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Planck is in Liège!


At the end of 2008, or perhaps early in 2009, the European space program will launch Herschel and Planck, two satellites designed to make scientific observations, into space with the help of powerful Ariane 5 rockets. Their mission is to probe the mysteries of the universe, such as the formation of galaxies, and to gather new information bearing on the origin and expansion of the universe by investigating the traces left by the initial explosion known as the Big Bang. In conducting these tests in a simulated outer space (vacuum and ultra-low temperature) environment, the CSL (Liège Space Center) will be playing a key role in the success of both satellite launch missions.

Planck-artistHerschel and Planck, (*) whose instruments have to be able to function in ultra-cold temperatures, will be placed in stationary orbits 1.5 million kilometers from Earth, a distance four times as great as that between the Earth and the moon. They will be installed at Lagrangian point L2, one of several points where gravitational equilibrium between the Sun and the Earth can be achieved (L2 actually keeps the Earth between it and the Sun), and they will observe the cosmos from there. Both satellites are being launched into space by the ESA (European Space Agency) on behalf of the scientific community.

The two missions are costing a total of 2 billion euros, half of which represents the cost of the satellite packages alone, but their missions are of the highest scientific interest. The director of the CSL until 2010, Jean-Marc Defise, is aware of the importance of the cryogenic tests to be carried out in his laboratory complex, which is one of the few in Europe that could perform them on such large objects as these satellites.

In the testing for the complete Planck observatory satellite launch module (without solar panels), Defise sees the ultimate validation of the technological efforts that the University, supported by the Belgian government via the Federal agency for scientific policy, has undertaken in response to the needs of the ESA. For him, these are new proofs of the excellence and the sterling reputation of the Liège facility and the expertise of its scientists in the area of space research.

A technological “first”

Financed by the Member States of the ESA (European Space Agency), Herschel (3.3 ton payload at launch) and Planck (1.8 tons) were built on the same platform by cooperating European industries, under the direction of the company Thales Alenia Space. This company’s affiliate ETCA in Charleroi worked on power conditioning units for the project, and the Nivelles firm EHP (Euro Heat Pipes ; member of WSL, Wallonia Space Logistics) provided the heat conduction pipes. But the know-how of the CSL was required for the tasks of estimating and calibrating the payload for Herschel – a monolithic telescope with a diameter of 3.5 meters – in the vertical space environment simulator FOCAL 6.5 (**). The three instruments in the focal plane of the satellite were also been tested for their abilty to withstand vibration under cryogenic conditions by the CSL, by means of a unique installation designed by Liège technicians. Now it is Planck’s turn to undergo testing in the horizontal simulator FOCAL 5 : the object of this series of tests, the last before the module is transferred to the Guiana Space Center (CSG) at Kourou, is to verify that the instrumentation for the 1.5 meter telescope functions perfectly under the ultra-cold conditions it will encounter in outer space.

The flight model for the Planck European observatory was transferred via special convoy from ESTEC, the European center for space research and technology in Noordwijk, the Netherlands, and arrived on April 23. This was the third time it had visited Liège: a prototype was tested during 65 days in an airless environment in 2005, and part of the flight model came to Liège in the first half of 2006 for 21 days of testing. Performance testing for the optical mirrors of Planck’s telescope took place separately at the CSL under ultra-cold conditions, using instrumentation specially designed by Liège technicians. These mirrors were tested before being mounted in the telescope, which focuses signals received toward complex cone-shaped detectors.

Planck-cruisingFrom now until the month of August, the entire satellite will undergo verification testing and calibration in FOCAL 5. Its core – the telescope and its sensors – will be placed in a vacuum during a sixty-day period, during which it will be exposed to a temperature of 0.1 °K, or 273,05°C. That means it will be in an environment where the temperature is very close to absolute zero! The satellite instrumentation includes two “seeing eyes” that will make observations of the heavens based on nine different wavelengths, in order to measure with unparalleled precision the temperature of the CBR (Cosmic microwave Background Radiation), the “background noise” of the universe. An LFI (Low Frequency Instrument) detector will make observations in the microwave range (30-77 GHz), while the HFI (High Frequency Instrument) package with 36 detectors will measure radiant energy based on infrared radiation from very distant sources, and also emissions in the 100-857 GHz portion of the microwave spectrum. The detectors have to pass qualifying tests so that the CSL can certify that in a vacuum they are sensitive to variations in temperature as small as a few millionths of a degree.

The unavoidable final exam

What is special about Planck, and the thing that makes its preparation so complicated, is that its scientific payload has to function at an ultra-low temperature during a period of 18 months, the time of its space mission. This is the necessary condition for measuring, based on precise observations and with unequaled clarity, the diffuse cosmic background radiation. CSL’s responsibility occurs at a crucial and delicate point of the process: the CSL teams of researchers, engineers and technicians, assisted by specialists from ESTEC and Thales Alenia Space, must give the final “OK” that will allow Planck to join its “buddy” Herschel on a flight to Guiana, soon afterward to be launched into space by an Ariane 5.

This final battery of tests for the Planck satellite is handled with all possible care. Industrial engineer Isabelle Domken has 20 years experience handling space systems testing. She is the director of the CSL team preparing the FOCAL 5 simulator. She is responsible for making sure that the satellite intended to spy out the very origins of the cosmos passes its final exam with flying colors. She reminds us that Planck first visited Liège in 2001, when a feasibility study showed that the CSL had the capacity to test an entire satellite. The facilities had to be adapted for these satellites (construction of an additional wing of the building, expansion of the receiving area), new equipment had to be purchased (helium liquefactor, railway system leading up to the entrance to the simulator enclosure to bring in the satellite, double thermal tent, security arrangements), and engineers had to receive additional training in cryogenic research.

A life filled with technological problems and challenges

Planck’s presence at the CSL during the summer season will be noticed. The Center’s priority is conducting the tests on the complete satellite before it is sent into space. The CSL was expecting Planck at the end of 2007, but unexpected difficulties and delays forced it to reschedule its work with the European cosmological observatory.

The FOCAL 5 simulator has been configured to test the satellite in operation at a temperature that is close to absolute zero; the coldest panels measuring minus 269°C! Taking every possible precaution, since the margin for error is extremely small, the CSL teams trained with the help of a mockup to bring Planck’s 1.8 tons into a thermal tent specially constructed for this purpose by the Liège-based company AMOS. The interior of the tent is actually covered with empty beehives attached to copper panels, a process inspired by a similar one employed by the Aerofleet company of Soumagne, Belgium.

The panels were painted black so as to absorb radiant heat flux, and to avoid any heat being reflected from the hot side (electronic) of the satellite, the side of the service module turned toward the sun (the temperature inside this module must remain constant at 20°C), onto the part that has to remain ultra-cold (the telescope and its associated instruments).

Another technological problem that had to be solved: the use of a panel indicating three reference sources that had to come down in front of the HFI instrument assembly and allow it to be calibrated with a heretofore unattained degree of precision. This panel is similar to a liquid helium bath, and was designed to function in a stable manner at temperatures around 4 Kelvin (– 269°C).

Support for the Big Bang theory

In order to keep the core of its payload temperature-controlled under extreme conditions, Planck uses a passive heat sink down to 50 K, and three active cooling units to maintain temperature at 20 K, 4 K, and 100 micro-Kelvin (mK). The latter value for the cooling of the focal plane of the HFI instrument group is achieved by a system of dilution of two isotopes of helium, helium 4 and helium 3. The first occurs naturally, but helium 3 is only produced as a byproduct of nuclear fission, and is one of the most expensive substances in the world (at about 1.2 million euros per kilogram). For Planck alone, the amount used represents two years worth of world production!

The CSL must therefore take every precaution in the testing of the fully operational refrigeration system of the space observatory tested in FOCAL 5.

A great deal is riding on the observations that will be made by Planck, in terms of clearing up certain mysteries of the cosmos. As of this moment, we are acquainted with only 4% of the matter of the Universe, namely visible matter. But we do not know the exact nature of the 23% of the universe that is made up of dark matter, and we know next to nothing about that which constitutes 73% of the mass of the universe, dark energy! From dark matter and dark energy there came, in a violent event, the spark of the Big Bang - according to theory. The hypothesis of a “let there be light” phenomenon is part of an attempt to explain why the universe is expanding. This theory is controversial, although the discovery of CBR, cosmic microwave background radiation, by means of detectors in space, has restored its reputation to some extent.

The American George Smoot, winner of the Nobel Prize in physics for 2006 for his work on data transmitted from the American satellite COBE (Cosmic Background Explorer), believes in the Big Bang. He believes that the European satellite observatory, Planck, will produce proof that the universe is expanding. Planck’s data, which is expected to be very precise, will include measurements of the universe accurate to 1%, and produce evidence of gravitational waves whose fluctuations should reveal the expansion of the universe, as well as the nature and quantity of the universe’s “missing mass” (dark matter and dark energy). George Smoot is already impatient to see the results of Planck’s observations. Stay tuned, for in 2010, thanks to the observations of the satellite Planck, we may enter a new era in cosmology.

(*) These are two great names from the European scientific pantheon: the astronomer William Herschel (1738-1822) discovered infrared radiation and sketched out the shape of our Galaxy; the name of physicist Max Planck (1858-1947) is commemorated in Planck’s constant, a number which lies at the basis of quantum mechanics.

(**) FOCAL (Facility for Optical Calibration At Liège) designates a family of simulators of vacuum environments at the space center of Liège. The number that follows the title indicates the diameter of the simulator enclosure in meters. The FOCAL 5 simulator required the construction (in 1984) of a building for space testing in the Liège Science Park in Sart Tilman.

© Université de Liège - - April 14, 2021