Science: Witness to the early universe
The Hubble Space Telescope is to be superseded by something even more powerful.
Friday 14 August 1998
Since Galileo, astronomy has leapt forward whenever a new and more powerful telescope has become available. One of the best examples was when the Hubble Space Telescope (HST) launched in April 1990. Astronomers obtained images showing that areas in the sky that had previously looked blank are in fact filled with thousands of distant, very faint galaxies.
Although astronomers plan to use the HST until the year 2010, the American National Aeronautics and Space Administration (Nasa), and the European Space Agency (ESA) are already planning Hubble's successor, the Next Generation Space Telescope (NGST). Since it will be much more powerful, astronomers expect the instrument to open up a new window of discovery. "It will see the first significant burst of star formation in the universe," says Gerry Gilmore, an astronomer at Cambridge University who has vast experience of using the HST.
The present plant is to launch the NGST in 2007. Europe has committed itself to contributing $200m towards the total price of about $1bn. Although ESA's science programme is short of funds, its readiness to participate is largely based on the extremely good experience over the collaboration with the HST, whereby European astronomers are obtaining more than 20 per cent of total available observation time.
The HST still looks like a conventional telescope - a tube with an opening at one end and a parabolic mirror at the other end - but the design of the NGST will be radically different. It can dispense with the tube, because it will be placed in a point in space much darker than the relatively low orbit of the HST.
It will circle the Sun in an orbit about a million miles wider than that of the Earth, but it will do this at the same speed as the Earth. The gravitational pulls of the Sun and Earth conspire to create a stable spot about a million miles away on the Sun-Earth axis, which will an be exploited by the new telescope. The main mirror will also be much larger than that of the HST: current plans call for a diameter of 8 metres - so will be able to gather about 10 times more light than the HST.
The NGST will observe distant celestial objects in visible light, but also infrared light. The human eye cannot detect infrared light, but we can feel it as the warmth of the Sun. Optical mirrors can focus infrared light just as well as visible light. However, the colder a telescope is, the higher the sensitivity of its detectors for infrared light, and therefore the NGST will be equipped with a heat shield to protect it from the Sun's radiation. This will cool it to a temperature of -243C (just 30C above absolute zero).
Infrared light is useful for observing dark objects, such as dust and protoplanetary rings around stars, because of their heat radiation. "Most of these things are typically at room temperature, and most of the energy is coming out at the near infrared, which is exactly where NGST will work well,'' says Gilmore.
However, astronomers will also need to use infrared light if they want to observe galaxies born shortly after the Big Bang about 12 billion years ago, when the universe began. Since the universe is still expanding, these galaxies are extremely distant and are moving away from the Earth at tremendous speeds.
Just as the pitch of a train's horn moving away from us at high speed is lowered - a phenomenon know as the Doppler effect - the wavelength of visible light from distant galaxies becomes extended, and so moves into the infrared. Astronomers call this effect "redshift".'
The NGST "will see directly the first generation of stars forming; it will see the blue light coming out from them that will be redshifted into the near infrared,'' says Gilmore. The infrared capabilities of the HST are much more limited.
Because the shift in wavelength is an indicator of velocity, the NGST will be also be able to study the rotation of stars and gases around the nuclei of galaxies. Because these stars move fast, the galaxies are suspected to contain massive, invisible black holes at their centres. The NGST "will probably be best at all at mapping black holes in normal galaxies,'' says Gilmore. "It will be able to tell us how many black holes there are.''
The challenges of placing an 8- metre mirror in space will be daunting. Although the optics are simple and will not require `'adaptive'' technology - there is no gravity to deform the mirror and no atmosphere to distort arriving light - the mirror will have to be sturdy yet lightweight. The designers are therefore planning to make the mirror out of beryllium.
Current plans give this mirror a central, 4-metre section and two folding sections to extend the central mirror to a diameter of 8 metres. It is in this respect that European industry will play an important role: "It is quite clear that in some areas, particularly in light-weight mirror technology and polishing, Europe is really playing a leading role,'' says Piero Benvenuti, an NGST project scientist at the European Space Agency. Benvenuti says that Nasa is already rewarding subcontracts for polishing test mirrors to a French company that has been contracted to build the four 8.2-metre mirrors of the largest telescope on Earth, now under construction in Chile by the European Southern Observatory (ESO). "Europe has the technological potential to participate at a very high level,'' says Benvenuti. And Peter Stockman, an NGST project scientist at the Space Telescope Science Institute in Baltimore, from where the NGST will be controlled, thinks it is possible that Europe may supply all the optics for the NGST: "European optics manufacturing is superb,'' he says.
Roger Davies, professor of astronomy at Durham University, who is a leading supporter of the next generation telescope, says the project has far-reaching implications for the study of the cosmos. `'We expect to see the birth of stars and galaxies. We will witness the act of creating the very stuff we are made of. This telescope will see back in time to when the universe was just a tenth of the age it is today.''
The NGST - "son of Hubble" - the most advanced telescope ever built, will be a sophisticated time machine for looking back at the violent birth of the universe.
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