Comet ISON may be the greatest comet of this century as it skims through the atmosphere of the Sun this autumn. The Hubble Space Telescope has just the best image yet of this emissary from the outer darkness.
When the image was made, the coma, the ‘head’ of the comet, was approximately 5000 km (3100 miles) across, about the same size as the planet Mercury. A dust tail extended about 92 000 km (57 000 miles) further, about a quarter of the distance from Earth to the Moon.
Even at its great distance the comet is already active. Faint rays of sunlight are warming the surface of the solid, icy nucleus causing frozen volatiles to sublimate. A close examination of the dusty coma surrounding the nucleus reveals a strong jet of escaping vapour blasting more dust particles off the sunward-facing side of the comet’s nucleus. Note that the nucleus is too small to be seen in the image and is completely obscured by the coma anyway.
So far the comet has been highly active yet preliminary measurements from the Hubble images suggest that the nucleus of ISON is no larger than four miles (6.5 km) across which is remarkably small. Astronomers are using these images to accurately estimate the size of the nucleus to better predict the comet’s activity when it skims within a million kilometres of the Sun’s incandescent surface on 28 November 2013.
(Article by Colin Johnston, Science Communicator)