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Tiny space engine to push back against sunshine

March 18, 2009
Courtesy European Space Agency
and World Science staff

Eu­ro­pe­an Space Agen­cy re­search­ers are pre­par­ing to test what they de­scribe as the small­est, yet most pre­cisely con­trol­la­ble en­gine ev­er built for space. It’s de­signed to be sen­si­tive enough to coun­ter­act the force of sun­shine.

Intense ion beams gen­er­ated by FEEP thrust­ers (cour­tesy ESA)


Meas­ur­ing 10 cen­time­tres (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, hold­ing the key to fu­ture suc­cess of an am­bi­tious mis­si­on of the agen­cy, re­search­ers de­clare.

“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 this one, “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 La­ser In­ter­fer­om­eter Space An­ten­na, or LI­SA, Path­find­er is to fly 1.5 mil­li­on km (900,000 miles) to a place called La­grange Point 1. The­re, the Sun and Earth’s gra­vi­ties can­cel each oth­er out, so that the be­hav­iour 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 per­turba­tions to over­come, most no­tably the slight but con­tin­u­ous pres­sure of sun­light it­self. That’s where FEEP comes in. It op­er­ates on a bas­ic prin­ci­ple fol­lowed by oth­er so-called ion en­gines: the ap­plica­tion of an elec­tric field serves to ac­cel­er­ate elec­tric­ally-charged atoms, pro­duc­ing thrust. 

But FEEP’s per­for­mance is meas­ured us­ing un­its called mi­cronew­tons, which are one-thousandth the size of the al­ready small un­its used for oth­er ion en­gines. The en­gine has a thrust range of 0.1 to 150 mi­cro­new­tons, with a resoluti­on ca­pa­bil­ity bet­ter than 0.1 mi­cronew­tons and a time re­sponse of one-fifth of a sec­ond or less, ac­cord­ing to proj­ect en­gineers.

The en­gine em­ploys the 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 would be mount­ed on the hull of LI­SA Path­find­er. Work­ing to­geth­er with a sep­a­rate NASA-de­signed pro­pul­si­on sys­tem, the thrusters should yield directi­onal con­trol at least 100 times more ac­cu­rate than any space­craft be­fore it—down to a mil­li­onth of a mil­li­me­tre, proj­ect en­gineers as­sert.

“We are over­see­ing the work here be­cause we have pre­vi­ous knowl­edge of FEEP tech­nol­o­gy,” said Pierre-Etienne Frigot of ESA’s Pro­pul­si­on Lab­o­r­a­to­ry. 

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 spec­ta­cu­larly suc­cess­ful the­o­ry of gen­er­al rel­a­ti­vity but so far un­de­tected. The waves would cause ti­ny varia­tions in the dis­tance meas­ured be­tween the satel­lites.

Once 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.


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European Space Agency researchers are preparing to test what they describe as the smallest, yet most precisely controllable engine ever built for space. It’s designed to be sensitive enough to counteract the force of sunshine. Measuring ten centimetres (4 inches) across and emitting 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 forceful thrusters, holding the key to future success of an ambitious science mission of the agency, researchers declare. “Most propulsion systems are employed to get a vehicle from A to B,” explains Davide Nicolini of the agency’s Scientific Projects Department, in charge of the engine project. But with this one, “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 Laser Interferometer Space Antenna, or LISA, Pathfinder is to fly 1.5 million km (900,000 miles) to an area in space called Lagrange Point 1. There, the Sun and Earth’s gravities cancel each other out, so that the behaviour 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 perturbations to overcome, most notably the slight but continuous pressure of sunlight itself. That’s where FEEP comes in. It operates on a basic principle followed by other so-called ion engines: the application of an electric field serves to accelerate electrically-charged atoms, producing thrust. But FEEP’s performance is measured using units called micronewtons, which are one-thousandth the size of of the already small units used for other ion engines. The engine has a thrust range of 0.1—150 micronewtons, with a resolution capability better than 0.1 micronewtons and a time response of one-fifth of a second or less, according to project engineers. The engine employs the 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 would be mounted on the hull of LISA Pathfinder. Working together with a separate NASA-designed propulsion system, the thrusters should yield directional control at least 100 times more accurate than any spacecraft before it—down to a millionth of a millimetre, project engineers assert. “We are overseeing the work here because we have previous knowledge of FEEP technology,” said Pierre-Etienne Frigot of ESA’s Propulsion Laboratory. LISA involves three satellites up to 5 million km (3 million miles) apart and linked by lasers, orbiting the Sun. They would form the single largest structure ever put in space by humans. The intention is to detect ripples in space-time known as gravitational waves, predicted by Einstein’s theory of general relativity but so far undetected from Earth. The waves would cause tiny variations in the distance measured between the satellites. Once 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.