When we talk about solar sails for space missions, we normally think of physical objects, vast but incredibly thin sheets of high-tech material pushed by the momentum imparted by solar photons. Someday we may use such sails to ply routes between the planets, but as researchers ponder such technologies, they’re also looking at the possibilities of magnetic sails using a different kind of propulsion. Rather than being pushed by photons, a magsail interacts with the plasma of the solar wind.
And that makes for some interesting possibilities. The solar wind is a stream of charged particles moving at high speeds — 500 kilometers per second and more — and if you can harness it through technologies like Robert Winglee’s Mini-Magnetospheric Plasma Propulsion (M2P2), you can ride that wind on a magnetic bubble hundreds of kilometers in diameter. Someday magsails may even provide deceleration capability for interstellar probes as they arrive in distant solar systems.
All that puts the doughty New Horizons spacecraft back in the spotlight. The probe, now enroute to Pluto and approaching Jupiter for its upcoming gravity assist, is contributing new data through its Solar Wind Around Pluto instrument, whose sensitivity has been turned down to deal with conditions in Jupiter space. Designed to measure the solar wind at 30 AU, the instrument is nonetheless returning good data much earlier, about the structures that form when solar wind streams compress and create discontinuities — shocks — in the outflowing particle stream.
“These solar wind structures collide with the magnetospheres of planets and, we believe, cause major variations in their structures,” says Dr. David McComas, SWAP principal investigator and senior executive director of the SwRI Space Science and Engineering Division. “Because it has the largest magnetosphere in the solar system, the effects of the solar wind at Jupiter could have significant implications for all the planets.”
Jupiter’s magnetosphere is itself a magnetic bubble that surrounds the planet. We’ll learn a lot more about how it works from these investigations, including what role the solar wind plays in Jupiter’s auroral phenomena as opposed to internal processes at the planet itself. The work will continue as New Horizons whips past the planet and explores the magnetotail (where the magnetosphere is pushed away from the Sun by the solar wind stream) in the months ahead.
All of which is wonderful for planetary science, but ultimately may prove useful for propulsion concepts and deep space missions built on them as well. Charting the movement of the solar wind is a prerequisite for testing magsail concepts in space. We may eventually find that Solar System exploration is faster and more economical with magsails created by injecting plasma into a magnetic field (this is the M2P2 idea). Or we may take magsails one gigantic step further, using particle beams to boost spacecraft into nearby interstellar space and beyond.
A provocative paper on magsail propulsion and interstellar missions is Dana Andrews’ “Interstellar Propulsion Opportunities Using Near-Term Technologies,” which appears in Acta Astronautica Vol. 55 (2004), pp. 443-451. Centauri Dreams originally discussed that one here.