Potential of balloon payloads for in flight validation of direct and nulling interferometry concepts

Olivier Demangeona, Marc Olliviera*, Jean-Michel Le Duigoub, Frédéric Cassaingce, Vincent Coudé du Forestode, Denis Mourardf, Pierre Kerng, Tien Lam Trongb, Jean Evrardb, Olivier Absilh, Denis

Defrèrei, Bruno Lopezf

aInstitut d’Astrophysique Spatiale, Université de Paris-Sud and CNRS (UMR 8617) – Bât 121, F-91405 Orsay Cedex France

bCentre National d’Etudes Spatiales, Centre spatial de Toulouse,18 avenue Edouard Belin F-31401 Toulouse Cedex 9 - France

cOffice National d’Etudes et de Recherches Aéronautiques, BP72 - 29 avenue de la Division Leclerc F-92322 Chatillon Cedex France

dLESIA Observatoire de Paris, 5 place J. Janssen, F-92195 Meudon – FranceeGroupement d’Intérêt Scientifique PHASE (Partenariat Haute résolution Angulaire Sol Espace)

between ONERA, Observatoire de Paris, CNRS and Université Paris DiderotfObservatoire de la Côte d’Azur, Boulevard de l’Observatoire, B.P. 4229, F-06304 Nice Cedex 4

FrancegLaboratoire d'Astrophysique Observatoire de Grenoble BP 53 F-38041 Grenoble Cedex 9

FrancehAstroph. extragalactique et observations spatiales (AEOS) Bât. B5C , allée du 6 Août 17

B-4000 Liège 1 BelgiumiMax Planck Institut für Radioastronomie – Auf dem Hügel 69

D-53121 Bonn Germany

ABSTRACT

While the question of low cost / low science precursors is raised to validate the concepts of direct and nulling interferometry space missions, balloon payloads offer a real opportunity thanks to their relatively low cost and reduced development plan. Taking into account the flight capabilities of various balloon types, we propose in this paper, several concepts of payloads associated to their flight plan. We also discuss the pros and cons of each concepts in terms of technological and science demonstration power.

1. CONTEXT OF SPACE INTERFEROMETRY

Space interferometry has clearly been identified for about twenty years as one of the most promising techniques allowing astrophysical observation at high angular resolution within spectral range that cannot be reached from the ground. In the particular case of thermal infrared, both roles of atmosphere (absorption, turbulence, thermal background) and thermal emission of the instruments prevent from getting high signal to noise ratio observations from the ground. In addition, space observations reaching angular resolution comparable to what is obtained in the visible spectral range requires, because of diffraction, the use of telescopes 8 to 20 times bigger than existing biggest space telescopes (at present the European Herschel telescope, waiting for the JWST).

* marc.ollivier@ias.u-psud.fr , tel : (+33) 1 69858630 fax : (+33) 1 69858675

Ground based interferometry has already proved its technical and scientific capabilities, mainly in the field of stellar physics: accurate measurements of stellar diameters, study of variable pulsating stars, imaging of giant stars surface, study of stellar environments and bright protoplanetary disks… Conversely, space interferometry stayed until now at the state of projects. At present, the most studied interferometric observatory projects is SIM in the United State, an astrometric observatory to identify earthlike planets around nearby solar like stars. This project is under development for the next decade1. In Europe, DARWIN2 has been proposed to ESA in 1993, pre-selected in 1995 for preliminary studies, studied in the laboratory and the industry for about 15 years. Finally, DARWIN has been postponed sine die mainly for technical feasibility reasons. Concepts that associate scientific program and (nulling) interferometry technological validation have been proposed to space agencies. PEGASE 3 in that context has been studied during 2005-2006 at CNES (French Space Agency), and then proposed to ESA as an answer to the first Cosmic Vision call for proposals in 2007. However, the technical uncertainties and the limits of the scientific program that could be affordable with a technique that has not been demonstrated in space are strong limits in the definition of a scientifically attractive and relatively cheap mission development. It appears thus necessary to validate the technique in space or in a similar environment, showing for instance how each sub-system can be operated without any strong consideration of scientific return.

In this context taking into account the lack of perspective in the use of Antarctica as an astronomical site for interferometry, It appears to us that the observation from the upper layers of the atmosphere using payloads carried by stratospheric balloons can be seen as a real opportunity to propose an instrumental concept that allows both the validation of instrumental concepts and the realization of a preliminary scientific program, as a first step towards more complex programs. Observation from balloons at high altitude (40 km) can be considered as representative of space environment and offer the opportunity to develop and validate technologies at reduced costs and development time. Balloon flight particularities (particularly residuals of gondola oscillations) can also be used as a first step simulation of perturbations that future generation of formation flying interferometers will have to face.

2. POSSIBLE SCIENCE CASES FOR A BALLOON CONCEPT

Initially, space interferometry projects, and particularly DARWIN, had ambitious scientific goals, that could not be achieved from the ground, as the spectral characterization of earthlike planets around nearby stars. However, it appeared during the assessment phase of such projects that, scientifically speaking, the primary goals could not been reached without a careful characterization of stellar environments themselves. The presence of giant planets or other sources, such as a debris disk as remnant of the formation phase can prevent from detecting small planets, which contrast to the stellar flux, can reach 10 -6 to 10-10 depending on the observation spectral range. The knowledge of intensity level of this exozodiacal emission is mandatory to design the instruments that will characterize the planets, whatever the spectral range - visible using a coronagraph 4 or thermal infrared using a nulling interferometer 5 - can be. The characterization of zodiacal environment (emission from the debris disk) of nearby candidates to planetary characterization should be done at the sensitivity level of a few times the integrated emission of our own zodiacal disk (later called “zodi”). The detection and characterization of these disks can be performed in the near infrared or in the thermal infrared spectral range. Assuming the performance of ground based instruments, reduced by infrared background due to thermal contribution of the instrument and the atmosphere, such a sensitivity cannot be reached by ground based facilities, so the idea of space instrumental concepts such as PEGASE 3 or FKSI6 (a single satellite similar concept, which future in the US is not certain too). Figure 1 compares the performance of several instruments from ground and space for the characterization of exozodiacal disks7. The reduced performance of PEGASE is due to the minimal length of the baseline (40m), which is not optimal in the case of nearby stars.

Figure 1: Performance comparison between ground-based (KIN-ALADDIN) and space-based (PEGASE-FKSI) observatories for the analysis of exozodiacal clouds 7. The performance difference between FKSI and PEGASE for nearby objects comes from the minimal

baseline of PEGASE (40m), which is not optimal for these objects.

A possible payload for a balloon based concept can thus be quite similar in its principle to PEGASE / FKSI, but should be developed at much reduced cost. This implies drastic reductions of constraints, and drives the choice of the spectral window (see next section). The payload may be composed of 2 siderostats on a 10 m boom (optimal distance to observe exozodis in the near infrared) and an interferometric beam combiner working in the nulling mode somewhere in the 1.5-5 µm spectral range for the characterization of exozodiacal environments. We wish to characterize N stellar systems (typically a few tens), N depending on the number and duration of flights. N also depends on the possible collaborations that can be developed on that project. The associated science has namely clearly been identified, by several roadmap groups (including Blue Dot 8), as a mandatory preliminary science before the characterization of exoplanets. The goal is to reach the sensitivity of several zodis on nearby stars. It would be an illusion to pretend that the same scientific goals in terms of number of targets can be reached either with a balloon or a dedicated but much more expensive spacecraft mission such as PEGASE or FKSI. However, one can observe a sample, statistically representative of solar neighborhood, and extract a first order evaluation of their mean environment.

3. STRATEGY AND ASSOCIATED INSTRUMENTAL CONCEPTS

3.1. StrategySeveral key technologies have to be developed and validated to allow the emergence of space interferometry concept that can be considered by space agencies. Let mention for instance, everything that concerns formation flying (micropropulsor allowing slow and small spacecraft drifts, GNC systems and associated metrology…) but also the interferometric beam combination itself (beam combiner, delay lines, fringe sensors, lateral and angular superimposition devices, data treatments…)

In the context of a balloon mission concept, we only wish to demonstrate the beam combination part of the technique. Taking into account the scientific goals, it seams necessary to develop an instrument working in the near to thermal infrared. The 2 sub apertures of the arrays will be made by two siderostats carried by a fixed length boom. All the instrumental difficulty is in the beam co-phasing in the balloon flight environment (vibrations, oscillation residuals…) and the control of thermal environment (thermo mechanical effects on the instrument, residual thermal background, working temperature of the instrument and detectors.)

We thus propose a 2-step strategy:- a first step, later called “preliminary mission” that is purely a technology demonstrator: its goal is to validate the

in-flight co-phasing of beams, with possible stabilization of interference pattern by the measurement of visibility function. As no astrophysical result is expected during this phase, one can work with a simple instrument, where the 2 siderostats are separated by several tens of centimeters and that observes in an “easy” spectral range, visible or near IR where thermal constraints are reduced and optical components are currently available and affordable. In that case, the experiment consists in the acquisition, the stabilization of the interferometric pattern, and the visibility measurement on a bright source (Betelgeuse, Vega, Deneb, Sirius, depending on the launch site and the season). The instrumental concept is developed in the next part.

- A second step, called later “main mission” consists of the flight of a nulling interferometer in the near IR to observe and measure the integrated intensity of zodiacal disks around nearby stars and in order to validate the performance of the instrument, bright well-known disks, already observed from the ground.

It is of course the main mission that justifies the development of the preliminary mission and the whole project should only be launched if one can demonstrate that the main mission is feasible at acceptable cost and development duration according with balloon standards. In any case, both steps should be carefully studied and critical points such as thermal constraints should be explained before the realization of the preliminary concept.

The main particularity of this program is its flexibility, modularity and the implementation of a step-by-step approach that allows a step-by-step technological and scientific validation. This concept is based on our expertise in the field of nulling interferometry thanks to the SYNAPSE 9 and PERSEE10,11 benches supported by CNES.

A real development plan has still to be written but we can yet estimate that the whole development duration for the 2 missions should be around 4 to 5 years. Concerning the cost of the project, the preliminary mission should be very cheap. The cost of the main mission should be evaluated taking into account all the possible options.

3.2. Preliminary mission

As mentioned before, the preliminary mission is a pure technological demonstration, with no scientific objective. The idea is to combine the beams from two 10-cm siderostats (2 mirrors on tip tilt mounts), separated by several tens of centimeters but on the same optical bench in the visible or near IR spectral range. The beam combiner is a very simple one and for stability reasons we propose a guided optics beam combiner (optical fiber coupler or integrated optics component). The interferogram is acquired by a scan of the optical path difference thanks to delay lines. Two options will be studied during the assessment phase: scan of the fringe while the configuration is frozen (scan faster than the oscillation residuals typical period) or fringe stabilization by a fringe sensor / delay line unit and scan of the optical path difference. We want to build a very compact instrument (not larger than a shoebox, excluding siderostats) screwed, as the siderostats, on the experience bench of the gondola of an open stratospheric balloon. This type of balloons is the best affordable trade off in terms of mass (several 100 kg) and altitude (about 40 km) of the payload. First studies have been done on a Swiss gondola previously used for infrared and sub-mm observations. These studies have shown that the 2 fringe stabilization options can be considered for several bright sources observable according to the season. A first schema of the instrument in its “fast scan” version is shown of Figure 2 below. Let’s mention that the pointing control is done using a servo control device based on the stellar light really injected in the fiber and monitor thanks to the coupler. The use of pressure balloons, for which the flight duration can be much longer is not adapted to this mission, because the payload mass should not exceed 20 kg on the gondola. In addition, the altitude reached by such balloons is much lower than the altitude reached by open stratospheric balloons.

Figure 2: preliminary schema of a beam combiner for the fast scan fringe acquisition in the context of an open stratospheric balloon flight.

3.3. Main mission

In its goals and principle, the instrument of the main mission is comparable to the instrument considered in the framework of PEGASE concept 3. It is a near IR beam combiner working in nulling mode. Its concept comes directly from laboratory demonstrator experience, SYNAPSE 9 at IAS but mainly PERSEE10,11. If the preliminary study confirms its feasibility, the design and realization of the instrument will directly benefit from past developments. It is also considered, in order to reduce the overall cost, to use several sub-systems developed for laboratory benches. Once again, the critical aspect of the instrument is the choice of the working spectral band that can be reduced compared to PEGASE’s one. This choice also strongly depends on preliminary mechanical and thermal studies of the instrument and the stability of the gondola. An open stratospheric balloon at its maximum altitude will carry out the payload, in order to reduce at best the influence of thermal background. The number of targets to observe and the performance of the instrument will determine the flight duration. During the preliminary study, we will consider the case where the payload can be used several times, including under a high capacity pressured balloon, if NASA develops them. The project is presently named BALEINE (“whale” in French) for BALloon Experiment for interferometric Nulling on Exosystems.

4. STATE OF THE ART AND FUTURE DEVELOPMENTS

Our mission concept is based on a more than 20-year experience in France in classical interferometry from the ground for the preliminary mission. This experience led to the study, the development and the operation of several ground-based instruments (I2T and GI2T at the Observatoire de la Côte d’Azur, VINCI, AMBER and MIDI on the VLTI, FLUOR on IOTA then CHARA).Concerning the main mission we can argue on a more than 15-year experience in nulling interferometry in the laboratory illustrated by the study and realization of several laboratory test benches such as PERSEE 10 developed thanks to the support of CNES (figure 3). This bench is under development and has already allowed co-phasing beams with a nanometric accuracy leading to a 10 -5 rejection using a monochromatic source 11. Let’s mention also the development of specific sub-systems such as symmetric beam combiners, achromatic phase shifters, fringe sensors and trackers, high accuracy delay lines, guided optics components including beam combiners and delay lines.

During the preliminary study of the main mission, we will try to determine if the instrument can be development at balloon project cost. If possible and necessary, several sub-systems developed for the laboratory will be re-used for the balloon instrument.

Figure 3: PERSEE test bench, to simulate beam combining in the presence en environmental perturbations.

5. CONCLUSION

Even if it appears as a difficult environment, the stratosphere may be a good alternative to space for technological validation of complex concept such as space-interferometry. Open stratospheric balloon can carry several 100 kg payloads in that goal. Nulling interferometry can benefit from this approach, if the study we started leads to the feasibility of a demonstrator at reduced cost. Next two years will tell us if this project can be converted into a real flying payload, or if another idea should be found to prepare the development of future space project.

REFERENCES

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