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Guest Post: Daniele Bortoluzzi, “LISA and its Challenges” July 29, 2007

Posted by dorigo in astronomy, news, physics, science.
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Daniele Bortoluzzi (daniele.bortoluzzi@ing.unitn.it) is researcher in Mechanism and Machine Theory at the Department of Mechanical and Structural Engineering of the Faculty of Engineering at the University of Trento (Italy). Since 2001 he is part of the Principal Investigator team of the LISA Pathfinder mission at the University of Trento and in this framework he is now involved in the development of the Caging Mechanism Assembly (responsibility of Thales Alenia Space) with the responsibility to verify its compliance to a mission critical phase.   

LISA (Laser Interferometer Space Antenna) is a joint space mission of ESA (European Space Agency) and NASA (National Aeronautics and Space Administration) for the in-flight detection of gravitational waves. LISA consists in a formation of three spacecrafts orbiting around the sun and makes it possible to reveal such time-dependent distortion of the space-time by carefully monitoring the relative displacement between pairs of free-floating test masses hosted on board of different spacecrafts. Just to give you an idea, each mass will be five million kilometers far from the other and the requested measurement accuracy on the displacement is of the order of the picometer! 

LISA is classified as a “cornerstone” mission thanks to its outstanding scientific relevance. It would provide an instrument that may measure the fluctuations of the gravity, that is the fundamental force that rules the evolution of the universe since the Big Bang. For instance, this means the possibility to detect black holes, super-massive black hole mergers and spiral descent of stars into black holes. Moreover, it will make it possible to “listen” the gravitational echo of the Big Bang. Details of the mission may be found in the ESA (http://www.esa.int/science/lisa) and NASA websites (http://lisa.jpl.nasa.gov/WHATIS/intro.html).  

Due to the technological criticalities of this mission, prior to LISA a risk-reduction mission has been scheduled by ESA, called LISA Pathfinder (formerly SMART-2). This mission aims at providing experimental evidence that it is possible, in orbit, to set a reference mass in a status that results free from any disturbing force different from the gravity produced by the celestial bodies.If we think of any solid body in our familiar Earth environment, for instance a pen on our desk, we may be induced to assume that, except for the weight force (due to the Earth gravity field), no other forces are applied on it. This is wrong. The surrounding air carries acoustic waves that exert a fluctuating force on the external surface of the pen. Some electric charges sit on the dielectric parts of the pen and interact with stray electric fields that are present anywhere, and some metallic parts may hide a permanent magnetization that also would interact with magnetic fields. The desk is moved by the natural micro-seismic activity of the ground and transmits motion to the pen through the contacting surfaces. The result is that an inertial observer would see the pen not only orbiting around the sun together with the Earth, but also rattling a little bit across such an ideal trajectory. Even if this deviation may seem irrelevant, a gravity wave would produce a motion of the “pen-probe” far smaller. This means that the proof body that is devoted to sense the gravity wave must be accurately shielded from any non-gravitational force, and the goal of the LISA Pathfinder mission is to demonstrate that this is feasible to a level of disturbance that results negligible as compared with the expected action of the gravity wave. 

Let us now think of the LISA (or equivalently the LISA Pathfinder) mission, from the launch phase to the final in-orbit condition. A 2kg cubic Gold-Platinum test mass is hosted in a Gravitational Reference Sensor, inside a housing with millimetre – order surrounding gaps, that can not be reduced as any facing surface constitutes a potential source of disturbing force. Clearly, during the launch phase it cannot be left free to shake, otherwise it would damage the enclosing housing. A mechanism is being developed to firmly cage it against the launch vibration, by means of a holding preload in the range of hundreds of kilograms. In orbit, the test mass must be injected in free-floating conditions (no contact with the housing nor with any other thing) in an orbit that is not much different from that of the spacecraft, otherwise sooner or later it would touch the housing. This means that the contact with any caging device must be removed without inducing additional velocity to the test mass.

This might look like a trivial task, if we think that every day we grab and lay down objects that remain still, like the pen on the desk. However, in the Earth environment many effects are present that help us to break the unavoidable adhesive forces that arise at the contact between the object and our fingers. For instance, the weight of the pen and the friction force between the desk surface and the pen itself keep it still on the desk while we retract our fingers. This task becomes more difficult when we handle a strip of bi-adhesive tape and we want to throw it away from our fingers. The only solution is to stick it to another surface, because its weight is not enough to break the adhesive force with our fingers.

In the space environment, we have no gravity force to help us detach the test mass from the holding mechanism, while the adhesion force is still present between the contacting surfaces. Even if at much lower level, this situation is similar to that of the bi-adhesive tape, though we have no other surface to attach the test mass to. The solution that is pursued is similar to what we do when we want to get rid of something that sticks to our fingers but we do not want to attach it to something else: we shake the fingers until the inertia force pulls it away. With the LISA test mass, this strategy consists in quickly retracting the locking device from the contact, breaking adhesion by means of the tendency of the mass to remain still.

We understand that by following this procedure we cannot avoid imposing a small velocity to the released test mass, given by the action of the adhesive force during the rupture of the surface bonds, and the resulting orbit would again differ from that of the spacecraft. However, I can reveal now that we are not completely free of authority on the test mass: by means of an electric field, we may apply a weak force on it (a millionth of a Newton), that can help us to bring it back to the required orbit, that is at the centre of the enclosing housing. Unfortunately, this force is not enough to break the adhesive bonds (results a thousandth of them) and can only help us to stop the test mass after it has been released as described above.

Being LISA Pathfinder a technology demonstration mission aimed at assessing the possibility of achieving in orbit the free falling condition of a test mass, its task is also to demonstrate that an injection phase in free falling condition may be performed as proposed. A test mass that remains attached to the caging device or to the inner surfaces of the housing is not suitable for the measurement of gravity waves, and would jeopardize the entire mission. For this reason, it has been studied how to verify on ground the possibility of detaching a test mass from a locking device without imposing an excessive velocity, in representative conditions of the in-orbit environment (that is, reducing all the effects that in the Earth environment helped us to leave the pen still on the desk). This task implies a novel approach to the study of the adhesion between surfaces, that is the characterization of what happens when an adhesive bond is dynamically broken.

At the Department of Mechanical and Structural Engineering of the University of Trento (click on the image to see the video clip ) we have developed a facility aimed at studying these phenomena and understanding if the mechanism designed to release the LISA test mass to free floating conditions is compliant with the strict requirement for the successful injection in the final orbit. The video shows a view of our laboratory and the vacuum chamber for the release experiment.

The concept of the experiment (see the layout above with surrounding pictures) is to focus on the contacting surfaces of the test mass and the holding finger and reproduce on ground what takes place in flight. A little part of the test mass and the finger comprising the contacting surfaces are set in free-falling like conditions by suspending them through two thin vertical wires, realizing two simple pendulums. Even though the gravity force is present, it is balanced by the suspending wires and the two bodies result weakly constrained to the ground in the horizontal plane, in a dynamical condition similar to the absence of gravity. The mock-up of the holding finger is actuated through a horizontal wire, which minimizes the external perturbation to the contacting pendulums. Once the holding finger mock-up is retracted, the dynamic effect of the rupture of the adhesive force is made observable by the following free oscillations of the test mass mock-up.

The second video clip (click on the image to play) shows the induced oscillations of the test mass mock-up (on the left) by the retraction of the finger mock-up (the small rectangle held by two wires on the right). Notice that the revealed adhesion force arose just by setting them into contact without any preload. The facility is being updated to introduce this additional feature and larger force impulses are expected. Some interesting aspects of the adhesion between gold-coated surfaces are studied though this experiment, made visible by the dynamic of the phenomena (for instance, conservative and dissipative forces play different roles on the resulting force impulse and may be intentionally emphasized or reduced).

Acknowledgements are due to the Experimental Gravitation Group headed by Prof. Stefano Vitale of the Department of Physics and to my colleagues at the Department of Mechanical and Structural Engineering: Dr. Matteo Benedetti, Prof. Mauro Da Lio, Prof. Mario De Cecco, Dr. Paolo Bosetti, Dr. Ilaria Cristofolini, Dr. Francesco Biral, Prof. Roberto Oboe, Dr. Enrico Bertolazzi, Francesco Tondini, Dr. Luca Baglivo (CISAS Padova), Marco Lapolla (Thales Alenia Space). The activity is financially supported by ESA, INFN, Thales Alenia Space.

Additional details on the experiment of release of the test mass may be found in the following publications:

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Comments

1. Markk - July 30, 2007

Fascinating detail on stickiness issues. “A 2kg cubic Gold-Platinum test mass”? Is gold and platinum really needed for this test mass? Sounds excesive, although, say, $100K is minor in the overall cost, but I wonder why for an inert mass with a little interaction one needs such expensive stuff.

2. carlbrannen - July 30, 2007

My instinct for setting a ball floating a mm away from its housing in space, but to keep it from bouncing around during lift-off would be to surround it with something, like ice, that would stay solid during the whole trip, but in vacuum would slowly sublimate away.

3. daniele - July 30, 2007

Hi Carl,

this solution would be cheap and effective indeed, however a mechanism is required to grab and release the test mass in orbit for the high-force phase (launch), one for the grabbing and positioning of the test mass for the medium force phase (on ground or in orbit), one for the release. Each mechanism engagest the test mass on dedicated contact surfaces.

Daniele

4. dorigo - July 30, 2007

Hi Markk,

my no brainer guess would be that the mechanical properties of the chosen metal are optimal, but we should ask Daniele… I think the cost of the cubes is ridiculously small if compared with the overall cost of the mission, but I agree I would rather send iron cubes and keep the platinum ones in a drawer, other factors being equal.

Cheers,
T.

5. carlbrannen - July 31, 2007

Tommaso, I agree that using platinum and gold feels a little wasteful. A good reason for not using it, even if it is optimal, is to avoid bad press.

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7. Mauro Da Lio - July 31, 2007

As for what concerns the caging mechanism.
The idea of having some material that holds the test mass in place during launch and then vanishes in vacuum was indeed proposed, it looked smaryt but lately it was abandoned because: 1) there were concerns not to be able to get perfect clean surfaces (which are mandatory to avoid disturbance forces); 2) the system was asked to be able to recage.

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