Overview

A Shkadov thruster is a type of megastructure which involves constructing a gargantuan orbital mirror next

to a star. In this lesson, we'll start off by discussing how such a stellar engine works. The orbital mirror is placed in a position next to a star where it acts as a statite: that is, the star's gravity acting on the mirror is canceled out by the star's radiation pressure acting on the mirror. This allows the mirror to stay in a position that is stationary relative to the star's surface. The mirror bounces some of the star's light back at itself; when that reflected light collides with the star, it exerts a thrust on the star which causes it to accelerate and move. This will bring us to the second main focus of this lesson: namely, what are the possible uses of a Shkadov thruster? As we'll discuss, since a Shkadov thruster can move the star and since all of the planets, moons, comets, and asteroids in the star system is gravitationally bound to the star, not only does the star move but the entire solar system moves away also. In the distant future, our Sun will eventually die. But we might be able to use a Shkadov thruster to move the Earth to another solar system, but this would take many millions of years.

How does a Shkadov thruster work?

"A look at the simplest type of Dyson Sphere, the Shkadov Thruster, a device able to move entire solar systems." This video was produced by Isaac Arthur.

Figure 1: An arc mirror (dark orange arc) subtended by an angle $$2\phi$$ reflects a star's radiation back towards itself. Photons collide against the portion of the star's surface which is subtended by the angle $$2\phi$$. These collisions result in a net force $$\vec{F}$$ exerted on the star causing the entire star to move in the direction of the arrow. The star's gravity pulls everything else in the star system along with it.

A Shkadov thruster is a type of megastructure and stellar engine first proposed by the physicist Leonid Shkadov in his 1987 paper entitled "Possibility of controlling solar system motion in the galaxy." To make one, an enormous arc mirror would have to be built near a star as depicted in Figure 2. The mirror would be placed in a position such that the star's gravitational force was equal-and-opposite to the total average force due to the pressure exerted on the mirror by that star's radiation. According to Newton's second law, the net force exerted on the mirror would be zero and, hence, the mirror's accelleration and change in velocity would also be zero. Thus, the mirror could remain stationary and "hover" about the star's surface without the distance between the mirror and star changing. A satelite which remains stationary relative to a massive body's surface is called a statite. Starlight would reflect off of the mirror and collide with the star while light emitted on the star's opposite side would disperse away from the star unhindered by any obstructions. For an isolated star without a giant mirror next to it, starlight is emitted radially in all directions exerting a net thrust of zero on the star (meaning it remains stationary); but for a star which is accompanied with a giant mirror statite that causes some of its radiation to get reflected to collide with the star, this asymmetry results in a net thrust being exerted on the star giving it a gentle continuous "push." But the star, being as gargantuon and massive as it is, would gravitationally pull that mirror statite (and, indeed, the planets and everything else as well) along with it as it moved away. The entire star-mirror system would move, continuously, at a steadily increasing velocity along a straight line in the direction of the arrow which passes through the center of the star and mirror illustrated in Figure 1.

Potential uses of a Shkadov thruster

Figure 2: Artist's depiction of a Shkadov thruster. Credit: https://www.artstation.com/artofsoulburn

If the remote descendants of humanity are a K2 civilization, they might decide to build a mirror statite above the Sun's surface and use the Sun-mirror system as a Shkadov thruster. Mercury is likely one of the last places in the solar system their human ancestors would have had wanted to colonize; it would therefore by likely that they would have had left this world untouched. The type and plentitude of Mercury's materials would make this world the ideal source of "construction materials" for building the Shkadov thruster. By disassembling and dismantling Mercury and utilizing its materials, our descendants would be able to build such a megastructure without even touching the asteroid belts or other worlds in the solar system.

Since all of the other worlds in the solar system are locked in their orbits by the realm of the Sun's gravity, as the Shkadov thruster traversed the enormous distances of interstellar space all of the planets, moons, asteroids and comets would come with it for the ride without their orbits getting altered. Spaceship solar system. If the mirror in the Shkadov thruster were hemisphereical (meaning, its total momentum changed due to the impulse exerted by half of the Sun's solar output) the solar system would speed up by an addition $$20m/s$$ and be displaced $$0.03$$ light years from its original position after one million years. That's not much but could still be potentially useful in certain situations. For example, if the Sun made a nearby-pass with another star, that star's gravitational field could perturb the orbits of the comets within the Oort cloud threatning the Earth and civilizations on many adjacent worlds. By using a Shkadov thruster to give a slight nudge to our solar system's path through space, such a catastrophe could be avoided.

In another lesson entitled Starlifting, we examined how by utilizing a technique called starlifting we could extend the Sun's lifetime. But an alternative to that would be to jettison the Sun entirely. Using a Shkadov thruster, we could send our solar system along a trajectory which comes very close to an infant Dwarf star; as our solar system whizzed by, the planet Earth could be gravitationally captured by that star and revolve around a new home star. Of course, $$0.03$$ light years isn't enough to reach another star. But after one billion years, the steady impulse exerted by the Sun's radiation upon the mirror would have sped up our solar system to $$20km/s$$ and, by then, we would have covered a distance of $$34,000$$ light years—about one third the diameter of the Milky Way galaxy. By planning many millions and maybe even close to a billion years in advance before our Sun turned into a Red Giant (which would "burn the Earth to a crisp or reduce it to a whirl of atoms"), the Earth could find a new home solar system. Dwarf stars can live for up to one-hundred trillion years giving our descendants a source of power for many eons.