WHEN is a cancer cure not a cancer cure? When it’s a satellite propulsion system. As riddles go, perhaps not that amusing. But as an example of the way scientific serendipity works, it is most enlightening. For it shows how that much-vaunted academic fashion, cross-disciplinary research, can actually work in practice.

Lyon King, of Michigan Technological University, works on nanosatellites. These are spacecraft, little larger than a smartphone, that can fly cheaply into orbit by piggybacking on rockets carrying bigger payloads. The power of modern electronics means that such tiny vessels can be equipped with lots of useful kit, including GPS trackers, cameras and radios. But their dependence on other people’s goodwill for their launch means that their orbits are not completely under their owners’ control, which restricts their usefulness. On top of this, atmospheric drag often brings them back to Earth in an untimely fashion. Dr King therefore wants to fit them with their own rocket motors. That would give them directional autonomy, and would also let them boost their orbits every so often, to escape the atmosphere’s clutches.

He is not alone in this desire. Several research groups have built miniature rockets that use electric fields to pull ions (atoms with a surfeit or shortage of electrons, and therefore an electric charge) out of a fluid and fling them away. This produces thrust in the same way that the exhaust of a chemical rocket does, by invoking Newton’s third law of motion, that to every action there is an equal and opposite reaction. But such rockets are finicky because the fuel must first be persuaded to travel up a small needle, from whose tip the ions are then sprayed off into space. A collection of such needles forms a thruster. Making them is hard. Every member of a cluster must be the same size. And the needles tend to be fragile. A bump or an unwanted bit of electrical arcing can destroy them.

Dr King’s idea was to dispense with solid needles and instead grow liquid ones out of the fuel itself. To do this he proposed employing something called a ferrofluid, which responds to magnetic fields. Place a magnet beneath such a fluid and it transforms into a simulacrum of an oily hedgehog, with dozens of spikes sticking out of its surface.

That is a promising start. But to make a motor out of this arrangement you have to use a ferrofluid that also contains enough ions to work as rocket fuel. Dr King tried various mixtures, but none was satisfactory. Then he heard about a team at the University of Sydney, in Australia, led by Brian Hawkett, which was working on a different problem in ferrofluidics in collaboration with Sirtex, an American biotechnology firm.

Dr Hawkett and his collaborators had developed exactly the kind of liquid Dr King needed—one that was both ionic and reactive to magnetic fields. They had done so, however, in the hope not of manoeuvring satellites but of treating liver cancer. Their idea was (and still is) to inject the fluid into a tumour, heat it up with a magnetic field and cook the tumour from within. So when Dr King got in touch to ask them about using their invention as rocket fuel they were understandably bemused.

That bemusement, though, has turned into collaboration, and with the help of Dr Hawkett’s ionic ferrofluid, Dr King’s team have built a prototype thruster that works, and seems more or less immune to the sort of accidents that might fry a more conventional sort of micro-rocket. Whether it will still work when it is in space remains to be seen. But if it does, then it will be one-up for scientific serendipity, and a reminder to researchers everywhere that, though specialisation has its advantages, a willingness to look outside one’s immediate field can sometimes pay dividends too.