Although I didn’t write about the so-called ‘Shkadov thruster’ yesterday, it has been on my mind as one mega-engineering project that an advanced civilization might attempt. The most recent post was all about moving entire stars to travel the galaxy, with reference to Gregory Benford and Larry Niven’s Bowl of Heaven (Tor, 2012), where humans encounter an object that extends and modifies Shkadov’s ideas in mind-boggling ways. I also turned to a recent Keith Cooper article on Fritz Zwicky, who speculated on how inducing asymmetrical flares on the Sun could put the whole Solar System into new motion, putting our star under our directional control.

The physicist Leonid Shkadov described a Shkadov thruster in a 1987 paper called “Possibility of Controlling Solar System Motion in the Galaxy” (reference at the end). Imagine an enormous mirror constructed in space so as to reflect a fraction of the star’s radiation pressure. You wind up with an asymmetrical force that exerts a thrust upon the star, one that Shkadov believed could move the star (with accompanying planets) in the event of a dangerous event, like a close approach from another star. Shkadov thrusters fall into the category of ‘stellar engines,’ devices that extract significant resources from the star in order to generate their effect.


Image: A Shkadov thruster as conceived by the artist Steve Bowers.

There are various forms of stellar engines that I’ll be writing about in future posts. But to learn more about the ideas of Leonid Shkadov, I turned to a recent paper by the always interesting Duncan Forgan (University of Edinburgh). Forgan points out that Shkadov thrusters are not in the same class as Dyson spheres, for the latter are spherical shells built so that radiation pressure from the star and the gravitational force on the sphere remain balanced, the purpose being to collect solar energy, with the additional benefit of providing vast amounts of living space.

Where Shkadov thrusters do remind us of Dyson spheres, as Forgan notes, is in their need for huge amounts of construction material. The scale becomes apparent in his description, which is clarified in the diagram below:

A spherical arc mirror (of semi-angle ψ) is placed such that the radiation pressure force generated by the stellar radiation field on its surface is matched by the gravitational force of the star on the mirror. Radiation impinging on the mirror is reflected back towards the star, preventing it from escaping. This force imbalance produces a thrust…

Here I’m skipping some of the math, for which I’ll send you to the preprint. But here is his diagram of the Shkadov thruster:


Figure 1: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2ψ. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow.

Forgan goes on to discuss the effects of the thruster upon the star:

In reality, the reflected radiation will alter the thermal equilibrium of the star, raising its temperature and producing the above dependence on semi-angle. Increasing ψ increases the thrust, as expected, with the maximum thrust being generated at ψ = π radians. However, if the thruster is part of a multi-component megastructure that includes concentric Dyson spheres forming a thermal engine, having a large ψ can result in the concentric spheres possessing poorer thermal efficiency.

The sheer size of Dyson spheres, Shkadov thrusters and other stellar engines inevitably makes us think about such constructions in the context of SETI, and whether we might be able to pick up the signature of such an object by looking at exoplanet transits. Richard Carrigan is among those who have conducted searches for Dyson spheres (see Archaeology on an Interstellar Scale), but Forgan thinks a Shkadov thruster should also be detectable. For the light curve produced by an exoplanet during transit would show particular characteristics if a Shkadov thruster were near the star, a signature that could be untangled by follow-up radial velocity measurements.

The chances that we might pick up a transit showing clear signs of extraterrestrial engineering seem remote, but Forgan’s point is that we have numerous exoplanet surveys in progress, ranging from analysis of the Kepler data (with a recent SETI component factored in) to future surveys using the TESS and PLATO instruments, each intended to undergo radial velocity scrutiny as a follow-up to any detections. The GAIA satellite will also provide useful data for possible follow-ups of transit candidates. With all this in the mix, Forgan wants to clarify what a Shkadov thruster would look like if by whatever chance we do find one in our data.

The presence of a Shkadov thruster, he demonstrates, can be flagged by study of the lightcurve of both transiting planet and thruster, with the possibility that both the primary and secondary eclipses can be affected. It would be a tricky catch even so, for transient phenomena like starspots can mask features in the lightcurve, and Forgan thinks that further radial velocity studies, along with interferometric imaging and asteroseismology would have to come into play to tease out the features of such a thruster. Missions designed to study exoplanet atmospheres — he mentions CHEOPS or EChO — could be used to confirm the thruster’s presence.

A long shot indeed, but it’s good to have this study of those features that would flag a lightcurve as anomalous and indicative of advanced engineering. For while the probabilities of finding a Shkadov thruster are remote, we’ll have a growing number of datasets from various exoplanet missions to draw on. Interstellar archaeology is all about digging into this rich stratum to see whether any observed events fit models that suggest the presence of artificial objects. And today’s exoplanet catalogue only hints at the volumes of information still to come.

The paper is Forgan, “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” accepted for publication in the Journal of the British Interplanetary Society (preprint). Leonid Shkadov’s paper on Shkadov thrusters is “Possibility of controlling solar system motion in the galaxy,” 38th Congress of IAF,” October 10-17, 1987, Brighton, UK, paper IAA-87-613. More on stellar engines in coming weeks.