Researchers from the European Space Agency are testing what they describe as the smallest, yet most precisely controllable engine ever built for space. Measuring 10 centimeters (4 inches) across and making a faint blue glow as it runs, the Field Emission Electric Propulsion, or FEEP, engine produces an average thrust equivalent to the force of one falling hair. But its thrust range and controllability are far superior to more potent thrusters, and will be important for a future space mission that will test Einstein’s General Theory of Relativity.
“Most propulsion systems are employed to get a vehicle from A to B,” explained Davide Nicolini of the agency’s Scientific Projects Department, in charge of the engine research. “But with FEEP, the aim is to maintain a spacecraft in a fixed position, compensating for even the tiniest forces perturbing it, to an accuracy that no other engine design can match.”
Watching how objects behave when separated from all outside influences is a long-time ambition of physicists, but it can’t be done within Earth’s gravity field. So a next-decade mission called the LISA Pathfinder (Laser Interferometer Space Antenna) will fly 1.5 million km (900,000 miles) to one of the Lagrangian points, L-1. There, the Sun and Earth’s gravities cancel each other out, so that the behavior of a pair of free-floating test objects can be precisely monitored.
But to detach the experiment fully from the rest of the Universe there will still be some remaining per-turbations to overcome, most notably the slight but continuous pressure of sunlight itself. That’s where FEEP comes in. It operates on the same basic principle as other ion engines flown aboard ESA’s SMART-1 Moon mission and other spacecraft: the application of an electric field serves to accelerate electrically-charged atoms (known as ions), producing thrust.
But while the thrust of other ion engines is measured in millinewtons, FEEP’s performance is assessed in terms of micronewtons – a unit one thousand times smaller. The engine has a thrust range of 0.1 – 150 micronewtons, with a resolution capability better than 0.1 micronewtons in a time response of one-fifth of a second (190 milliseconds) or better.
The engine uses liquid metal caesium as propellant. Through capillary action—a phenomenon associated with surface tension—caesium flows between a pair of metal surfaces that end in a razor-sharp slit. The caesium stays at the mouth of the slit until an electric field is generated. This causes tiny cones to form in the liquid metal which have charged atoms shooting from their tips to create thrust.
Twelve thrusters will be used for the LISA Pathfinder. Working together with another propulsions system designed by NASA, the thrusters should yield directional control at least 100 times more accurate than any spacecraft before; down to a millionth of a millimeter.
LISA involves three satellites up to five million km (three million miles) apart and linked by lasers, orbiting the Sun. The aim is to detect ripples in space and time known as gravitational waves, predicted by Einstein’s theory of general relativity but so far undetected. The waves would cause tiny variations in the distance measured between the satellites.
The engine was tested last month, and once the tests are analyzed and the concept is proven, the FEEP technology has been earmarked for a broad range of other missions, including precision formation flying for astronomy, Earth observation and drag-free satellites for mapping variations in Earth’s gravity.