LITTLE more than ten centimetres across, so-called CubeSats are cheap-to-launch satellites that are able to carry out many scientific and commercial tasks at a fraction of the cost of conventional orbiters. This miniaturisation is mainly thanks to electronics developed for smartphones.

CubeSats can be launched as additional payload alongside bigger satellites, sent up during deliveries to the International Space Station or put aloft on small, purpose-built rockets like NASA’s new Venture Class. Being cheap means that once their orbits decay and they burn up on re-entering the atmosphere, CubeSats can simply be replaced with newer versions. But it can pay to reposition them, both to operate more effectively and prolong their service life. That, however, is a challenge because for reasons of safety a CubeSat cannot carry conventional rocket fuel. They typically rely for manoeuvrability on heating an inert liquid and ejecting small jets of the resulting vapour, which is a poor substitute for the energetic burning of rocket fuel. So better means of motion are required—and at least two are on offer.

One, developed by Paulo Lozano and his colleagues at the Massachusetts Institute of Technology, is a miniature version of the ion drives used on larger satellites. An ion drive accelerates charged particles derived from a liquid propellant to very high speeds. This creates thrust more efficiently than ejecting heated vapour, but without the explosive chemical reactions of a conventional rocket.

At the moment, ion drives are not used on CubeSats because both space and weight are at a premium in such small devices and an ion drive needs pumps to move the propellant around. Dr Lozano, however, has been able to dispense with these large and heavy components. He and his team have developed a porous glass emitter that has hundreds of tiny channels running through it. These suck the propellant up by capillary action, obviating the need for pumps. The propellant itself is a substance known as an ionic liquid, that consists of positive and negative ions which can be separated by passing a current through the liquid and then, because they are electrically charged, accelerated by an electric field. (Both current and field are supplied by a battery on board the satellite.) The ions are then fired from separate thrusters, one for the positive ions and one for the negative ones.

Individual modules, containing emitter, thrusters and enough propellant for an hour of thrust, are about the size of a sugar cube and weigh less than four grams. Dr Lozano’s team has shown in the laboratory that eight can produce enough impulse to shift a typical CubeSat sufficiently to extend its life in orbit from months to years. Three are now in the hands of NASA, awaiting flight-testing later this year.

The other approach, that of Young Bae of Advanced Space and Energy Technologies in Tustin, California, is to use light. Dr Bae’s device, which he dubs a photonic laser thruster, was developed with NASA funding. His thruster works because light exerts pressure when it hits something. In theory, it is possible to move an object like a CubeSat by nudging it with a laser beam. In practice, however, the pressure which light exerts is so small that a device able to do a useful amount of nudging would require a laser of unfeasibly large power.

Dr Bae has overcome this limitation by bouncing light repeatedly between the source laser and the satellite, to multiply the thrust. In his latest experiments, Dr Bae has managed to amplify the thrust imparted by a single nudge of the laser by a factor of 1,500, which is big enough to manoeuvre a CubeSat as well as a conventional thruster would. This brings two advantages. First, since no on-board propellant is required, there is more room for instruments. Second, there being no fuel to run out, a CubeSat’s orbit can be boosted as many times as is desired, and its working life prolonged indefinitely.

The flip side of these advantages is that a suitable laser is required to provide the thrust. Dr Bae thinks it could be in orbit as well. The laser would be powered by solar cells and shepherd a veritable flock of CubeSats, providing the propulsion needed to move and arrange them as required. That is still some years away so, unlike Dr Lozano’s system, Dr Bae’s is not yet ready to fly. But his vision of an orbiting Bo Peep, cajoling and directing her charges by flashing lights at them, is a pleasing one.