Centauri Dreams regular James Jason Wentworth wrote recently with some musings about Bracewell probes, proposed by Ronald Bracewell in a 1960 paper. Bracewell conceived the idea of autonomous craft that could monitor developments in a distant solar system, perhaps communicating with any local species that developed technology. Pondering how such a craft might manage station-keeping over the aeons, Jason hit on the idea of using a natural effect that would draw little attention to itself, one he explains below. An amateur astronomer and interstellar travel enthusiast who worked at the Miami Space Transit Planetarium and volunteered at the Weintraub Observatory atop the adjacent Miami Museum of Science, Jason now makes his home in Fairbanks (AK). He was the historian for the Poker Flat Research Range sounding rocket launch facility near Fairbanks. His space history and advocacy articles have appeared in Quest: The History of Spaceflight magazine and Space News.
by James Jason Wentworth
Dreams, daydreams, and flights of fancy have far greater value than most modern people realize. (Centauri Dreams are *not* just two nice-sounding words, but instead constitute a vital and necessary prelude to, and continuing inspiration for, interstellar space flight. Without Centauri dreams, there will be no “Centauri do’s,” as in visiting that stellar system via robotic probes or crewed starships!) Besides being pleasant forms of mental play, dreams can also bring insights that are of great practical importance. Such activities are usually considered the province of poets, storytellers, and songwriters, but scientists have also been helped by them. The most well-known example of this involved Friedrich August Kekulé, a 19th century German chemist, who gained answers he was seeking about molecular configurations from two dreams that he had . The more famous of these two dreams–which involved a snake-like string of atoms that formed a circle, which then transformed into a snake eating its tail–led him to the ring structure of the benzene molecule.
In January of last year, such an insight came to me regarding a new form of fuel-less spacecraft propulsion and attitude control – one that, to my knowledge, no one has suggested before. It would be something that used the forces of nature, and it would also be something subtle and non-polluting. A light sail would fit these preferences well, but it occurred to me that there was another alternative (particularly for use within star systems) which would also employ starlight but would be more subtle than a sail, not blazing forth in the skies of nearby planets. Moreover, it would have other advantages, which would be useful to long-life spacecraft of all kinds, from unmanned Earth satellites to mobile space colonies to Bracewell interstellar messenger probes. I shall explore these advantages below.
The Power of Emitted Photons
The Yarkovsky effect  was discovered by Ivan Yarkovsky (1844-1902), a Russian civil engineer who worked on scientific problems in his spare time. The effect imparts a very small but constant thrust to small, rotating bodies in orbit around the Sun, via the heating of the bodies’ surfaces by sunlight. As such an object rotates, its “afternoon” quadrant emits infrared photons as it cools, and this photon emission imparts an asymmetrical thrust force to the object. The Yarkovsky effect affects the orbits of meteoroids and asteroids between about 10 cm and 10 km across. (Smaller objects are heated more uniformly via internal heat transfer, which precludes the asymmetrical infrared photon emission, and larger asteroids are too massive to be affected appreciably by the infrared photon thrust.) A prograde-rotating meteoroid or asteroid (one that is rotating in the same direction that it is orbiting the Sun, counter-clockwise in the case of our solar system) gradually spirals outward away from the Sun due to the Yarkovsky effect, while a retrograde-rotating body spirals inward toward the Sun.
A related phenomenon, the Yarkovsky-O’Keefe-Radzievskii-Paddack effect (YORP effect) , affects the rotation rate, the rotational axis tilt, and the rotational axis precession rate in small asymmetric meteoroids and asteroids. These two effects could also be utilized by spacecraft, for fuel-less propulsion as well as attitude control.
Image: The Yarkovsky Effect: An asteroid is warmed by sunlight, its afternoon side becoming hottest. As a result, that face of the asteroid re-radiates most thermal radiation, creating a recoil force on the asteroid and causing it to drift a little. The direction of the radiation depends on whether the asteroid is rotating in a prograde (anticlockwise) manner (a) or in a retrograde (clockwise) manner (b). Credit: “Planetary science: Spin control for asteroids,” by Richard Binzel in Nature 425 (11 September 2003), 131-132.
Putting Yarkovsky to Work
The now-solved Pioneer anomaly was an unintentional demonstration of the Yarkovsky Effect’s ability to impart measurable thrust to a spacecraft. The Pioneer 10 and 11 spacecrafts’ Radioisotope Thermoelectric Generators (RTGs), rather than the Sun, supplied the infrared photons, which produced a tiny thrust toward the Sun by bouncing off the back of the probes’ dish antennas. A spacecraft that was purposely designed to utilize the Yarkovsky effect (and also the YORP effect, if desired) could move (and maneuver) much more quickly than either massive, rock/metal asteroids or the “accidentally-propelled” Pioneer spacecraft. The rate of acceleration of such a spacecraft would likely be comparable to that of a solar sail, although a higher thrust/mass ratio would increase its possible acceleration rate. A spacecraft of this type might be designed as follows:
Picture a black, rotating, drum-shaped vehicle, whose spin axis is perpendicular to the plane of its orbit around the Sun. (The drum could be a “stand-off” cylinder, like Skylab’s lost meteoroid shield, which could be deployed from a central spacecraft via centrifugal force.) The vehicle would spiral away from the Sun if it rotated in a prograde direction, and it would spiral inward toward the Sun if it rotated in a retrograde direction, just as asteroids (those which are small enough to be affected by the Yarkovsky effect) do. It could also change the plane of its orbit, by tilting its spin axis to inclinations other than perpendicular to its orbit plane. Changing its spin rate and spin direction would alter the magnitude and direction of its infrared photon thrust. Reversing the vehicle’s spin direction could be accomplished either by stopping the spin and re-starting it in the opposite direction or by precessing the spin axis 180 degrees around (the latter method would be preferable for large spacecraft). Like a solar sail, a Yarkovsky/YORP effect propelled spacecraft would have a low rate of acceleration, but it could achieve very high velocities over time.
The YORP effect could be utilized, if desired, to control the spacecraft’s spin rate, spin axis tilt, and spin axis precession rate (using no moving parts) by equipping the drum-shaped vehicle with short, wedge-shaped “blades” (which could, optionally, be made retractable) that would protrude from its sides. The blades could also have electronically-variable light reflectivity and absorption, like the variable-reflectivity liquid crystal steering panels on JAXA’s IKAROS solar sail. These blades would create an asymmetrical total vehicle solar illumination, which is the cause of the multiple YORP effects. As an alternative, the spacecraft’s spin rate control and spin axis pointing might be handled – again without any moving parts – by using selectively-charged wires (or other vehicle parts) to interact with the local planetary or solar magnetic fields. Or the vehicle might use magnetically-levitated, internal torque flywheels to control its spin rate and direction.
The black drum could be a soft (“quilted” quartz cloth, optionally rigidized by a vacuum-hardening pre-impregnated resin) or rigid (a folding metal or composite) outer cylinder standing off from the surface of the spacecraft, held there either by rigid struts or by tensioned cables or cords, in concert with centrifugal force. Either type could contain thermovoltaic cells to generate electricity for the spacecraft’s systems.
Photovoltaic solar cells could, however, be utilized by such a vehicle if desired. Its instruments, imaging system (if any – perhaps a spin-scan camera), and solar cells could be mounted on parts of the spacecraft “bus” that protrude above and below the ends of the black drum. Or, by using angled circumferential mirrors on the exposed ends of the bus (and metallized Kapton or other such material on the inside of the black drum), solar cells on the drum-obscured parts of the bus could be illuminated by sunlight. If a soft fabric drum were used, it would absorb some of the solar and cosmic radiation that degrades solar cells, and so would enable them to last longer.
Such a spacecraft could even use thermocouples in order to utilize the solar heat on the black drum (and the cold in the shadowed areas at its ends, by placing circumferential “heat shades” between the inside wall of the black cylinder and the cold sides of the thermocouples) to generate electricity for its onboard systems. Thermocouples made of dissimilar refractory metals might be very long-lived electricity generating devices for spacecraft of this type.
Station-Keeping for the Long Haul
Such a capability, combined with the ability to change orbits, maintain orbits, and perform Lagrangian point station-keeping without using any propellant (and with no moving parts), would enable Yarkovsky/YORP effect-utilizing spacecraft to operate for very long periods, whether in orbit around the Earth, other planets, the Sun, or other stars. The black drums used by these spacecraft would likely also have at least three advantages over solar sails. Over long periods of time, it is more difficult for a reflective object to remain reflective than for a black object to remain black. Unlike a sail, the drum could be more compact as well as have greater thickness and strength, and its rotation would increase its stiffness. Spacecraft using this method of propulsion should also be able to maneuver more effectively closer to a planet (especially one possessing an atmosphere) than a sail could.
A further possible advantage – for human-dispatched Bracewell probes sent to “loiter” in their target star systems for decades, centuries, or even millennia – would be that such black spacecraft wouldn’t attract visual attention as a sail-equipped probe would. An infrared search could find such a probe, but with dark asteroids and black, extinct comet nuclei likely being as common in other stellar systems as in our own, it might escape positive identification as an alien visitor, at least for some time.
Image: One of many science fiction treatments of Bracewell probes occurs in Michael McCollum’s Life Probe (Del Rey, 1983).
As Robert Freitas  has written, any civilization – perhaps even our own – might consider an alien Bracewell probe in its star system to be a threat, at least initially. Providing such probes with a measure of protection would “buy them time to explain themselves” by making them less-than-easy to find. This, and their ability to move between broadcasts, would better enable them to establish contact and demonstrate their peaceful purposes before they might otherwise be attacked by a wary race.
While brute-force methods got humanity into space, it is increasingly obvious that for far journeys and long sojourns there, harnessing the subtle natural forces that are freely available just above our heads is the only way that humanity can truly thrive and prosper in that realm.
 “Kekulé’s Dream” (see: http://web.chemdoodle.com/kekules-dream)
 “Yarkovsky Effect” (see: http://en.wikipedia.org/wiki/Yarkovsky_Effect)
 “Yarkovsky–O’Keefe–Radzievskii–Paddack Effect” (YORP Effect, (see: http://bit.ly/1fsVRRl)
 Robert Freitas Official Website (see: http://www.rfreitas.com/)