experiments on fundamental physics on the space station

Class. Quantum Grav.14 (1997) 2971–2989. Printed in the UK PII: S0264-9381(97)80809-X

Experiments on fundamental physics on the space station∗

A Spallicci† 3, A Brillet‡, G Busca§, G Catastini‖ 4, I Pinto¶, I Roxburgh+,C Salomon], M Soffel1 and C Veillet2 5

† European Space Research and Technology Centre ESTEC, Noordwijk of the European SpaceAgency ESA, Paris, France‡ CNRS, Orsay, France§ Observatoire de Neuchatel, France‖ Dip. Matematica, Gruppo Meccanica Spaz., Pisa University, Italy¶ Gravitation Research Group, DIIIE, Salerno University at Benevento, Italy+ Astronomy U., School of Mathematical Sciences, Queen Mary and Westfield College,London, UK] Ecole Normale Superieure, Paris, France1 Techn. University of Dresden and Lohrmann Observatory, Germany2 CERGA, Observatoire de la Cote d’Azur, Grasse, France

Received 30 December 1996, in final form 11 September 1997

Abstract. Original proposals and experiments on gravitation and fundamental metrology onthe space station are described. These experiments were formulated in the Metrology andGravitation Science Team, in two ESA industrial study contracts, on microsatellites and ontime and frequency science, within the space station scenario. Although limited by the designconstraints of the space station, the experiments range from clock-based tests on special andgeneral relativity to, with additional infrastructure, the equivalence principle and the detection ofgravitational waves. Supporting technology, such as damping systems and microgravity cooledatom clocks, is also described. Finally, the major scientific goals, the experiments, hardware andthe status are summarized. This work represents the first coordinated attempt, at least withinthe European space programmes, to consider experiments on relativity and fundamental physicswithout resorting to experiment dedicated space missions. For details on specific issues a largebibliography is referred to.

PACS numbers: 0480, 0420, 9530, 9555, 0230

1. The international space station

The international space station (ISS) (table 1) has been promoted as a multi-disciplinaryscientific research institute in space for fundamental and applied research in low Earth orbit(LEO) (ESA D/MSM 1995, Larter and Gonfalone 1996). Future facilities may include theassembly, launch and retrieval of small satellites (Aleniaet al 1995); liquid helium servicing(Breon 1988); a drop tower (Reynolds unpublished, Everitt 1986), consisting of an evacuated

∗ Presented at the Symposium on Fundamental Physics in Space, 16–19 October 1995, London. For the otherpapers refer to: 1996Class. Quantum Grav.13 (11A).3 Now at: Salerno University at Benevento, Faculty of Engineering, Gravitation Research Group, E-mail:[email protected] and Kamerlingh Onnes Laboratory, Department of Physics and Astronomy, Faculty ofMathematics and Natural Science, University of Leiden, E-mail: [email protected] With acknowledgements to A M Nobili (Pisa).5 Now at Canada–France–Hawaii Telescope Corporation, Hawaii.

0264-9381/97/112971+19$19.50c© 1997 IOP Publishing Ltd 2971

2972 A Spallicci et al

module, e.g. a Shuttle tank, in which freely floating experiments are run; tether systems forgravitational waves detection (Braginsky and Thorne 1985) and relativistic frequency shifts(Stalio and Shvartsburg 1993).

The disadvantage of ISS with respect to dedicated missions is obvious, as ISS placesconstraints on mission requirements, and thus on the range of scientific experimentation.Nevertheless, there are several reasons which lead one to consider utilization of ISS forfundamental physics: the ever tighter budget for pure science missions; the cheaper accessvia ISS to space as opposed to dedicated missions; the ESA historical record (in the field offundamental physics, only externally funded or low-cost experiments have been approved†);the opportunity of tests of technologies on ISS in view of future dedicated missions; theadvantages offered by maintenance and servicing of the experiments, unique feature of ISS;the possibility of gradual growth in hardware and in project experience of the European spacefundamental physics community, thereby avoiding the risks associated with the launch oflarge and ambitious projects without previous small–medium programmes; the competitionwith traditional space science communities, which play a significant role in the selectioncommittees.

The research interests in the field of fundamental physics within ESTEC, the intentof enlarging the spectrum of disciplines within the space station programme and theproposition of experiments by the outside scientific community were the grounds for newspecific activities at ESA since 1991 (Spallicci 1993, CMGST 1991a, b), together withthe recommendations of scientific bodies (SSD 1988, Blaser 1994, ELGRA 1995), a largenumber of references (Davies 1986, Everitt 1986, Friedman 1984, Gursky 1986, Naugle1973, Reynolds 1989) and the planning of a large experiment such as the antimatterspectrometer (AMS 1994) in the NASA programme. Finally, the importance of fundamentalphysics for ISS was recognized and officially included in the ISS payloads manifest (ESAD/MSM 1995, ESA 1995).

Table 1. Details of the ISS (ESA D/MSM 1995).

Dimensions 108× 74 mMass 415 tonnesElectrical power 110 kW (46 kW for research)Total pressurized volume 1140 m3

External payload surface > 50 m2

Orbital altitude 335–460 kmdepending on solar activity and dragreboosting every 90 days

Orbital period ≈ 90 mOrbital inclination 51.6◦Attitude deviations 5◦ axis−1 absolute, 0.02◦ axis−1 s−1 rateMicrogravity 10−6g up to 0.1 Hz

10−5g × f for 0.16 f 6 100 Hzwith active crew below 10−3g

Data transmission uplink 72 kb s−1, downlink 32 Mb s−1

Two classes of experiments were identified: time and frequency, and mechanical. Fortime and frequency experiments some extra hardware may be needed for accurate timeand frequency transfer links. Instead, for mechanical experiments a very low level of

† Ulysses gravitational waves experiment (Bertotti 1983) and CRONOS, originally proposed for M3 by Buscaetal (1993), and subsequently approved by ESA as a mission of opportunity on the Russian satellite Radioastron(see also Buscaet al 1995, 1997).

Experiments on fundamental physics on the space station 2973

noise (unwanted accelerations) is required; since ISS has a noisy environment, such a goal,accomplished with drag-free systems on dedicated missions, requires some extra hardware,such as a noise attenuator, a drop tower or even a microsatellite in the vicinity of ISS.

The following sections describe the experiments with reference to the activitiesundertaken at ESTEC in which they originated.

2. The Columbus metrology and gravitation science team

The team (Brillet, Busca, Fuligni†, Nobili, Roxburgh, Chairman, Spallicci, ScienceSecretary) operated in 1991–1992 (Spallicci 1993). The main propositions were PGB andNEWTON (Nobili), GRAVCON (Roxburgh) and dual maser (Busca and Vessot). The teamproduced several short publications (e.g. Spallicciet al 1993, Spallicci and Busca 1996),a draft final report (Roxburghet al 1992), two proceedings (CMGST 1991a, b) and itsactivities were reported by Bertotti (1994). The final recommendations of the CMGSTwere:

(i) For mechanical experiments, the development and test of a noise attenuator system ismandatory.

(ii) Time and frequency experiments are promising on ISS.

2.1. Picogravity box

A noise attenuator, picogravity box (PGB) or other design, is essential to mechanicalexperiments (e.g. NEWTON, GG, GRAVCON). PGB (Nobiliet al 1991a–c, 1993b, for acomparison with ESA MGIM system see Catastiniet al 1992) is a passive (possibly multi-stage) noise attenuator of compact design that can cope with the small acceleration levels inorbit. The attenuators have to sustain a ‘weight’ ofm×adrag/g, whereg = 9.8 m s−2, suchthat 100 kg requires a very soft spring, and thus limits the dimensions of the apparatus.

PGB, based on the same principles as the seismic noise attenuators in bars andinterferometers used for gravitational wave detection, is made by simple mechanicaloscillators which reduce noise on all the degrees of freedom. A box is appended atone end of the attenuator, allowing a small laboratory inside the box, where the noiseabove a threshold frequency (i.e. 0.001 Hz) is reduced below 10−12gHz−1/2, and the localgravitational acceleration is cancelled out.

The feature of a passive attenuator of reducing the noise above a frequency thresholdis not affected by the spectral distribution of the noise in a spacecraft, which is not peakedat low frequency like Earth seismic noise.

The suspension reduces noise and damps the noise peaks in correspondence to the naturalfrequency (the harmonic frequency and all the other proper frequencies) due to its lowQ.The threshold frequencyνt of the attenuator is about

√k/mpl wherek is the equivalent

spring constant of the suspension, andmpl the mass of the attached payload.The orbit flight test would require an accelerometer located inside the PGB and a second

accelerometer for the transfer function. The PGB could be directly connected to emptyspace to obtain a good level of vacuum (no pressure disturbances act on the experimentaldevice inside the suspended laboratory), while internal accommodation allowing access tothe experimental apparatus would require vacuum pumping.

The concept is very simple, but the promised performances appear exceptionally high.Since a noise attenuator is essential for the success of mechanical experiments, and is of

† Prematurely deceased in the summer of 1995.


potential benefit to a larger number of disciplines than fundamental physics, a space flighttest is considered mandatory.

2.2. NEWTON

NEWTON consists of a small ‘planetary system’ in orbit (Nobili 1989, Nobiliet al1989, 1990), with a test mass of 1 kg orbiting around a central mass of, e.g., 100 kg. Theexperiment is aimed at the determination of the universal constant of gravityG to 10−5 viapre-launch masses weight determination and masses separation measurements via a telescopelooking inward to the payload.G is the least known of the fundamental constants: today’saccuracy is of the order of 10−3.

The experiment, if upgraded with a laser measuring system, could also test theequivalence principle (EP) at short range (Spallicci 1990) up toα = 10−5.5 for λ = 10 cm,whereα is the exponent coefficient e−αr/λ, measuring deviation from the Newtonian potentialGM/r andλ represents the range of action of the deviation.

The experiment could be performed inside a small satellite launched from ISS, butthe gravitational scattering due to tidal forces (Farinellaet al 1987) would complicate thedata analysis considerably. Thus the requirement of a geostationary orbit, where close orbitswithin a sufficiently Roche lobe of the primary mass are possible, seems to place NEWTONout of the ISS scenario.

2.3. GRAVCON

GRAVCON is an experiment to measure the universal constant of gravityG to 10−5

(Roxburghet al 1989). The principle of the experiment is to measure the periodic variationsin the difference between the accelerations induced by a rotating asymmetric body of knownmass and dimensions, using a gradiometer sensitive in at least one axis. The rotating massescould be a bar or dumbbell; in order to have a source with net zero angular momentum (toeliminate disturbances to theµg environment), two counter-rotating dumbbells or bars needto be used. Since a rotating source will give a signal with a known frequency, the inducedaccelerations can be more easily separated from background noise.

Considering a dumbbell consisting of two spheres each of massM = 100 kg, ofdensityρ = 20 g cm−3, and separated by 40 cm, the masses rotate about an axis throughtheir common centre of mass. The acceleration, induced by one of the masses on anaccelerometer at a height ofrb = 15 cm directly above its centre of mass, is of the orderGM2/rb = 3 × 10−5 cm s−2. The acceleration on the accelerometer from the rotatingdumbbell varies with a frequency of twice the rotation period of the dumbbell and has anamplitude of this order.

To determineG to an accuracy of 10−5, the acceleration difference between thetwo accelerometers has to be measured to 10−10 cm s−2, which appears within thegoal for the sensitivity of the electrostatic GRADIO instrument for the ARISTOTELESmission (Aguirre-Martinezet al 1992) and comparable to the sensitivity goal of theaccelerometers in the gradiometer under development at CNR-Frascati and elsewhere(Onera, Stanford).

One way to solve the problem of the acceleration noise within ISS would be to placethe experiment inside a PGB or other noise attenuator. An alternative solution would beto place the experiment in a drag free environment (i.e. a small satellite), or perhaps to‘float’ the experiment inside a flying drop tower. GRAVCON could also be targeted at ashort-range equivalence principle test.


2.4. Maser clock experiments

The proposal, originally for EURECA, was to place two H-masers†, one from Neuchateland the other from Harvard–Smithsonian with a laser and microwave time and frequencytransfer systems (Busca 1991a), for independent evaluations of both masers, and of bothtime and frequency transfer systems.

2.5. Relativistic measurements

A circular LEO orbit is not adequate for time dilation and gravitational shift measurements,but is optimal for measuring the first-order Doppler asymmetry term and the Sagnac effect(CMGST 1991b, Spallicci and Busca 1996). These measurements could be carried out withmasers, cooled atom clocks or other advanced clock technologies.

2.5.1. Doppler asymmetry.A satellite at positionr1 with velocity v1 emits a signal (f1)towards Earth. The signal (f1g) is transponded by a station (rg, vg) and then received (f2)by the same satellite (r2, v2). The measured shift frequency (ECI) is:

1f = (f1g − f0)− 12(f2− f1) (1)

wheref0 is the ground station clock frequency,f1g − f0 is the one-way measurement atthe ground station andf2− f1 is the two-way measurement at the satellite and transmittedto ground station. A one-way measurement implies that the transmitter and receiver useindependent clocks. This technique is sensitive to relative time fluctuations between the twoclocks. Instead, for a two-way measurement the transmitter signal is coherently transpondedback to the transmitter, at a different frequency, by the receiver. The latter technique issensitive to time fluctuations accumulated by the clock during propagation. The relativefrequency shift is:

1f

f1= f1g

f1− δff1− 1

2

f2

f1− 1

2(2)

whereδf is the offset between the satellite clock and the ground station clock (f0 = f1+δf ).The frequencyf1g of the satellite signal at the ground station is:

f1g

f1= 1− βg · r1g − φg/c2+ 1

2β2g

1− β1 · r1g − φ1/c2+ 12β

21

(3)

whereβ = v/c andφ ' −GM/r is the gravitational potential. The ratiof2/f1 is obtainedfrom (f2/f1g) × (f1g/f1). Assuming negligible change of altitudeφ1 ' φ2 and negligiblechange of velocity|β1| ' |β2|, up to second order, the relative frequency shift is given by

1f

f1= D2+DA1+DA2+GS − δf

f1(4)

where the time dilation is:

D2 = 12

(β2g − β2

1

)(5)

† An H-maser is based on the hyperfine transition of atomic H at 1.42 GHz; the beam of H atoms passes througha magnetic selector and those (> 1012 atoms s−1) of the selected energy state sustain the permanent oscillation.


the first-† and second-order‡ terms in 1/c Doppler asymmetries (DA) are:

DA1 = 12

[ (βg − β1

)· r1g −

(βg − β2

)· rg2

](6)

DA2 = 12

[(β1 · r1g)

2− (βg · rg2)2− (βg · r1g)(β1 · r1g)− (βg · r1g)(β2 · rg2)

+ (βg · r1g)(βg · rg2)+ (βg · rg2)(β2 · rg2)− (βg · rg2)(β1 · r1g)

− (β1 · r1g)(β2 · rg2)]

(7)

and the gravitation shift is given by

GS = φg − φ1

c2. (8)

The precise quantification of the contributions (5)–(8) necessitates the specific locationof the ground station and it depends on the changing altitude of ISS. The contributions are,for a possible case, shown in table 2, with the exception ofDA2, which is too small for theISS orbit and clock accuracy.

Table 2. Comparison between ISS and GP-A.

ISS GP-A

Time dilation 3.5× 10−10 3× 10−10

First-order Doppler asymmetry 6× 10−10 2.2× 10−12

Gravity shift 9.5× 10−11 4.5× 10−10

On GP-A (Vessot and Levine 1979, Vessotet al 1980), due to the ballistic orbit and theuse of the satellite clock as the transponder,DA1 was smaller. Instead, in a circular LEO,it shows a high dynamic change and it deserves consideration for potential measurements(isotropy of light, frame dragging). In circular LEO,GS andD2 are nearly constants;this situation differs greatly from the GP-A experiment where they significantly changedmagnitude, again due to the ballistic orbit.

The first-order Doppler asymmetry term measurement accuracy is 1×10−15/6×10−10 '2× 10−6.

Third-order terms. The third-order terms§ originate from mixed, classical and relativistic(third-order purely relativistic terms, such as(v/c)3 or

√φ

3, do not appear in this

formulation) and purely classical contributions. The detailed expressions have been derived,but an evaluation of theorder of magnitudeis adequate for this paper. It is deduced byposing some geometrical simplifications asr1g = −rg2 and |β1| = |β2|. For the mixedterms we find:

− 12

(β2g − β2

1

) (βg − β1

)r1g − φg − φ1

c2

(βg − β1

)r1g (9)

† Jimenez, a Spanish graduate trainee at ESTEC Utilisation Office, showed the relation between the GP-A approachand this formulation (Jimenez 1991). Later, correspondence with one of the referees of this paper brought us to theconclusion that the fractional output frequency variation (Vessot 1991) does not include the second-order Dopplerasymmetry term and that the term(r1g · a)/c2 therein corresponds to the first-order Doppler asymmetry termherein.‡ DA2 herein has been calculated exactly without previously used geometrical simplifications (CMGST 1991b,Spallicci and Busca 1996).§ Acknowledgements for checking the third-order calculations are due to P Cicchiello, F Lauro and M de Luca,students of the numerical analysis course at Salerno University at Benevento.


while for the purely classical terms:

+(βg · r1g)3+ (β1 · r1g)

2(βg · r1g)− 2(βg · r1g)2(β1 · r1g). (10)

The third-order terms (9) consist of two main terms: one is the product of time dilationand first-order Doppler, the other of gravitational shift and first-order Doppler. If suchhigher-order effects are visible, they will anyhow not contribute to scientific insight as eachis a relativistic sub-term visible at lower order. It is worthwhile considering such effects incombination with an analysis of the tropospheric and ionospheric errors and especially inthe case of the utilization of more accurate clocks.

The terms (10) originate from the third-order Doppler asymmetry term and thus aresmaller thanDA2, which is already not visible.

2.5.2. The Sagnac effect.The Sagnac effect is a special relativistic effect due to finite lightvelocity in rotating frames. It is given by

1t = 2

c2ω ×A (11)

whereA = 12rA ∧ rB is the area swept out by the radius vector from the Earth’s centre to

the electromagnetic pulse as it propagates from the point of transmissionA to the point ofreceptionB, andω is the Earth’s rotational angular speed.

The measurement would be carried out as follows. A ground station at a defined time,i.e. the occurrence of the zero of the one-way Doppler shift, makes a measurement of thetime offset and frequency offset between its clock and the space clock and appliesall therelativistic corrections, except the Sagnac effect. One orbital period later, again at the zeroof the one-way Doppler shift, the same comparison is made. Assuming an equatorial circularorbit, a spherical Earth moving at constant angular velocity, example data are provided intable 3.

Table 3. The Sagnac effect.

Light time from station to satellite 2× 10−3 sLight time for a full circle around the Earth 1.462 509× 10−1 sLight time from satellite to station after one revolution 2× 10−3 sTotal light time 1.502 509× 10−1 sMotion of the Earth station during light time 69.881 88 mSagnac effect 233.1008 ns

For a time error of 23 ps, the Sagnac effect accuracy is 2.3×10−11/2.33×10−7 ' 10−4,about two orders of magnitude better than currently obtained with GPS (Allan and Weiss1985). The effect of the satellite velocity is a large difference between the times measuredon the ground, the difference being(2r⊕ωv + v2)t/2c2, wherer⊕ is the Earth radius,v thesatellite velocity,t is the time of circumnavigation (about 90 minutes). This contribution isof the order of 1.6× 10−6 s and must be deducted for the calculation of the, one order ofmagnitude smaller, Sagnac effect of the Earth.

2.5.3. Relativistic corrections on a GPS space receiver on a LEO spacecraft.Each GPSunit’s clock is slowed by 4.465× 10−10 for time dilation and gravitational frequency shiftbefore launch. Terrestrial users have only to process the GPS units’ orbital eccentricities, asinusoidal type function.


For a spacecraft, including ISS, its LEO velocity and its location within the Earthgravitational field determine a largely different effect than for terrestrial users (Spalliccietal 1992). The effect of drag and the orbital eccentricity of the spacecraft further complicatethe in-orbit processing.

3. Time and frequency study

ESTEC initiated in 1995 a study on time and frequency multidisciplinary applications forISS (DASA 1995). The external scientists involved in fundamental physics were Salomon,Soffel and Veillet and the major conclusions were:

(i) Confirmation of CMGST findings in terms of time and frequency experiments feasibilityand measurements;

(ii) Recommendation of the cooled atomic clock;(iii) Proposition of ACES, an ensemble of clocks.

Finally, there has been an additional suggestion, based on ACES. For the gravitationalshift defined by:

1f

f= 1+ α1φ

c2(12)

and measured with two different clocks:1fa

fa− 1fb

fb= 1α1φ

c2(13)

a null test could be performed, see the work by Soffel (DASA 1995). Improvements shouldbe obtained with respect to the actual best upper limit on1α = 4× 10−5 (Vucetich etal 1988), achieved with a differential Mossbauer measurement. Soffel (DASA 1995) alsoconsiders Doppler tracking of ISS for gravitational waves detection, the Shapiro effect andthe issue of reference frames, but a promising experiment has not been conceived. Thelast topic, together with the measurement of the Earth spacetime curvature, was analysedpreviously (Bernacca and Lattanzi 1989, de Felice 1989, White and Lestrade 1989).

3.1. Cooled atom clock

The deflection of atoms by light was experimentally observed sixty years ago by Frisch.Ashkin raised the possibility that radiation pressure from laser light could be used totrap atoms. The physical principle that underlies the laser cooling mechanism is theDoppler effect. A laser retro/reflector combination is arranged such that the moving atomis illuminated from both sides with counter-propagating beams. The frequency of the laseris slightly detuned below the resonance frequency of the atom. For the Doppler effect, thecounter-propagating wave gets closer to resonance and exerts a stronger radiation pressureon the atom than the co-propagating wave. The result of that force is the decrease of thevelocity of the atom, as if the atom was moving in a viscous medium (optical molasses).

Caesium clocks have the best long term stability (10−14, from 103 s to several years)and accuracy, but caesium clocks using laser cooled atoms could operate with atoms attemperatures in theµK range, allowing a 100-fold improvement in the observation timewith respect to conventional 400 K caesium clocks. On Earth, a gas of cold caesium atoms ata temperature of 2.5µK, corresponding to an rms velocity of 12 mm s−1, allows interactiontimes approaching 1 s, when using an atomic fountain geometry. As the atomic resonancelinewidth is inversely proportional to the interaction time, an improvement of two orders of


magnitude is obtained. The resolution increases with the timeT spent by the atoms abovethe cavity, i.e. the square root of the fountain height. Beyond a 1 s fountain time, it requiresexpensive and bulky apparatus.

In a reduced gravity environment, a measurement time of several seconds can beenvisaged using a smaller and simpler apparatus, leading to a factor of 10 increase ininteraction time and clock performances. Not only is the long term stability of fountainclocks and microgravity clocks very good (10−16 d−1), but also the accuracy (1016 range) isimproved. The collisional shift scales asT −1/2 and, due to the constant velocity of atomsin a microgravity clock, it is much easier than on Earth to increase the duty cycle of thefountain by preparing successive clouds of atoms filling the space between the cold atomsource and the microwave cavity. This enables one to increase the signal-to-noise ratio.

Proposals to ESA were submitted (Busca 1991b, Salomon 1991, Claironet al 1992,Louniset al 1993, Laurentet al 1995) and developed up to the state of the art (see Salomonin DASA 1995, Salomonet al 1996, Busca and Thomann 1996).

The observation strategy should be carefully analysed. Because of the low orbit ofISS, the useful observation time by a given Earth station is limited. For a 1 ps stabilityof a specially designed transfer system over this time, it allows a frequency comparison of10−14 per path. Averaging 10 successive and independent measurements per day gives anuncertainty of 3× 10−15 per day, a factor 30 short of the expected 10−16 cold atom clockstability, but nevertheless it is an interesting and unexplored domain. Perhaps, the very lowdrift rate of ISS might allow better averaging strategies.

Concerning sources of noise, residual acceleration can be divided into two components,one parallel to the atomic drift motion between the source and the microwave cavity and theother orthogonal to the beam tube. The latter amounts only to a loss of atoms in the detectionregion, i.e. an amplitude fluctuation of the signal. Accelerations greater than 2× 10−6 g inthe frequency range of 5× 10−2–10 Hz may begin to degrade the clock performances bychanging the number of transmitted atoms by more than 20%. Instead, the former affectsthe transit time of the atoms in the microwave cavity and therefore enters the linewidth ofthe caesium resonance: it is equivalent to frequency noise of the microwave source. Fortypical operating conditions (velocity 5 cm s−1, transit time of 5 s in the cavity), the residualparallel acceleration must be less than 2× 10−5g in the range 5× 10−2–10 Hz.

First experiments on laser cooling of atoms in microgravity were recently performed,while PHARAO (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite)is under development at CNES. It involves ENS (Ecole Normale Superieure), LPTF(Laboratoire Primaire du Temps et des Frequences) and LHA (Laboratoire de l’HorlogeAtomique). The objectives of an experiment on ISS are also:

(i) demonstration of a clock running with laser cooled atoms in microgravity conditions andthe determination of performances (better than 10−13 τ−1/2 and for one day of operationthe stability better than 3× 10−16), corresponding to an absolute time fluctuation of30 ps. This stability would be two orders of magnitude better than present caesiumclocks;

(ii) relativity tests;(iii) preparation for future missions.

3.2. T 2L2 time transfer by laser link

T2L2 (see Veillet in DASA 1995, Thomaset al 1994) is a possible optical link between ISSclock(s) and the ground. A space clock must be compared to a ground time scale for:


(i) qualification of its performances (the reference clock could be on board, but it wouldnot be considered reliable unless a sure way of interrogating it from the ground in nearreal time was established);

(ii) provision of a time scale to potential users in the world, imposing time transfer betweenthe satellite and the Earth;

(iii) measurements in space, since when the scientific data are represented by frequencymeasurements, a reference in a different location, the Earth generally, and thus a timeand frequency transfer is needed for clock comparison;

(iv) operational needs and thus the performances must be checked by comparison with areference, generally on the ground.

Both time and frequency can be transferred between two clocks. Frequency is transferredby monitoring the phase of two clocks, one on the ground. However, such a continuousmonitoring would require a continuous view of the flying clock from the ground clock, whichis not applicable to ISS orbit. From one pass to the next, the phase is lost which impliestime transfer rather than frequency. But for time metrology, the satellite and the groundstations must have similar equipment: a laser detector and a timer record the departure,arrival or return time of a laser light pulse on a time scale provided by the associated clock.A given laser pulse will then permit three timings: the start at the ground station, the arrivalon board and the return back to the ground. A telescope is needed on the ground fortransmitting the light, and a retroreflector on ISS for returning the light back to the ground.The most critical part of the experiment is the calibration of the ground stations.

T2L2 (Fridelance 1994) is a new generation of optical link, and a continuation of LASSO(laser synchronization from the stationary orbit), flown on a geosynchronous satellite,Meteosat-3. It is presently under study by CNES for providing a link between the flyingcooled atom clock and the ground with a 10 ps precision, and an accuracy better than 50 ps.A ground version of the event timer is already completed and is used operationally at theLunar Laser Ranging Station at CERGA.

3.3. ACES atomic clock ensemble in space

The characterization and comparison of three different clocks (see the work by Salomonand Veillet in DASA 1995, Salomon and Veillet 1996), H-maser, cooled atom clock andtrapped ion clock, would bring an invaluable insight to their behaviour and capabilities.

The second goal is to provide, through radio and optic links, an unique flying time scalebased on an ensemble of very stable clocks, on a worldwide basis. Relativity tests couldalso be carried out, especially a null gravitation shift.

The components could be tuned from the ground allowing the modification of somecharacteristics and on-board stability tests. The two links would also provide an externalcheck by comparison with ground clocks.

The duration of the experiment should be at least one year.

4. Microsatellite study

ESTEC initiated in 1995 a study on microsatellites, including small satellites, within an ISSscenario, i.e. assembly, launch and retrieval with ISS†. The external scientists involved in

† For the mission engineering as orbital mechanics, project design and for experiments in earth observation, spacescience and technology see Alenia (1995).


fundamental physics were Catastini, Iess, Nobili and Pinto. The major conclusions of thisstudy were:

(i) future utilization of microsatellites in the ISS scenario is a viable option when payloadrequirements cannot be accomplished by internal or external accommodation with ISS;

(ii) launch of small satellites, already performed on MIR, is a small step toward the originalambitious ISS goal as a station for interplanetary exploration.

4.1. ODF—optical drag free

The aim of ODF (see the article by Catastini and Nobili in Alenia (1995)) is thedemonstration of the feasibility of the compensation of non-gravitational forces down toa level of few 10−10 cm s−2, by means of the cylindrical symmetry of the spacecraft, anoptical device and FEEP (field emission electric propulsion) thrusters covering a large rangeof forces from less than a 0.1µN to tenths of mN with a negligible mass of propellant (liquidCs).

ODF is a small satellite with azimuthal symmetry to give a favourable behaviour of thegravitational field of the spacecraft inside itself. The test mass is kept close to the centreof mass, which is also the geometrical centre of the satellite, and thus it is freely falling inthe gravitational field of the Earth at the level of the residual non-gravitational accelerationof ODF. By control with an optical device, the test mass is kept close to the centre of massup to 2–3 cm distance and thus the only source of noise is the gravity field of ODF itself.

The sphere is optically marked and its motion with respect to the centre of mass of thespacecraft is observed by cameras and an on-board computer, giving input to the controlsystem of the FEEP. The drag free control must reposition ODF with respect to the testmass before its gravitational perturbations becomes bigger than a few 10−13g. It requiresabout 100 s to move 1 cm away from the centre of mass of the spacecraft. This conceptcould be used for geodesy and aeronomy studies.

Although a bigger spacecraft allows a better level of drag compensation and makes thecontrol system easy, a small satellite can also be used where the constraints on the volumeare very strict. Finally, the advantage of ODF in flying in a ISS scenario as a microsatelliteis mainly related to servicing, retrieval and assembly.

4.2. GG—Galileo Galilei

The aim of GG (see Catastini and Nobili in Alenia 1995, Bramantiet al 1992, Catastiniet al 1996, Nobili et al 1993a, 1994, 1995, 1996) is the test of the equivalence principle.The mission performed within the ISS scenario also allows a short-range test (< 50 km) inwhich the gravitational source is ISS itself. The measurement accuracy ofη, the Eotvosratio†, would be around 10−16, the same as the long-range test (' 6700 km), where theEarth is the reference. Both would give improvements with respect to ground results (validat 1 AU range, the Sun being the reference mass) of about four orders of magnitude.

The technological goals are the testing of: (i) the FEEP thrusters for accurate dragcompensation; (ii) the capacitive sensor developed for relative displacements of 10−10 cm;and (iii) the supercritical rotation in space.

GG is low weight, one axis stabilized (5 Hz spin frequency) and it does not requireany active pointing. The experiment is designed to detect a differential force, due to

† η is a parameter whose magnitude indicates the different behaviour between two test bodies of their gravitationaland inertial masses.


a violation of the equivalence principle, acting between two cylindrical, concentric testmasses, made of different material, which are suspended with springs inside the PGB. Thecapacity read out of the experiment can detect a differential force (for the long-range test)F = mgη = 10−10 dyn wherem = 10 kg is the mass of the test bodies,g = 840 cm s−2 isthe gravitational acceleration at 500 km altitude,η = 10−16 is the smallest value that canbe measured as a deviation from the equivalence principle.

The main point of the experiment is that the test masses are in rotation, since thesatellite has a 5 Hz frequency spin. In the case of the long-range test, the signal of apossible equivalence principle violation appears at this high frequency with advantages forits detection. The rotation of test masses implies centrifugal force on board the satellite,acting on the test bodies, and thus rotation reduces the centrifugal forces and aligns therotational axes of the cylinders. The absence of weight allows active damping of the whirlingmotions of the test masses by an electrostatic system without requiring a significant force.The noise at the expected signal frequency is reduced inside the PGB laboratory, and alarge number of perturbations appear as a DC effect or can be separated in frequency fromthe expected signal.

The second point of GG is the possibility of reducing the non-gravitational perturbationsby means of the FEEP for the six months duration of the mission with a few grams of liquidCs propellant and a few watts of power. FEEP may be electrically tuned to smoothly providethe required thrust for air drag compensation without affecting the experimental apparatus.The area-to-mass ratio of the spacecraft isA/M ' 2.4× 10−2 cm2 g−1 so that the dragacting on the satellite is about 2 dyn. The FEEP thrusters are required to reduce the dragby a factor of 103, which is a resolution of 10−3 dyn.

Although perturbations are partially compensated by the FEEP, the resulting inertialforces on the suspended test masses inside are still larger than the signal of interest.However, the equivalence principle violation signal is differential, while the effect of inertialforces is in common mode, which means that it does not gives rise to a relative displacementof the test bodies, as long as their masses are equal and their suspensions respond in thesame manner.

GG has been approved for a first year study by the Italian Space Agency ScientificCommittee. The configuration proposed is based on body-mounted solar cells, a dipole-array antenna, a data rate of 1 kb s−1 for a few minutes each orbit, an orbit of 520 kmaltitude almost equatorial and circular, passive stabilization on one axis rotation (5 Hz)almost perpendicular to orbit plane, a pointing accuracy of 1–3◦ and a lifetime of sixmonths.

The ISS scenario offers two advantages: (i) the short-range test of the equivalenceprinciple, where ISS acts as signal source; and (ii) retrievability and servicing: the possibilityof changing the test masses improves enormously the quality of the scientific results of thelong-range experiment.

In the framework of the short-range test, we have to consider the ‘composition infrequency’ of the spin frequency of the satellite and of the frequency of the relative motionbetween the satellite and ISS.

4.3. ALGA—astronautical laboratory for gravitational astrophysics

This consists of a narrowband tunable spaceborne detector for low-frequency gravitationalwaves in the frequency range 10−2–102 Hz, based on parametric mode conversion in coupledmicrowave resonators (see Pinto in Alenia 1995, Cappettaet al 1995, 1996). The aim ofthe experiment is the detection of gravitational waves, yet unachieved, from specific and


pre-defined sources.Ground-based laser interferometers, resonant bars and spheres, should be able, within

the next ten years, to detect gravitational waves produced by coalescing neutron stars andnon-massive black holes and supernovae, alloccasionalphenomena. Since seismic noiseimpedes Earth detectors from working below 1 Hz, wherecontinuousbinaries of knownsky location and waveform radiate, at the main or one of the harmonic frequencies, a spaceexperiment launched in the near future could be the first to detect gravitational waves†.

Since microwave Doppler experiments are not adequately sensitive and presentlybaselined space laser interferometers will not fly before 2017, it is legitimate to ask whether acheaper and simpler (less ambitious) space mission is possible. This simpler mission wouldnot be an extraordinary observatory in space as LISA, but would ‘guarantee’ detection bylooking at known sources.

The ALGA working principles are not new and are relatively well understood.Preliminary laboratory evaluations several years ago showed the potential of the experimentand look encouraging.

The detector consists of two identical microwave cavities with proper resonant frequencyω0 and quality factorQ0, coupled, e.g., through a small iris. For cylindrical cavitiesthe T E011 (pure azimuthal wall currents) resonant mode could be chosen. As a resultof the coupling, two resonant field patterns are possible (symmetric and antisymmetric),and the input impedance of the whole system correspondingly exhibits two, approximatelyLorentzian, narrow resonance peaks atω±. The cavities are connected to the side-arms ofa magic-T junction whose port is fed by a microwave oscillator, delivering a powerPrfand sustaining the symmetrical mode atω−. A time-harmonic gravitational wave field, ofangular frequency�gw, affects both the dielectric constant of the vacuum inside the cavityand the boundary conditions. These effects can be equivalently described by volume andsurface currents in terms of the dimensionless quantityhgw. If �gw = ω+ − ω−, i.e. thegravitational wave frequency is the difference between the symmetric and antisymmetricmode frequencies, the electromagnetic energy is pumped into the normally unexcited modeat ω+. The system thus acts as a parametric converter.

The limit sensitivity is determined by the noise represented by: (i) the power leakinginto the unexcited mode through the tails of the resonance peak of the excited mode; (ii) thethermal power of the unexcited mode; and (iii) the thermal noise of the front end of themicrowave detector. A further sensitivity limit is the gravitational wave induced boundarydisplacement to the rms Brownian vibration of the cavity walls〈1l2〉1/2.

For better performance, higherQ0 and largePrf are needed. For commercial Nb andNb3Sn coated surfaces very large (unloaded) quality factors> 1012 can be obtained atCERN, below the critical temperatureTc. A more or less obvious constraint is that the RFmagnetic induction close to the superconducting cavity walls should not exceed the criticalvalue where transition to normal conductivity occurs.

The following sensitivity estimate summarizes present technology:

h ' 7.5× 10−19 1

�gw

(1014

Q0

)2f0

3× 108(14)

for a typical 〈1l2〉1/2 ' 10−24. Large improvements appear necessary to achieve lowerfrequencies for detectability of binaries such as 1913+16 or 1534+12, but other binaries arealready potential candidates (Cappettaet al 1995).

Other potentially dangerous noise sources include satellite vibrations and cryostat noise.

† Continuous sources detectable on Earth require a very long integration time, due to their modest amplitude.


Vibrations could be eliminated by a multi-stage vibration isolation (e.g. PGB) or via a drag-free system. In both cases it appears that the design would be enormously simplified by:(i) the narrow band of the detector that implies attenuation and drag-free control also ina narrow band; and (ii) the knowledge of the signal coming from a pre-selected binary.Cryocooler noise, mainly due to He boil-off and the mechanical compressor, could berelevant above some 40 Hz.

Payload technologies require: (i) superconducting microwave cavities (significantadvances have been made in this area, in the particle-accelerator field); (ii) ultralow-noise microwave detectors, including Schottky, HEMT and superconducting (SIS) devices;(iii) stable, spectrally clean microwave sources; and (iv) compact, lightweight, quietcryostats suited to the space environment (in this connection, reverse Stirling cycle coolerslook promising and reliable).

The experimental apparatus looks potentially compact, relatively cheap and light-weight,and could eventually emerge as the most promising space-borne gravitational wave detectorfor the near future.

In the ISS scenario, the benefits would be an easier communications scheme, servicing,launch of the microsatellite when a burst phenomena occurs, and last but not least the testof technology.

Recent considerations have brought up the possibility of external accommodation forALGA, rather than a microsatellite mission, greatly shortening the time to realization. Otheroptions include the exploration of higher frequency, large band operations and coupling toa small resonant mass.

5. Other experiments

Beside the interest for time and frequency measurements (Vessot 1989), NASA plansantimatter search as proposed by Ting. The detector (AMS 1994), a permanent magnet,makes mass and charge determination via the particle’s curvature in the magnetic fieldof the detector. The goal is the detection of positrons, antiprotons and antinuclei fromcosmic rays. Measurement of their abundance and spectra over a large energy range wouldallow determination of their origin (black holes, interstellar material). Astromag was also aproposal for antimatter search (see papers in Bernacca and Ruffini 1989).

A large x-ray telescope (Wood and Michelson 1989) (XLA 100 m2 surface and50 Mb s−1), is unlikely to be realized in the short term, but the investigations into thephysics of neutron stars, black hole candidates, and millisecond pulsars constitute an idealcomplement to gravitational waves research. XLA would analyse the x-ray emission inthe fast processes of accreting neutron stars or black holes that with the companion starconstitute coalescing binaries, i.e. sources of gravitational waves.

In the general physics microgravity programme, four experiments were identified(ELGRA 1995): (i) cooled atoms and microgravity clocks; (ii) kinetic theory of thermaldiffusion effects in liquid systems; (iii) plasma crystals; and (iv) dust aggregation.

Among NASA experiments is the analysis of the lambda transition point of He4.

6. Conclusions and future outlook

Table 4 shows the 12European experiments and the facilities in fundamental physicsconsidered for ISS. They include specificscientific measurements and instrumentationprovision as hardware support to future space missions. In most cases the facilities could


Tabl

e4.

Sum

mar

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expe

rimen

ts.

ND

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peA

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Table 4. (continued)

N Mass (kg) Volume (cm) Power (W) Critical technologies Readiness

1 ∼ 300 ∼ 100× 100× 100 PGB Medium, but test of PGB isrequired first

2 N/A N/A N/A N/A N/A

3 PGB Medium–high, but test ofPGB is required first

4 150 h = 100 φ = 70 85 PGB, and other newtechnologies for space

Medium, but test of PGB isrequired first and needsµsatprogramme

5 70–80 h = 90–100φ = 80–100

80–350 High quality factorsuperconducting cavities,cryogenics, and thermalrequirements issues are to beanalysed.

Medium, but depending onµsat programme

6 40–70 h = 80–100φ = 80–100

80–350 High quality factorsuperconducting cavities,cryogenics, and thermalrequirements issues are to beanalysed.

Medium, recommended forfurther evaluation

7 ∼ 100 140 litres 150 No relevant critical aspects High

8 20 25× 25× 25 20 None High

9 Sum Sum Sum None as ensemble Medium–high

10 150 h = 150 φ = 150 1000 A concern onaccommodation on SS mayrise in terms of longitudinalaccelerations along the driftchamber, but no other effectthan less than optimalperformance is expected

High, recommended forfurther evaluation

11 Low 60× 60 Low The concept is very simple,but the proposedperformances areexceptionally high, thusraising concern on theengineering feasibility

The benefits of PGB wouldbe to a large number ofdisciplines, beyond FP,which urgently demands aspace flight test

12 50–150 h = 60 φ = 80 Low ODF is mainly a proof of aconcept. FEEP thrustershave never flown but theyhave been under ESA studyfor more than 20 years

High, but requiresµsatprogramme

be consideredtechnologyexperiments supporting multi-disciplinary scientific utilization,i.e. precise clocks and noise attenuators.

These experiments do not represent a complete picture of ISS potential. Proposals areunder development and some experiments will hopefully fly. As ISS still poses doubts onthe quality of science it will produce (Beardsley 1996), fundamental physics experimentswill not justify ISS on their own (the rationale to build ISS lies mostly outside pure science),but certainly contribute to a more proper scientific utilization.

The three activities described were carried out with a small budget from the ESTECISS Utilization Office but, while opening new frontiers, have stimulated the proposition of


experiments certainly among the most scientifically outstanding within the ISS scenario,and with reasonable expectations of flying in the next few years (the microgravity clockand ACES).

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