Millimetron Project

1. Introduction

The goal of the project is to construct space observatory operating in millimeter, sub-millimeter and infrared wavelength ranges using 12-m cryogenic telescope in a single-dish mode and as an interferometer with the space-ground and space-space baselines (the later after the launch of the second identical space telescope). The observatory will provide possibility to conduct astronomical observations with super high sensitivity (down to nanoJansky level) in a single dish mode, and observations with super high angular resolution in an interferometric mode.

It is well known that the spectrum of the sky in the wavelength range of 0.4-20 mm is dominated by the cosmic microwave background radiation (CMB), which follows Planck law with the temperature of 2.73 K. This is an absolute minimum of the brightness temperature over the whole spectrum of the background radiation.

Fig.1. The spectrum of a electromagnetic cosmic background for high galactic latitudes (R.C. Henry, Ap.J., 516, L49-L52, 1999).

At the wavelength of 250 – 500mm there is a deep minimum both for spectral intensity I_n and total intensity I~n I_n of the background radiation(emission) at the frequency n. This minimum separates the region with the CMB domination from the region where the emission of interstellar and interplanetary dust dominates. If one adopts that the flux sensitivity of the telescope is restricted only by background radiation, the estimates show that the sensitivity of the 12 meter space telescope at the wavelength 300m is equal to the sensitivity of 1 kilometer ground radio telescope at the wavelength 2 cm. Two or more such telescopes operating as interferometer with space baselines (SBLI), will provide angular resolution by thousands times better than ground VLBI network.

The millimeter and submillimeter wavelength ranges are now under intensive development. Utilization of these wavelength ranges has already brought very important information for cosmology and extragalactic astronomy as well as for the investigations of our Galaxy and the objects in the Solar system. Ultra high angular resolution and high sensitivity possible in these wavelength ranges will enable to investigate many stars with planetary systems. Negligible effects of scattering waves on clouds of the interstellar plasma in these wavelength ranges will facilitate imaging of super compact objects and will enable high accuracy measurements of coordinates and motions of such objects. Both continuum and spectral line (atoms and molecules) investigation will be possible, as well as polarimetric observations and observations of variable sources.

Millimetron Project is included into the Space Research Program of Russian Federation for the term till the 2015 year. The launch date for the first spacecraft is planned for 2016 year.

2. Main parameters and scientific objectives of the Mission

Preliminary analysis has demonstrated the possibilities to achieve the following parameters.

• The sensitivity may be close to the values limited only by the spectrum of the background radiation (Fig.2).

Fig.2. The ultimate flux density sensitivity of the 12-m dish limited by the sky background and by the quantum noise. 1) space telescope with the bolometer (quantum counter), or Space-to-Space interferometer in signal transmission mode without amplification (Mikelson interferometer), Dn=n, Dt=1 s; 2) the same for Dn=10 GHz, Dt=300 s; 3) the same for Dn=10 GHz, Dt=1 day; 4) the same Dn=n, Dt=1 day; 5) Space-to-Space VLBI mode with the account of zero quantum noize of the amplifier, Dn=10 GHz, Dt=1 day; 6) Space-to-Space intensity interferometer, Dn=n, Dt=1 day. The vertical line designates high frequency border of the optimum frequency range (1.5 THz).

• The guaranteed operation life time of the mission will be three years with the ultimate sensitivity provided by the both active and passive cooling system of the whole dish and science payload (total power consumption is about 3.5 kWt)
• Subsequent years (up to 7-10 years) the telescope will operate with the passive cooling system.
• Automatically deploying 12-m dish of the telescope will provide obtaining images in the wavelength ranges down to 250 mm with the quality limited by the diffraction.

· The main and the secondary dishes of the telescope will be cooled down to 4 K by the cryogenic cooling system, providing the inherent radiation of the telescope to be below the background radiation. The passive cooling system only will keep the dish at the temperature about 50 K.

• The design of the telescope utilizes three-mirror configuration: the deep main mirror (which simultaneously will act as a heat screen), the 60-cm diameter Cassegrain mirror, and the flat mirror used for switching between different wavelength ranges and for fine pointing to the source (0.3 arc seconds RMS). Surface accuracy of the main dish after its deployment is expected to be better than 10 mm, and surface accuracy of its separate elements as well as the accuracy of the Cassegrain mirror and the flat mirror will be better than 3 mm (RMS).
• The accuracy of pointing of the main reflector will be equal to 1 arc second.
• The telescope will be equipped with bolometers (and/or photon counters), radiometers, spectrometers, and polarimeters in the wavelength range 10 mm – 3 mm (single or multiple beam operation with detectors cooled down to 0.1 K or even lower).
• The range 0.3-20 mm will be used for interferometry “Ground-to-Space and Space-to-Space with the second space telescope”.
• The range of 0.2-3 mm will be used for Space-to-Space interferometry only.
• The frequency band transmitted in interferometric mode will be 10 GHz and higher. The possibility of using fiber optics data transmission is considered as well as the processing the data on line.
• We consider three options for the orbit of the first spacecraft: the elliptic orbit with period of about 9-days and with the perigee of 75 000 km and the apogee of 300 000 km (similar to the orbit for RadioAstron mission); the halo orbit around the Lagrange point L₂ (1.5 10⁶ km from the Earth in the opposite to the Sun direction), elliptic orbit with the perigee of 75 000 km and the apogee located near the Lagrange point L₂.

The combination of very high sensitivity and angular resolution in very important for modern astronomy submillimeter wavelength range offers outstanding possibilities for all kinds of investigations in continuum, spectral lines, polarimetry, and monitoring of variability of different types of objects including measurements with super high angular resolution. Below is the list of science objectives, which shall stimulate the discussion on the most perspective studies in order to rectify the technical specifications for the project.

1. Molecular content and physical conditions in the atmosphere of planets and their satellites.
2. Asteroids and comets.
3. Dust component of the interplanetary medium, Van Allen and Oort belts.
4. Spectral polarimetry, mapping, rotation and variability of stars of different kinds (from giants, WR stars, cepheids, dwarfs, neutron and quark stars, galactic black holes).
5. Planets and dust envelopes of stars, detection and investigation of star, planetary system and even single planets formation & evolution regions, submillimeter masers, search of life in the Universe.
6. Content, structure and dynamics of the most cold gas-dust clouds.
7. Structure and dynamics of the matter near supermassive black hole in the Galactic Center.
8. Dynamics of the Galaxy using radial velocity and proper motion super high precision data.
9. Dynamics and masses of galaxies in the Local Group.
10. Distribution of Dark Matter inside our Galaxy and in the Local Group of galaxies.
11. Structure and dynamics of gas-dust component in galaxies and quasars, merge of galaxies, bursts of star formation. Megamasers.
12. Structure and physical processes in galactic nuclei, cosmic ray acceleration.
13. Structure and dynamics of clusters and superclusters of galaxies, distribution of dark matter inside these objects.
14. Extended structure near radio galaxies: synchrotron emission and scattering of the nuclei emission.
15. Dynamics of galaxies collision.
16. Early galaxies: detection of galaxy’s birth event, search of their further evolution, including evolution of dark matter and gas-dust content.
17. Extragalactic supernovae and cosmology.
18. Gravitational lenses, as natural telescopes.
19. Chemical evolution and cosmology.
20. Syunaev-Zeldovich effect at submillimeter wavelength and cosmology.
21. Hubble diagram and cosmology.
22. Angular dimension – redshift dependence and cosmology.
23. Proper motion – redshift dependence, relic proper motion and cosmology.
24. Superluminal motion – redshift dependence and cosmology.
25. Spatial fluctuations of CMB at submillimeter wavelength an cosmology.
26. Physical processes during the explosion due star’s merging, observing expanding shell for determination of cosmological parameters.
27. Search of pre-galactic objects, investigation of early stage evolution of the Universe, from the time moment of recombination (recombination lines) to the beginning of stars and galaxies formation, search of primordial black holes.
28. Evolution of matter and vacuum, dark matter and dark energy state equation, inflation relics, worm holes, multi element model of the Universe, additional dimensions of space.
29. Gravitational emission in the Galaxy and in the Universe.
30. Astroengineering activity in the Galaxy and in the Universe.
31. Construction of high precision astronomical coordinate system.
32. Construction of high precision model of the Earth’s gravitational field.

Further development of the project suppose a more detail discussion on each item of this list, it’s possible extension or reducing of the list, after discussions among high qualified experts and leading scientists.

3. The content of the Space Observatory Millimetron

The observatory will include the following systems.

1. Spacecraft bus of the type of “SPECTR” module providing all service functions like power supply, pointing and tracking of the sources, control and monitoring of science payload, and communication with the Earth.
2. Space cryogenic telescope.
3. Active and passive (screens) cooling system of the telescope.
4. Bolometers (or photon counters), radiometers with the spectrometers, polarimeters, and additional deep cooling systems.
5. Frequency standards and synthesizers to provide high stability frequencies and clock for interferometric investigations.
6. Digital data acquisition system to provide data recording, memorization and processing.
7. Super broad band coherent data transmission line providing communication between Space-to-Space and Space-to-Ground system elements.
8. On-board system of high accuracy orbit determination including range and range-rate measurements, three-axes accelerometer, phase-locked and laser distance measuring devices.

4. Design and main parameters of the Space Observatory

4.1. The main mirror of the telescope

The main 12-meter reflector consists of solid 3-meter diameter central part and 24 petals which will be automatically deployed in space after launch. The design is identical to that which used in RadioAstron project, but the surface accuracy and its stability will be kept within 10 mm (RMS). All construction will be cooled and shielded from radiation of the Sun, the Earth, and the Moon by the special screens. The screens position will be controlled by the special control system. General view of the telescope with screens-radiators is shown in Fig. 3.

Schematic diagram of the main components of the telescope is presented in Fig. 4.

Surface accuracy and its stability is provided by the high accuracy of fabrication of the elements, by the design of the unfolding and fixating system and by cryogenic thermostating.

4.2. Cooling system of the telescope

Complicated cooling system will be used to achieve the highest sensitivity. The cryogenic system will include two stage: passive radiation cooling which releases the heat into space providing the cooling of the telescope down to 50 K, and the cryogenic machine to be used for the deep cooling of all telescope mirrors (made of solid aluminium or carbonsilicon) through the capillaries in the segments of its construction, which will provide cooling down to 4 K. Some devices of the telescope will be cooled below 0.1 K.

4.3. Complex of the scientific devices

Total spectral range 10 mm – 2 cm is separated into 10 bands by octave. Switching between the bands will be possible by the selected command with the turn of the flat mirror when it necessary. There will be the following bands: 1) 15-30 GHz (1-2 cm), 2) 30-60 GHz, 3) 60-120 GHz, 4) 120-240 GHz, 5) 240-480 GHz, 6) 480-960 GHz, 7) 0.96-1.92 TGz, 8) 1.92-3.84 TGz, 9) 3.84-7.68 TGz, 10) 7.68-15.36 TGz (9.8-19.5 mm).

To achieve the highest sensitivity in flux density the scientific complex will be optimized for the range of 200-400 mm. The scientific complex will include the following devices:

1. broad band matrix bolometer (or photon counter) – deep survey camera (»100 elements, wavelength range 0.2-3 mm),
2. polarimetric spectral system of medium resolution (R=l/lD » 10³) for investigation of active galactic nuclei,
3. polarimetric spectral system of high resolution (R » 10⁶) to study sources with low temperatures and maser sources in the range of 10 mm – 3 mm,
4. polarimetric spectral system of low resolution (R » 5) with the possibility to cover broader wavelength range (10-800 mm, with constant beam size (angular resolution) for wavelengths shorter than diffraction limit near 200 mm),
5. interferometric complex Space-to-Ground and Space-to-Space (frequency ranges 0.1, 0.2, 0.4, 0.65, and 0.85 THz) will operate in the ranges of the best transparency for ground radio telescopes, in particular for the multi element aperture synthesis system ALMA in Chili,
6. interferometric complex Space-to-Space for the whole range of 0.2-1 mm (0.3-1.5 THz) with the direct signal transmission from one spacecraft to another (without amplification) and with data processing on-board (Michelson interferomer with very high sensitivity, but with limited baseline extension).

5. Perspectives

Potential parameters of space telescopes for millimeter and submillimeter wavelength ranges open up outstanding opportunities for astrophysical investigations. The research program proposes increase in number of simultaneously operating satellites, longer operation time, and extension of their orbits. There may be three modes of operation for such space telescopes:

1. autonomous operation of space telescope with the optimization for highest sensitivity in wavelength range of 0.2-0.4 mm,
2. interferometer Space-to-Ground for observations optimized in wavelength range 0.35-3 mm in conjunction with large ground telescopes (for example, ALMA, Attakama desert in Chili, or 70-m radio telescope, at Suffa plateau in Uzbekistan),
3. interferometer Space-to-Space consisting of two space telescopes optimized in the same range (this mode anticipates development of space array in a future).

In a case of Moon-perturbed elliptic orbit with the apogee up to 300 000 km the interferometer fringe width will be equal to 0.2 mas. However, in order to achieve better conditions for cooling the telescope and the receiver system, galo-orbit or orbit with the apogee near the Lagrange point L₂ may be selected for space telescope. In this case the interferometer fringe width will be equal to 0.045 mas at 350 mm for the baseline of 1.5x10⁶ km.

It may be advisable to place telescopes in triangle Lagrange points L₄ and L₅ to achieve even higher angular resolution of the interferometer. In this case interferometer fringe width will be equal to 0.4 nanoseconds of arc at 300 mm for the baseline of 1.5x10⁸ km.

Possible scenario of development of the system of space telescopes is shown in Fig.5. The system begins with the orbits within the Moon orbit, and it ends with the array near Lagrange point L₂, accompanied with very distant antennas located in the triangle Lagrange points.

Fig.5. Scenario of development of the system of space telescopes. The system begins with the orbits within the Moon orbit, and it ends with the array near Lagrange point L₂, accompanied with distant antennas located in the triangle Lagrange points.