Name: Shkadov Thruster

Named for: Russian physicist Leonid Shkadov, who presented the megastructure concept in a 1987 paper at the 38th Congress of the International Astronautical Federation.

Selected science fiction portrayals: Star Maker, a 1937 novel by Olaf Stapledon, and the 2012 novel Bowl of Heaven and its sequel Shipstar by Gregory Benford and Larry Niven.

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.

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Mirror Placement

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.

Strip-Mining Mercury

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."

Within Our Grasp?

The scope of the operation—fashioning Mercury-sourced hematite into thin sheets, connecting them in space, and orienting them into a growing megastructure at a considerable distance from Earth—certainly exceeds our logistical abilities, to put it mildly. Yet building a Shkadov Thruster would not be an unfathomable technological and engineering leap. "At small scale, present-day technology is able to manage all the operations needed," Badescu says.

The bigger issue, Badescu said, is how humanity would go about agreeing to pursue such an immense project as the best bet for our survival as a species. "Taking such a major decision is possible only in critical situations, making possible some consensus," Badescu says. "When the time for such a decision comes, probably the engineering aspects will be accessible."

A trip with a Shkadov Thruster would be slow going at first, at least in relative terms. The sun is already moving around the center of the Milky Way at a relative speed of about 500,000 miles per hour. The first few million years of Shkadov thrusting "may only slightly change the usual trajectory of the sun," Badescu says.

Over the course of geological time, however, that bonus oomph adds up as the sun accelerates along its new path. "After 200 million years," Badescu says, "the distance between the perturbed and unperturbed positions of the sun is on the order of 10 to 40 parsecs," or 30 to 130 light-years.

Within the billion years we have left before the warming sun wipes us out, that level of displacement would be plenty to save Earth's bacon. Many dozens of reachable stars suitable for hosting Earth surely could be found within a few hundred light-years.

Lining up our speeding solar system with the target star so that Earth seamlessly transfers into a nice, circular orbit around its new host would surely make for some hairy course correcting en route. Exquisite timing would be needed so that Earth, in its regular orbit, is situated on the side of our sun as near as possible to the other star for gravitational capture during the stars' close pass. Shkadov's original paper, however, suggested it could be done.

Maybe the ultimate motivation for forging a Shkadov Thruster would not be survival but curiosity. After the passage of a few billion years, with the continued quickening of the Shkadov Thruster's pace, our solar system ensemble could traverse the Milky Way—or even leave our galaxy behind.

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