In about a billion years our aging sun will become hot enough to boil off Earth’s oceans. But we needn’t let our world bake to death. By devising a megastructure called a Shkadov Thruster, we could cruise our solar system - sun, planets, and all - close enough to a younger star for it to gravitationally capture Earth. By enabling us to swap our sun for another, the Shkadov Thruster could give the planet’s biota a brand new lease on life.
"Shkadov Thrusters are kind of awesome," says Anders Sandberg, a research fellow at Oxford University’s Future of Humanity Institute who has studied Shkadov Thrusters amongst other megastructure concepts. "You can use it to move the whole solar system."
The Shkadov Thruster setup is simple (in theory): It’s just a colossal, arc-shaped mirror, with the concave side facing the sun. Builders would place the mirror at an arbitrary distance where gravitational attraction from the sun is balanced out by the outward pressure of its radiation. The mirror thus becomes a stable, static satellite in equilibrium between gravity’s tug and sunlight’s push.
Solar radiation reflects off the mirror’s inner, curved surface back toward the sun, effectively pushing our star with its own sunlight - the reflected energy produces a tiny net thrust. Voilà, a Shkadov Thruster, and humanity is ready to hit the galactic trail.
If humanity were ever crazy or desperate enough to build a Shkadov, the first order of business would be deciding where to place the megastructure. Leonid Shkadov, the megastructure concept’s inventor, figured placement in the temperate orbital band where Earth resides would be fine. Still, much of the rear, space-facing side of the thruster would probably still need to be lined with cooling fins. These fins would radiate away excess solar heat in order to keep the mirror from deforming or melting, depending upon its material.
The thruster, of course, could not be positioned in Earth’s orbital path. A logical spot for it would be above or below the plane of Earth’s orbit, with the reflective mirror beaming energy mostly perpendicularly. For a thruster with a mirror angle of 30 degrees, the usual presumed curvature, Earth would still catch some extra rays. But the influence on Earth’s temperature is expected to be small, says Viorel Badescu, a thermodynamicist at the Polytechnic University of Bucharest in Romania, who has investigated so-called stellar engines, which include Shkadov Thrusters and Dyson Spheres.
The second problem is simply acquiring enough material to build the behemoth structure. Badescu estimated that 1/10,000 of Earth’s mass would be required - probably about a sextillion pounds. Shkadov’s figure is a bit higher, more like septillion pounds. Either way, it’s hefty.
Although the thruster would be vast - perhaps on the order of a few hundred million miles in diameter, or greater than the distance between the Earth and the sun - most of it would be thin, reflective material. “It’s probably going to be a lot of thin foil,” Sandberg says.
An excellent readily available material is hematite, which humans have been polishing into mirrors for millennia. A simple iron oxide, hematite could be obtained on a grand scale by essentially strip-mining the entire planet of Mercury. Although dismantling even a smallish planet like Mercury sounds tough, it still beats rounding up scattered asteroids, which wouldn’t provide enough usable material anyway.
"It’s much easier to disassemble existing planets than to collect bodies distributed over huge spaces," Badescu says. "The inner planets of the solar system probably would be the first source of material."