wind is neither constant nor uniform. Combining the spurious nature of the solar wind flux with the fact that controlled reflection of solar wind ions is a technique we have not yet mastered, the notion of sailing in this manner becomes akin to tossing a bottle into the surf at high tide, hoping the currents will carry the bottle to where you want it to go.
Interstellar Tradewinds and the Local Cloud
We have much to learn about the solar wind, but missions like Ulysses and the Advanced Composition Explorer have helped us understand its weakenings and strengthenings and their effect upon the boundaries of the heliosphere, that vast bubble whose size depends upon the strength of the solar wind and the pressures exerted by interstellar space. For we’re not just talking about a wind from the Sun. Particles are also streaming into the Solar System from outside, and data from four decades and eleven different spacecraft have given us a better idea of how these interactions work.
A paper from Priscilla Frisch (University of Chicago) and colleagues notes that the heliosphere itself is located near the inside edge of an interstellar cloud, with the two in motion past each other at some 22 kilometers per second. The result is an interstellar ‘wind,’ says Frisch:
“Because the sun is moving through this cloud, interstellar atoms penetrate into the solar system. The charged particles in the interstellar wind don’t do a good job of reaching the inner solar system, but many of the atoms in the wind are neutral. These can penetrate close to Earth and can be measured.”
Image: The solar system moves through a local galactic cloud at a speed of 50,000 miles per hour, creating an interstellar wind of particles, some of which can travel all the way toward Earth to provide information about our neighborhood. Credit: NASA/Adler/U. Chicago/Wesleyan.
We’re learning that the interstellar wind has been changing direction over the years. Data on the matter go back to the 1970s, and this NASA news release mentions the U.S. Department of Defense’s Space Test Program 72-1 and SOLRAD 11B, NASA’s Mariner, and the Soviet Prognoz 6 as sources of information. We also have datasets from Ulysses, IBEX (Interstellar Boundary Explorer), STEREO (Solar Terrestrial Relations Observatory), Japan’s Nuzomi observatory and others including the MESSENGER mission now in orbit around Mercury.
Usefully, we’re looking at data gathered using different methods, but the flow of neutral helium atoms is apparent with each, and the cumulative picture is clear: The direction of the interstellar wind has changed by some 4 to 9 degrees over the past forty years. The idea of the interstellar medium as a constant gives way to a dynamic, interactive area that varies as the heliosphere moves through it. What we don’t know yet is why these changes occur when they do, but our local interstellar cloud may experience a turbulence of its own that affects our neighborhood.
The interstellar winds show us a kind of galactic turbulence that can inform us not only about the local interstellar medium but the lesser known features of our own heliosphere. Ultimately we may learn how to harness stellar winds, perhaps using advanced forms of magnetic sails to act as brakes when future probes enter a destination planetary system. As with solar sails, magsails give us the possibility of accelerating or decelerating without carrying huge stores of propellant, an enticing prospect indeed as we sort through how these winds blow.
The paper is Frisch et al., “Decades-Long Changes of the Interstellar Wind Through Our Solar System,” Science Vol. 341, No. 6150 (2013), pp. 1080-1082 (abstract)