Radio Astronomers Link Up With Japanese Satellite
Radio astronomy is set to make a giant leap into the future on 11 February with the launch of a Japanese radio telescope known as Muses-B. UK astronomers based at the Nuffield Radio Astronomy Laboratories, University of Manchester, Jodrell Bank, are among those looking forward to taking advantage of the significant improvement in spatial resolution which the satellite will allow.
The 830 kg satellite is scheduled for launch from Kagoshima Space Centre on the maiden flight of the M-V rocket. If all goes well, its 8-metre diameter umbrella-shaped antenna will unfold two weeks after launch. Following further tests, astronomical observations will begin about two months after launch. Muses-B is the first stage in an international programme known as VSOP - the VLBI (Very Long Baseline Interferometry) Space Observatory Programme. The project relies on the fact that the angular resolution, or amount of detail, which can be seen is determined by the size of the telescope. For example, by combining signals from two telescopes 500 km apart, astronomers can obtain results equivalent to those obtained from a single telescope whose dish is 500 km across. This is known as Very Long Baseline Interferometry. Until now, the length of this baseline has been limited by the size of the Earth. However, since Muses-B will be placed in an elliptical orbit whose altitude varies between 1,000 and 20,000 km, the effective size of the VSOP radio 'dish' will be three times bigger than Earth.
For VSOP, the data from the orbiting telescope will be combined with those from the ground-based instruments by first recording the signals on magnetic tape, then flying the tapes to special processing centres in Japan, USA and Canada. The long wavelength obser-vations made from the UK will involve the famous 76-metre Lovell dish at Jodrell Bank while shorter wavelength observations will be made with the 25-metre Mk2 telescope.
According to Dr Ralph Spencer of Jodrell Bank, Muses-B will improve the spatial resolution of radio telescopes by a factor of three. VSOP will enable resolutions of 1/1000 of an arcsecond at long wavelengths and one sixteenth of this (60 micro- arcseconds) at the shortest wavelengths that VSOP can operate. This gives a resolution fine enough to distinguish a football- sized object on the surface of the Moon and is almost 1,000 times better than the angular resolution of the Hubble Space Telescope at optical wavelengths.
"This extremely high angular resolution will provide a unique view of the most compact objects in the Universe," said Dr. Spencer. "It will be possible to penetrate the innermost regions of distant quasars, the most energetic objects known. The environs of the black holes supposed to exist in the centres of active galaxies will be probed at many wavelengths, including those of maser molecules OH and H2O. Stars are also a focus for rewarding investigations."
UK astronomers from Jodrell Bank are involved in six projects using VSOP, mainly related to the properties of so-called gravitational lenses. These are very distant, massive objects such as galaxies which bend the light arriving from even more distant quasars located along the same line of sight. The result is that several images of the same quasar become visible in optical or radio telescopes.
The projects involving UK astronomers are as follows: Dr Peter Wilkinson is PI for an investigation of three gravitational milli-lens candidates. If these exist, they are probably intermediate-sized between stars and galaxies with perhaps a few million solar masses. Verification of their existence would have important impli-cations for studies of the missing dark matter and the future expansion of the Universe.
Dr Michael Garrett (now at the Joint Institute for VLBI in Europe at Dwingeloo in the Netherlands) is PI for two projects. The first is an investigation of 0223+341, a possible small separation gravitational lens candidate. A comparison of the spectra and core-jet structure observed in the two distinct radio components of the object will allow the lensing hypothesis to be tested. The second is to produce high resolution radio images of the gravitational lens PKS 1830-211. The main aim is to search for the presence of very small but massive objects in the halo of the main lensing galaxy by looking for distortions in the lensed images.
Dr Richard Davis is PI for high resolution observations of the jet being emitted by quasar 3C273. The jet is currently known to extend out to distance of about 650 light years and to be travelling at close to the speed of light.
Co-investigator for a programme to observe the gravitational lens system B0218+357 is Dr Ian Browne. His team will be investigating the two lensed images, looking for structures which can help refine our understanding of the object which is causing the lensing.
Co-investigator for VLBI observations of the quasar 3C395 is Dr Tom Muxlow. His team intends to study the compact core of the quasar and its associated jets.
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