Small Engine For the Big Job of Testing Theory of Relativity

Article written: 10 Apr , 2009
Updated: 24 Dec , 2015

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.

Source: ESA

16 Responses

  1. Jorge says

    This technology also seems very promising to build space interferometers, if I got the overall idea right.

  2. Jon Hanford says

    This technology seems very promising to build all sorts of space vehicles. Most science spacecraft require only tiny adjustments to their velocity or orientation, and these micro-sized, light weight thrusters seem to fill the bill when it comes to compact microthrusters for spacecraft.

  3. Andre Mestre says

    Imagine hwo cool will it be with a spacecraft having dozens of these dotted around it’s hull to provide precise manouvering!

  4. Don Alexander says

    If they had called it the Field Emission Electric Propulsion Light Engine…

    FEEPLE! 😀

  5. UKDave says

    Very impressive design work. Testing GR keeps requiring ever more imaganitive and brilliantly engineered experiments.

    A beautiful piece of work!

  6. Tyler Durden says

    Am I the only one who looked at that and saw a tiny warp nacelle?

    I really need to stop watching old Next Generation reruns, but it’s the only thing left on SciFi network that doesn’t suck now that Battlestar Galactica is over.

  7. Jon Hanford says

    @ Don Alexander, maybe NASA should consider you as head of its’ acronym division. I really like your idea of the FEEPLE drive and can’t wait to see FEEPLE integrated into our next-gen astronomy satellites.:)

  8. Jorge says

    This ain’t NASA. It’s ESA. Europe, not North America.

  9. Bravehart says

    This Ion engine is only half the story?
    What will be the real story is getting five sat’s
    up there in line with the space curvature.
    From what I do understand from the L1 location is that it really is not a point where
    two elipse though each other? It really is a space between the overlapping curvatures.
    Since this is an area in 3D and it is in constant flux, where do they line up these satelites?
    Is there anyone that can explain how this
    can work? What I’m looking for is the astro phisical data? The Ion engine can be made
    even smaller and have better trust than the
    one mentioned here! It is the material that is used as fuel, which determine the agility of the response. We worked on this for a long time, it is dishartening to see so little progress on this!

  10. Treebeard says

    What do gravity waves do for String Theory?

  11. Olaf says

    What do you mean Treebeard ?
    These gravity waves are large scale gravity waves. It is part of the theory of relativity.

    String theory is more closely related to quantum mechanics, and is not proven yet.
    According to string theory gravity does not originate in this dimension but close to this dimension (it is complicated to explain). And string theory suggests that if we are lucky, then we could measure the gravity within 1/10th of a mm and should not be the inverse square law.

  12. Olaf says

    Treebeard, you have to look at this way.

    Even though gravity in string theory, relativity theory and newtonion are one and the same, depending on the scale you look some formula’s work best.

    Like scales about our solar system, the Newtonian formulas work like charm, simple to use and you can actually predict the location of the planets, the comets and send space craft to them with just the basic Newtonian formula’s.

    But when you come close to very high speeds, or very big gravity well’s then it is better to use Relativity theory gravity models. These formula’s are more complex but more accurate than Newtonian formula’s.

    At the very small scale, this gravity an be ignored since it is the weakest force of the 3 others so you can ignore it’s effect. For example, an electron moves around a nucleaus but it is not there because of the gravity attraction at all. Electro magnetic forces are too strong here to have any measurable effect on the attraction by gravity.

  13. Astrfiend says

    Awww… Isn’t it CUTE! I want one.

  14. Lawrence B. Crowell says

    A gravity wave in string theory is a heterotic E_8xE_8 or SO(32) string coincident (parallel motion in the bulk) with the 3D-brane. On a target map the image is spacetime, or the universe if you will.

    What these tiny motors are used for is to keep the mirror in the LISA interferometer on pure geodesics. Any force due to solar wind or some drag force pushes the spacecraft on a path which is not purely determined by curvature. So you have to detect this nongeodesic motion (accelerometers) and compensate for that with this little engines. A techonology analogue might be adaptive optics for telescopes.

    A millionth of a millimeter is 10 times smaller than the diameter of an average atom! This is a tricky thing to accomplish. With LIGO similar tolerances are acheived with adaptive-like optics. The laser light is in a squeezed state where the quantum uncertainty in position is near zero. In this way the small quadrupole motion induced by a gravavity wave can be measured. Those mirrors have to be very cold, no thermal flutuations, and all nongravitational motion must be removed.

    Lawrence B. Crowell

  15. Treebeard says

    Thank you Olaf and Lawrence. My understanding is far below Lawrence’s first paragraph, but somewhat above Olaf’s reasonable assumptions of his audience. I guess my question was, does String Theory account for gravity waves? It sounds like the answer is yes. Olaf, you said: “String theory is more closely related to quantum mechanics, and is not proven yet.”, but certainly it seems fair to say that no one would be working on String Theory if they couldn’t bring gravity into the fold (no pun intended). Thus, if the LISA project is yet another test for Relativity is it also a test for String Theory given that there are currently so few tests for String Theory (like, none with the LHC down, right)? If no gravity waves are found, besides the blow to Einstein’s perfect record, would it also hurt String theory. Again it sounds like the answer to this is yes.

  16. Lawrence B. Crowell says

    String theory is a bit strange with respect to gravity. It imposes a Ricci curvature on top of a flat background. General relativists find this distrubing, and frankly it is not entirely satisfactory to me. However, it appears that the universe is flat, and so cosmology might be telling us that this is not too bad. The curvature imposed on top of this flat background does reproduce the graviton pretty well (a quantized gravity wave) and in a certain sense the black hole as well. The stringy black hole is what is termed BPS, which means it is built up with lots of stringy gauge terms. Some people don’t like this, but the idea is suggestive or close enough that it is at least a reasonable approximation.

    String theory gets into M-theory of p-branes. Strings are 1-branes or one dimensional parameterized fields. Well as you can imagine this goes up dimensions to 2-branes, 3-branes and so forth. The spatial surface of the universe is mapped to a 3-brane. A type of 3-brane is a black brane, which has a black hole. The structure is a bit curious for it leads to a T duality symmetry between an an anti-de Sitter spacetime and conformal field theory. This is a major result, which gives some sense that string theory is at least smewhat reasonable.

    The loop variable guys might object, and frankly there are things to be said for that approach. I suspect string theory and loop theory are looking at the problem through different “windows,” and a greater synthesis of things might be in store.

    Lawrence B. Crowell

    Lawrence B. Crowell

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