Cosmic vision

Published : Jul 03, 2009 00:00 IST

THE human eye can register only visible light a small wavelength band in the electromagnetic spectrum. It is blind to the rest of the wavelengths radio, microwave, infrared, ultraviolet, X-rays and gamma rays (see graphic 1). Because each kind of light reveals different natural phenomena, we see only a part of nature. The same is true of telescopes. Optical telescopes detect only visible light. A view of the cosmos as revealed by the most sophisticated and sensitive of them, such as the Hubble Space Telescope (HST; see Part I), is thus only partial.

For obvious reasons, the sky was first explored in the visible wavelengths. But with the evolution of technology, astronomy with other parts of the spectrum too became prominent initially in the radio, and later at high energies such as X-rays and gamma rays, and finally the sensitive infrared. Telescopes sensitive to different wavelengths reveal different facets of the universe.

On May 14, the European Space Agency (ESA) successfully launched two of the most ambitious astronomy missions to unveil the secrets of the darkest, coldest and oldest parts of the universe by mapping the cosmos in wavelengths outside the visible with unprecedented sensitivity and resolution. The two most complex science satellites ever built in Europe the far-infrared telescope Herschel and the cosmic microwave background mapper Planck were launched aboard the Ariane 5 ECA rocket from Europes launch complex at Kourou in French Guyana.

ESA also designed and built the two spacecraft under a common engineering programme and both underwent a joint development process aimed at optimising resources, by using the same industrial teams and shared designs of spacecraft components wherever possible. [It] is a crowning of some 20 years of hard work for the scientists who conceived these missions, the engineers who designed these satellites and the firms that built them, said ESAs Director-General, Jean-Jacques Dordain, soon after the launch.

About 26 minutes after launch, Herschel and Planck were released separately from the upper stage of the rocket, Herschel first and Planck two minutes later, on an escape trajectory toward their far-earth orbits at L2, a virtual point in space about 1.5 million kilometres from the Earth (about four times the Earth-Moon distance) in the direction away from the Sun, at velocities of about 10 km/second. L2 is one of the so-called Lagrangian points where all the gravitational forces acting between the Earth and the Sun cancel each other out, thus resulting in a gravitationally stable point. L2 can therefore be used by the spacecraft to hover where it will orbit the Sun at the same rate as the Earth. In fact, L2 is rapidly establishing itself as a pre-eminent location for advanced space probes. A spacecraft then does not have to make constant orbits of the Earth, which result in it passing in and out of the Earths shadow, causing it to heat up and cool down and distorting its view.

Currently, both the satellites are on a highly elongated orbit and are reportedly in nominal condition. When they are in the vicinity of L2, which will be about 60 days from launch, the two will be put in separate orbits around L2. The optimum orbit for Herschel is a quasi-halo orbit with an average distance of 800,000 km from L2, and that for Planck is a Lissajous orbit with an average distance of 400,000 km. (The former resembles a halo orbit, which is a three-dimensional orbit that does not technically orbit the Lagrange point itself but travels in a closed, repeating path near the Lagrange point. The latter is a quasi-periodic natural orbital motion around an L-point.) (see graphic 2). Since the Launcher set the satellites directly on the path to L2, no injection manoeuvre was necessary for Herschel; the satellite was close to its halo orbit right from separation. However, since Plancks orbit amplitude is half that of Herschel, a manoeuvre was required to reduce its amplitude.

Orbits around L2 are dynamically unstable. Small departures from equilibrium occur, which require orbit maintenance manoeuvres every month. Being the natural motion of a satellite around L2, Lissajous orbits require less momentum change than halo orbits for station keeping. Once in their respective operational orbits, satellites will be undisturbed by thermal, magnetic and radiation interference caused by the Sun, the Earth and the Moon as these will always be located behind the payload that is shielded. While Herschel will observe pre-selected celestial targets, Planck will carry out a continuous survey of the complete sky.

According to the Big Bang picture of the cosmos, in the past the universe was far denser and hotter than it is today, and that it started to cool and expand about 13.7 billion years ago a process that is still on . But this picture is far from complete. Questions such as what triggered the Big Bang, how stars and galaxies came to be formed, what is the density of matter in the universe and what is the true nature of this matter, how will this matter drive universes evolution in the future and will the process come to an end or will go on forever, remain unanswered. The Herschel and Planck space observatories are designed to answer to some of these questions.

Planck is designed to map tiny irregularities in the fossil radiation left over from the very first light of the universe, emitted shortly after the Big Bang. The sensitivity of Planck reaches the limits of what can be experimentally observed. It will thus enable a peek into the early universe as well as a study of the elusive dark matter and dark energy that today constitute major cosmological puzzles. Herschel, equipped with the largest mirror ever launched into space, will observe in mostly unexplored part of the electromagnetic spectrum so as to study the birth of stars and galaxies as well as dust clouds and planet-forming discs around stars. In addition, it will also serve to look for the presence of water in remote parts of the universe (see chart).

About 380,000 years after the Big Bang, the universe had cooled down to about 3,000 degrees Kelvin (273.15 K corresponds to 0 degree Celsius). This was cold enough for hydrogen atoms to bind with free electrons and form neutral hydrogen. Before this, there were freely streaming atomic nuclei and electrons a plasma state which interacted strongly with radiation. As radiation and matter were strongly coupled, no light was emitted from this dense early universe. Now in the absence of free electrons, which have the highest interaction cross-section with photons, light could travel freely for the first time. But as the cooling and expansion continued, this light was stretched to higher wavelengths and today it is in the microwave, whose wavelength is typically a few millimetres, corresponding to radiation from a cold blackbody at 2.7 K (about 270 oC). This all-pervasive free streaming first glow is called the Cosmic Microwave Background (CMB) radiation. Though this relic radiation was detected in 1964, and many ground- and space-based experiments have studied it, cosmologists have not yet been able to extract all the information it holds. Planck is a survey telescope that is designed to provide the most precise and reliable data on CMB.

The CMB comes from every direction with almost equal brightness. But the measurement of its apparent temperature all over the sky has revealed that tiny differences do exist from place to place. These CMB anisotropies, which can be as small as one part in a million, are interpreted as the leftover imprints of matter from an epoch when matter and radiation were strongly coupled. At that time, matter already contained the seeds of the huge structures in the large scale the galaxies, galaxy clusters, and so on that we see today. The anisotropies perhaps harbour information about these clots of matter of the early universe. Much of this information will be contained in the precise shape and intensity of these anisotropies, which Planck will measure with unprecedented accuracy.

The period up to a millionth of a second after the universe was born is full of uncertainties. According to the widely accepted scenario, a very brief inflation process took place at the beginning of this epoch when the universe expanded extremely quickly by a huge factor after which it expanded and cooled down much more slowly. If true, the in-homogeneities in CMB will reflect some details of this event and Planck should be able to provide the most reliable information about it.

The first evidence of CMB anisotropy was obtained by the Russian space mission Relikt, launched in 1983, whose first results were published in January 1992. In April 1992, data from the COBE (Cosmic Background Explorer) satellite of the United States space agency, National Aeronautics and Space Agency or NASA, also detected these anisotropies. (Nevertheless, the Nobel Prize in Physics for 2006 was awarded only to COBEs team of American scientists.) The measurements indicated that over angular scales larger than 10o, the CMB temperature varies by about one part in 100000 from the average value of 2.73 K. In 2003, COBEs successor, NASAs Wilkinson Microwave Anisotropy Probe (WMAP) obtained maps of these anisotropies that have started to reveal their detailed properties (Frontline, September 14, 2001).

The objective of Planck is to complete the picture by mapping these features as fully and as accurately as possible. Simulations of observations of the CMB show the dramatic improvement that can be achieved by Plancks increased angular resolution and sensitivity. Planck will be able to recover about 15 times more relevant information than WMAP. Planck was selected as the third Medium-sized Mission (M3) of ESAs Horizon 2000 Programme and was formerly called COBRA/SAMBA. After the mission was approved in 1996, it was renamed after the German scientist Max Planck (who won the Nobel Prize for Physics in 1918). The total cost of the mission is about 700 million ($974 million). This includes the spacecraft, its scientific payload, the launch and the operations.

Planck will study the CMB by measuring its temperature all over the sky. Its large telescope will collect the light from the CMB and focus it onto two highly sensitive broadband (25-1000 GHz) radio detectors, which will convert the signal data into temperature. Its principal objective is to produce whole sky maps in different frequency channels. The Planck instruments are also designed to provide information on the polarisation state of the CMB that will also be useful for the improved estimation of cosmological parameters, such as the density and Hubble constant with a few per cent uncertainty. Plancks angular resolution, a measure of sharpness of vision, is higher than any of the earlier space-based CMB missions. Its angular resolution is three times better than WMAP, its detectors have the ability to detect signals 10 times fainter and its wavelength coverage allows it to examine wavelengths ten times smaller.

The spacecraft consists of two main elements: a warm satellite bus, or service module, and a cold payload module consisting of the two scientific instruments and the telescope. The octagonal service module houses data handling systems and subsystems essential for the spacecraft to function and to communicate with the Earth, and the electronic and computer systems. At the base of the service module is a flat circular solar panel, which is permanently illuminated and generates power for the spacecraft and also protects it from direct solar radiation. Three reflective thermal shields isolate the service module from the payload module.

The baffle is an important component of the payload module and is a part of the passive cooling system before active cooling takes over. It surrounds the telescope, limiting the amount of stray light interference from the Earth, Sun and Moon. It also helps radiate excess heat into space, cooling the instruments and telescope to a stable temperature of about -223 oC (50 K). As a result, the temperature difference between the warm and cold ends of the spacecraft is an astounding 300 degrees C. The active system is a three-stage refrigeration chain that brings down the temperature of the detectors to as low as 0.1 K, just one-tenth of a degree from the lowest temperature in the universe, the absolute zero (-273.17 oC). Indeed, this is a key requirement of Plancks instruments so that the heat of the detectors themselves does not swamp the signal of the CMB, which is at 2.7 K. These detectors may, in fact, be the coldest points in space. The cooling system had presented one of the biggest technological challenges in the design of Planck.

The Planck detectors are specifically designed to detect microwaves in nine wavelength bands from the radio to the infrared 0.3 mm to 1 cm. This includes wavelengths that have not been observed before with Plancks precision and resolution. Wide coverage is required in order to be able to differentiate between scientifically useful signals and the many other undesirable noises arising from many other objects, such as our own galaxy, that emit radiation at the same wavelengths as the CMB. Seen in the microwave range, the CMB is only 1 per cent as luminous as Earth, so stray light is a particular concern for any space-based telescopes observing the CMB in the microwave. Locating Planck strategically at L2 helps avoid that. Planck will use several of its wavelength channels to measure signals other than the CMB to obtain the cleanest CMB signal ever.

As it is monitoring signals other than the CMB, the Planck sky survey will be used to study in detail the very sources of emission that contaminate the CMB signal and will result in a wealth of information on the properties of extragalactic sources, and on the dust and gas in our own galaxy, with unprecedented accuracy in the microwave making it the best astrophysical observatory ever in the microwave. One specific result will be the all-sky survey of the Sunyaev-Zeldovich (SZ) effect in many thousands of galaxy clusters. The SZ effect is the contamination of CMB radiation due to the scattering of high-energy electrons with the low energy CMB photons when some of the electron energy is transferred to the CMB (inverse Compton Scattering). Observed distortions can provide information about galaxy clusters.

The telescopes 1.9 m x 1.5 m primary mirror is large for any space mission, but it is still smaller than the companion missions Herschel telescope. Made of CFRP (carbon fibre reinforced plastic), coated with a thin reflective layer of aluminium (reflectivity over 99.5 per cent), the mirror weighs only 28 kg (of the telescope mass of 205 kg) but is robust enough to withstand stresses of launch as well as the temperature difference between 300 K at launch and 40 K during operations.

Planck carries two complimentary scientific instruments, the High Frequency Instrument (HFI) and the Low Frequency Instrument (LFI). The HFI is designed for high-sensitivity measurements of the diffuse radiation permeating the sky in all directions at six wavelengths 3.6 mm-0.3 mm (corresponding to 84 -1000 GHz frequency). It includes an array of 52 bolometric detectors placed in the focal plane of the telescope and cooled to 0.1 K. Bolometers convert radiation into heat, and are capable of detecting and measuring small amounts of thermal radiation. The LFI is designed to measure with high sensitivity microwave at three wavelength bands in the range of 11.1 mm to 3.9 mm (27-77 GHz). It consists of an array of 22 tuned radio receivers located at the focal plane that will operate at 253 oC. These receivers will convert the detected microwave radiation from the sky into an estimate of its intensity at each frequency.

Planck will slowly rotate and sweep a large swath of the sky each minute. In about 15 months, it will cover the sky fully twice over. It will operate continuously and mostly autonomously, and will send data acquired each day to the ground station at New Narcia, Australia, where ESAs deep-space antenna is located, over a three-hour period.

The prime contractor for the 4.2 m x 4.2 m spacecraft, which weighed 1.9 tonne at launch, was Thales Alenia Space (France), leading a consortium of European industries with Tales Alenia Space (Italy) being responsible for the service module. The telescope mirrors were fabricated under collaboration between ESA and a Danish consortium of institutes led by the Danish Space Centre. HFI was built by a consortium of 20 institutes, led by the Institut d Astrophysique Spatiale, Orsay, France. LFI was built by a consortium of more than 20 institutes, led by the Instituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Italy. Several European agencies and the NASA funded the fabrication of the instruments. But, sometime in 2002, LFI ran into some financial trouble causing some delay, which was subsequently resolved.

Mission Planck will nominally end in 2010 after 15 months of routine science operations but a one-year extension is possible. In 2012, the processed scientific data will be archived at ESAs European Space Operation Centre (ESOC) in Darmstadt, Germany, and made available to the global astronomical community.

Cosmologists often call the epoch before first light and galaxy formation the dark ages. Since no light was emitted, no current instrument can peek into that era. However, not long after the Big Bang, possibly small clusters of stars began to form with light atoms only, which were possibly very massive and, therefore, had a short life and died a violent death. But these first stars perhaps created heavy atoms from which the next-generation stars were made. The clusters of stars would have merged resulting in more stars and more raw material future stars. Vast collections of stars became galaxies which may have collided and merged to form bigger galaxies causing intense bursts of star formation, and the expanding universe became filled with the galaxies of the universe that we see.

Space observatories have contributed a great deal to our present understanding. The HST, for example, has provided fundamental insights into galaxy formation and evolution. However, there are a lot of gaps in this scenario of galaxy formation and evolution. Astronomers hope to be able to answer the many open questions that still remain with the help of an advanced and sensitive space observatory such as Herschel that can observe in the infrared.

Why the infrared? Although our eyes cannot see infrared, we can feel it as infrared radiation in heat. All objects, even the coldest ones even ice emit a certain amount of heat. In fact, stellar objects with surface temperatures of about 2000 oC, which is cold compared to the Sun with a surface at 5500 oC, radiate most of their energy in the infrared. The universe actually abounds in cold objects ageing stars, planets and dust discs around stars, asteroids, brown dwarfs (failed stars) and stars being born none of which generally shines brightly enough in the visible wavelengths and could not be observed directly until the advent of infrared detectors. The cool universe is best studied in the infrared. Herschels sensitivity will enable their detailed observation.

Infrared radiation also tells us a lot more about the young, distant universe. This is because the expansion of the universe stretches every wavelength of light travelling through it (called redshift). Much of the visible and ultraviolet light emitted in the early universe would have now been stretched into the infrared. It is known that when the universe was half its present age, star formation was much more vigorous than it is today. So the rate of star formation varies with time. The Infrared Astronomical Satellite (IRAS), for example, made the completely unexpected discovery of galaxies the infrared-dominated galaxies emitting almost all their energy in the infrared. These are star-burst galaxies that are forming stars at about 1,000 times faster than the Milky Way today. Herschel will be able to observe these galaxies through the cosmic epochs when most of the stars that ever existed in the universe were being formed.

The far-infrared and sub-millimetre wavelengths that Herschel will study are crucial to understand the physical properties and processes of the interstellar medium. Low-metallicity dwarf galaxies exhibit dust and gas properties different from metal-rich galaxies such as the Milky Way. Herschel will provide the opportunity to study extreme low-metallicity environments that have not yet experienced repeated recycling. The data will help understand primordial conditions in the interstellar medium and star formation in the young universe. By observing in the infrared we can study how things get formed, the very early steps, because formation processes very often happen in cool and dusty places, explains Goran Pilbratt, ESAs Herschel Project scientist.

Dust is the bane of optical astronomy as it blocks the view of many interesting objects such as dust-enshrouded stars. The universe is full of dust made of microscopic particles of diverse materials such as carbon, silicon, water ice, minerals, frozen carbon monoxide, organic compounds and silicates whose sizes are usually less than one micrometre. Since the visible light has wavelength of the same order, it is easily blocked. But dust is transparent to longer wavelength radiation like infrared. And in the far-infrared, which Herschel is designed to probe, the glow from the dust as dust absorbs starlight and re-emits it in the infrared itself becomes visible.

The Andromeda galaxy, located about two million light years away, is one of the galaxies closest to us and a good example of how infrared can reveal secrets. It is considered a typical spiral galaxy. But ESAs Infrared Space Observatory (ISO), launched in 1995, showed that it is made of several concentric rings. The rings are made of dust at about 260 oC, much colder than previous estimates. The chemical make-up of dust clouds and other regions can also be studied by looking at the spectra of their molecules. Radiation emitted by atoms and molecules during their rotation and vibration is usually in the infrared. In dust clouds, more complex compounds such as organic molecules are often found. Infrared astronomy in the past has made many discoveries of such complex molecules in space.

The reason for going to space to do all this is for two important reasons: the Earths atmosphere blocks most infrared wavelengths; and, the Earth also produces its own infrared radiation that would swamp the celestial infrared.

Herschel was conceived to build on the legacy of previous infrared satellites with its substantially larger telescope and extended spectral coverage. Space-based infrared astronomy was pioneered by IRAS, a joint venture between the Netherlands, the United Kingdom and the U.S., which was launched in 1983. IRAS resulted in the first infrared maps of the entire sky at four wavelengths. ESAs ISO followed IRAS and it was the first general-purpose infrared satellite. It improved on IRAS discoveries and operated until 1998.

The Japanese mission Akari was the next in which ESA participated. During 2006-07, it mapped more than 94 per cent of the sky in greater detail than IRAS. NASAs Spitzer Space telescope, a general-purpose infrared observatory with a slightly bigger telescope than ISO, is currently in orbit. Herschel marks not just the next step but a giant leap in infrared astronomy. Herschel also bridges the gap between the wavelengths seen by previous infrared missions and those studied by radio telescopes on ground. In the process, it will discover a large number of unknown objects within and without our galaxy.

The telescope is named after the British astronomer William Herschel who discovered infrared radiation in 1800 while studying the Sun. It was originally named FIRST (Far Infra-red and Sub-millimetre Telescope). With a total mission cost of about 1100 m ($1530 m), it is the largest infrared telescope ever launched. Its 3.5-m diameter primary mirror, the telescopes light collector, is four times bigger than any previous infrared space telescope and one-and-a-half times the HST. The captured light will be directed towards a smaller secondary mirror. The two mirrors work together focussing the light and directing it to the three on-board instruments, where the light is detected and analysed and the results recorded by an on-board computer.

The size of the primary mirror determines the telescopes sensitivity as the larger light collecting surface will enable detection of fainter objects as well. With a significantly larger reflector than before, Herschel will provide astronomers with the best view yet of the universe at far-infrared and sub-millimetre wavelengths in much more detail. The primary mirror has been constructed almost entirely of silicon carbide. It has been made out of 12 segments brazed together to form a monolithic mirror which was machined and polished to the required thickness (3 mm), shape and surface accuracy within thousandth of a millimeter.

The three instruments of Herschel that turn the telescope from a mere large light collector into a powerful infrared eye are: the Heterodyne Instrument for the Far Infrared (HIFI), the Photoconductor Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE). The three instruments are complimentary. Each instrument is designed to study gas and dust, but at different temperatures and states. The large wavelength range covered enables the instruments to see the entire process of star formation, from the earliest stages of condensation to the moment at which the protostar emerges from its cocoon, and is born.

HIFI, a high-resolution spectrometer, is designed to observe unexplored wavelengths. It covers two bands (157-212 micrometre and 240-265 micrometre), and uses superconducting mixers as detectors. Its spectral resolution is the highest in the wavelength range it covers. HIFI can produce high resolution spectra of thousands of wavelengths simultaneously. It can reveal an unprecedented level of detail and identify individual molecular species in the enormity of space and study their motion, temperature and other physical properties. It was built by a nationally funded consortium led by the Netherlands Institute for Space Research (SRON).

PACS consists of a colour camera and an imaging spectrometer. With its wavelength range (55-210 micrometre), PACS is the first instrument capable of obtaining the complete image of a target at once. The PACS spectrometer has a lower resolution than HIFI but can still see young galaxies and star-forming gas clouds. It was built by a nationally funded consortium led by the Max Planck Institute for Extraterrestrial Physics, Germany. SPIRE also consists of a colour camera and an imaging spectrometer, and it covers a range of wavelengths (194-672 micrometre) complementary to PACS. These wavelengths have never been studied before. SPIRE also performs photometry (measuring the intensity of the radiation) on astronomical objects in three bands, centred at 250, 350 and 500 micrometre, simultaneously. It was built by a nationally funded consortium led by Cardiff University.

The 7.5-m high and 4.5-m diameter Herschel spacecraft is basically a tall tube. Like Planck, it also comprises three sections. First is the 3.5-m diameter mirror and the sunshade that protects it from direct solar illumination. The sunshade is also covered by solar cells to provide power to the spacecraft. The mirror sits atop a cryostat a giant thermos flask that houses the instrument detectors. The cryostat provides a stable cooling environment for the instrument package to within a fraction of absolute zero ( -273.15 oC), which makes the detectors as sensitive as possible. It contains 2,300 litres of superfluid helium at 1.65 K (271.5 oC). Further cooling, down to 0.3 K, is required for PACSs and SPIREs bolometric detectors. The instruments, the detectors and the cryostat constitute the second section, the payload module. The role of the cryostat is fundamental because it determines the observatorys lifetime. Superfluid helium evaporates at a constant rate. It is expected to completely evaporate about four years after launch.

The third section is the service module located below the payload module. Like in Planck, it houses instrument electronics and the components responsible for satellite function, such as communication hardware. The service module also houses the payload electronics that do not need cooling and provides the necessary subsystems. The prime contractor for building Herschel was also Thales Alenia Space, France, with Astrium, Germany responsible for the Extended Payload Module and Thales Alenia Space, Italy, responsible for the service module. The spacecraft weighed 3.4 tonnes at launch, which included the 2,300 litres of liquid helium.

The operational lifetime for routine science observations is three years. The Herschel Science Centre (HSC) located at ESAs European Space Astronomy Centre (ESAC) in Villanueva de la Canada, Spain, is responsible for the science from Herschel and is the centre for all interaction with worldwide astronomical community. That infrared astronomy is becoming increasingly important for the future is evident from the joint NASA plans to launch the James Webb Space Telescope (JWST), an even more sensitive infrared telescope designed to look into the very farthest reaches of the universe, in 2014.

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