Raphael Thys
Filtered light from an eclipse in Oregon, 2017. The red light is emitted by charged iron particles at 1 million °C, and the green at 2 million °C.
(M. Druckmuller/ Habbal et al., ApJL, 2021)
Sailing to the stars on the scale of human lifetimes could be a matter of choosing the right kind of wind.
Researchers from McGill University in Canada and the Tau Zero Foundation in the US have proposed a new way to cross the extraordinary distances of interstellar space, using a whole lot of nothing and a touch of inspiration from seabirds.
Until now, one of the most promising solutions to space travel makes use of the spectrum of starlight streaming from the Sun. Though small in impact, sheer numbers and high speeds make photons an intriguing source of power for building up the high velocity needed to cross light-years of emptiness in a short time.
Innovations in solar sail technology have progressed considerably over the years, with models going as far as being tested in the hostile environments of our inner Solar System.
Though functional, solar sails all have one downside in common: the sail itself. Solar sails must stretch meters across to catch the photons needed to propel a craft.
They also need the right shape and material to turn every photon's tiny bit of momentum into movement. And they need to shed heat well enough not to distort and break.
This isn't just a headache in materials science; all of these requirements add mass. Even using the lightest materials known, the fastest speeds we might achieve using our Sun's radiation would be just over 2 percent the speed of light, meaning a trip to the nearest star would still take a few centuries.
Needless to say, sailing to the stars would be a lot easier if we could ditch the sails part.
Fortunately, another kind of gale blows from the solar surface, one made not of photons but a plasma of ions whipped into a frenzy by the snap and crackle of the Sun's magnetic fields.
Though there are far fewer high-speed electrons and protons blasting from the Sun than photons, their charged masses pack a greater punch.
Such particles would usually be a problem for typical sails, imparting their charges on the material's surface like static on a woolen jumper in winter, creating drag and changing the sail's shape.
Yet as anybody who has ever tried pushing the poles of magnets together knows all too well, an electromagnetic field can provide resistance without requiring a large, solid surface.
And so it's goodbye shiny material, and hello superconductor. A cable just a few meters long could, in theory, produce a field wide enough to deflect the Sun's charged wind on the scale of tens to hundreds of kilometers.
The system would act more like a magnetic parachute, one that is being dragged by a flow of particles moving at speeds of close to 700 kilometers (about 430 miles) a second, or just under a quarter of a percent of the speed of light.
This isn't bad, but as birds like the albatross know, the winds don't set the speed limits when it comes to flying high.
By looping in and out of air masses moving at different velocities, seabirds can pick up the energy of a headwind, using what's known as dynamic soaring to gain speed before returning to their original trajectory.
Using a similar trick in the 'headwind' of the termination shock – a turbulent zone of contrasting stellar winds used by astronomers to define the edge of our Solar System – a magnetic sail could exceed the solar wind's speeds, potentially bringing it within reach of solar sails based on radiation alone.
Though the technology might not initially seem much faster than the 'traditional' solar sails method, other forms of turbulence at the fringes of interstellar space might provide a bigger boost.
Even without a gentle nudge from dynamic soaring, feasible plasma-based technology could put cube-sat satellites around Jupiter within months rather than years.
Like the age of sail of yore, there are plenty of ways we might be able to take advantage of the currents that wash through the vastness of space.
And still, the seabirds show us the way.